LPTENS–07/16
April 2007
A Wave-function for Stringy Universes∗
Costas Kounnas,1 Nicolaos Toumbas2 and Jan Troost1
1 Laboratoire de Physique Théorique, Ecole Normale Supérieure,†
24 rue Lhomond, F–75231 Paris Cedex 05, France
2 Department of Physics, University of Cyprus, Nicosia 1678, Cyprus.
Abstract
We define a wave-function for string theory cosmological backgrounds. We give
a prescription for computing its norm following an earlier analysis within general
relativity. Under Euclidean continuation, the cosmologies we discuss in this paper
are described in terms of compact parafermionic worldsheet systems. To define
the wave-function we provide a T-fold description of the parafermionic conformal
field theory, and of the corresponding string cosmology. In specific examples, we
compute the norm of the wave-function and comment on its behavior as a function
of moduli.
∗ Research partially supported by the EU under the contracts MRTN-CT-2004-005104, MRTN-
CT-2004-512194 and ANR (CNRS-USAR) contract No 05-BLAN-0079-01 (01/12/05).
† Unité mixte du CNRS et de l’Ecole Normale Supérieure associée à l’université Pierre et Marie
Curie 6, UMR 8549.
http://arxiv.org/abs/0704.1996v3
1 Introduction
Our goal in this paper is to embed the Hartle-Hawking no-boundary proposal for a wave-
function description of the quantum state of the universe [1][2] in a string theoretic frame-
work. The Hartle-Hawking proposal pertains in particular to de Sitter–like universes in
general relativity. A partial list of interesting recent work on related topics is [3][4][5] [6][7][8].
One motivation for embedding the Hartle-Hawking proposal into string theory is that it
provides us with a calculable quantity in de Sitter-like compactifications of a quantum theory
of gravity. These quantities are hard to come by (see e.g. [9][10] and references thereto).
Two essential properties which a cosmological background must fulfill in order to admit
a wave-function description under the no-boundary proposal are the following. First the
cosmology must be spatially closed. More importantly, the cosmology should admit a con-
tinuation to a positive definite Euclidean geometry that is compact and has no boundaries
or singularities. The most familiar example is the case of n-dimensional de Sitter space, dSn,
where these properties are satisfied. In global coordinates, the dSn metric is given by
ds2 = R2(−dt2 + cosh2 t dΩ2), (1)
where dΩ2 is the metric on a round unit (n − 1)-sphere and R is the radius of curvature.
The spatial slices of constant time t are (n− 1)-spheres of radius R cosh t. We can rotate to
Euclidean signature by setting t = iτ = i(π/2− θ), upon which we obtain an n-sphere Sn of
radius R. The Euclidean continuation is a compact smooth manifold.
In a field theoretic framework, the quantum state of a de Sitter cosmology can be ex-
pressed as a functional of fields, including both matter fields and metric fluctuations, on a
spatial slice of time-reversal symmetry. For the reversal t→ −t, this is the slice t = 0 in the
de Sitter space dSn. That slice is also the equator θ = π/2 of the corresponding Euclidean
sphere Sn. Imagine cutting de Sitter space along this slice and gluing smoothly one half of
it to half a sphere Sn. Under the Hartle-Hawking proposal, we express the wave-function as
a Euclidean path integral over half the sphere Sn with the condition that the metric gij and
the matter fields, collectively denoted by φ, take specific values (hij , φ0) on the boundary
equator θ = π/2:
Ψ(hij, φ0) =
[dg][dφ]e−SE(g,φ). (2)
No other boundary condition needs to be specified due to the compactness of the Euclidean
manifold. Here, SE is the Euclidean gravitational action in the presence of matter fields and
a positive cosmological constant Λ.
The norm of the wave-function is given by the full Euclidean path integral on Sn. It can
be computed in the semi-classical approximation by evaluating the Euclidean action for a
given solution to the classical equations of motion. One solution is empty de Sitter space of
radius R ∼ Λ−1/2. In this approximation, and in the case of four dimensions, the norm is
given by [1]:
||ΨHH ||2 ∼ e
3λ , (3)
where the dimensionless parameter λ is proportional to the cosmological constant:
λ = 2GΛ/9π .
The compactness of the Euclidean manifold ensures that the full path integral is free of
any infrared divergences. However the field theory in question is non-renormalizable, and to
go beyond the semi-classical approximation, we need to impose an ultraviolet cutoff. One
way to deal with the ultraviolet ambiguities is to embed the calculation in a string theoretic
framework, where we expect the ultraviolet divergences to be absent. Unfortunately there
are no known classical de Sitter solutions in string theory to begin with. Therefore, we seek
other cosmological backgrounds which are exact solutions to string theory and for which we
can generalize the Hartle–Hawking computation.
To this end, notice that any tachyon free, compact Euclidean string background provides
us with a finite, calculable quantity, namely the string partition function Zstring. Associated
to the classical string background is a two-dimensional worldsheet conformal field theory
(CFT). At the perturbative level, the string partition function can be computed as usual as
a sum of CFT vacuum amplitudes over compact worldsheets of all topologies. Our proposal
is that when such Euclidean string backgrounds admit a continuation to a Lorentzian cosmol-
ogy, the Hartle-Hawking construction can be generalized with the norm of the wave-function
given by
||Ψcosm.||2 = eZstring . (4)
We will motivate this formula by working out specific examples in string perturbation theory.
As we will explain, the relevant string partition function has to be thermal.
Given the discussion above, a first candidate to consider is a Euclidean model for which
the two-dimensional CFT is of the form SU(2)k × K, the first factor corresponding to an
SU(2) Wess-Zumino-Witten (WZW) model at level k and the second factor K corresponding
to a suitable internal compact conformal field theory. The WZW factor is equivalent to a
sigma model on a 3-sphere of radius (kα′)1/2 and with k units of NSNS 3-form flux through
the sphere. The dilaton field is constant and by choosing this to be small we can apply
string perturbation theory. Unfortunately, however, the continuation to Lorentzian signature
results in a dS3 cosmology with imaginary flux, and it is not clear whether such a Lorentzian
background is physical. (See [11] for an alternative non-compact, time-like Liouville model
for which the SU(2) WZW factor describes the internal space.)
The only known string theory example which satisfies all the criteria we described so far is
based on the parafermionic SU(2)|k|/U(1) coset model [12], which can be realized as a gauged
SU(2) WZW model at level |k|. We consider Euclidean backgrounds corresponding to a two-
dimensional CFT of the form SU(2)|k|/U(1) × K where K is again an internal compact
conformal field theory. Such a Euclidean background admits a Lorentzian continuation to
a cosmological background belonging to a class of models studied in [13][14], and which are
described by two-dimensional CFTs of the form SL(2, R)−|k|/U(1) × K. To avoid having
to deal with the tachyonic instabilities of bosonic string theory, we consider solutions of this
form in superstring theory. The total central charge must be ctot = 15 (ĉ = 10) in order for
worldsheet (super-)gravitational anomalies to cancel. When we fix the internal conformal
field theory K, the level |k| is determined by anomaly cancellation.
The non-trivial time-dependence of the cosmology necessarily breaks space-time super-
symmetry. As in the de Sitter case, the Euclidean path integral can be interpreted as a
thermal ensemble. Thus from the point of view of the Euclidean N = 2 worldsheet super-
conformal system, space-time supersymmetry will be broken by specific boundary conditions,
analogous to the thermal co-cycles that appear in the partition function of superstring the-
ories on flat space at finite temperature [15]. For large level |k|, the effective temperature of
the models is of order T ∼ 1/
|k|α′ [14][28]. In this paper, we will explore some low level
|k| models. In order for the corresponding cosmological wave-function to be computable in
string perturbation theory, the effective temperature must be below the Hagedorn tempera-
ture. A Hagedorn temperature would signal a phase transition, as proposed in [15][16][17].
We will construct explicitly low level |k| models for which the effective temperature is below
the Hagedorn temperature and so string perturbation theory can be applied.
It is well known that the geometric sigma model approach to the parafermionic coset
model (and to the corresponding Lorentzian cosmology) leads to a metric with curvature
singularities and strong coupling. However, the underlying CFT is perfectly well behaved
at these apparently singular regions, and by using T-duality a weakly coupled description
of these regions can be obtained [18]. Using this fact, we construct an almost geometrical
description of the CFT in terms of a compact, non-singular T-fold [19][20] with a well–
defined partition function. These considerations allow us to define the wave-function of the
Lorentzian cosmology.
Our paper is organized as follows. In section 2, we review properties of the two-dimensional
SL(2, R)−|k|/U(1) × K conformal field theory that corresponds to a cosmological back-
ground. It is the analogue of the de Sitter universe. In section 3, we describe how to ana-
lytically continue the cosmology to a compact Euclidean space-time described at the string
level by a two-dimensional parafermionic model of the form SU(2)|k|/U(1) × K. Then, we
discuss in section 4 how to obtain an almost geometrical description of these backgrounds
in terms of T-folds. We discuss in sections 5 and 6 how to calculate a wave-function and its
norm for the cosmology. In section 7 we discuss the thermal nature of the wave-function. In
section 8 we apply the definition of the wave-function to some particular compact models
and for which perturbation theory can be used to compute its norm. Finally we discuss
interpretations of the results in the concluding sections.
2 The cosmological solution
In this section, we review in some detail the cosmological solution of string theory which is
based on an SL(2, R)/U(1) gauged WZW model at level k [13].
We can define a WZW conformal field theory on the group manifold SL(2, R), at least
classically. The worldsheet action is given by
d2zTr(g−1∂gg−1∂̄g) +
Tr(g−1dg ∧ g−1dg ∧ g−1dg), (5)
where Σ is the worldsheet Riemann surface, M is a 3-manifold whose boundary is Σ and g
is an element of SL(2, R).
For concreteness, we parameterize the SL(2, R) group manifold as follows
with ab+ uv = 1. The conformal field theory has an SL(2, R)× SL(2, R) global symmetry.
We choose to gauge an axial U(1) subgroup under which g → hgh. In particular, we consider
the non-compact U(1) subgroup generated by
δg = ǫ
g + ǫg
. (7)
Infinitesimally, we have the transformations δa = 2ǫa, δb = −2ǫb, δu = δv = 0. To gauge
this U(1) symmetry, we introduce an Abelian gauge field and render the action invariant.
The action is quadratic and non-derivative in the gauge field, and so this can be integrated
out in a straightforward way [21].
In the region 1−uv > 0, we can use the gauge freedom to set a = b and integrate out the
gauge field. The resulting action is expressed in terms of gauge invariant degrees of freedom
only, and it turns out to be
S = − k
∂u∂̄v + ∂v∂̄u
1− uv
, (8)
while a non-trivial coupling to the worldsheet curvature is generated corresponding to a
dilaton background [21]. This action can be identified with a non-linear sigma-model action
with background metric
ds2 = −kα′ dudv
1− uv
. (9)
The non-trivial dilaton is given by
e2Φ =
1− uv
. (10)
The metric (9) is a Lorentzian metric whose precise causal structure, however, depends
on the sign of k. For positive level k, u and v are Kruskal-like null coordinates of a 2-
dimensional black hole. In this case, the time-like coordinate is given by u + v, and the
metric has space-like singularities in future and past times at uv = 1.
For negative level k, one obtains a cosmological solution [13]. It consists of a singularity-
free light-cone region, and there are (apparent) time-like singularities in the regions outside
the light-cone horizons. Indeed, for negative level k we may set u = −T +X and v = T +X
and the metric becomes
ds2 = |k|α′−dT
2 + dX2
1 + T 2 −X2
. (11)
The surfaces of constant time T intersect the singularities at X = ±
1 + T 2. Even though
the singularities follow accelerated trajectories, their proper distance remains finite with
respect to the string frame metric
L = (|k|α′)
1+T 2
1+T 2
1 + T 2 −X2
= π(|k|α′)
2 . (12)
So with respect to stringy probes, the cosmology is spatially closed.
The singularity-free light-cone region is the region T 2 − X2 ≥ 0 (or uv ≤ 0). The
future part of this region describes an expanding, asymptotically flat geometry with the
string coupling vanishing at late times. See e.g. [11][13][14][23][24][25][26][27][28][29] for
some discussions of these types of models. To see this, we parameterize the region uv ≤ 0
with coordinates (x, t) such that
u = −tex, v = te−x (13)
and the metric becomes
ds2 = |k|α′−dt
2 + t2dx2
1 + t2
, (14)
while the dilaton field becomes
e2Φ =
1 + t2
. (15)
The scalar curvature is given by
|k|α′(1 + t2)
. (16)
Initially the curvature is set by the level |k| and it is positive. No matter how small the
level |k| is, asymptotically the scalar curvature vanishes. An observer in this region never
encounters the singularities. These are hidden behind the visible horizons at T = ±X .
However signals from the singularities can propagate into the region uv < 0, and therefore
influence its future evolution.
Thus when |k| is small the early universe region t ∼ 0 is highly curved, with curvature
of order the string scale. In this sense, it is similar to a big-bang cosmology. Despite the
regions of large curvature, this cosmological background has a well defined CFT description
and can be described in a string theoretic framework.
The cosmological background can also be realized as a solution of superstring theory by
generalizing the worldsheet theory to a superconformal SL(2, R)/U(1) model. The central
charge of the superconformal SL(2, R)/U(1) model at negative level k, is given by
c = 3− 6
|k|+ 2
, ĉ = 2− 4
|k|+ 2
In superstring theory, we must tensor it with other conformal field theories so as to satisfy
the condition ĉtot = 10 for worldsheet gravitational anomalies to cancel.
An interesting case considered in [13] is the case where we add two large (however com-
pact) free super-coordinates (y, z) together with a compact, superconformal CFT of central
charge δĉ = 6 + 4/(|k| + 2). The resulting background is a four dimensional cosmological
background whose metric in Einstein frame is given by
ds2E = |k|α′(−dt2 + t2dx2) + (1 + t2)(R2ydy2 +R2zdz2). (18)
This is an anisotropic cosmology which at late times however, and for large Ry ∼ Rz,
asymptotes to an isotropic flat Friedman-Robertson cosmology.
The cosmological region t2 = −uv ≥ 0 is non-compact, and when Ry,z are large it has the
desired four-dimensional interpretation. This is so irrespective of how small the level k is. In
the region uv > 0 (t2 < 0), sigma-model time-like singularities appear at uv = 1 (t2 = −1).
As we propose later in this work, these singularities are resolved at the string level, since the
structure of the space-time manifold is replaced by a non-singular T -fold.
The string partition function depends crucially on the extra ĉ = 6+4/(|k|+2) supercon-
formal system, which is taken to be compact. In contrast to the four dimensional part defined
by (t, x, y, z), for the internal, Euclidean ĉ = 6+4/(|k|+2) system, the naive six-dimensional
interpretation, which is valid for large level |k| with curvature corrections of order 1/(|k|α′),
is not valid for small values of |k| [18]. For example, for |k| = 2, the system can be taken to be
a seven-dimensional torus. In general, small |k| implies that the generalized curvatures (i.e.
including dilaton gradients etcetera) are large and the moduli/radii are small. We remind
the reader of the example of the SU(2)k=1 Wess-Zumino-Witten model which is equivalent
to a (one-dimensional) compact boson at self dual radius. For large level |k|, however, the
sigma model manifold is a large three-dimensional sphere with NSNS 3-form flux.
3 The Euclidean continuation
Let us consider the region 1− uv ≥ 0 of the two-dimensional cosmology, and set u = −T +
X, v = T +X . We can rotate to Euclidean signature by setting T → −iTE . The Euclidean
continuation is a disk of unit coordinate radius parameterized by Z = X+ iTE , Z̄ = X− iTE
such that |Z|2 ≤ 1. The metric (9) becomes
ds2 = |k|α′ dZdZ̄
1− ZZ̄
= |k|α′dρ
2 + ρ2dφ2
1− ρ2
and the dilaton
e2Φ =
1− ZZ̄
1− ρ2
, (20)
where we have also set Z = ρeiφ with 0 ≤ ρ ≤ 1. The singularity becomes the boundary
circle ρ = 1.
The radial distance of the center to the boundary of the disk is finite, but the circum-
ference of the boundary circle at ρ = 1 is infinite. Geometrically the space looks like a bell.
This Euclidean background corresponds to a well defined worldsheet conformal field theory
based on an SU(2)/U(1) gauged WZW model at level |k|.
From the point of view of the WZW worldsheet theory, the Euclidean continuation can
be understood as a double analytic continuation as follows. We parameterize the SL(2, R)
group manifold as in equation (6). Let us also set a = X̃ − T̃ , b = X̃ + T̃ so that the group
element becomes
X̃ − T̃ X − T
−X − T X̃ + T̃
X̃2 +X2 − T̃ 2 − T 2 = 1. (22)
This parameterization shows that the SL(2, R) group manifold is a 3-dimensional hyper-
boloid. Then it is clear that upon the double analytic continuation T → −iTE , T̃ → −iT̃E
the group element becomes the following SU(2) matrix
−Z̄ W̄
with WW̄ + ZZ̄ = 1. After the analytic continuation we also have that a → W = X̃ +
iT̃E , b→ W̄ = X̃ − iT̃E .
A useful parameterization of the SU(2) group manifold for our purposes is
W = cos θeiχ, Z = sin θeiφ (24)
and the metric on S3 in these coordinates becomes
ds2 = dθ2 + sin2 θdφ2 + cos2 θdχ2. (25)
The ranges of the angles are as follows 0 ≤ θ ≤ π, 0 ≤ χ, φ,≤ 2π.
The original global SL(2, R) × SL(2, R) symmetry naturally continues to the SU(2) ×
SU(2) global symmetry of the resulting SU(2) WZW model. The non-compact U(1) axial
symmetry subgroup that we gauge continues to a compact U(1) subgroup generated by
δg = iǫ
g + iǫg
, (26)
which amounts to the following infinitesimal transformations δW = 2iǫW , δW̄ = −2iǫW̄ and
δZ = δZ̄ = 0. In the parameterization (24), the U(1) symmetry corresponds to shifts of the
angle χ. Gauging this symmetry results in the SU(2)/U(1) coset model. In the Euclidean
set-up, we take the level |k| to be an integer for the WZW model to be well-defined.
After the analytic continuation described, we end up with the action (see e.g. [31] for a
review):
d2z∂θ∂̄θ + tan2 θ∂φ∂̄φ
+cos2 θ(∂χ + tan2 θ∂φ + Az)(∂̄χ− tan2 θ∂̄φ+ Az̄).
In the Euclidean theory the gauge freedom can be fixed by setting the imaginary part of
W (equivalently the angle χ) to zero. The equations of motion for the gauge field can then
be used to integrate the gauge field out. This amounts to setting the last term in (27) to
zero and producing a dilaton e2Φ = e2Φ0/ cos2 θ. We end up with a sigma model action with
metric
ds2 = |k|α′(dθ2 + tan2 θdφ2) (28)
which is equivalent to the metric (19) after the coordinate transformation Z = sin θeiφ.
The curvature singularity occurs at θ = π/2. The procedure of fixing the gauge χ = 0
and using the equations of motion to integrate the gauge field out is not valid near θ = π/2,
since it results into singular field configurations on the worldsheet. However, the full action
(27) is perfectly well behaved at θ = π/2. To see this, we expand the Lagrangian in (27)
around θ = π/2. Setting θ = π/2− θ̃, we obtain that
d2zφFzz̄ +O(θ̃
2), (29)
where we expressed the action in terms of manifestly gauge invariant degrees of freedom.
The leading term in this expansion describes a simple topological theory, which shows that
an alternative, non-geometric description of the theory can be given including the region
near θ = π/2. We return to this point later on.
From the form of the action near θ = π/2, we also learn that the U(1) symmetry cor-
responding to shifts of the angle φ is quantum mechanically broken to a discrete symmetry
Z|k|. This is because compact worldsheets can support gauge field configurations for which
Fzz̄ = 2πin, with n an integer, and such configurations must be summed over in the full
path integral. It is clear then that the path integral is only invariant under discrete shifts of
the angle φ: δφ = 2πm/|k|.
This breaking of the classical U(1) symmetry to Z|k| is in accordance with the algebraic
description of the SU(2)/U(1) coset in terms of a system of Z|k| parafermionic currents
ψ±l(z), l = 0, 1 . . . |k| − 1 [with ψ0 = 1, ψ†l ≡ ψ−l = ψk−l], of conformal weights hl =
l(|k| − l)/|k| (see also [32]). These satisfy the OPE relations
ψl(z)ψl′(0) = cll′z
−2ll′/|k|(ψl+l′(0) + . . . )
ψl(z)ψ
l (0) = z
−2hl(1 + 2hlz
2T (0)/c+ . . . ) (30)
which are invariant under the Z|k| global symmetry: ψl → e2πil/kψl. Here T is the energy
momentum tensor of the parafermions, c the central charge (which is the same as the central
charge of the coset model) and the coefficients cll′ are the parafermionic fusion constants [12].
In the infinite level |k| limit, the conformal weights of the parafermion fields become integers.
In this limit the sigma model metric is flat, and we recover the full rotational invariance of
flat space [33]. The system can be also generalized to an N = 2 superconformal system by
tensoring the Z|k| parafermions with a free compact boson as described in [36].
Finally we can check that the central charge remains the same after the analytic contin-
uation. Indeed, it is the very fact that the central charge of the conformal field theory is
smaller than the central charge corresponding to two macroscopic flat dimensions that codes
the de Sitter nature of the two-dimensional cosmology.
4 The cosmological T-fold
The parafermionic T-fold
It is interesting to take a closer look at the geometry that we associate to the parafermionic
model SU(2)|k|/U(1). As we already discussed, we describe it in terms of a metric and
dilaton profile:
ds2 = |k|α′(dθ2 + tan2 θdφ2)
cos θ
, (31)
where φ ∼ φ+2π and θ takes values in the interval [0, π/2]. This description breaks down near
θ = π/2. Nevertheless, the parafermionic conformal field theory is perfectly well-behaved,
and we can wonder whether there is a more appropriate, almost-geometrical description. We
argue that such a description exists in terms of a T-fold.
To obtain it, we perform a T-duality along the angular direction φ on the geometry
described above:
ds2 = |k|α′dθ2 +
cot2 θdφ̃2
sin θ
. (32)
By changing variables θ̃ = π/2− θ, we see that this is equivalent to:
ds2 = |k|α′dθ̃2 +
tan2 θ̃dφ̃2
cos θ̃
. (33)
This description is therefore at weak curvature (apart from an orbifold–like singularity) and
weak coupling near θ = π/2. Moreover, we can identify it as a Z|k| orbifold of a vectorially
(or axially) gauged SU(2)/U(1) coset. Indeed, it is true for the parafermionic theory that
the T-dual and the Z|k| orbifold give two models with identical spectrum due to the coset
character identity χj,m = χj,−m (see e.g. [30][31] for reviews).
We now use these facts to give an almost geometrical description of the parafermionic
theory, in terms of a T-fold [19][20]. We use the description in terms of the first geometry
(31) near θ = 0. We cut it just past θ = π/4, where the radius of the circle is
|k|α′.
We glue it to the T-dual geometry which we consider near θ̃ = 0, or θ = π/2, and which
we cut just past θ̃ = π/4, where we have radius
α′/|k|. We glue the circles (and their
environments) using the T-duality transformation described above. In the gluing process, it
is crucial to realize that we glue a patch with a direction of increasing radius to a T-dual
patch which in the same direction has decreasing radius. That gives us the parafermionic
T-fold. The associated partition function is (see e.g. [31] for a review) :
χj,m(τ)χj,m(τ̄ ). (34)
One aspect of the model that is rendered manifest by the T-fold description is the breaking
of the U(1) rotation symmetry to a discrete Z|k| symmetry, due to the Z|k| orbifolding. This is
consistent with our previous discussion of the breaking due to worldsheet instantons. The T-
fold yields an almost-geometrical picture of the symmetry breaking. The T-fold description
is indeed everywhere regular modulo a benign orbifold singularity.
The cosmological T-fold
In the case of the two-dimensional cosmology as well, we can obtain a regular T-fold de-
scription of the target space of the conformal field theory. We recall that under T-duality
(the metric can be obtained by analytically continuing the metric (32) in the direction φ̃),
the light-cone and the singularities get interchanged. Consider the cosmology, and cut it at
a hyperbola at radius
|k|α′, in between the light-cone and the time-like singularities in
the Penrose diagram (see the upper part of figure 1). Consider then its T-dual, and cut it
along a similar line. Glue the two parts of the T-dual cosmologies along these cuts to obtain
the T-fold cosmology. The description we obtain is particularly nice as we no longer need
a microscopic origin of a would-be source associated to the time-like singularities, nor do
we need to define boundary conditions associated to them. There is no singularity in, nor
is there a boundary to the T-fold cosmology. Indeed, the almost-geometrical description is
very much like dS2, which we can think of as a hyperboloid embedded in three-dimensional
space. The difference is that the T-fold cosmology has two patches glued together via a
Figure 1: The analytic continuation of the T-fold. The upper part of the diagram shows
the two T-dual descriptions of the cosmology in which the horizon and the (apparent) sin-
gularities are exchanged (in bold blue). The (striped black) cut along which they are glued
is indicated, as well as the (thin black) line along which the cosmology is cut to obtain a
space-like slice (see later). Analytic continuation then gives rise to the lower part of the
figure, in which we have sketched the T-fold description of the parafermionic conformal field
theory. In bold blue we have the center and the boundary of the disk, and (in black stripes)
the T-dual circles along which we glue.
T-duality transformation (instead of an ordinary coordinate transformation in the case of
two-dimensional de Sitter space).
In figure 1 we show how the T-fold description of the parafermions and the two-dimensional
cosmology continue into one another after analytic continuation.
5 Defining the wave-function of the universe
Later on, we will consider string theory backgrounds which are product models and in which
one factor consists of the two-dimensional cosmology discussed in sections 2, 3 and 4. For
these models, we wish to define a wave-function of the universe in string theory following
ideas of [1] which define a wave-function of dSn universes within a field theoretic context.
We consider a time-reversal symmetric space-like slice of the cosmology, within the bound-
aries of the (seeming) singularities. See figure 2. This is the slice T = 0. In the past of
the space-like slice, we glue half of the target space of an SU(2)/U(1) coset conformal field
theory – a half disk. By the analytic continuation discussed in the previous section, this
Figure 2: The continuous gluing of the half-disk into the cosmology, when cutting the cos-
mology along a space-like slice, and analytically continuing. The figure should be viewed as
a simplified version of the previous T-fold picture.
gluing is continuous in the backgrounds fields, and moreover in the exact conformal field
theory description.
A crucial feature of the proposal of [1] for the definition of the wave-function of the
universe is that the corresponding Euclidean space is without boundary. In our set-up as
well, the Euclidean conformal field theory has a target with no boundary. It is important
in this respect that we have obtained an almost-geometric description of the parafermionic
conformal field theory1. It is intuitively clear from the T-fold description given in the previous
section that the parafermionic theory does not have a boundary2. When we cut the Euclidean
T-fold into half, it is clear (from figure 1) that we can glue the boundary of that half-T-fold
into the initial surface of the cosmological T-fold.
Thus we have determined the precise gluing of T-folds necessary in order to define a
wave-function depending on initial data. We now define the wave-function of the universe by
performing a “half T-fold” Euclidean path integral over all target space fields with specified
values on the boundary:
Ψ[h∂ , φ∂, . . . ] =
[dg][dφ] . . . e−S(g,φ,... ) , (35)
where the path integral is such that the metric, the dilaton and all other space-time fields
1A traditional description of the target space as a disk, which is singular,would lead to the faulty conclusion
that the target space has a boundary.
2Since a T-fold is non-geometric, one needs to define the concept of boundary precisely. We believe that
a reasonable definition will match our intuition.
satisfy g = h∂, φ = φ∂, . . . on the boundary of the half T-fold that we glued into the
cosmological solution. The path integral above can in principle be performed off-shell, in
a second-quantized string field theory context, where we may also express it as an integral
over a single string field Φ. (See e.g. [34] for a concise review).
Let us be more specific. The initial space-like slice of the T-fold cosmology has two
patches. On each patch, we define a boundary metric h1 and h2, and similarly for other
fields. The boundary metrics satisfy the condition that on the overlap of the patches, they
match up to a T-duality transformation, symbolically: h1
∂1∩∂2 = T (h
∂1∩∂2). This is the way
in which we can specify boundary data precisely. In the following, we do not emphasize this
important part of the definition of the path integral further, not to clutter the formulas.
In principle, a T-fold path integral can be computed as follows. Consider again the two
patches. Each patch has a non-singular geometric description. Over each patch the path
integral reduces to an ordinary field theory path integral, and can be performed in the usual
way giving rise to a functional of boundary data. The full path integral can be obtained by
integrating the two functionals together over data that belong to the common boundary of
the two patches. Since at the common boundary of the patches their fields are related by
a T-duality transformation, to do the final integral we would need to perform a T-duality
transformation on one of the two functionals.
We remark here that this particular feature of definitions of path integrals over T-folds
with boundaries is generic. The above description is easily extended to a generic description
of T-fold boundary data. Although we do not need a general prescription in this paper,
we believe it would be interesting to develop the path integral formalism for T-folds with
boundary further.
The prescription for the wave-function of the universe we outlined above should have an
analogue, via the relation between string oscillators and the target space fields, to a first
quantized prescription. Notice that the initial-time data allow multi closed string configura-
tions. Summing over histories that lead to them, would allow worldsheets with boundaries
(and other topology features), including disconnected ones. The wave-function would take
the form
Ψ[X∂(σ, τ)] =
topologies
[dX ]e−S[X(σ,τ)], (36)
where the worldsheet path integrals are performed over string configurations X(σ, τ) that
satisfy a specified boundary condition at given values of the zero-modes of the string con-
figuration, i.e. at a given position of the target space. The equivalence of these descriptions
is far from obvious, but it is made plausible by the fact that for two-dimensional string
worldsheets, the first quantized description automatically comes with a prescription for the
proper weighting of interaction vertices. The initial-time closed string configurations could
be specified in terms of macroscopic loop operators discussed for example in [35]. The first
quantized prescription considers fluctuations around a given background. A full second
quantized prescription also integrates over backgrounds as in general relativity [1].
The wave-function so defined is hard to compute, although it may be obtained presumably
for very particular boundary conditions. An example would be boundary conditions that
are fixed by taking a Z2 orbifold that folds over the disk onto itself – in that case, one may
be able to compute the value of the wave-function for a particular argument. In order to
understand better some global properties, we again follow [1] and concentrate on calculating
the norm of the wave-function.
6 The norm of the wave-function
The norm of the wave-function is easier to compute. It is given by the following calculation:
||Ψ||2 =
[dΦ∂]
half T−fold
[dΦ]e−S(Φ) ×
conj half T−fold
[dΦ]e−S(Φ)
T−fold
[dΦ]e−S(Φ), (37)
where we have expressed it as a string field theory path integral in terms of a string field Φ.
The final integral is an integral over all possible string field configurations on the Euclidean
T-fold. No boundary conditions need to be specified.
We can do this calculation by considering the fluctuations around an on-shell closed string
background, in a first quantized formalism:
||Ψ||2 =
topologies
[dX ]e−S[X(σ,τ)] (38)
where X(σ, τ) is any mapping from the string worldsheet into the target space. The sum
is over all closed worldsheet topologies, and includes a sum over disconnected diagrams. In
fact it is equal to the following exponential of a sum of connected diagrams:
||Ψ||2 = exp(Ztotal), (39)
where the function Ztotal is the total string theory partition function, which is defined as a
sum over Euclidean worldsheet topologies:
Ztotal =
ZS2 + ZT 2 + g
sZgenus=2 +
g2g−2s Zgenus=g. (40)
Therefore, to evaluate the norm of the wave-function perturbatively, we need to evaluate
the partition function for string theory on the Riemann surfaces of genus 0, 1, 2, . . . and
add their contributions with the appropriate power of the string coupling constant. The
first contribution is akin to the tree level contribution in ordinary gravity, the second to the
one-loop contribution, etc.
7 Thermal nature of the wave-function
A natural way to perform the Euclidean path integral in equation (35) over half the space
is as follows. The origin X = 0 in one T-fold patch (and similarly for the other), divides
the T = 0 slice into two halves: the left half corresponding to X < 0 and the right part
corresponding to X > 0. We denote the boundary data on X < 0 by φL and on X > 0 by
φR. See figure 3. By dividing the space into angular wedges spanning an overall angle equal
to π, we can evaluate the path integral in terms of the generator of angular rotations. This
generator is given by the analytic continuation of iHω, where Hω = i∂ω is the Hamiltonian
conjugate to “Rindler” time in the region uv > 0 of the Lorentzian cosmology. Indeed in
this region, we may set u = ρe−ω, v = ρeω, with the string frame metric and dilaton given
ds2 = |k|α′
dρ2 − ρ2dω2
1− ρ2
e2Φ =
1− ρ2
. (41)
In this patch, the background metric is static, invariant under time translations, and the
dilaton field is space-like. Rotating to Euclidean signature amounts to setting ω = −iφ.
Figure 3: The thermal interpretation of the wave-function is obtained by thinking of the
path integral as being performed along angular wedges from an initial (right) to a final (left)
configuration.
So Rindler time translations correspond to angular rotations in the Euclidean space. As
we have already discussed, only discrete angular rotations are true symmetries of the string
theory background.
The boundary data can then be viewed as specifying initial and final conditions for the
path integral evolution. This is clearly reviewed for the case of flat Rindler space and black
hole spaces in [37]. In particular, the path integral measures the overlap between the data
on the right φR, evolved for a Euclidean time π, and the data specified on the left φL (see
figure 3), and it can be written as an amplitude
Ψ(φL, φR) = 〈φL|e−πHω |φR〉. (42)
If we integrate over φL we obtain a thermal density matrix appropriate for the Rindler
observer [37]
[dφL]Ψ(φL, φR)Ψ
∗(φL, φ
R) = 〈φ′R|e−2πHω |φR〉, (43)
with dimensionless temperature Tω = 1/2π. The norm of the wave-function is given by the
trace
||Ψ||2 = Tre−2πHω (44)
and so it can be interpreted as a thermal space-time partition function. The genus-1 string
contribution is a thermal one-loop amplitude.
In the full Euclidean path integral equation (37), the contributions of the fermionic fields
are positive. To perform the full path integral over the whole T-fold, we divide it into angular
wedges spanning an angle equal to 2π. Since the space has no non-contractible cycles, the
space-time fermionic fields have to be taken anti-periodic in the angular variable and they
contribute positively to the path integral.
The T-fold patches are glued along the hyperbola ρ = 1/
2 (see section 4). Near this
region, the curvature is low for large enough level |k|. Thus for large level |k|, we may use the
metric equation (41) to conclude that observers moving near the region ρ ∼ 1/
2 measure
a proper temperature T ∼ 1/(2π
|k|α′). In the cosmological region uv ≤ 0, there is also an
effective temperature of the same order as a result of particle production [14][28]. For small
level |k|, we need a string calculation to deduce the proper temperature of the system.
8 Specific Examples
As we discussed above in order to derive the wave-function of the cosmology, we need to
compute the total string partition function for the corresponding Euclidean background.
When conformal field theories are compact, the genus-zero contribution to the total string
partition function vanishes. This is because the spherical string partition function is divided
by the infinite volume of the conformal Killing group. This fact is a first important difference
with the calculation in general relativity where the classical contribution is non-zero. In
perturbative string theory the leading contribution is the genus-1 amplitude.
The Euclidean examples we shall describe here in detail belong to the family of ĉ = 10
superconformal, compact systems. In order for them to admit a Lorentzian continuation to
a cosmological (time-dependent) background, space-time supersymmetry must be broken.
Moreover, the models must be free of tachyons. The presence of tachyonic modes would
indicate that the system undergoes a phase transition. The only known examples with the
above properties are of the form
SU(2)|k|
× K ←→
SL(2, R)−|k|
× K , (45)
where we indicated the analytic continuation from the Euclidean to the Lorentzian space-
time. The level |k| can be taken to be small.
As we already discussed, the relevant genus-1 string amplitude has to be thermal. The
total superstring model has transverse central charge equal to c = 12 (or ĉ = 8). As a
consequence, it has a Hagedorn transition at the fermionic radius RH =
2α′. In order for
the genus-1 string amplitude to be finite, the physical temperature of the model has to be
below the Hagedorn temperature: T < TH = 1/(2π
2α′). Let us give an argument that this
can be realized for any |k| ≥ 2. In writing the norm of the wave-function as a thermal space-
time partition function, the role of the Hamiltonian is taken up by the generator of rotations
on the disk. For the superconformal SU(2)/U(1) model, the corresponding U(1) current is
at level |k| + 2 [36]. Thus we expect the physical temperature of the model to be set by
the radius associated to this isometry generator, namely
(|k|+ 2)α′. The corresponding
temperature is given by
(|k|+ 2)α′
and is below the Hagedorn temperature for any positive (integer) level |k|. We will find that
this temperature arises naturally in a level |k| = 2 model below. At level |k| = 0, where
the minimal model has zero central charge, and consists only of the identity operator (and
state), the cosmology disappears. When we reach the Hagedorn transition, the cosmology
becomes so highly curved that it is no longer present in the string theory background.
8.1 Compact models
In a first class of specific examples that we will discuss in this section, we choose the level
|k| = 2, and we take the internal conformal field theory K to be:
K = T 2 ×
i=1,...,7
SU(2)ki
where all ki’s are taken equal to 2, so that ĉK = 9 (representing the central charge equivalent
of nine flat directions). In the sequel, we set α′ = 1.
For this choice, the supersymmetric characters of the whole system are defined in terms
of eight level k = 2 parafermionic systems (which are nothing but eight real fermions ψi and
eight bosons φi compactified at the self-dual radius R = 1), and also a complex fermion ΨT
and a complex boson ΦT for the torus T
2. The N = 2 superconformal operators TF , J are:
i=0,1,...7
2φi + iΨT∂ΦT
i=0,1,...7
∂φi +ΨT Ψ̄T . (48)
It is convenient to pair the (0, 1), (2, 3), (4, 5), (6, 7) systems respectively in order to
obtain four copies of ĉ = 2 systems. For the first copy we define the bosons H0, H1 at radius
2 (or R = 1√
, fermionic T -dual points):
(H0 +H1), φ1 =
(H0 −H1) (49)
and similarly for the others. Then the currents are given by
i=0,2,4,6
i(Hi+Hi+1) + ψi+1 e
i(Hi−Hi+1) + i∂ΦT e
J = i∂H0 + i∂H2 + i∂H4 + i∂H6 + i∂HT , (50)
where i∂HT = ΨT Ψ̄T , which is also defined at the fermionic point. Observe that the N = 2
current is given in terms of H0, H2, H4, H6 and HT only and is normalized correctly for a
system with ĉ = 10.
The N = 2 left-moving characters of a particular ĉ = 2 system (e.g of the one containing
H0), are expressed in terms of the usual level-2 Θ-functions 3:
η(τ)3
2 ΘH0
α+2H0
β+2G0
γ0−2H0
δ0−2G0
3γ0−4H0
3δ0−4G0
where the arguments (γ0, δ0) and (4H0, 4G0) are integers. The later exemplifies the chiral
Zk+2-symmetry of the superconformal parafermionic characters (k + 2 = 4 in our case).
Similar expressions are obtained for the other three ĉ = 2 parafermionic systems by replacing
(γ0, δ0) with (γi, δi), and (H0, G0) by (Hi, Gi), i = 1, 2, 3.
The global existence of the N = 2 superconformal world-sheet symmetry and thus the
existence of the left space-time supersymmetry imply
(HT , GT ) +
i=0,1,2,3
(Hi, Gi) = ǫ mod 2 , (52)
3Our convention for the level-2 Θ function is Θ [
iπτ(n+
)2+2iπ(n+
and similarly for the right supersymmetry. The arguments (ǫ, ǭ) define the chirality of the
space-time spinors. A simple choice is to set (HT , GT ) = (0, 0), (Hi, Gi) = (H,G) for i = 0, 1
and (Hi, Gi) = (−H,−G) for i = 2, 3 (and similarly for the right arguments). Then if
(ǫ, ǭ) = (1, 1), space-time supersymmetry is broken. For the other choices there is some
amount of supersymmetry preserved. In this class of models the only remaining possibility
consistent with the global N = 2 super-parameterization consists of shifts on ΦT of the T
torus.
Z4 orbifold models
Using the chiral Z4 symmetry defined above and its subgroups, we can obtain four classes
of models:
(H,G) =
(h, g), where (h, g) = integers, M = 1, 2, 3, 4. (53)
In particular, if we orbifold by Z4 (M = 1 or M = 3 ), the ψ0 parafermion decouples from
the rest, especially from ψ1. This is clear since in this case, the arguments of Θψ0 and Θψ1
become independent.
Initially, the arguments (γi, δi) are taken to be identical for the left- and right-moving
characters. In this case the modular invariance of the partition function is manifest. Indeed,
using the periodicity property of Θ-functions
∣ = |Θ [γδ ]| , ǫ, ζ integers, (54)
and orbifolding by Z4, (M = 1), the genus-1 modular invariant partition function becomes:
dτdτ̄
(Imτ)2
η(τ)12η̄(τ̄ )12
h,g=0,1,2,3
γ0,δ0
γ0−h2
δ0− g2
γ1,δ1
γ1−h2
δ1− g2
γ2,δ2
γ3,δ3
(α,β)
eiπ(α+β+ǫαβ) Θ
(ᾱ,β̄)
eiπ(ᾱ+β̄+ǭᾱβ̄) Θ̄
β̄− g
. (55)
The arguments (α, β) and (ᾱ, β̄) are those associated to the N = 2 left- and right-moving
supercurrents. If (ǫ, ǭ) = (1, 1), supersymmetry is broken. For all other choices this partition
function is identically zero and there is some amount of supersymmetry preserved. To obtain
the above result we have used the fact that the contribution of the superconformal ghosts
cancel the oscillator contributions of the T 2 supercoordinates (ΦT ,ΨT ). This is the reason
for choosing the Z4 not to act on ΨT , having set (HT , GT ) = (0, 0). The only remnant from
the torus contribution is the Γ2,2 lattice, which can be possibly shifted by (Lh/2, Lg/2) with
either L = 0 or L = 1, 2, 3 (as we will see later).
To proceed we need to identify and insert the thermal co-cycle S
q , (α+ ᾱ)
p, (β+β̄)
associated to
the time direction of the cosmology:
q, (α+ᾱ)
p, (β+β̄)
= eiπ( p(α+ᾱ)+ q(β+β̄) ), (56)
where p and q are the lattice charges associated to the compactified Euclidean time. Here,
Fα = (α + ᾱ) and Fβ = (β + β̄) define the spin of space-time particles: Fα = 1 modulo
2 for fermions and Fα = 0 modulo 2 for bosons. To impose this co-cycle insertion, it is
necessary to rewrite the partition function in a form that reveals the charge lattice (p, q) of
the Euclidean time direction. To this end, it is convenient to separate the partition function
into the “untwisted sector” (h, g) = (0, 0) and “ twisted sectors” (h, g) 6= (0, 0),
dτdτ̄
(Imτ)2
Zunt +
(h,g)6=(0,0)
Ztwist
 . (57)
To isolate the relevant (p, q) charge lattice, we use the identity
|Θ [γδ ]|
(m,n)
2 |m+τn|
Imτ eiπ(mγ+nδ+mn) , R2 =
. (58)
Although in the above identity the radius is fixed to the fermionic point R2 = 1/2, we note
that the modular transformation properties are the same for any R2, and in particular for
the dual-fermionic point with R2 = 2.
Using the above identity for the conformal block involving the parafermions ψ0,1 and the
H1 field, we obtain
(γ,δ)
∣Θψ0,1 [
2 |ΘH1 [
(γ,δ)
(m1,n1),(m2,n2)
|m1+τn1|
−πR22
|m2+τn2|
Imτ eiπ((m1+m2)γ+(n1+n2)δ+m1n1+m2n2) (59)
where R21 = R
2 = R
2 = 1/2. Since the arguments (γ, δ) do not appear elsewhere (in equation
(55)), summing over them forces (m1+m2) and (n1+n2) to be even integers. This constraint
can be solved if we take
m1 = p1 + p2, m2 = p1 − p2, n1 = q1 + q2, n2 = q1 − q2 (60)
so that,
(γ,δ)
∣Θψ0,1 [
2 |ΘH1 [
= Γ1,1(R+) Γ1,1(R−) =
(p1,q1)
2 R+ e
−πR2+
|p1+τq1|
(p2,q2)
2 R− e
−πR2−
|p2+τq2|
 (61)
with R2+ = R
− = 2R
2 = 1. Therefore, the partition function of the bosonic part of
parafermions factorizes in two Γ1,1 lattices, both of them with twice the initial radius squared.
The charge lattices (p1, q1) and (p2, q2) are associated to the ψ0 and ψ1 parafermions.
We can see this as follows. Consider the left-charge operators which are well defined in the
untwisted sector:
Q+ = i
dz(ψ0ψ1 + ∂H1), Q− = i
dz(ψ0ψ1 − ∂H1) (62)
and similarly for the right-moving ones (Q̄±). Then,
(Q + Q̄)+ = m1 +m2 = 2p1, (Q− Q̄)+ = n1 + n2 = 2q1
(Q+ Q̄)− = m1 −m2 = 2p2, (Q− Q̄)− = n1 − n2 = 2q2 (63)
where we have used the constraint (60). We identify the charges (p1, q1) as the momenta
that enter in the thermal co-cycle, and associate the lattice to the Euclidean time direction.
Before we proceed further, let us stress the following point. We started with a diagonal
modular invariant combination and with initial radii R21 = R
2 = R
2 = 1/2. The anti-diagonal
choice implies that the initial values for the radii are at the fermionic T-dual points, namely
R21 = R
2 = R
2 = 2. Notice that in all we perform two T-dualities simultaneously so that
we remain in the same type II theory. Thus the conformal block, equation (59), can be
replaced with the T-dual one with R2 = 2.4 For the anti-diagonal choice, the radii of the
corresponding factorized lattices are given by R2+ = R
− = 4 instead of unity for the diagonal
combination.
Thus in the untwisted sector, Zunt has to be replaced with
Zunt −→ Zthermalunt =
(p1,q1)
(α,β),(ᾱ,β̄)
Zthermalunt
q1, α+ᾱ
p1, β+β̄
eiπ(p1(α+ᾱ)+q1(β+β̄)). (64)
Performing a similar factorization for the remaining three copies of ĉ = 2 superconformal
parafermionic blocks, we can write the thermal untwisted contribution in a compact form:
Zthermalunt =
ImτΓ9,9
η12η̄12
(α,β),(ᾱ,β̄)
Γ1,1(R+)
eiπ(α+β+ǫαβ) Θ
eiπ(ᾱ+β̄+ǭᾱβ̄) Θ̄
The Γ9,9 lattice factor is composed of a product of lattices: the initial Γ2,2 lattice of the
torus T 2 with radii Ry, Rz, the Γ1,1 lattice of the first parafermionic block at radius R−, and
the product of three pairs Γ1,1(R
+) Γ1,1(R
−) for the other three parafermionic blocks. The
Γ1,1(R+) lattice is the thermally shifted lattice
Γ1,1(R+)
(p1,q1)
2 R+ e
−πR2+
|p1+τq1|
Imτ eiπ((α+ᾱ)p1+(β+β̄)q1). (66)
Its coupling with the space-time spin structure breaks space-time supersymmetry so that
both bosons and fermions give positive contributions to the thermal partition function. The
radius R+ sets the temperature of the system: 2πT = 1/R+. The form of the thermal
coupling in equation (65) is similar to the one that appears in the familiar flat type II
superstring theories at finite temperature. The difference here is that the temperature is
fixed. Since we have succeeded to factorize out the thermal lattice, we can now treat all other
radii parameterizing the Γ9,9 lattice as independent moduli. To obtain the four dimensional
4In Gepner’s formalism [36] the anti-diagonal combination corresponds to exchanging (m, m̄)→ (m,−m̄).
interpretation we discussed in section 2, we take the radii Ry,z to be large keeping all other
ones small.
As we already remarked, there are only two choices consistent with the cosmological
interpretation of the partition function corresponding to the two values of the radius R+.
For the diagonal choice we have a radius R2+ = 1 corresponding to a temperature 2πT = 1/R+
higher than Hagedorn: 2πTH = 1/RH = 1/
2. This model is tachyonic and so unstable in
perturbation theory. For the second anti-diagonal choice R2+ = 4, and the temperature is
below Hagedorn: 2πT = 1/2 < 2πTH . This is precisely the temperature that we gathered
from general arguments, equation (46) for level |k| = 2. That model gives rise to a well
defined, integrable partition function and a finite norm for the wave-function at one loop.
The integral is difficult to perform analytically but it can be estimated. We will not carry
out this computation here.
The remaining part consists of the twisted sectors of the theory, (h, g) 6= 0. Here we shall
find new stringy phenomena associated with the fact that we are orbifolding the Euclidean
time circle. Since this is twisted, the thermal co-cycle has to be extended consistently. It
takes the general form, valid for both the untwisted and the twisted sectors:
(q+h), (α+ᾱ)
(p+g), (β+β̄)
= eiπ( (p+g)(α+ᾱ)+ (q+h)(β+β̄) ). (67)
That is, the relevant lattice is augmented by the quantum numbers (g, h) that label the
twisted sectors. Again, the thermal co-cycle insures that fermions contribute positively to
the partition function. In the twisted sectors, there is no momentum charge and we can set
(p, q) = (0, 0).
In the twisted sectors, each of the ĉ = 2 superconformal blocks is equivalent to a system
described by a free complex boson and a free complex fermion twisted by Z4. This equivalence
implies topological identities for each N = 2 twisted superconformal block [38][39]:
2|η|4
(γi,δi)
γi−h2
δi− g2
= 22 sin2(
πΛ(h, g)
where Λ(h, g) = Λ(g, h) depend on the (h, g)-twisted sector. Λ(h, g) = 2 when (h, g) =(0,2),
(2,0) and (2,2) while for the remaining 12 twisted sectors Λ(h, g) = 1. Although the above
orbifold expressions are derived at the fermionic point, they remain valid for any other point
of the untwisted moduli space.
Using the above orbifold identity, the “twisted” part of the thermal partition function
simplifies to:
Zthermaltwist =
(α,β,ᾱ,β̄)
(h,g)6=(0,0)
Imτ Γ2,2 e
iπ(α+β+ǫαβ)eiπ(ᾱ+β̄+ǭᾱβ̄)eiπ(α+ᾱ)g+iπ(β+β̄)h
× 28 sin8(
πΛ(h, g)
β̄− g
. (69)
Furthermore, by using the left- (and right-) Jacobi identities
(α,β)
eiπ(α+β+ǫαβ)Θ
= −|ǫ|Θ
, (70)
the twisted part of the thermal partition function simplifies further:
Zthermaltwist =
(h,g)6=(0,0)
Imτ Γ2,2 2
8 sin8(
πΛ(h, g)
) φ((g, h), (ǫ, ǭ)). (71)
The factor φ depends on the initial choice of the left- and right-chirality coefficients ǫ, ǭ
φ((g, h), (ǫ, ǭ)) =
|ǫ|+ (1− |ǫ|)(1− eiπgh)
|ǭ|+ (1− |ǭ|)(1− eiπgh)
. (72)
It should be noted that expression (71) has a field theoretic interpretation in terms of a
momentum lattice only.
If the initial non-thermal model was not supersymmetric, |ǫ| = |ǭ| = 1, then the number
of massless bosons and fermions will not be equal, nb 6= nf . For all other choices nb =
nf . This situation is reflected in the factor φ((g, h), (ǫ, ǭ)), which distinguishes the four
different possibilities. In the non supersymmetric case there is a non-vanishing contribution
to the partition function even in the absence of the thermal co-cycle. This is equivalent,
in field theory, to the one-loop zero temperature contribution to the effective action. This
contribution is zero in supersymmetric theories. In the later cases, the corrections are coming
from the massive thermal bosons and fermions plus a contribution from massless bosons. We
will display here a number of typical examples. The first class is when the Γ2,2 lattice is
unshifted by (h, g) and so it factorizes out from the sum over (g, h).
Unshifted Γ2,2 lattice
In the unshifted case, the sum over (g, h) can be performed easily so that the only remaining
dependence is that of ǫ, ǭ. We obtain [40][41],
Sthermaltwist =
dτdτ̄
(Imτ)2
Zthermaltwist = −C[ǫ, ǭ] log
|η(T )|4 |η(U)|4 ImT ImUµ2
where µ2 is an infrared cut-off and T, U parameterize the Kähler and complex structure
moduli of the target space torus T 2. The coefficient C[ǫ, ǭ] depends on the initial chiralities
of the spinors:
C[1, 1] = 240, C[1, 0] = C[0, 1] = 32, C[0, 0] = 64 . (74)
Actually, C[ǫ, ǭ] is nothing but the number of the massless bosonic degrees of freedom of the
theory. Equation (73) is invariant under the full target space T-duality group acting on the
T and U moduli separately.
For large volume, ImT ≫ 1, the leading behavior is linear in ImT ∼ RyRz. Assuming
iU ∼ Ry/Rz fixed and µ2 ∼ γ/RyRz, we have
Sthermaltwist = C[ǫ, ǭ]
RyRz − log
γ |η(U)|4 ImU
. (75)
Note that in the large volume limit, the twisted sector contribution to one-loop amplitude
depends both on the Kähler and complex structure moduli.
Shifted lattice
Another illustrating example is when the Z4 action shifts the Γ2,2 lattice simultaneously with
the twist we described before. In this case the lattice is replaced by a shifted Γ2,2 lattice.
The (Lh/2, Lg/2) shifted lattice, L = 1, 2, 3, is given by 5:
(R) =
(m,n)
2 |(4m+Lg)+(4n+Lh)τ |
Imτ . (76)
Here, we will examine in more detail the L = 1 case which corresponds to a 1/4-shifted
lattice.
5For brevity we have given the expression for the shifted Γ1,1 lattice but the generalization to the Γ2,2
lattice is straightforward.
When (ǫ, ǭ) = (1, 1), the contribution of the twisted sector to the partition function
becomes [39][42]:
Sthermaltwist = 240
dτdτ̄
(Imτ)2
Γ2,2[T, U ]−
Γ2,2[4T, 4U ]
. (77)
To obtain the above expression we have used the identities
(h,g)
= Γ2,2(T, U), Γ2,2[
0 ] =
Γ2,2[4T, 4U ] (78)
and we subtracted the contribution of the untwisted sector, (g, h) = (0, 0). Integrating over
τ we obtain [39] [42]
Sthermaltwist = − 60 log
|η(T )|16 |η(U)|16
|η(4T )|4 |η(4U)|4
ImT 3 ImU3µ6
. (79)
There is no volume factor in the large ImT limit (and this is generic in the case of freely
acting orbifolds [39][42]). So for large ImT , (and setting ImTµ2 ∼ γ), we obtain
Sthermaltwist = −60 log
|η(U)|16
|η(4U)|4
. (80)
In the large volume limit, Sthermaltwist only depends on the complex structure modulus of the
torus.
Comments
The total Euclidean one-loop amplitude in both the shifted and unshifted lattice cases is
given by:
Sthermal = Sthermalunt + S
thermal
twist (81)
The Sthermalunt is nothing but one-quarter of the thermal partition function of type II super-
string theory on S1 × T 9 with all nine spatial radii arbitrary, while that of Euclidean time
fixed by the temperature: 2πT = 1/R+ = 1/2. By exponentiating S
thermal, with the twisted
sector contributions in our examples given by equations (75) and (80), we obtain the norms
of the corresponding cosmological wave-functions as functions of all moduli.
The difference between the shifted and the ordinary model discussed previously can be
understood as follows. It is known that the freely acting orbifolds are related to gravitational
and gauge field backgrounds with fluxes [43]. This indicates the different interpretations of
the two cosmological models with shifted and unshifted Γ2,2. In the shifted model there are
non-vanishing magnetic fluxes [43] while in the unshifted case such fluxes are absent.
Let us stress here that the thermal Z4-orbifold described involves a twisting that leaves
two moduli parameterizing a Γ2,2 lattice, which in the large moduli limit gives us the four
dimensional cosmological model discussed in section 2. Many other orbifold-like models
can be constructed, which may factorize bigger lattices, admitting a higher dimensional
interpretation. In all those cases, the partition function is computable as a function of the
moduli. However its analytic form in terms of the moduli in various limits depends crucially
on whether the orbifold is freely, or even partially freely acting (or in other words, on the
different structure of magnetic fluxes). Models based on asymmetric orbifolds can also be
constructed giving rise to a rich family of calculable models. A more detailed analysis would
be interesting so as to understand the classification of the low level cosmologies, as well as
the characteristic dependence of the norm of the wave-function on the various features of
the large class of models. To illustrate the above points, we offer one further simple example
based on a Z2 instead of the Z4 orbifold.
Z2 orbifold models
In the Z2 orbifold models (M = 2 in equation (53)), the factorization of the cosmological
CFT factor is not so explicit as it was in the Z4 examples. However the cosmological
interpretation remains the same.
For the untwisted sector, the genus-1 contribution Sthermalunt is now one half of the thermal
partition function of type II theory on S1 × T 9. We proceed to analyze the twisted sector
contribution to the genus-1 amplitude. Following similar steps as in the Z4 orbifold case,
and now setting (2H, 2G) = (h, g) to be integers defined modulo 2, we obtain for the case
(ǫ, ǭ) = (0, 0)
Sthermaltwist =
dτdτ̄
(Imτ)2
(h,g)6=(0,0)
28 Imτ Γ2,2 . (82)
To arrive at the result notice that since the characters (h, g) are defined to be integers modulo
2, the factor of 1/4 in the first line of equation (55) now becomes 1/2; no other modifications
in this formula are needed. All the other steps carry through as before. Here also we may
classify the models in two classes; in the first class the lattice is taken to be unshifted, while
in the second class the Γ2,2 is half-shifted. We will use the definition:
(R) =
(m,n)
2 |(2m+Lg)+(2n+Lh)τ|
Imτ . (83)
For the unshifted Γ2,2 Z2-model, and when (ǫ, ǭ) = (0, 0), we obtain:
Sthermaltwist = −384 log
|η(T )|4 |η(U)|4 ImT ImUµ2
. (84)
As in the Z4 models, for large volume, ImT ≫ 1, the leading behavior is linear in ImT ∼
RyRz. Assuming iU ∼ Ry/Rz fixed and µ2 ∼ γ/RyRz we have:
Sthermaltwist = 384
RyRz − log
γ |η(U)|4 ImU
. (85)
The Z2-model with half-shifted lattice, Γ2,2[
g ], (g, h = 0, 1), yields [39][42]
Sthermaltwist = 384
dτdτ̄
(Imτ)2
Γ2,2[T, U ]−
Γ2,2[ 2T, 2U ]
. (86)
Here also the contribution coming from the untwisted sector (h, g) = (0, 0), is subtracted.
To obtain the above expression we have used the identities
(h,g)
Γ2,2[
g ] = Γ2,2[T, U ], Γ2,2[
0 ] =
Γ2,2[ 2T, 2U ]. (87)
Integrating over τ we obtain:
Sthermaltwist = −192 log
|η(T )|8 |η(U)|8
|η(2T )|4 |η(2U)|4
ImT ImUµ2
. (88)
There is no volume factor in the large ImT limit. Thus, for large ImT , (and setting ImTµ2 ∼
γ), we obtain for the half-shifted lattice contribution to Sthermaltwist :
Sthermaltwist = −192 log
γ |η(U)|8
|η(2U)|4
. (89)
As we see, we can obtain explicit expressions for the genus-1 approximation to the norm
of the wave-function for these particular Z2-models. The twisted sector contribution is given
explicitly by equations (88) and (89).
In the above family of models, we have always considered a two-dimensional cosmology
at small level. That choice is mainly due to two obstructions that are difficult (but not
necessarily impossible) to circumvent. One is associated to the difficulty of continuing from
Lorentzian to Euclidean signature in the presence of electric fluxes. The other is that it is
difficult to construct compact models with positive central charge deficit (negatively curved
Euclidean backgrounds) in string theory, or alternatively, a compact version of linear dilaton
type models. We make some further comments on this in the next section.
8.2 Liouville type models
Consider string theory cosmological backgrounds based on worldsheet CFTs of the form (see
e.g. [13][22][14][23][25][26][27][28][29]):
SL(2, R)−|k|
SL(2, R)|k|+4
×K. (90)
A nice feature of such models is that the combined central charge of the two SL(2, R)/U(1)
factors is independent of |k|, and so this can be taken to be an independent, varying param-
eter. Although the partition function of the euclidean cigar background is known [44], we
need to deal first with the fact that for such a background the cosmological wave-function is
non-normalizable due to the infinite volume of the cigar factor SL(2, R)|k|/U(1). To produce
a normalizable wave-function we must face the problem of consistently compactifying this
factor, as alluded at the end of the previous section. Moreover, it would be interesting to
obtain a compactification scheme which leaves |k| a free parameter. Compactifying the cigar
would amount to discretizing its continuous modes keeping at the same time the unitarity
and the modular invariance of the torus partition function intact.
An interesting aspect of these models is that now the sphere contribution to the string
partition function is finite since the volume of the conformal Killing group cancels against
the volume of the SL(2, R)|k|/U(1) conformal field theory factor. Nevertheless, we can see
that the torus contribution dominates (at any finite string coupling) due to the volume
divergence. If a consistent way of cutting off the volume of the cigar is found, we could
interpret the torus contribution as a finite thermal correction to the tree-level contribution,
realizing a stringy version of the computation in [5][7][45].
A further suggestion for developing our formalism in linear dilaton spaces, is to view
the wave-function of the universe as also depending on a boundary condition in the space-
like linear dilaton direction of the cigar, in order to obtain a linear dilaton holographic
interpretation [46]. Part of the interpretation of the wave-function of the universe would
then be as in the Hartle-Hawking picture, and part would be holographic. Finally in these
models supersymmetry can be restored asymptotically in the large |k| limit, making contact
with linear dilaton models in null directions (see e.g. [47] for recent progress).
9 Discussion
We have outlined a framework generalizing the Hartle-Hawking no boundary proposal of
the wave-function of the universe to string theory cosmological backgrounds. The class of
example cosmologies considered here are described by worldsheet conformal field theories
of the general form SL(2, R)−|k|/U(1) × K, where K is an internal, compact CFT. In
order to define the analogue of the Hartle-Hawking wave-function, we had to surmount
the technical hurdle of realizing that such cosmologies (like the corresponding Euclidean
parafermion theories) have an almost geometrical description in terms of a compact non-
singular T-fold. We then defined the wave-function of the universe via a Euclidean string
field theory path integral (generalizing the no-boundary proposal). For specific examples we
computed the norm of the wave-function to leading order in string perturbation theory, as
a function of moduli parameters. There are many interesting similar examples to which we
can generalize our analysis.
In a probabilistic interpretation, with a normalizable wave-function at hand, one can
attempt to compute vacuum expectation values for particular physical quantities in various
cosmological models, and analyze their properties in various regions of the moduli space. Our
purpose in this paper was to provide the framework for such a discussion, which promises
to be interesting. In particular, it is an open problem to identify preferred regions in the
moduli space in the large class of models to which our analysis applies.
More concretely, we believe our construction points out the good use that can be made
of T-folds, and generalized geometry, in stringy cosmologies (allowing to evade various no-go
theorems in pure geometry). Moreover, we have been able to define a sensible calculation
in a de Sitter-like compactification of string theory, after analytically continuing to the
Euclidean theory. These calculations are generically hard to come by in de Sitter gravity
after quantization, so any well-defined cosmological quantity, like the norm of the wave-
function of stringy universes, merits scrutiny. Finally, we calculated a quantity akin to an
entanglement entropy in de Sitter space, and showed that it only gets contributions starting
at one loop, and we gave its microscopic origin.
Acknowledgements
We thank Constantin Bachas, James Bedford, Ben Craps, John Iliopoulos, Dieter Luest,
Hervé Partouche, Anastasios Petkou, Giuseppe Policastro and Marios Petropoulos for useful
discussions. N. T. thanks the Ecole Normale Supérieure and C. K. and J. T. thank the
University of Cyprus for hospitality. This work was supported in part by the EU under
the contracts MRTN-CT-2004-005104, MRTN-CT-2004-512194 and ANR (CNRS-USAR)
contract No 05-BLAN-0079-01 (01/12/05).
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	Introduction
	The cosmological solution
	The Euclidean continuation
	The cosmological T-fold
	Defining the wave-function of the universe
	The norm of the wave-function
	Thermal nature of the wave-function
	Specific Examples
	Compact models
	Liouville type models
	Discussion

  We define a wave-function for string theory cosmological backgrounds. We give
a prescription for computing its norm following an earlier analysis within
general relativity. Under Euclidean continuation, the cosmologies we discuss in
this paper are described in terms of compact parafermionic worldsheet systems.
To define the wave-function we provide a T-fold description of the
parafermionic conformal field theory, and of the corresponding string
cosmology. In specific examples, we compute the norm of the wave-function and
comment on its behavior as a function of moduli.

Introduction
Our goal in this paper is to embed the Hartle-Hawking no-boundary proposal for a wave-
function description of the quantum state of the universe [1][2] in a string theoretic frame-
work. The Hartle-Hawking proposal pertains in particular to de Sitter–like universes in
general relativity. A partial list of interesting recent work on related topics is [3][4][5] [6][7][8].
One motivation for embedding the Hartle-Hawking proposal into string theory is that it
provides us with a calculable quantity in de Sitter-like compactifications of a quantum theory
of gravity. These quantities are hard to come by (see e.g. [9][10] and references thereto).
Two essential properties which a cosmological background must fulfill in order to admit
a wave-function description under the no-boundary proposal are the following. First the
cosmology must be spatially closed. More importantly, the cosmology should admit a con-
tinuation to a positive definite Euclidean geometry that is compact and has no boundaries
or singularities. The most familiar example is the case of n-dimensional de Sitter space, dSn,
where these properties are satisfied. In global coordinates, the dSn metric is given by
ds2 = R2(−dt2 + cosh2 t dΩ2), (1)
where dΩ2 is the metric on a round unit (n − 1)-sphere and R is the radius of curvature.
The spatial slices of constant time t are (n− 1)-spheres of radius R cosh t. We can rotate to
Euclidean signature by setting t = iτ = i(π/2− θ), upon which we obtain an n-sphere Sn of
radius R. The Euclidean continuation is a compact smooth manifold.
In a field theoretic framework, the quantum state of a de Sitter cosmology can be ex-
pressed as a functional of fields, including both matter fields and metric fluctuations, on a
spatial slice of time-reversal symmetry. For the reversal t→ −t, this is the slice t = 0 in the
de Sitter space dSn. That slice is also the equator θ = π/2 of the corresponding Euclidean
sphere Sn. Imagine cutting de Sitter space along this slice and gluing smoothly one half of
it to half a sphere Sn. Under the Hartle-Hawking proposal, we express the wave-function as
a Euclidean path integral over half the sphere Sn with the condition that the metric gij and
the matter fields, collectively denoted by φ, take specific values (hij , φ0) on the boundary
equator θ = π/2:
Ψ(hij, φ0) =
[dg][dφ]e−SE(g,φ). (2)
No other boundary condition needs to be specified due to the compactness of the Euclidean
manifold. Here, SE is the Euclidean gravitational action in the presence of matter fields and
a positive cosmological constant Λ.
The norm of the wave-function is given by the full Euclidean path integral on Sn. It can
be computed in the semi-classical approximation by evaluating the Euclidean action for a
given solution to the classical equations of motion. One solution is empty de Sitter space of
radius R ∼ Λ−1/2. In this approximation, and in the case of four dimensions, the norm is
given by [1]:
||ΨHH ||2 ∼ e
3λ , (3)
where the dimensionless parameter λ is proportional to the cosmological constant:
λ = 2GΛ/9π .
The compactness of the Euclidean manifold ensures that the full path integral is free of
any infrared divergences. However the field theory in question is non-renormalizable, and to
go beyond the semi-classical approximation, we need to impose an ultraviolet cutoff. One
way to deal with the ultraviolet ambiguities is to embed the calculation in a string theoretic
framework, where we expect the ultraviolet divergences to be absent. Unfortunately there
are no known classical de Sitter solutions in string theory to begin with. Therefore, we seek
other cosmological backgrounds which are exact solutions to string theory and for which we
can generalize the Hartle–Hawking computation.
To this end, notice that any tachyon free, compact Euclidean string background provides
us with a finite, calculable quantity, namely the string partition function Zstring. Associated
to the classical string background is a two-dimensional worldsheet conformal field theory
(CFT). At the perturbative level, the string partition function can be computed as usual as
a sum of CFT vacuum amplitudes over compact worldsheets of all topologies. Our proposal
is that when such Euclidean string backgrounds admit a continuation to a Lorentzian cosmol-
ogy, the Hartle-Hawking construction can be generalized with the norm of the wave-function
given by
||Ψcosm.||2 = eZstring . (4)
We will motivate this formula by working out specific examples in string perturbation theory.
As we will explain, the relevant string partition function has to be thermal.
Given the discussion above, a first candidate to consider is a Euclidean model for which
the two-dimensional CFT is of the form SU(2)k × K, the first factor corresponding to an
SU(2) Wess-Zumino-Witten (WZW) model at level k and the second factor K corresponding
to a suitable internal compact conformal field theory. The WZW factor is equivalent to a
sigma model on a 3-sphere of radius (kα′)1/2 and with k units of NSNS 3-form flux through
the sphere. The dilaton field is constant and by choosing this to be small we can apply
string perturbation theory. Unfortunately, however, the continuation to Lorentzian signature
results in a dS3 cosmology with imaginary flux, and it is not clear whether such a Lorentzian
background is physical. (See [11] for an alternative non-compact, time-like Liouville model
for which the SU(2) WZW factor describes the internal space.)
The only known string theory example which satisfies all the criteria we described so far is
based on the parafermionic SU(2)|k|/U(1) coset model [12], which can be realized as a gauged
SU(2) WZW model at level |k|. We consider Euclidean backgrounds corresponding to a two-
dimensional CFT of the form SU(2)|k|/U(1) × K where K is again an internal compact
conformal field theory. Such a Euclidean background admits a Lorentzian continuation to
a cosmological background belonging to a class of models studied in [13][14], and which are
described by two-dimensional CFTs of the form SL(2, R)−|k|/U(1) × K. To avoid having
to deal with the tachyonic instabilities of bosonic string theory, we consider solutions of this
form in superstring theory. The total central charge must be ctot = 15 (ĉ = 10) in order for
worldsheet (super-)gravitational anomalies to cancel. When we fix the internal conformal
field theory K, the level |k| is determined by anomaly cancellation.
The non-trivial time-dependence of the cosmology necessarily breaks space-time super-
symmetry. As in the de Sitter case, the Euclidean path integral can be interpreted as a
thermal ensemble. Thus from the point of view of the Euclidean N = 2 worldsheet super-
conformal system, space-time supersymmetry will be broken by specific boundary conditions,
analogous to the thermal co-cycles that appear in the partition function of superstring the-
ories on flat space at finite temperature [15]. For large level |k|, the effective temperature of
the models is of order T ∼ 1/
|k|α′ [14][28]. In this paper, we will explore some low level
|k| models. In order for the corresponding cosmological wave-function to be computable in
string perturbation theory, the effective temperature must be below the Hagedorn tempera-
ture. A Hagedorn temperature would signal a phase transition, as proposed in [15][16][17].
We will construct explicitly low level |k| models for which the effective temperature is below
the Hagedorn temperature and so string perturbation theory can be applied.
It is well known that the geometric sigma model approach to the parafermionic coset
model (and to the corresponding Lorentzian cosmology) leads to a metric with curvature
singularities and strong coupling. However, the underlying CFT is perfectly well behaved
at these apparently singular regions, and by using T-duality a weakly coupled description
of these regions can be obtained [18]. Using this fact, we construct an almost geometrical
description of the CFT in terms of a compact, non-singular T-fold [19][20] with a well–
defined partition function. These considerations allow us to define the wave-function of the
Lorentzian cosmology.
Our paper is organized as follows. In section 2, we review properties of the two-dimensional
SL(2, R)−|k|/U(1) × K conformal field theory that corresponds to a cosmological back-
ground. It is the analogue of the de Sitter universe. In section 3, we describe how to ana-
lytically continue the cosmology to a compact Euclidean space-time described at the string
level by a two-dimensional parafermionic model of the form SU(2)|k|/U(1) × K. Then, we
discuss in section 4 how to obtain an almost geometrical description of these backgrounds
in terms of T-folds. We discuss in sections 5 and 6 how to calculate a wave-function and its
norm for the cosmology. In section 7 we discuss the thermal nature of the wave-function. In
section 8 we apply the definition of the wave-function to some particular compact models
and for which perturbation theory can be used to compute its norm. Finally we discuss
interpretations of the results in the concluding sections.
2 The cosmological solution
In this section, we review in some detail the cosmological solution of string theory which is
based on an SL(2, R)/U(1) gauged WZW model at level k [13].
We can define a WZW conformal field theory on the group manifold SL(2, R), at least
classically. The worldsheet action is given by
d2zTr(g−1∂gg−1∂̄g) +
Tr(g−1dg ∧ g−1dg ∧ g−1dg), (5)
where Σ is the worldsheet Riemann surface, M is a 3-manifold whose boundary is Σ and g
is an element of SL(2, R).
For concreteness, we parameterize the SL(2, R) group manifold as follows
with ab+ uv = 1. The conformal field theory has an SL(2, R)× SL(2, R) global symmetry.
We choose to gauge an axial U(1) subgroup under which g → hgh. In particular, we consider
the non-compact U(1) subgroup generated by
δg = ǫ
g + ǫg
. (7)
Infinitesimally, we have the transformations δa = 2ǫa, δb = −2ǫb, δu = δv = 0. To gauge
this U(1) symmetry, we introduce an Abelian gauge field and render the action invariant.
The action is quadratic and non-derivative in the gauge field, and so this can be integrated
out in a straightforward way [21].
In the region 1−uv > 0, we can use the gauge freedom to set a = b and integrate out the
gauge field. The resulting action is expressed in terms of gauge invariant degrees of freedom
only, and it turns out to be
S = − k
∂u∂̄v + ∂v∂̄u
1− uv
, (8)
while a non-trivial coupling to the worldsheet curvature is generated corresponding to a
dilaton background [21]. This action can be identified with a non-linear sigma-model action
with background metric
ds2 = −kα′ dudv
1− uv
. (9)
The non-trivial dilaton is given by
e2Φ =
1− uv
. (10)
The metric (9) is a Lorentzian metric whose precise causal structure, however, depends
on the sign of k. For positive level k, u and v are Kruskal-like null coordinates of a 2-
dimensional black hole. In this case, the time-like coordinate is given by u + v, and the
metric has space-like singularities in future and past times at uv = 1.
For negative level k, one obtains a cosmological solution [13]. It consists of a singularity-
free light-cone region, and there are (apparent) time-like singularities in the regions outside
the light-cone horizons. Indeed, for negative level k we may set u = −T +X and v = T +X
and the metric becomes
ds2 = |k|α′−dT
2 + dX2
1 + T 2 −X2
. (11)
The surfaces of constant time T intersect the singularities at X = ±
1 + T 2. Even though
the singularities follow accelerated trajectories, their proper distance remains finite with
respect to the string frame metric
L = (|k|α′)
1+T 2
1+T 2
1 + T 2 −X2
= π(|k|α′)
2 . (12)
So with respect to stringy probes, the cosmology is spatially closed.
The singularity-free light-cone region is the region T 2 − X2 ≥ 0 (or uv ≤ 0). The
future part of this region describes an expanding, asymptotically flat geometry with the
string coupling vanishing at late times. See e.g. [11][13][14][23][24][25][26][27][28][29] for
some discussions of these types of models. To see this, we parameterize the region uv ≤ 0
with coordinates (x, t) such that
u = −tex, v = te−x (13)
and the metric becomes
ds2 = |k|α′−dt
2 + t2dx2
1 + t2
, (14)
while the dilaton field becomes
e2Φ =
1 + t2
. (15)
The scalar curvature is given by
|k|α′(1 + t2)
. (16)
Initially the curvature is set by the level |k| and it is positive. No matter how small the
level |k| is, asymptotically the scalar curvature vanishes. An observer in this region never
encounters the singularities. These are hidden behind the visible horizons at T = ±X .
However signals from the singularities can propagate into the region uv < 0, and therefore
influence its future evolution.
Thus when |k| is small the early universe region t ∼ 0 is highly curved, with curvature
of order the string scale. In this sense, it is similar to a big-bang cosmology. Despite the
regions of large curvature, this cosmological background has a well defined CFT description
and can be described in a string theoretic framework.
The cosmological background can also be realized as a solution of superstring theory by
generalizing the worldsheet theory to a superconformal SL(2, R)/U(1) model. The central
charge of the superconformal SL(2, R)/U(1) model at negative level k, is given by
c = 3− 6
|k|+ 2
, ĉ = 2− 4
|k|+ 2
In superstring theory, we must tensor it with other conformal field theories so as to satisfy
the condition ĉtot = 10 for worldsheet gravitational anomalies to cancel.
An interesting case considered in [13] is the case where we add two large (however com-
pact) free super-coordinates (y, z) together with a compact, superconformal CFT of central
charge δĉ = 6 + 4/(|k| + 2). The resulting background is a four dimensional cosmological
background whose metric in Einstein frame is given by
ds2E = |k|α′(−dt2 + t2dx2) + (1 + t2)(R2ydy2 +R2zdz2). (18)
This is an anisotropic cosmology which at late times however, and for large Ry ∼ Rz,
asymptotes to an isotropic flat Friedman-Robertson cosmology.
The cosmological region t2 = −uv ≥ 0 is non-compact, and when Ry,z are large it has the
desired four-dimensional interpretation. This is so irrespective of how small the level k is. In
the region uv > 0 (t2 < 0), sigma-model time-like singularities appear at uv = 1 (t2 = −1).
As we propose later in this work, these singularities are resolved at the string level, since the
structure of the space-time manifold is replaced by a non-singular T -fold.
The string partition function depends crucially on the extra ĉ = 6+4/(|k|+2) supercon-
formal system, which is taken to be compact. In contrast to the four dimensional part defined
by (t, x, y, z), for the internal, Euclidean ĉ = 6+4/(|k|+2) system, the naive six-dimensional
interpretation, which is valid for large level |k| with curvature corrections of order 1/(|k|α′),
is not valid for small values of |k| [18]. For example, for |k| = 2, the system can be taken to be
a seven-dimensional torus. In general, small |k| implies that the generalized curvatures (i.e.
including dilaton gradients etcetera) are large and the moduli/radii are small. We remind
the reader of the example of the SU(2)k=1 Wess-Zumino-Witten model which is equivalent
to a (one-dimensional) compact boson at self dual radius. For large level |k|, however, the
sigma model manifold is a large three-dimensional sphere with NSNS 3-form flux.
3 The Euclidean continuation
Let us consider the region 1− uv ≥ 0 of the two-dimensional cosmology, and set u = −T +
X, v = T +X . We can rotate to Euclidean signature by setting T → −iTE . The Euclidean
continuation is a disk of unit coordinate radius parameterized by Z = X+ iTE , Z̄ = X− iTE
such that |Z|2 ≤ 1. The metric (9) becomes
ds2 = |k|α′ dZdZ̄
1− ZZ̄
= |k|α′dρ
2 + ρ2dφ2
1− ρ2
and the dilaton
e2Φ =
1− ZZ̄
1− ρ2
, (20)
where we have also set Z = ρeiφ with 0 ≤ ρ ≤ 1. The singularity becomes the boundary
circle ρ = 1.
The radial distance of the center to the boundary of the disk is finite, but the circum-
ference of the boundary circle at ρ = 1 is infinite. Geometrically the space looks like a bell.
This Euclidean background corresponds to a well defined worldsheet conformal field theory
based on an SU(2)/U(1) gauged WZW model at level |k|.
From the point of view of the WZW worldsheet theory, the Euclidean continuation can
be understood as a double analytic continuation as follows. We parameterize the SL(2, R)
group manifold as in equation (6). Let us also set a = X̃ − T̃ , b = X̃ + T̃ so that the group
element becomes
X̃ − T̃ X − T
−X − T X̃ + T̃
X̃2 +X2 − T̃ 2 − T 2 = 1. (22)
This parameterization shows that the SL(2, R) group manifold is a 3-dimensional hyper-
boloid. Then it is clear that upon the double analytic continuation T → −iTE , T̃ → −iT̃E
the group element becomes the following SU(2) matrix
−Z̄ W̄
with WW̄ + ZZ̄ = 1. After the analytic continuation we also have that a → W = X̃ +
iT̃E , b→ W̄ = X̃ − iT̃E .
A useful parameterization of the SU(2) group manifold for our purposes is
W = cos θeiχ, Z = sin θeiφ (24)
and the metric on S3 in these coordinates becomes
ds2 = dθ2 + sin2 θdφ2 + cos2 θdχ2. (25)
The ranges of the angles are as follows 0 ≤ θ ≤ π, 0 ≤ χ, φ,≤ 2π.
The original global SL(2, R) × SL(2, R) symmetry naturally continues to the SU(2) ×
SU(2) global symmetry of the resulting SU(2) WZW model. The non-compact U(1) axial
symmetry subgroup that we gauge continues to a compact U(1) subgroup generated by
δg = iǫ
g + iǫg
, (26)
which amounts to the following infinitesimal transformations δW = 2iǫW , δW̄ = −2iǫW̄ and
δZ = δZ̄ = 0. In the parameterization (24), the U(1) symmetry corresponds to shifts of the
angle χ. Gauging this symmetry results in the SU(2)/U(1) coset model. In the Euclidean
set-up, we take the level |k| to be an integer for the WZW model to be well-defined.
After the analytic continuation described, we end up with the action (see e.g. [31] for a
review):
d2z∂θ∂̄θ + tan2 θ∂φ∂̄φ
+cos2 θ(∂χ + tan2 θ∂φ + Az)(∂̄χ− tan2 θ∂̄φ+ Az̄).
In the Euclidean theory the gauge freedom can be fixed by setting the imaginary part of
W (equivalently the angle χ) to zero. The equations of motion for the gauge field can then
be used to integrate the gauge field out. This amounts to setting the last term in (27) to
zero and producing a dilaton e2Φ = e2Φ0/ cos2 θ. We end up with a sigma model action with
metric
ds2 = |k|α′(dθ2 + tan2 θdφ2) (28)
which is equivalent to the metric (19) after the coordinate transformation Z = sin θeiφ.
The curvature singularity occurs at θ = π/2. The procedure of fixing the gauge χ = 0
and using the equations of motion to integrate the gauge field out is not valid near θ = π/2,
since it results into singular field configurations on the worldsheet. However, the full action
(27) is perfectly well behaved at θ = π/2. To see this, we expand the Lagrangian in (27)
around θ = π/2. Setting θ = π/2− θ̃, we obtain that
d2zφFzz̄ +O(θ̃
2), (29)
where we expressed the action in terms of manifestly gauge invariant degrees of freedom.
The leading term in this expansion describes a simple topological theory, which shows that
an alternative, non-geometric description of the theory can be given including the region
near θ = π/2. We return to this point later on.
From the form of the action near θ = π/2, we also learn that the U(1) symmetry cor-
responding to shifts of the angle φ is quantum mechanically broken to a discrete symmetry
Z|k|. This is because compact worldsheets can support gauge field configurations for which
Fzz̄ = 2πin, with n an integer, and such configurations must be summed over in the full
path integral. It is clear then that the path integral is only invariant under discrete shifts of
the angle φ: δφ = 2πm/|k|.
This breaking of the classical U(1) symmetry to Z|k| is in accordance with the algebraic
description of the SU(2)/U(1) coset in terms of a system of Z|k| parafermionic currents
ψ±l(z), l = 0, 1 . . . |k| − 1 [with ψ0 = 1, ψ†l ≡ ψ−l = ψk−l], of conformal weights hl =
l(|k| − l)/|k| (see also [32]). These satisfy the OPE relations
ψl(z)ψl′(0) = cll′z
−2ll′/|k|(ψl+l′(0) + . . . )
ψl(z)ψ
l (0) = z
−2hl(1 + 2hlz
2T (0)/c+ . . . ) (30)
which are invariant under the Z|k| global symmetry: ψl → e2πil/kψl. Here T is the energy
momentum tensor of the parafermions, c the central charge (which is the same as the central
charge of the coset model) and the coefficients cll′ are the parafermionic fusion constants [12].
In the infinite level |k| limit, the conformal weights of the parafermion fields become integers.
In this limit the sigma model metric is flat, and we recover the full rotational invariance of
flat space [33]. The system can be also generalized to an N = 2 superconformal system by
tensoring the Z|k| parafermions with a free compact boson as described in [36].
Finally we can check that the central charge remains the same after the analytic contin-
uation. Indeed, it is the very fact that the central charge of the conformal field theory is
smaller than the central charge corresponding to two macroscopic flat dimensions that codes
the de Sitter nature of the two-dimensional cosmology.
4 The cosmological T-fold
The parafermionic T-fold
It is interesting to take a closer look at the geometry that we associate to the parafermionic
model SU(2)|k|/U(1). As we already discussed, we describe it in terms of a metric and
dilaton profile:
ds2 = |k|α′(dθ2 + tan2 θdφ2)
cos θ
, (31)
where φ ∼ φ+2π and θ takes values in the interval [0, π/2]. This description breaks down near
θ = π/2. Nevertheless, the parafermionic conformal field theory is perfectly well-behaved,
and we can wonder whether there is a more appropriate, almost-geometrical description. We
argue that such a description exists in terms of a T-fold.
To obtain it, we perform a T-duality along the angular direction φ on the geometry
described above:
ds2 = |k|α′dθ2 +
cot2 θdφ̃2
sin θ
. (32)
By changing variables θ̃ = π/2− θ, we see that this is equivalent to:
ds2 = |k|α′dθ̃2 +
tan2 θ̃dφ̃2
cos θ̃
. (33)
This description is therefore at weak curvature (apart from an orbifold–like singularity) and
weak coupling near θ = π/2. Moreover, we can identify it as a Z|k| orbifold of a vectorially
(or axially) gauged SU(2)/U(1) coset. Indeed, it is true for the parafermionic theory that
the T-dual and the Z|k| orbifold give two models with identical spectrum due to the coset
character identity χj,m = χj,−m (see e.g. [30][31] for reviews).
We now use these facts to give an almost geometrical description of the parafermionic
theory, in terms of a T-fold [19][20]. We use the description in terms of the first geometry
(31) near θ = 0. We cut it just past θ = π/4, where the radius of the circle is
|k|α′.
We glue it to the T-dual geometry which we consider near θ̃ = 0, or θ = π/2, and which
we cut just past θ̃ = π/4, where we have radius
α′/|k|. We glue the circles (and their
environments) using the T-duality transformation described above. In the gluing process, it
is crucial to realize that we glue a patch with a direction of increasing radius to a T-dual
patch which in the same direction has decreasing radius. That gives us the parafermionic
T-fold. The associated partition function is (see e.g. [31] for a review) :
χj,m(τ)χj,m(τ̄ ). (34)
One aspect of the model that is rendered manifest by the T-fold description is the breaking
of the U(1) rotation symmetry to a discrete Z|k| symmetry, due to the Z|k| orbifolding. This is
consistent with our previous discussion of the breaking due to worldsheet instantons. The T-
fold yields an almost-geometrical picture of the symmetry breaking. The T-fold description
is indeed everywhere regular modulo a benign orbifold singularity.
The cosmological T-fold
In the case of the two-dimensional cosmology as well, we can obtain a regular T-fold de-
scription of the target space of the conformal field theory. We recall that under T-duality
(the metric can be obtained by analytically continuing the metric (32) in the direction φ̃),
the light-cone and the singularities get interchanged. Consider the cosmology, and cut it at
a hyperbola at radius
|k|α′, in between the light-cone and the time-like singularities in
the Penrose diagram (see the upper part of figure 1). Consider then its T-dual, and cut it
along a similar line. Glue the two parts of the T-dual cosmologies along these cuts to obtain
the T-fold cosmology. The description we obtain is particularly nice as we no longer need
a microscopic origin of a would-be source associated to the time-like singularities, nor do
we need to define boundary conditions associated to them. There is no singularity in, nor
is there a boundary to the T-fold cosmology. Indeed, the almost-geometrical description is
very much like dS2, which we can think of as a hyperboloid embedded in three-dimensional
space. The difference is that the T-fold cosmology has two patches glued together via a
Figure 1: The analytic continuation of the T-fold. The upper part of the diagram shows
the two T-dual descriptions of the cosmology in which the horizon and the (apparent) sin-
gularities are exchanged (in bold blue). The (striped black) cut along which they are glued
is indicated, as well as the (thin black) line along which the cosmology is cut to obtain a
space-like slice (see later). Analytic continuation then gives rise to the lower part of the
figure, in which we have sketched the T-fold description of the parafermionic conformal field
theory. In bold blue we have the center and the boundary of the disk, and (in black stripes)
the T-dual circles along which we glue.
T-duality transformation (instead of an ordinary coordinate transformation in the case of
two-dimensional de Sitter space).
In figure 1 we show how the T-fold description of the parafermions and the two-dimensional
cosmology continue into one another after analytic continuation.
5 Defining the wave-function of the universe
Later on, we will consider string theory backgrounds which are product models and in which
one factor consists of the two-dimensional cosmology discussed in sections 2, 3 and 4. For
these models, we wish to define a wave-function of the universe in string theory following
ideas of [1] which define a wave-function of dSn universes within a field theoretic context.
We consider a time-reversal symmetric space-like slice of the cosmology, within the bound-
aries of the (seeming) singularities. See figure 2. This is the slice T = 0. In the past of
the space-like slice, we glue half of the target space of an SU(2)/U(1) coset conformal field
theory – a half disk. By the analytic continuation discussed in the previous section, this
Figure 2: The continuous gluing of the half-disk into the cosmology, when cutting the cos-
mology along a space-like slice, and analytically continuing. The figure should be viewed as
a simplified version of the previous T-fold picture.
gluing is continuous in the backgrounds fields, and moreover in the exact conformal field
theory description.
A crucial feature of the proposal of [1] for the definition of the wave-function of the
universe is that the corresponding Euclidean space is without boundary. In our set-up as
well, the Euclidean conformal field theory has a target with no boundary. It is important
in this respect that we have obtained an almost-geometric description of the parafermionic
conformal field theory1. It is intuitively clear from the T-fold description given in the previous
section that the parafermionic theory does not have a boundary2. When we cut the Euclidean
T-fold into half, it is clear (from figure 1) that we can glue the boundary of that half-T-fold
into the initial surface of the cosmological T-fold.
Thus we have determined the precise gluing of T-folds necessary in order to define a
wave-function depending on initial data. We now define the wave-function of the universe by
performing a “half T-fold” Euclidean path integral over all target space fields with specified
values on the boundary:
Ψ[h∂ , φ∂, . . . ] =
[dg][dφ] . . . e−S(g,φ,... ) , (35)
where the path integral is such that the metric, the dilaton and all other space-time fields
1A traditional description of the target space as a disk, which is singular,would lead to the faulty conclusion
that the target space has a boundary.
2Since a T-fold is non-geometric, one needs to define the concept of boundary precisely. We believe that
a reasonable definition will match our intuition.
satisfy g = h∂, φ = φ∂, . . . on the boundary of the half T-fold that we glued into the
cosmological solution. The path integral above can in principle be performed off-shell, in
a second-quantized string field theory context, where we may also express it as an integral
over a single string field Φ. (See e.g. [34] for a concise review).
Let us be more specific. The initial space-like slice of the T-fold cosmology has two
patches. On each patch, we define a boundary metric h1 and h2, and similarly for other
fields. The boundary metrics satisfy the condition that on the overlap of the patches, they
match up to a T-duality transformation, symbolically: h1
∂1∩∂2 = T (h
∂1∩∂2). This is the way
in which we can specify boundary data precisely. In the following, we do not emphasize this
important part of the definition of the path integral further, not to clutter the formulas.
In principle, a T-fold path integral can be computed as follows. Consider again the two
patches. Each patch has a non-singular geometric description. Over each patch the path
integral reduces to an ordinary field theory path integral, and can be performed in the usual
way giving rise to a functional of boundary data. The full path integral can be obtained by
integrating the two functionals together over data that belong to the common boundary of
the two patches. Since at the common boundary of the patches their fields are related by
a T-duality transformation, to do the final integral we would need to perform a T-duality
transformation on one of the two functionals.
We remark here that this particular feature of definitions of path integrals over T-folds
with boundaries is generic. The above description is easily extended to a generic description
of T-fold boundary data. Although we do not need a general prescription in this paper,
we believe it would be interesting to develop the path integral formalism for T-folds with
boundary further.
The prescription for the wave-function of the universe we outlined above should have an
analogue, via the relation between string oscillators and the target space fields, to a first
quantized prescription. Notice that the initial-time data allow multi closed string configura-
tions. Summing over histories that lead to them, would allow worldsheets with boundaries
(and other topology features), including disconnected ones. The wave-function would take
the form
Ψ[X∂(σ, τ)] =
topologies
[dX ]e−S[X(σ,τ)], (36)
where the worldsheet path integrals are performed over string configurations X(σ, τ) that
satisfy a specified boundary condition at given values of the zero-modes of the string con-
figuration, i.e. at a given position of the target space. The equivalence of these descriptions
is far from obvious, but it is made plausible by the fact that for two-dimensional string
worldsheets, the first quantized description automatically comes with a prescription for the
proper weighting of interaction vertices. The initial-time closed string configurations could
be specified in terms of macroscopic loop operators discussed for example in [35]. The first
quantized prescription considers fluctuations around a given background. A full second
quantized prescription also integrates over backgrounds as in general relativity [1].
The wave-function so defined is hard to compute, although it may be obtained presumably
for very particular boundary conditions. An example would be boundary conditions that
are fixed by taking a Z2 orbifold that folds over the disk onto itself – in that case, one may
be able to compute the value of the wave-function for a particular argument. In order to
understand better some global properties, we again follow [1] and concentrate on calculating
the norm of the wave-function.
6 The norm of the wave-function
The norm of the wave-function is easier to compute. It is given by the following calculation:
||Ψ||2 =
[dΦ∂]
half T−fold
[dΦ]e−S(Φ) ×
conj half T−fold
[dΦ]e−S(Φ)
T−fold
[dΦ]e−S(Φ), (37)
where we have expressed it as a string field theory path integral in terms of a string field Φ.
The final integral is an integral over all possible string field configurations on the Euclidean
T-fold. No boundary conditions need to be specified.
We can do this calculation by considering the fluctuations around an on-shell closed string
background, in a first quantized formalism:
||Ψ||2 =
topologies
[dX ]e−S[X(σ,τ)] (38)
where X(σ, τ) is any mapping from the string worldsheet into the target space. The sum
is over all closed worldsheet topologies, and includes a sum over disconnected diagrams. In
fact it is equal to the following exponential of a sum of connected diagrams:
||Ψ||2 = exp(Ztotal), (39)
where the function Ztotal is the total string theory partition function, which is defined as a
sum over Euclidean worldsheet topologies:
Ztotal =
ZS2 + ZT 2 + g
sZgenus=2 +
g2g−2s Zgenus=g. (40)
Therefore, to evaluate the norm of the wave-function perturbatively, we need to evaluate
the partition function for string theory on the Riemann surfaces of genus 0, 1, 2, . . . and
add their contributions with the appropriate power of the string coupling constant. The
first contribution is akin to the tree level contribution in ordinary gravity, the second to the
one-loop contribution, etc.
7 Thermal nature of the wave-function
A natural way to perform the Euclidean path integral in equation (35) over half the space
is as follows. The origin X = 0 in one T-fold patch (and similarly for the other), divides
the T = 0 slice into two halves: the left half corresponding to X < 0 and the right part
corresponding to X > 0. We denote the boundary data on X < 0 by φL and on X > 0 by
φR. See figure 3. By dividing the space into angular wedges spanning an overall angle equal
to π, we can evaluate the path integral in terms of the generator of angular rotations. This
generator is given by the analytic continuation of iHω, where Hω = i∂ω is the Hamiltonian
conjugate to “Rindler” time in the region uv > 0 of the Lorentzian cosmology. Indeed in
this region, we may set u = ρe−ω, v = ρeω, with the string frame metric and dilaton given
ds2 = |k|α′
dρ2 − ρ2dω2
1− ρ2
e2Φ =
1− ρ2
. (41)
In this patch, the background metric is static, invariant under time translations, and the
dilaton field is space-like. Rotating to Euclidean signature amounts to setting ω = −iφ.
Figure 3: The thermal interpretation of the wave-function is obtained by thinking of the
path integral as being performed along angular wedges from an initial (right) to a final (left)
configuration.
So Rindler time translations correspond to angular rotations in the Euclidean space. As
we have already discussed, only discrete angular rotations are true symmetries of the string
theory background.
The boundary data can then be viewed as specifying initial and final conditions for the
path integral evolution. This is clearly reviewed for the case of flat Rindler space and black
hole spaces in [37]. In particular, the path integral measures the overlap between the data
on the right φR, evolved for a Euclidean time π, and the data specified on the left φL (see
figure 3), and it can be written as an amplitude
Ψ(φL, φR) = 〈φL|e−πHω |φR〉. (42)
If we integrate over φL we obtain a thermal density matrix appropriate for the Rindler
observer [37]
[dφL]Ψ(φL, φR)Ψ
∗(φL, φ
R) = 〈φ′R|e−2πHω |φR〉, (43)
with dimensionless temperature Tω = 1/2π. The norm of the wave-function is given by the
trace
||Ψ||2 = Tre−2πHω (44)
and so it can be interpreted as a thermal space-time partition function. The genus-1 string
contribution is a thermal one-loop amplitude.
In the full Euclidean path integral equation (37), the contributions of the fermionic fields
are positive. To perform the full path integral over the whole T-fold, we divide it into angular
wedges spanning an angle equal to 2π. Since the space has no non-contractible cycles, the
space-time fermionic fields have to be taken anti-periodic in the angular variable and they
contribute positively to the path integral.
The T-fold patches are glued along the hyperbola ρ = 1/
2 (see section 4). Near this
region, the curvature is low for large enough level |k|. Thus for large level |k|, we may use the
metric equation (41) to conclude that observers moving near the region ρ ∼ 1/
2 measure
a proper temperature T ∼ 1/(2π
|k|α′). In the cosmological region uv ≤ 0, there is also an
effective temperature of the same order as a result of particle production [14][28]. For small
level |k|, we need a string calculation to deduce the proper temperature of the system.
8 Specific Examples
As we discussed above in order to derive the wave-function of the cosmology, we need to
compute the total string partition function for the corresponding Euclidean background.
When conformal field theories are compact, the genus-zero contribution to the total string
partition function vanishes. This is because the spherical string partition function is divided
by the infinite volume of the conformal Killing group. This fact is a first important difference
with the calculation in general relativity where the classical contribution is non-zero. In
perturbative string theory the leading contribution is the genus-1 amplitude.
The Euclidean examples we shall describe here in detail belong to the family of ĉ = 10
superconformal, compact systems. In order for them to admit a Lorentzian continuation to
a cosmological (time-dependent) background, space-time supersymmetry must be broken.
Moreover, the models must be free of tachyons. The presence of tachyonic modes would
indicate that the system undergoes a phase transition. The only known examples with the
above properties are of the form
SU(2)|k|
× K ←→
SL(2, R)−|k|
× K , (45)
where we indicated the analytic continuation from the Euclidean to the Lorentzian space-
time. The level |k| can be taken to be small.
As we already discussed, the relevant genus-1 string amplitude has to be thermal. The
total superstring model has transverse central charge equal to c = 12 (or ĉ = 8). As a
consequence, it has a Hagedorn transition at the fermionic radius RH =
2α′. In order for
the genus-1 string amplitude to be finite, the physical temperature of the model has to be
below the Hagedorn temperature: T < TH = 1/(2π
2α′). Let us give an argument that this
can be realized for any |k| ≥ 2. In writing the norm of the wave-function as a thermal space-
time partition function, the role of the Hamiltonian is taken up by the generator of rotations
on the disk. For the superconformal SU(2)/U(1) model, the corresponding U(1) current is
at level |k| + 2 [36]. Thus we expect the physical temperature of the model to be set by
the radius associated to this isometry generator, namely
(|k|+ 2)α′. The corresponding
temperature is given by
(|k|+ 2)α′
and is below the Hagedorn temperature for any positive (integer) level |k|. We will find that
this temperature arises naturally in a level |k| = 2 model below. At level |k| = 0, where
the minimal model has zero central charge, and consists only of the identity operator (and
state), the cosmology disappears. When we reach the Hagedorn transition, the cosmology
becomes so highly curved that it is no longer present in the string theory background.
8.1 Compact models
In a first class of specific examples that we will discuss in this section, we choose the level
|k| = 2, and we take the internal conformal field theory K to be:
K = T 2 ×
i=1,...,7
SU(2)ki
where all ki’s are taken equal to 2, so that ĉK = 9 (representing the central charge equivalent
of nine flat directions). In the sequel, we set α′ = 1.
For this choice, the supersymmetric characters of the whole system are defined in terms
of eight level k = 2 parafermionic systems (which are nothing but eight real fermions ψi and
eight bosons φi compactified at the self-dual radius R = 1), and also a complex fermion ΨT
and a complex boson ΦT for the torus T
2. The N = 2 superconformal operators TF , J are:
i=0,1,...7
2φi + iΨT∂ΦT
i=0,1,...7
∂φi +ΨT Ψ̄T . (48)
It is convenient to pair the (0, 1), (2, 3), (4, 5), (6, 7) systems respectively in order to
obtain four copies of ĉ = 2 systems. For the first copy we define the bosons H0, H1 at radius
2 (or R = 1√
, fermionic T -dual points):
(H0 +H1), φ1 =
(H0 −H1) (49)
and similarly for the others. Then the currents are given by
i=0,2,4,6
i(Hi+Hi+1) + ψi+1 e
i(Hi−Hi+1) + i∂ΦT e
J = i∂H0 + i∂H2 + i∂H4 + i∂H6 + i∂HT , (50)
where i∂HT = ΨT Ψ̄T , which is also defined at the fermionic point. Observe that the N = 2
current is given in terms of H0, H2, H4, H6 and HT only and is normalized correctly for a
system with ĉ = 10.
The N = 2 left-moving characters of a particular ĉ = 2 system (e.g of the one containing
H0), are expressed in terms of the usual level-2 Θ-functions 3:
η(τ)3
2 ΘH0
α+2H0
β+2G0
γ0−2H0
δ0−2G0
3γ0−4H0
3δ0−4G0
where the arguments (γ0, δ0) and (4H0, 4G0) are integers. The later exemplifies the chiral
Zk+2-symmetry of the superconformal parafermionic characters (k + 2 = 4 in our case).
Similar expressions are obtained for the other three ĉ = 2 parafermionic systems by replacing
(γ0, δ0) with (γi, δi), and (H0, G0) by (Hi, Gi), i = 1, 2, 3.
The global existence of the N = 2 superconformal world-sheet symmetry and thus the
existence of the left space-time supersymmetry imply
(HT , GT ) +
i=0,1,2,3
(Hi, Gi) = ǫ mod 2 , (52)
3Our convention for the level-2 Θ function is Θ [
iπτ(n+
)2+2iπ(n+
and similarly for the right supersymmetry. The arguments (ǫ, ǭ) define the chirality of the
space-time spinors. A simple choice is to set (HT , GT ) = (0, 0), (Hi, Gi) = (H,G) for i = 0, 1
and (Hi, Gi) = (−H,−G) for i = 2, 3 (and similarly for the right arguments). Then if
(ǫ, ǭ) = (1, 1), space-time supersymmetry is broken. For the other choices there is some
amount of supersymmetry preserved. In this class of models the only remaining possibility
consistent with the global N = 2 super-parameterization consists of shifts on ΦT of the T
torus.
Z4 orbifold models
Using the chiral Z4 symmetry defined above and its subgroups, we can obtain four classes
of models:
(H,G) =
(h, g), where (h, g) = integers, M = 1, 2, 3, 4. (53)
In particular, if we orbifold by Z4 (M = 1 or M = 3 ), the ψ0 parafermion decouples from
the rest, especially from ψ1. This is clear since in this case, the arguments of Θψ0 and Θψ1
become independent.
Initially, the arguments (γi, δi) are taken to be identical for the left- and right-moving
characters. In this case the modular invariance of the partition function is manifest. Indeed,
using the periodicity property of Θ-functions
∣ = |Θ [γδ ]| , ǫ, ζ integers, (54)
and orbifolding by Z4, (M = 1), the genus-1 modular invariant partition function becomes:
dτdτ̄
(Imτ)2
η(τ)12η̄(τ̄ )12
h,g=0,1,2,3
γ0,δ0
γ0−h2
δ0− g2
γ1,δ1
γ1−h2
δ1− g2
γ2,δ2
γ3,δ3
(α,β)
eiπ(α+β+ǫαβ) Θ
(ᾱ,β̄)
eiπ(ᾱ+β̄+ǭᾱβ̄) Θ̄
β̄− g
. (55)
The arguments (α, β) and (ᾱ, β̄) are those associated to the N = 2 left- and right-moving
supercurrents. If (ǫ, ǭ) = (1, 1), supersymmetry is broken. For all other choices this partition
function is identically zero and there is some amount of supersymmetry preserved. To obtain
the above result we have used the fact that the contribution of the superconformal ghosts
cancel the oscillator contributions of the T 2 supercoordinates (ΦT ,ΨT ). This is the reason
for choosing the Z4 not to act on ΨT , having set (HT , GT ) = (0, 0). The only remnant from
the torus contribution is the Γ2,2 lattice, which can be possibly shifted by (Lh/2, Lg/2) with
either L = 0 or L = 1, 2, 3 (as we will see later).
To proceed we need to identify and insert the thermal co-cycle S
q , (α+ ᾱ)
p, (β+β̄)
associated to
the time direction of the cosmology:
q, (α+ᾱ)
p, (β+β̄)
= eiπ( p(α+ᾱ)+ q(β+β̄) ), (56)
where p and q are the lattice charges associated to the compactified Euclidean time. Here,
Fα = (α + ᾱ) and Fβ = (β + β̄) define the spin of space-time particles: Fα = 1 modulo
2 for fermions and Fα = 0 modulo 2 for bosons. To impose this co-cycle insertion, it is
necessary to rewrite the partition function in a form that reveals the charge lattice (p, q) of
the Euclidean time direction. To this end, it is convenient to separate the partition function
into the “untwisted sector” (h, g) = (0, 0) and “ twisted sectors” (h, g) 6= (0, 0),
dτdτ̄
(Imτ)2
Zunt +
(h,g)6=(0,0)
Ztwist
 . (57)
To isolate the relevant (p, q) charge lattice, we use the identity
|Θ [γδ ]|
(m,n)
2 |m+τn|
Imτ eiπ(mγ+nδ+mn) , R2 =
. (58)
Although in the above identity the radius is fixed to the fermionic point R2 = 1/2, we note
that the modular transformation properties are the same for any R2, and in particular for
the dual-fermionic point with R2 = 2.
Using the above identity for the conformal block involving the parafermions ψ0,1 and the
H1 field, we obtain
(γ,δ)
∣Θψ0,1 [
2 |ΘH1 [
(γ,δ)
(m1,n1),(m2,n2)
|m1+τn1|
−πR22
|m2+τn2|
Imτ eiπ((m1+m2)γ+(n1+n2)δ+m1n1+m2n2) (59)
where R21 = R
2 = R
2 = 1/2. Since the arguments (γ, δ) do not appear elsewhere (in equation
(55)), summing over them forces (m1+m2) and (n1+n2) to be even integers. This constraint
can be solved if we take
m1 = p1 + p2, m2 = p1 − p2, n1 = q1 + q2, n2 = q1 − q2 (60)
so that,
(γ,δ)
∣Θψ0,1 [
2 |ΘH1 [
= Γ1,1(R+) Γ1,1(R−) =
(p1,q1)
2 R+ e
−πR2+
|p1+τq1|
(p2,q2)
2 R− e
−πR2−
|p2+τq2|
 (61)
with R2+ = R
− = 2R
2 = 1. Therefore, the partition function of the bosonic part of
parafermions factorizes in two Γ1,1 lattices, both of them with twice the initial radius squared.
The charge lattices (p1, q1) and (p2, q2) are associated to the ψ0 and ψ1 parafermions.
We can see this as follows. Consider the left-charge operators which are well defined in the
untwisted sector:
Q+ = i
dz(ψ0ψ1 + ∂H1), Q− = i
dz(ψ0ψ1 − ∂H1) (62)
and similarly for the right-moving ones (Q̄±). Then,
(Q + Q̄)+ = m1 +m2 = 2p1, (Q− Q̄)+ = n1 + n2 = 2q1
(Q+ Q̄)− = m1 −m2 = 2p2, (Q− Q̄)− = n1 − n2 = 2q2 (63)
where we have used the constraint (60). We identify the charges (p1, q1) as the momenta
that enter in the thermal co-cycle, and associate the lattice to the Euclidean time direction.
Before we proceed further, let us stress the following point. We started with a diagonal
modular invariant combination and with initial radii R21 = R
2 = R
2 = 1/2. The anti-diagonal
choice implies that the initial values for the radii are at the fermionic T-dual points, namely
R21 = R
2 = R
2 = 2. Notice that in all we perform two T-dualities simultaneously so that
we remain in the same type II theory. Thus the conformal block, equation (59), can be
replaced with the T-dual one with R2 = 2.4 For the anti-diagonal choice, the radii of the
corresponding factorized lattices are given by R2+ = R
− = 4 instead of unity for the diagonal
combination.
Thus in the untwisted sector, Zunt has to be replaced with
Zunt −→ Zthermalunt =
(p1,q1)
(α,β),(ᾱ,β̄)
Zthermalunt
q1, α+ᾱ
p1, β+β̄
eiπ(p1(α+ᾱ)+q1(β+β̄)). (64)
Performing a similar factorization for the remaining three copies of ĉ = 2 superconformal
parafermionic blocks, we can write the thermal untwisted contribution in a compact form:
Zthermalunt =
ImτΓ9,9
η12η̄12
(α,β),(ᾱ,β̄)
Γ1,1(R+)
eiπ(α+β+ǫαβ) Θ
eiπ(ᾱ+β̄+ǭᾱβ̄) Θ̄
The Γ9,9 lattice factor is composed of a product of lattices: the initial Γ2,2 lattice of the
torus T 2 with radii Ry, Rz, the Γ1,1 lattice of the first parafermionic block at radius R−, and
the product of three pairs Γ1,1(R
+) Γ1,1(R
−) for the other three parafermionic blocks. The
Γ1,1(R+) lattice is the thermally shifted lattice
Γ1,1(R+)
(p1,q1)
2 R+ e
−πR2+
|p1+τq1|
Imτ eiπ((α+ᾱ)p1+(β+β̄)q1). (66)
Its coupling with the space-time spin structure breaks space-time supersymmetry so that
both bosons and fermions give positive contributions to the thermal partition function. The
radius R+ sets the temperature of the system: 2πT = 1/R+. The form of the thermal
coupling in equation (65) is similar to the one that appears in the familiar flat type II
superstring theories at finite temperature. The difference here is that the temperature is
fixed. Since we have succeeded to factorize out the thermal lattice, we can now treat all other
radii parameterizing the Γ9,9 lattice as independent moduli. To obtain the four dimensional
4In Gepner’s formalism [36] the anti-diagonal combination corresponds to exchanging (m, m̄)→ (m,−m̄).
interpretation we discussed in section 2, we take the radii Ry,z to be large keeping all other
ones small.
As we already remarked, there are only two choices consistent with the cosmological
interpretation of the partition function corresponding to the two values of the radius R+.
For the diagonal choice we have a radius R2+ = 1 corresponding to a temperature 2πT = 1/R+
higher than Hagedorn: 2πTH = 1/RH = 1/
2. This model is tachyonic and so unstable in
perturbation theory. For the second anti-diagonal choice R2+ = 4, and the temperature is
below Hagedorn: 2πT = 1/2 < 2πTH . This is precisely the temperature that we gathered
from general arguments, equation (46) for level |k| = 2. That model gives rise to a well
defined, integrable partition function and a finite norm for the wave-function at one loop.
The integral is difficult to perform analytically but it can be estimated. We will not carry
out this computation here.
The remaining part consists of the twisted sectors of the theory, (h, g) 6= 0. Here we shall
find new stringy phenomena associated with the fact that we are orbifolding the Euclidean
time circle. Since this is twisted, the thermal co-cycle has to be extended consistently. It
takes the general form, valid for both the untwisted and the twisted sectors:
(q+h), (α+ᾱ)
(p+g), (β+β̄)
= eiπ( (p+g)(α+ᾱ)+ (q+h)(β+β̄) ). (67)
That is, the relevant lattice is augmented by the quantum numbers (g, h) that label the
twisted sectors. Again, the thermal co-cycle insures that fermions contribute positively to
the partition function. In the twisted sectors, there is no momentum charge and we can set
(p, q) = (0, 0).
In the twisted sectors, each of the ĉ = 2 superconformal blocks is equivalent to a system
described by a free complex boson and a free complex fermion twisted by Z4. This equivalence
implies topological identities for each N = 2 twisted superconformal block [38][39]:
2|η|4
(γi,δi)
γi−h2
δi− g2
= 22 sin2(
πΛ(h, g)
where Λ(h, g) = Λ(g, h) depend on the (h, g)-twisted sector. Λ(h, g) = 2 when (h, g) =(0,2),
(2,0) and (2,2) while for the remaining 12 twisted sectors Λ(h, g) = 1. Although the above
orbifold expressions are derived at the fermionic point, they remain valid for any other point
of the untwisted moduli space.
Using the above orbifold identity, the “twisted” part of the thermal partition function
simplifies to:
Zthermaltwist =
(α,β,ᾱ,β̄)
(h,g)6=(0,0)
Imτ Γ2,2 e
iπ(α+β+ǫαβ)eiπ(ᾱ+β̄+ǭᾱβ̄)eiπ(α+ᾱ)g+iπ(β+β̄)h
× 28 sin8(
πΛ(h, g)
β̄− g
. (69)
Furthermore, by using the left- (and right-) Jacobi identities
(α,β)
eiπ(α+β+ǫαβ)Θ
= −|ǫ|Θ
, (70)
the twisted part of the thermal partition function simplifies further:
Zthermaltwist =
(h,g)6=(0,0)
Imτ Γ2,2 2
8 sin8(
πΛ(h, g)
) φ((g, h), (ǫ, ǭ)). (71)
The factor φ depends on the initial choice of the left- and right-chirality coefficients ǫ, ǭ
φ((g, h), (ǫ, ǭ)) =
|ǫ|+ (1− |ǫ|)(1− eiπgh)
|ǭ|+ (1− |ǭ|)(1− eiπgh)
. (72)
It should be noted that expression (71) has a field theoretic interpretation in terms of a
momentum lattice only.
If the initial non-thermal model was not supersymmetric, |ǫ| = |ǭ| = 1, then the number
of massless bosons and fermions will not be equal, nb 6= nf . For all other choices nb =
nf . This situation is reflected in the factor φ((g, h), (ǫ, ǭ)), which distinguishes the four
different possibilities. In the non supersymmetric case there is a non-vanishing contribution
to the partition function even in the absence of the thermal co-cycle. This is equivalent,
in field theory, to the one-loop zero temperature contribution to the effective action. This
contribution is zero in supersymmetric theories. In the later cases, the corrections are coming
from the massive thermal bosons and fermions plus a contribution from massless bosons. We
will display here a number of typical examples. The first class is when the Γ2,2 lattice is
unshifted by (h, g) and so it factorizes out from the sum over (g, h).
Unshifted Γ2,2 lattice
In the unshifted case, the sum over (g, h) can be performed easily so that the only remaining
dependence is that of ǫ, ǭ. We obtain [40][41],
Sthermaltwist =
dτdτ̄
(Imτ)2
Zthermaltwist = −C[ǫ, ǭ] log
|η(T )|4 |η(U)|4 ImT ImUµ2
where µ2 is an infrared cut-off and T, U parameterize the Kähler and complex structure
moduli of the target space torus T 2. The coefficient C[ǫ, ǭ] depends on the initial chiralities
of the spinors:
C[1, 1] = 240, C[1, 0] = C[0, 1] = 32, C[0, 0] = 64 . (74)
Actually, C[ǫ, ǭ] is nothing but the number of the massless bosonic degrees of freedom of the
theory. Equation (73) is invariant under the full target space T-duality group acting on the
T and U moduli separately.
For large volume, ImT ≫ 1, the leading behavior is linear in ImT ∼ RyRz. Assuming
iU ∼ Ry/Rz fixed and µ2 ∼ γ/RyRz, we have
Sthermaltwist = C[ǫ, ǭ]
RyRz − log
γ |η(U)|4 ImU
. (75)
Note that in the large volume limit, the twisted sector contribution to one-loop amplitude
depends both on the Kähler and complex structure moduli.
Shifted lattice
Another illustrating example is when the Z4 action shifts the Γ2,2 lattice simultaneously with
the twist we described before. In this case the lattice is replaced by a shifted Γ2,2 lattice.
The (Lh/2, Lg/2) shifted lattice, L = 1, 2, 3, is given by 5:
(R) =
(m,n)
2 |(4m+Lg)+(4n+Lh)τ |
Imτ . (76)
Here, we will examine in more detail the L = 1 case which corresponds to a 1/4-shifted
lattice.
5For brevity we have given the expression for the shifted Γ1,1 lattice but the generalization to the Γ2,2
lattice is straightforward.
When (ǫ, ǭ) = (1, 1), the contribution of the twisted sector to the partition function
becomes [39][42]:
Sthermaltwist = 240
dτdτ̄
(Imτ)2
Γ2,2[T, U ]−
Γ2,2[4T, 4U ]
. (77)
To obtain the above expression we have used the identities
(h,g)
= Γ2,2(T, U), Γ2,2[
0 ] =
Γ2,2[4T, 4U ] (78)
and we subtracted the contribution of the untwisted sector, (g, h) = (0, 0). Integrating over
τ we obtain [39] [42]
Sthermaltwist = − 60 log
|η(T )|16 |η(U)|16
|η(4T )|4 |η(4U)|4
ImT 3 ImU3µ6
. (79)
There is no volume factor in the large ImT limit (and this is generic in the case of freely
acting orbifolds [39][42]). So for large ImT , (and setting ImTµ2 ∼ γ), we obtain
Sthermaltwist = −60 log
|η(U)|16
|η(4U)|4
. (80)
In the large volume limit, Sthermaltwist only depends on the complex structure modulus of the
torus.
Comments
The total Euclidean one-loop amplitude in both the shifted and unshifted lattice cases is
given by:
Sthermal = Sthermalunt + S
thermal
twist (81)
The Sthermalunt is nothing but one-quarter of the thermal partition function of type II super-
string theory on S1 × T 9 with all nine spatial radii arbitrary, while that of Euclidean time
fixed by the temperature: 2πT = 1/R+ = 1/2. By exponentiating S
thermal, with the twisted
sector contributions in our examples given by equations (75) and (80), we obtain the norms
of the corresponding cosmological wave-functions as functions of all moduli.
The difference between the shifted and the ordinary model discussed previously can be
understood as follows. It is known that the freely acting orbifolds are related to gravitational
and gauge field backgrounds with fluxes [43]. This indicates the different interpretations of
the two cosmological models with shifted and unshifted Γ2,2. In the shifted model there are
non-vanishing magnetic fluxes [43] while in the unshifted case such fluxes are absent.
Let us stress here that the thermal Z4-orbifold described involves a twisting that leaves
two moduli parameterizing a Γ2,2 lattice, which in the large moduli limit gives us the four
dimensional cosmological model discussed in section 2. Many other orbifold-like models
can be constructed, which may factorize bigger lattices, admitting a higher dimensional
interpretation. In all those cases, the partition function is computable as a function of the
moduli. However its analytic form in terms of the moduli in various limits depends crucially
on whether the orbifold is freely, or even partially freely acting (or in other words, on the
different structure of magnetic fluxes). Models based on asymmetric orbifolds can also be
constructed giving rise to a rich family of calculable models. A more detailed analysis would
be interesting so as to understand the classification of the low level cosmologies, as well as
the characteristic dependence of the norm of the wave-function on the various features of
the large class of models. To illustrate the above points, we offer one further simple example
based on a Z2 instead of the Z4 orbifold.
Z2 orbifold models
In the Z2 orbifold models (M = 2 in equation (53)), the factorization of the cosmological
CFT factor is not so explicit as it was in the Z4 examples. However the cosmological
interpretation remains the same.
For the untwisted sector, the genus-1 contribution Sthermalunt is now one half of the thermal
partition function of type II theory on S1 × T 9. We proceed to analyze the twisted sector
contribution to the genus-1 amplitude. Following similar steps as in the Z4 orbifold case,
and now setting (2H, 2G) = (h, g) to be integers defined modulo 2, we obtain for the case
(ǫ, ǭ) = (0, 0)
Sthermaltwist =
dτdτ̄
(Imτ)2
(h,g)6=(0,0)
28 Imτ Γ2,2 . (82)
To arrive at the result notice that since the characters (h, g) are defined to be integers modulo
2, the factor of 1/4 in the first line of equation (55) now becomes 1/2; no other modifications
in this formula are needed. All the other steps carry through as before. Here also we may
classify the models in two classes; in the first class the lattice is taken to be unshifted, while
in the second class the Γ2,2 is half-shifted. We will use the definition:
(R) =
(m,n)
2 |(2m+Lg)+(2n+Lh)τ|
Imτ . (83)
For the unshifted Γ2,2 Z2-model, and when (ǫ, ǭ) = (0, 0), we obtain:
Sthermaltwist = −384 log
|η(T )|4 |η(U)|4 ImT ImUµ2
. (84)
As in the Z4 models, for large volume, ImT ≫ 1, the leading behavior is linear in ImT ∼
RyRz. Assuming iU ∼ Ry/Rz fixed and µ2 ∼ γ/RyRz we have:
Sthermaltwist = 384
RyRz − log
γ |η(U)|4 ImU
. (85)
The Z2-model with half-shifted lattice, Γ2,2[
g ], (g, h = 0, 1), yields [39][42]
Sthermaltwist = 384
dτdτ̄
(Imτ)2
Γ2,2[T, U ]−
Γ2,2[ 2T, 2U ]
. (86)
Here also the contribution coming from the untwisted sector (h, g) = (0, 0), is subtracted.
To obtain the above expression we have used the identities
(h,g)
Γ2,2[
g ] = Γ2,2[T, U ], Γ2,2[
0 ] =
Γ2,2[ 2T, 2U ]. (87)
Integrating over τ we obtain:
Sthermaltwist = −192 log
|η(T )|8 |η(U)|8
|η(2T )|4 |η(2U)|4
ImT ImUµ2
. (88)
There is no volume factor in the large ImT limit. Thus, for large ImT , (and setting ImTµ2 ∼
γ), we obtain for the half-shifted lattice contribution to Sthermaltwist :
Sthermaltwist = −192 log
γ |η(U)|8
|η(2U)|4
. (89)
As we see, we can obtain explicit expressions for the genus-1 approximation to the norm
of the wave-function for these particular Z2-models. The twisted sector contribution is given
explicitly by equations (88) and (89).
In the above family of models, we have always considered a two-dimensional cosmology
at small level. That choice is mainly due to two obstructions that are difficult (but not
necessarily impossible) to circumvent. One is associated to the difficulty of continuing from
Lorentzian to Euclidean signature in the presence of electric fluxes. The other is that it is
difficult to construct compact models with positive central charge deficit (negatively curved
Euclidean backgrounds) in string theory, or alternatively, a compact version of linear dilaton
type models. We make some further comments on this in the next section.
8.2 Liouville type models
Consider string theory cosmological backgrounds based on worldsheet CFTs of the form (see
e.g. [13][22][14][23][25][26][27][28][29]):
SL(2, R)−|k|
SL(2, R)|k|+4
×K. (90)
A nice feature of such models is that the combined central charge of the two SL(2, R)/U(1)
factors is independent of |k|, and so this can be taken to be an independent, varying param-
eter. Although the partition function of the euclidean cigar background is known [44], we
need to deal first with the fact that for such a background the cosmological wave-function is
non-normalizable due to the infinite volume of the cigar factor SL(2, R)|k|/U(1). To produce
a normalizable wave-function we must face the problem of consistently compactifying this
factor, as alluded at the end of the previous section. Moreover, it would be interesting to
obtain a compactification scheme which leaves |k| a free parameter. Compactifying the cigar
would amount to discretizing its continuous modes keeping at the same time the unitarity
and the modular invariance of the torus partition function intact.
An interesting aspect of these models is that now the sphere contribution to the string
partition function is finite since the volume of the conformal Killing group cancels against
the volume of the SL(2, R)|k|/U(1) conformal field theory factor. Nevertheless, we can see
that the torus contribution dominates (at any finite string coupling) due to the volume
divergence. If a consistent way of cutting off the volume of the cigar is found, we could
interpret the torus contribution as a finite thermal correction to the tree-level contribution,
realizing a stringy version of the computation in [5][7][45].
A further suggestion for developing our formalism in linear dilaton spaces, is to view
the wave-function of the universe as also depending on a boundary condition in the space-
like linear dilaton direction of the cigar, in order to obtain a linear dilaton holographic
interpretation [46]. Part of the interpretation of the wave-function of the universe would
then be as in the Hartle-Hawking picture, and part would be holographic. Finally in these
models supersymmetry can be restored asymptotically in the large |k| limit, making contact
with linear dilaton models in null directions (see e.g. [47] for recent progress).
9 Discussion
We have outlined a framework generalizing the Hartle-Hawking no boundary proposal of
the wave-function of the universe to string theory cosmological backgrounds. The class of
example cosmologies considered here are described by worldsheet conformal field theories
of the general form SL(2, R)−|k|/U(1) × K, where K is an internal, compact CFT. In
order to define the analogue of the Hartle-Hawking wave-function, we had to surmount
the technical hurdle of realizing that such cosmologies (like the corresponding Euclidean
parafermion theories) have an almost geometrical description in terms of a compact non-
singular T-fold. We then defined the wave-function of the universe via a Euclidean string
field theory path integral (generalizing the no-boundary proposal). For specific examples we
computed the norm of the wave-function to leading order in string perturbation theory, as
a function of moduli parameters. There are many interesting similar examples to which we
can generalize our analysis.
In a probabilistic interpretation, with a normalizable wave-function at hand, one can
attempt to compute vacuum expectation values for particular physical quantities in various
cosmological models, and analyze their properties in various regions of the moduli space. Our
purpose in this paper was to provide the framework for such a discussion, which promises
to be interesting. In particular, it is an open problem to identify preferred regions in the
moduli space in the large class of models to which our analysis applies.
More concretely, we believe our construction points out the good use that can be made
of T-folds, and generalized geometry, in stringy cosmologies (allowing to evade various no-go
theorems in pure geometry). Moreover, we have been able to define a sensible calculation
in a de Sitter-like compactification of string theory, after analytically continuing to the
Euclidean theory. These calculations are generically hard to come by in de Sitter gravity
after quantization, so any well-defined cosmological quantity, like the norm of the wave-
function of stringy universes, merits scrutiny. Finally, we calculated a quantity akin to an
entanglement entropy in de Sitter space, and showed that it only gets contributions starting
at one loop, and we gave its microscopic origin.
Acknowledgements
We thank Constantin Bachas, James Bedford, Ben Craps, John Iliopoulos, Dieter Luest,
Hervé Partouche, Anastasios Petkou, Giuseppe Policastro and Marios Petropoulos for useful
discussions. N. T. thanks the Ecole Normale Supérieure and C. K. and J. T. thank the
University of Cyprus for hospitality. This work was supported in part by the EU under
the contracts MRTN-CT-2004-005104, MRTN-CT-2004-512194 and ANR (CNRS-USAR)
contract No 05-BLAN-0079-01 (01/12/05).
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	Introduction
	The cosmological solution
	The Euclidean continuation
	The cosmological T-fold
	Defining the wave-function of the universe
	The norm of the wave-function
	Thermal nature of the wave-function
	Specific Examples
	Compact models
	Liouville type models
	Discussion
