Project Summary for the Particle Physics Project Prioritization Panel

**February 7, 2003**

**The Supernova/Acceleration Probe (SNAP) Collaboration**

G. Aldering, C. Bebek, J. Bercovitz, M. Bester, E. Commins, W.
Carithers, C. Day, R. DiGennaro, G. Goldhaber, D. Groom, S. Harris, P.
Harvey, H. Heetderks, S. Holland, D. Huterer, R. W. Kadel, A. Karcher,
A. Kim, W. Kolbe, J. Lamoureux, R. Lafever,

M. Lampton, M. Levi, E. Linder, S. Loken, R. Miquel, P. Nugent, H.
Oluseyi, N. Palaio,

D. Pankow, S. Perlmutter, K. Robinson, N. Roe, M. Sholl, G. Smoot, A.
Spadafora,

H. von der Lippe, J-P. Walder, G. Wang

**Lawrence Berkeley National Laboratory, University of California
Berkeley, and University of California Space Sciences Laboratory**

C. Akerlof, D. Levin, T. McKay, S. McKee, M. Schubnell, G. Tarlé, A.
Tomasch

**University of Michigan**

R. Ellis, J. Rhodes

**California Institute of Technology**

C. Bower, N. Mostek, J. Musser, S. Mufson

**Indiana University**

A. Fruchter, R. Bohlin

**Space Telescope Science Institute**

G. Bernstein

**University of Pennsylvania**

S. Deustua

**American Astronomical Society**

P. Astier, E. Barrelet, A. Bonissent, A. Ealet, J-F. Genat, R. Malina,
R. Pain, E. Prieto,

A. Refregier, G. Smadja, D. Vincent

**France: IN2P3/INSU/CEA/LAM**

R. Amanullah, L. Bergström, M. Eriksson, A. Goobar, E. Mörtsell

**University of Stockholm**

C. Baltay, W. Emmet, J. Snyder, A. Szymkowiak, D. Rabinowitz, N. Morgan

**Yale University**

Supernova studies have demonstrated that, contrary to all expectations,
the expansion of the Universe is not slowing, but rather accelerating.
Combining these results with measurements of the cosmic microwave
background shows that two-thirds of the energy density of the universe
must be a previously unknown component, termed "dark energy," whose
distinguishing characteristic is that its pressure is negative. Nearly
all the remainder is dark matter, an unidentified substance not made of
protons, neutrons, and electrons. Together, these sources appear to
provide just enough density to render the universe flat, in accord with
the predictions of the inflationary model. Evidently, the successful
model of particle physics describes only a tiny fraction of the
composition of the universe, since neither dark matter nor dark energy
is encompassed in our current picture: We are ignorant of the dominant
constituents of the universe, those that shape its structure and
determine its ultimate destiny. The Supernova/Acceleration Probe will
explore these fundamental mysteries.

SNAP is a major step beyond the ground-based supernova studies that
revealed the existence of dark energy, and is designed to study the
properties of dark energy in detail. Its power derives from the
simplicity with which its results can be interpreted. The Type Ia
supernovae are nearly uniform beacons, distributed through space and
time. By observing their brightness and redshifts, we map the history of
the expanding universe. Results first reported in January 1998 showed
that distant supernovae were dimmer than was expected for their
redshifts. It was in this way that the unexpected acceleration was
discovered. Detailed study of the brightness as a function of the
redshift measures the ratio, *w*, of the dark energy pressure *P* to its
energy density, ρ. To fulfill its role as the source of expansion, *w*
must be less than -1/3. By observing thousands of Type Ia supernovae
with better precision than any single supernova has ever been measured
so far, SNAP will measure the history of the universe's growth over the
past 10 billion years -- and thus determine the value of *w* to ±5%, if
*w* does not vary in time.

But SNAP can do even more. The traditional view of the expanding
universe was overthrown only when the supernova measurements revealed
the time variation of that expansion -- the acceleration. In the same
way, real insight into dark energy may come only with measurement of the
time variation of *w*. If dark energy is actually due to a cosmological
constant, invented, then rejected by Einstein, and abhorred by
theorists, then *w* will be -1 always and its time derivative *w\'* will
vanish. If dark energy is dynamical, *w* need not be -1 and will likely
show a non-zero time derivative. Measurements of *w* and *w\'* will
provide the basis for understanding dark energy.

Dramatic reduction in systematic errors is critical to the measurements
of *w* and *w\'*. This will be possible by working beyond the earth\'s
atmosphere with SNAP's wide-field 2-m telescope, half-billion pixel
imager, and spectrograph, all specifically designed for this
comprehensive study. The resulting instrument will also have broad
capabilities for complementary measurements, especially the study of the
spatial distribution of dark matter through weak gravitational lensing.
This will enable a determination of the total matter density to high
precision and help constrain *w\'*. Moreover, SNAP will survey an area
of sky several thousand times larger than the Hubble Deep Field, and
with somewhat greater depth.

SNAP is prepared to move forward, working from extensive experience
acquired in ground-based supernova searches, exploiting advances in
instrumentation, such as the development of the high-quantum efficiency
CCD, and benefiting from DOE's substantial computing infrastructure. The
SNAP team brings together the expertise in physics, astrophysics, space
science, and engineering needed to carry out this unique project. The
Standard Model of particle physics is an exquisite model, which
successfully accounts for our tangible world and for sub-atomic
phenomena. But evidently, we have not yet explored the dominant
constituents of the universe. SNAP will be the first big step in this
new exploration.

> **Supernova/Acceleration Probe (SNAP)**

## I. The Importance of the Science

***Introduction:*** Our picture of the nature of the universe and
fundamental physics has been transformed in recent years by the first
precision measurements of cosmological parameters. Contrary to
expectations that the matter density would equal the critical density,
with exactly the gravitational force to balance the universe between
eternal expansion and ultimate contraction, supernova studies supported
by the DOE Office of Science concluded that the expansion of the
universe is accelerating.^1^ This revolutionary result was confirmed
independently, and other cosmological measurements such as studies of
the cosmic microwave background (CMB) power spectrum,^2^ large-scale
structure surveys,^3^ and weak gravitational lensing distortion maps
provide strong corroboration.^4^ This concordance has established that
roughly two-thirds of the energy density of the universe is due to a
previously unknown source, "dark energy." The nature of this dark energy
has become one of *the* forefront physics questions, with direct impact
on fundamental theories of particle physics, cosmology, and gravitation.
The Supernova/Acceleration Probe (SNAP) is an experiment dedicated to
exploring dark energy: its place among the fundamental particles and
forces, and its consequence for the fate of the universe.

***The evidence for dark energy:*** The study of supernovae provided the
first evidence of the accelerating universe, and hence of dark energy.
The simple and direct relation between the observations and their
interpretation led quickly to wide acceptance of the result, despite its
revolutionary nature. The brightness of a well-calibrated source
("standard candle") like a Type Ia supernova provides a direct measure
of its distance through the inverse square law. Distance can
equivalently be thought of as the light transit time, or "look-back
time" to the explosion. But while the light is traveling towards us, its
wavelength is stretched ("redshifted") as space expands. The scale
factor *R*(*t*) defines the expansion as a function of time and is
directly related to the spectroscopically observed redshift, *z*, by
*R*(*t~e~*) / *R*(*t*~o~ )= 1/(1+*z*), where *t~e~* and *t*~o~ are the
times of emission and observation, and 1+*z* ~ ~=  λ~ο~/λ~e~. Supernova
measurements of brightness and redshift thus directly determine the
scale factor as a function of time, providing a very clear mapping of
the expansion history of the universe.

The traditional Hubble diagram shows brightness in units of magnitude,
which is proportional to the negative log of the flux as a function of
redshift. The current supernova data are displayed in Fig. 1a; the most
distant observed supernovae, with redshifts around *z* \~ 1, have larger
magnitudes (that is, they are dimmer) than expected for the line that
corresponds to a Universe expanding with constant velocity. In other
words, the light traveled farther and took longer to reach us than
expected based on the present expansion rate. This observation leads to
the conclusion that the universe was expanding less rapidly in the past
than it is today. Rather than slowing down, as expected from
gravitational attraction due to matter, the expansion rate of the
Universe is now increasing with time; the acceleration of the expansion
shows itself straightforwardly in the curvature of the data in the
Hubble diagram. The mysterious repulsive force that is causing the
Universe to expand faster and faster has been termed dark energy.

![](media/image1.png){width="3.25in" height="2.5347222222222223in"}

**Figure 1.** (a) Left Panel: Hubble diagram for the current Supernova
Cosmology Project data set. (b) Right Panel: Simulated SNAP data set;
the data are binned in redshift and plotted as the difference in
observed magnitude from a flat Universe with Ω~Λ~=0.7. The curves
correspond to predictions of various dark energy models.

The modern use of supernovae as standard candles to measure the
expansion history of the Universe was pioneered at LBNL with DOE
support, and has roots in the methods of particle physics. Supernovae,
whose discovery had previously been a random and relatively rare
occurrence, are now systematically found by using a batch discovery
method. Wide images of the sky showing thousands of distant galaxies are
digitally recorded, then revisited a short time later. By subtracting
the images, supernovae that had exploded in the intervening time are
revealed. The Type Ia supernovae rate is only a few per thousand
galaxies per year, so the discovery of very many requires a wide survey,
a deep survey, or both. Supernovae reach their peak intensity within a
couple of weeks, and the observation timing is design to discover them
early enough to measure their peak brightness and fully study and
calibrate them.

Despite being rare and transitory objects, Type Ia supernovae make
particularly good standard candles for the determination of relative
distances because of their nearly uniform intrinsic peak brightness. The
precursor star, a white dwarf whose structure is determined completely
by nuclear physics and gravity, accretes mass from a companion star
until it approaches the so-called Chandrasekhar mass where gravity just
overcomes the electron degeneracy pressure, at \~1.4 times the solar
mass. This triggers an explosion that follows a consistent pattern,
insensitive to the details of the initial conditions.

Supernovae are extremely bright, outshining the billions of stars in
some host galaxies, so it is possible to observe very distant ones whose
light has been traveling towards us for billions of years. Furthermore,
they exhibit a rich spectroscopy, as heavier elements are forged and
blown off in the explosion. Each event provides a rich stream of
observational data in the form of its spectrum and flux over time that
includes signatures, such as the Si II line, which can be used not only
to identify the Type Ia class of supernovae, but also to characterize
them precisely and control small systematic differences.

The Hubble diagram of supernovae, the CMB power spectrum, and
measurements of the matter density all point to an accelerating
universe. At least two of the three would need to be incorrect for this
conclusion to be mistaken. The discovery of dark energy raises profound
questions for particle physics and cosmology. There is no more
fundamental challenge facing physical science than discovering the
nature of the dominant energy constituent of the Universe.

***Theoretical implications of dark energy**:* The initial supernova
measurements were undertaken to measure the *deceleration* parameter,
the rate at which the expansion of the Universe was slowing down due to
gravitational attraction. There were in fact strong theoretical
arguments that there should be just enough matter in the universe, the
so-called critical density, to gradually slow the expansion of the
universe and bring it asymptotically to rest. The supernova results
demonstrating an *acceleration* rather than a deceleration came as a
complete surprise.

What could cause such bizarre behavior? In fact there was already one
well-known possibility originally suggested by Einstein. Believing the
universe to be static, Einstein posited a new force to counter gravity
and embodied it in his cosmological constant, Λ, in order to keep the
universe from collapsing. Once the Hubble expansion was discovered,
ruling out a static Universe, Einstein discarded the cosmological
constant and declared its introduction his biggest blunder. The
reintroduction of the cosmological constant would pervade all space with
a substance having a negative pressure to density ratio, *P*/ρ = −1,
leading to a repulsive force.

According to General Relativity, the gravitational force acting on the
universe is proportional not simply to the energy density ρ, but rather
to ρ + 3*P*. The pressure, *P*, of a substance is determined by its
"equation of state," a familiar example being the ideal gas law,
*PV=NkT*, where *N* is the number of molecules of an ideal gas, *P* its
pressure, *V* its volume, *k* the Boltzmann constant, and *T* the
temperature. Thus *P=nkT*, where~ ~*n* is the number density. From
kinetic theory we can write (3/2)*kT*=(1/2)*mv*^2^, leading to an
equation of state parameter *w*≡*P*/ρ=(1/3)(*v/c*)^2^. Galaxies, for
example, have typical internal velocities a thousandth the speed of
light, so *w* is very close to zero for matter in the universe.
Radiation or relativistic particles have *w*=1/3. But to explain the
supernova results requires a repulsive gravitational effect with *w* \<
-1/3.

The cosmological constant has such a negative equation of state, but it
does not provide a completely satisfactory solution. Although it cannot
be calculated in any generally accepted way, dimensional analysis shows
that it ought be some number -- not too big or too small -- times a mass
scale to the fourth power. Since this is an issue of gravity, the
logical guess is that the mass scale is the Planck mass. But if we guess
*Λ = M~P~^4^* we find a value that is a factor of 10^120^ too big to be
compatible with what we know about the energy density in the universe.
This has been termed the worst scientific prediction ever made, and is
widely regarded as the greatest challenge facing theories that seek to
unify gravity with the other forces.

Moreover, the cosmological constant is an extremely special case, with a
particular value of *w*, which also remains perfectly constant.
Physicists have struggled to understand how a cosmological constant of
the necessary minuscule magnitude could come to be dynamically important
to the universe just at the epoch we are observing. To date, all
attempts at any natural explanation have failed, although the concept of
dynamical scalar fields, also called "quintessence," can alleviate some
of the problems. Generically this also requires the equation of state to
be a dynamical variable, characterized by a time variation, *w´.* This
time variation is a crucial clue. We have known for 75 years that the
universe is expanding, but it was the discovery of how that expansion
varies with time -- that it is accelerating -- that rocked the standard
paradigm. So too for the nature of dark energy: what will open new
windows on fundamental physics is not just the particular value of *w*
today, but its dynamical aspects - its time variation *w´*.

At present there are no constraints on *w´*, yet it has profound
consequences for the fate of the universe, as it is capable of halting
the accelerating expansion and even causing eventual re-collapse. But to
see such a critical indicator of fundamental physics will require a
comprehensive, carefully designed set of observations covering a broad
redshift range with tight control of systematic errors. As a dedicated
space experiment, SNAP can achieve this next generation advance using
supernova cosmology to provide a window on fundamental physics (see Fig.
1b).

***The SNAP satellite**:* The Supernova/Acceleration Probe (SNAP) is a
space-based experiment designed to obtain a high statistics, calibrated
data set of \~2000 Type Ia supernovae with redshifts out to *z*\~1.7
(see Figure 2 and [http://snap.lbl.gov](http://snap.lbl.gov/)). The
supernovae measured by SNAP will be used as cosmic markers of the scale
of the universe over time to construct a landmark fundamental
measurement, a clear history of the universe\'s growth going back some
10 billion years, or more than 70% of the age of the universe. The
proposed SNAP satellite and instrumentation suite have been designed for
an optimal "harvest" of supernova. It has a 2-meter telescope, powerful
enough to detect light from very distant supernovae, and a wide (0.7
square degree) field of view instrumented with a unique one-half billion
pixel camera to discover and follow many supernovae at once. The camera
has both optical and near-infrared detectors to observe supernova light
that has been redshifted up to *z*=1.7. This reach in *z* will enable
SNAP to break degeneracies in cosmological parameters and discriminate
between dark energy models.

Because measurements of the highly redshifted light from very distant
supernovae require sensitivity into the near-infrared, this program can
only be achieved in space, unhindered by absorption in the earth's
atmosphere. SNAP will also have the power to eliminate selection bias
and constrain many potential sources of systematic error, including the
influence of the host galaxy environment, intergalactic dust, and
variations in the composition of the precursor star. In addition to
collecting a high statistics sample, SNAP will obtain supernova flux
measurements frequently and accurately over a wide range of wavelengths,
and will take a detailed spectrum of each supernova at peak brightness.
In fact, each supernova will be better characterized than any
previously-studied supernova to date. By matching like to like among
supernova subsamples, we can construct independent Hubble diagrams for
each set, which when compared, test systematic uncertainties down to the
level of 1% in distance.

![](media/image3.wmf){width="3.140277777777778in"
height="2.259027777777778in"}**Figure 2.** (a) Left Panel: cutaway view
of the SNAP satellite. (b) Right Panel: the optical assembly, which is a
three-mirror anastigmat that directs light to the focal plane, and the
0.6 Gigapixel camera, which is passively cooled by a radiator.

Launched into an elliptical high-earth orbit, SNAP will systematically
scan regions of the sky located near the north and south ecliptic poles
totaling 15 square degrees, returning to each field every four days. The
camera, with fixed filters covering nine wavelength bands, will sweep
across the entire field and record every supernova in every color, with
sufficiently long exposures that supernovae with *z* \< 1.7 will be
discovered within a few restframe days of explosion. The data will be
periodically downlinked to a ground station and analyzed to identify
Type Ia supernovae for follow-up measurements with the spectrograph at
peak brightness.

With this wealth of detailed data, SNAP will construct a Hubble diagram
with unprecedented control over systematic uncertainties, addressing all
known and proposed sources of error. The first goal is to provide
precision measurements of the cosmological parameters: the matter
density, Ω~M~, will be measured to ±0.02, while the dark energy density,
Ω~Λ~, and the curvature parameter, Ω~k~, will both be determined to an
accuracy of ±0.04. The SNAP measurements will be largely orthogonal to
the CMB measurements in the Ω~M~ -Ω~Λ  ~plane, and the curvature
measurement at *z* \~ 1 will test cosmological models by comparison with
the CMB determination at *z*\~1000.

SNAP's science reach will then extend to an exploration of the nature of
the dark energy, measuring the present equation of state, *w,* to 5%. Of
even more interest is a determination of *w* as a function of redshift.
SNAP's tight control of systematics and high statistics in each redshift
bin allows determination of the dynamical variation *w´*. The data can
also be used to test the framework of general relativity and cosmology
theory. The shape of the Hubble diagram will provide a map of the
universe's expansion history; each spurt of growth and slowdown might
give an indication of the potential-energy curve of the dark energy
scalar field and point us to the underlying physics (see Fig. 1b).

***Other science with SNAP:*** SNAP is specifically designed to answer
the frontier questions about dark energy and fundamental physics. But it
is also a forefront science facility, an optical and near infrared
observatory in space well suited for a broad exploration of topics in
astrophysics and cosmology. SNAP has been described as "wide, deep, and
colorful," creating a data set 9000 times the area of the revolutionary
Hubble Deep Field, twice as deep, time-sampled, and in nine separate
wavelength bands. As part of the science mission, SNAP will carry out a
weak-lensing survey covering several hundred square degrees to a depth
comparable to that of the Hubble Deep Field. With these data, the dark
matter distribution can be mapped through its gravitational effects,
providing cosmological measurements that are complementary to the
supernova results. Weak lensing can measure the matter density, Ω~M~,
with uncertainties as low as ±0.01-0.02, with little dependence on *w.*
Such a constraint will play a crucial role in the supernova exploration
of *w′*. Furthermore, SNAP will detect a different class of supernovae
called Type II, involving different physics and giving an independent,
though less accurate, measure of the expansion history. This provides an
important crosscheck on systematic effects.

The supernovae and weak lensing surveys will provide a rich data trove
available to all scientists. After completion of its primary mission,
SNAP can serve a broader community through a guest survey program,
supporting investigations of high redshift quasars, galaxy evolution,
the epoch of reionization, outer solar system objects, and other
compelling new science is proposed.

In addition, the SNAP mission offers a superb opportunity to bring
cutting edge research to the public. Cosmology and the "big questions"
of the universe hold a unique fascination for both school children and
the general public. Dozens of articles on dark energy have appeared in
newspapers and magazines ever since the initial discovery of cosmic
acceleration was named by *Science* as "Breakthrough of the Year." SNAP
already has an experienced education and outreach coordinator, and has
plans to develop an outstanding program to engage and educate the
public.

***Other approaches to studying dark energy:*** Supernovae are the
optimal method to study dark energy; they are the best understood,
simplest, and most accurate probe. Other methods can add complementary
information if their systematics can be addressed: the
Sunyaev-Zel'dovich effect, weak and strong gravitational lensing, galaxy
cluster counts, etc. They can provide important crosschecks and break
further degeneracies among cosmological and dark energy parameters. In
particular, the cosmic microwave background, though rather insensitive
to the form of dark energy and nearly blind to *w′*, is strongly
complementary to the supernova method; SNAP combined with Planck
Surveyor data improves its constraints on the time variation of *w* by a
factor of two.

The recent HEPAP subpanel roadmap showed SNAP construction beginning in
2005, with launch in 2009. This mission timeline would position SNAP
after completion of the next major ground-based wide-field telescopes.
SNAP is in fact designed specifically to pick up where ground-based
facilities leave off when they hit systematic uncertainty limits imposed
by the nature of ground-based observing. Furthermore, SNAP has a
marvelous synergism with the next generation James Webb Space Telescope
(JWST), scheduled for launch in 2010. Taken together, these two missions
can accomplish far more than either would achieve alone.

Many projects are currently planned to advance our knowledge of
cosmological parameters, one generation of experiment at a time.
Together all these will contribute to our understanding of dark energy,
but for the critical clue of time variation of the equation of state,
SNAP as the third generation of the supernova method offers the best
path, free from many kinds of systematic uncertainties. In the words of
a recent DOE review committee report: "SNAP will...provide the first
precision cosmology measurements by directly addressing the nature of
the dark energy... SNAP will have a unique ability to measure the
variation in *w* with redshift."

***Community assessment and interest**:* Several national panel survey
reports from the National Academy of Sciences and the High Energy
Physics Advisory Panel have recently set priorities in the fields of
astronomy, fundamental particle physics, and their intersection. Dark
energy may be unique in being addressed prominently by all of them.

-   HEPAP's 20 year planning report states: "One of the most exciting
    developments of recent years has been the convergence of particle
    physics and cosmology... Among the most powerful \[probes\] are
    measurements of the expansion rate of the universe from observations
    of Type Ia supernovae, and measurements of the large scale
    distribution of dark matter from observations of weak gravitational
    lensing... We endorse R&D funding for SNAP from the high energy
    physics program."

-   NAS Committee on Physics of the Universe declares: "Deciphering the
    nature of dark matter and dark energy is one of the most important
    goals in the physics of the universe. The solutions to these
    problems will cast light not only on the fate of the universe but
    the very nature of matter, space, and time." In their final report
    they recommend "Use supernovae to probe the expansion history to get
    at the nature of dark energy, e.g., SNAP."

-   NASA's Beyond Einstein initiative identified dark energy as one of
    three compelling questions: "To understand this energy, we must
    measure the expansion of the universe with high precision...for
    example, precision measurement of distant supernovae by a wide-field
    space telescope." NASA has shown serious interest in a partnership
    with DOE to carry forward such a project and recently posted an
    Announcement of Opportunity for an Einstein Probe mission concept
    study of dark energy.

The strong community interest in SNAP is perhaps best exemplified by the
*Resource Book on Dark Energy*
(<http://supernova.lbl.gov/~evlinder/sci.html>), produced after Snowmass
2001 with 41 contributed articles from almost a hundred authors. At the
April 2003 joint meeting of the American Physical Society and the
Division of Particles and Fields a special session is planned on "Dark
Energy in the Next Generation." Articles describing the project and its
science program have appeared in *Physical Review Letters*, *Physical
Review D*, *Astrophysical Journal*, and other journals by both
collaboration and community researchers.

The SNAP collaboration includes over a dozen institutions, with several
others (including FNAL and SLAC) actively involved in discussions to
join. SNAP also has international participation, with the largest
contributions coming from IN2P3 and LAM in France and substantial
interest from the French space agency CNES. Currently some 78 scientists
and engineers work on the project, bringing together high-energy
physicists, detector experts, space system experts, and theoretical and
observational astrophysicists in a unified quest to pursue fundamental
physics.

> **II. Mission Readiness**

***Formal assessments:*** Numerous reviews of the SNAP mission have been
conducted on both conceptual and technical issues. Most recently, DOE
conducted a comprehensive three-day study by 18 external scientists
convened for an enhanced Lehman review in July 2002. This panel was
charged to answer "does the proposed project meet the scientific
objectives described by the \[NAS\] Turner panel report" and to "conduct
an integrated examination of each subsystem, as well as assessing the
technical progress overall, and the cost, schedule, and management
planning of the R&D program, in the context of the entire proposed
experiment." The review committee concluded: "The need for a space based
large field of view mission like SNAP to elucidate the nature of dark
energy via the study of Type Ia supernovae has been convincingly
established.... The proposed instruments and observing strategy is
appropriate and sufficient to carry out the scientific objectives of the
mission." (The final report is at
<http://doe-hep.hep.net/general_reports.htm> and the review materials
are available at <http://snap.lbl.gov/review>.) Additional reviews of
the project include two assessments by SAGENAP and studies by the
National Academy of Sciences. Comprehensive technical studies by NASA
Goddard's Integrated Mission Design Center and the Instrument Synthesis
Analysis Lab, and by JPL's Team-X confirm the overall mission design
practicality and identified "no tall poles," or technological
show-stoppers.

***Technical challenges and R&D status:*** The DOE technical review
mentioned above also found no major technology obstacles and recommended
that SNAP proceed with R&D. The R&D is focused on the instrumentation
suite, which brings state-of-the-art HEP technology to address the
challenges of imaging, data acquisition and data analysis. SNAP's
half-billion pixel camera will employ unique radiation-tolerant CCDs
originally developed at the LBNL Micro Systems Laboratory as an
outgrowth of R&D for vertex detectors at the SSC. These thick,
back-side-illuminated CCDs have high quantum efficiency extending
further into the infrared than the traditional thinned CCDs used in
astronomy. LBNL CCDs are in use at the UCO/Lick Observatory and Kitt
Peak National Observatory; the September 2001 newsletter of the National
Optical Astronomy Observatory highlighted the science produced with an
early LBNL device, calling it the "new red hot CCD" (Fig. 3a). CCDs that
meet SNAP noise and dark current requirements have been fabricated in a
2k x 4k format, and a new production is planned soon in the SNAP 3.5k x
3.5k format. A program to industrialize the fabrication process is well
underway (Fig 3b).

![](media/image5.png){width="2.75in" height="2.58125in"}

**Figure 3. (**a) Left Panel: SNAP focal plane instrumentation showing
the shutter, light shield/baffle, visible and near infra-red detectors
mounted to a cold-plate. The spectrograph is seen extending out the
back. (b) Right Panel: LBNL-designed, commercially fabricated CCDs on a
150 mm wafer; the three 3k x 3k CCDs in the middle are an early SNAP
version.

A custom, radiation-tolerant front-end integrated circuit for SNAP CCD
readout was recently submitted for fabrication in a commercial deep
sub-micron CMOS process. For near-infrared observations, SNAP will
employ the HgCdTe detector technology that is being developed for the
Hubble Space Telescope's Wide Field Camera 3. The spectrograph will
utilize image slicer technology being developed for the JWST, and will
employ the same CCD and HgCdTe technology used in the imager.

***Status of project cost estimate:*** In addition to the scientific and
technical aspects, a major focus of the DOE Lehman review in July 2002
was management and budget. The report concluded that the project was
well-managed and that the estimated costs for the R&D phase were
reasonable and included adequate contingency. An important next step,
following the successful conclusion of the R&D phase, will be the
development of a Conceptual Design Report including a credible mission
cost baseline. If DOE and NASA decide to partner in the development of
SNAP, it will also be necessary to develop a set of costing guidelines
that are acceptable to both agencies.

***Summary of mission readiness:*** The major conclusion of the July
2002 DOE Lehman review was very simple: "SNAP is ready to go to CD-0."
This places not only the project, but also the supernova/weak lensing
method, in a unique position in its state of readiness and its
sensitivity to the nature of the dark energy. In addition to the
realization of this important milestone, the success of SNAP depends on
adequate R&D funding this year and next year. With the continuing
support of the DOE Office of Science, SNAP will complete a Conceptual
Design and will be ready to initiate construction in two years, with the
goal to launch in 2009.

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