String Cosmology — Large-Field Inflation in String Theory

title: Gauge unification in extra dimensions: power corrections vs. higher dimension operators
abstract: Power-like loop corrections to gauge couplings are a generic feature of higher-dimensional field theories. In supersymmetric grand unified theories in d = 5 dimensions, such corrections arise only in the presence of a vacuum expectation value of the adjoint scalar of the gauge multiplet. We show that, using the analysis of the exact quantum effective action by Intriligator, Morrison and Seiberg, these power corrections can be understood as the effect of higher dimension operators. Such operators, both classical and quantum, are highly constrained by gauge symmetry and supersymmetry. As a result, even non-perturbatively large contributions to gauge coupling unification can be unambiguously determined within 5d low-energy effective field theory. Since no massive hypermultiplet matter exists in 6 dimensions, the predictivity is further enhanced by embedding the 5d model in a 6d gauge theory relevant at smaller distances. Thus, large and quantitatively controlled power-law contributions to gauge couplings arise naturally and can, in the most extreme case, lead to calculable TeV-scale power law unification. We identify a simple 5d SU(5) model with one massless 10 in the bulk where the power-law effect is exactly MSSM-like.

title: Evading the Lyth Bound in Hybrid Natural Inflation
abstract: Generically, the gravitational-wave or tensor-mode contribution to the primordial curvature spectrum of inflation is tiny if the field-range of the inflaton is much smaller than the Planck scale. We show that this pessimistic conclusion is naturally avoided in a rather broad class of small-field models. More specifically, we consider models where an axion-like shift symmetry keeps the inflaton potential flat (up to non-perturbative cosine-shaped modulations), but inflation nevertheless ends in a waterfall-regime, as is typical for hybrid inflation. In such hybrid natural inflation scenarios (examples are provided by Wilson line inflation and fluxbrane inflation), the slow-roll parameter ǫ can be sizable during an early period (relevant for the CMB spectrum). Subsequently, ǫ quickly becomes very small before the tachyonic instability eventually terminates the slow-roll regime. In this scenario, one naturally generates a considerable tensor-mode contribution in the curvature spectrum, collecting nevertheless the required amount of e-foldings during the final period of inflation. While non-observation of tensors by Planck is certainly not a problem, a discovery in the medium to long term future is realistic.

title: Study of the neutron quantum states in the gravity field
abstract: We have studied neutron quantum states in the potential well formed by the earth’s gravitational field and a horizontal mirror. The estimated characteristic sizes of the neutron wave functions in the two lowest quantum states correspond to expectations with an experimental accuracy. A position-sensitive neutron detector with an extra-high spatial resolution of $\sim 2 \mathrm{\mu} $m was developed and tested for this particular experiment, to be used to measure the spatial density distribution in a standing neutron wave above a mirror for a set of some of the lowest quantum states. The present experiment can be used to set an upper limit for an additional short-range fundamental force. We studied methodological uncertainties as well as the feasibility of improving further the accuracy of this experiment.

title: $R^2\log R$ quantum corrections and the inflationary observables
abstract: We study a model of inflation with terms quadratic and logarithmic in the Ricci scalar, where the gravitational action is $f(R)=R+\alpha R^2+\beta R^2 \ln R$. These terms are expected to arise from one loop corrections involving matter fields in curved space-time. The spectral index $n_s$ and the tensor to scalar ratio yield $10^{-4}\lesssim r\lesssim0.03$ and $0.94\lesssim n_s \lesssim 0.99$. i.e. $r$ is an order of magnitude bigger or smaller than the original Starobinsky model which predicted $r\sim 10^{-3}$. Further enhancement of $r$ gives a scale invariant $n_s\sim 1$ or higher. Other inflationary observables are $d n_s/d\ln k \gtrsim -5.2 \times 10^{-4},\, \mu \lesssim 2.1 \times 10^{-8} ,\, y \lesssim 2.6 \times 10^{-9}$. Despite the enhancement in $r$, if the recent BICEP2 measurement stands, this model is disfavoured.

title: Power Spectrum of Inflationary Attractors
abstract: Inflationary attractors predict the spectral index and tensor-to-scalar ratio to take specific values that are consistent with Planck. An example is the universal attractor for models with a generalised non-minimal coupling, leading to Starobinsky inflation. In this paper we demonstrate that it also predicts a specific relation between the amplitude of the power spectrum and the number of e-folds. The length and height of the inflationary plateau are related via the non-minimal coupling: in a wide variety of examples, the observed power normalisation leads to at least 55 flat e-foldings. Prior to this phase, the inflationary predictions vary and can account for the observational indications of power loss at large angular scales.

title: Observation of muons using the polar ice cap as a Cerenkov detector
abstract: DETECTION of the small flux of extraterrestrial neutrinos expected at energies above 1 TeV, and identification of their astrophysical point sources, will require neutrino telescopes with effective areas measured in square kilometres—much larger than detectors now existing1–3. Such a device can be built only by using some naturally occurring detecting medium of enormous extent: deep Antarctic ice is a strong candidate. A neutrino telescope could be constructed by drilling holes in the ice with hot water into which photomultiplier tubes could be placed to a depth of 1 km. Neutrinos would be recorded, as in underground neutrino detectors using water as the medium, by the observation of Cerenkov radiation from secondary muons. We have begun the AMANDA (Antarctic Muon and Neutrino Detector Array) project to test this idea, and here we describe a pilot experiment using photomultiplier tubes placed into Arctic ice in Greenland. Cerenkov radiation from muons was detected, and a comparison of count rate with the expected muon flux indicates that the ice is very transparent, with an absorption length greater than 18 m. Our results suggest that a full-scale Antarctic ice detector is technically quite feasible.

title: AMANDA: Antarctic Muon And Neutrino Detector Array
abstract: Enormous detectors with unprecedented sensitivity will be required to search for astrophysical sources of neutrinos. If the transparency of deep polar ice is similar to that measured in the laboratory, then AMANDA (Antarctic Muon And Neutrino Detector Array) may be the most cost-effective way to reach the detector volumes required to search for astrophysical sources of high energy neutrinos or WIMP annihilation within the sun.

title: Energy loss of relativistic heavy ions in matter
abstract: We have investigated the theory of energy loss of charged particles in matter due to ionization of the medium, integrating the work of other authors over several decades. We describe the most important corrections to standard energy-loss formulae. We compare our calculations to an improved measurement of the range of 1 A GeV U ions and to other codes and tabulations. We show that calculations based on the theory of ionization energy loss are at least as accurate as tabulations extrapolated from empirical data for the stopping of uranium ions in matter. We have made available the computer code resulting from our investigations.

title: Amanda South Pole neutrino detector
abstract: High energy neutrino radiation provides us with a tomographic tool to scan the Universe. Sites shielded by a few hundred grams of matter are not revealed by light, whatever its wavelength. High energy neutrinos are decay products of pions and therefore are a signature of the most energetic cosmic processes. It is proposed to instrument the polar ice cap as a low‐noise particle detector sensing the Cherenkov light from neutrino‐induced muons. This paper describes the successful operation of a prototype string of optical modules that were deployed on January 17, 1992 at the South Pole. The advantages of the method are intriguing. The ice has good optical transmission and provides a mechanical structure to support the instrument. Ice is a quiet, sterile medium where photomultiplier tubes experience low background counting rates. The low background allows us to develop electronics and triggering schemes that are simple and can be installed at the surface and thus remain accessible after deployment of the PMTs...

title: Comet 81P/Wild 2 Under a Microscope
abstract: The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.