Probing Dark Energy in the Accelerating
Universe with SNAP
Michael Schubnell (for the SNAP Collaboration)
1
Physics Department, University of Mic higan, Ann Arbor, MI 48109
Abstract.It has now been ?rmly established that the Universe is expanding at an accelerated rate,
driven by a presently unknown form of dark energy that appears to dominate our Universe today.
A dedicated satellite mission has been designed to precisely map out the cosmological expansion
history of the Universe and thereby determine the properties of the dark energy. The SuperNova /
Acceleration Probe (SNAP) will study thousands of distant supernovae, each with unprecedented
precision, using a 2-meter aperture telescope with a wide ?eld, large-area optical-to-near-IR imager
and high-throughput spectrograph. SNAP can not only determine the amount of dark energy with
high precision, but test the nature of the dark energy by examining how its equation of state evolves.
The images produced by SNAP will have an unprecedented combination of depth, solid-angle,
angular resolution, and temporal sampling and will provide a rich program of auxiliary science.
INTRODUCTION
Recent measurements of luminosity distance versus redshift of nearby Type Ia super-
novae by the Supernova Cosmology Project and the High-z Supernova Team have deter-
mined that the expansion of the Universe is accelerating [1, 2]. Furthermore, the results
constrain the mass density, ?
M
, and the density of an unknown form of negative pressure
energy, ?
?
, characterized by an equation of state w p ? 1 3 causing the accelera-
tion. This additional energy component, coined dark energy, appears to dominate energy
density and dynamics of the Universe at the present epoch.
The evidence for dark energy is in remarkable concordance with other observations.
Measurements of small scale ?uctuations in the cosmic microwave background (CMB)
radiation support the supernova results and have determined that the Universe is nearly
?at [3]. Observations of galaxy clustering [4] have shown that the fraction of the critical
density consisting of matter is ?
M
0.3, also consistent with the results obtained from
the supernova measurements (Figure 1). Combined, these results strongly suggest that -
at the present epoch - a t least 70% of the Universe’s density is in the form of dark energy
and only approximately 30% in some form of matter (which is mostly dark).
In its simplest form, dark energy might well be Einstein’s cosmological constant in
the form of a vacuum energy but numerous other theories have been proposed including
the possibility of slowly evolving scalar ?elds (so-called quintessence models [5, 6].)
When combined with CMB and galaxy cluster measurements, a tight bound on the dark
energy equation of state w can be extracted (assuming it is constant over the expansion
1
for a list of SNAP collaboration members see http://snap.lbl.gov
1 2 0 1 2 3
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ec ol la
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ua
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Supernovae
CMB + weak H
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mass density
vacuum energy densi
ty
(cosmological constant)
open
flat
network of cosmic strings
w = –
1/3
range of
Quintessence
models
cosmological constant
w = –
1
90%
95%99%
68
%
0.0
0.0 0.2 0.4 0.6 0.8 1.0
–0.2
–0.4
–0.6
–0.8
–1.0
?
?
equation of state w = p / ?
Flat Universe
Constant w
SNAP
Target Statistical Uncertainty
SNAP
TARGET
Clusters
No Big Bang
FIGURE 1. Left ?gure – Con?dence regions in the?
?
?
M
plane for supernovae [1], galaxy cluster
[4], and CMB [3] data. The consistent overlap is compelling evidence for a geometrically ?at, dark energy
dominated Universe. Also shown (in both ?gures) is the expected con?dence region from the SNAP
satellite for a ?at ?
M
0 28 Universe. Right ?gure – Constraints on the equation-of-state parameter w
from the Supernova Cosmology Project [1 ]. Shown are con?dence regions in the?
?
w plane for an
energy density component ?
?
characterized by w p ? . If the dark energy is Einstein’ s cosmological
constant, then w
1. Also shown are w predictions for other dark energy models.
time, i.e. dw dz 0). Current constaints on w are consistent with dark energy being
a cosmological constant b ut also allow for a wide range of alternate models, including
those with a time dependent value of w (Figure 1).
A precise measurement of dark energy properties requires a much larger data set of
supernovae than currently available and a signi?cant improvement of the systematic un-
certainties in the measurements over current experiments. A de?nitive program to study
dark energy with supernovae must provide a high degree of statistical and systematic
rigor [7]. Furthermore, the greatest sensitivity to cosmological parameters is obtained
with measurements extending from the present epoch of acceleration into the matter
dominated deceleration phase [8]. Because measurements of the highly redshifted light
from very distant supernovae require sensitivity into the near infrared (NIR), such a pro-
gram can only be achieved in space, unhindered by absorption in the earth’s atmosphere.
SATELLITE AND MISSION
The primary goal of the SNAP mission is to measure cosmological parameters with a
precision that will allow to distinguish between different dark energy models. For this
supernova observations provide a proven and well understood cosmological tool. The
FIGURE 2. Left ?gure – Cross-sectional view of the SNAP satellite. Right ?gure – The SNAP focal
plane. For detailed description of satellite and anstrument see [9, 10]
essential measurement for this purpose is a comparison of luminosity distance to redshift
providing information on the scale size as a function of expansion time. With a precisely
calibrated data set of several thousand Type Ia SNe with redshift 0.1 – 1.7 the expansion
history of the Universe can be reconstructed back to more than 70% of its age.
It has been shown that Type Ia supernova e have uniform peak B -band brightness when
their light curves are corrected for a stretch factor which describes the relation between
absolute brightness and explosion [1]. However, to fully standardize the SN peak bright-
ness, a variety of additional observ ations must be made. Color measurements throughout
the light curve for instance provide constraints on host-galaxy environment and galac-
tic extinction and spectra obtained near maximum brightness allow identi?cation of the
explosion as a Type Ia through characteristic features (e.g. SiII at 6150 Ĺ).
The SNAP satellite and mission design has been optimized for ef?cient supernova
detection and high quality follow-up measurements. The combination of a three mirror
2-meter telescope and a
600 million pixel optical to near infrared imager with a large
0.7 square degree ?eld of view will allow discovery and follow up of many supernovae at
once. The imaging system comprises 36 large format (3.5k
3.5k) CCDs and the same
number of 2k
2k HgCdTe infrared sensors. Both the CCDs and the NIR detectors are
placed in four symmetric 3
3 arrangements as shown in Figure 2. Both the imager and
a low resolution (R
100) high-throughput spectrograph cover the waveband from 350
to 1700 nm, allowing detailed characterization of supernovae out to z
1 7. This deep
reach in redshift is essential to the mission as it will allow to resolve degeneracies in
cosmological parameters and to discriminate between models of dark energy.
Nine special ?lters ?xed above the imaging sensors will provide overlapping red-
shifted B -band coverage from 350 – 1700 nm. As SNAP repeatedly steps across its
target ?elds in the north and south ecliptic poles, every supernova will be seen in every
?lter in both the visible and NIR. Because of their larger linear size, each NIR ?lter will
be visited with twice the exposure time of the visible ?lters. This, combined with the
time-dilated light curve, will ensure that Type Ia supernovae out to redshift 1.7 will be
detected with a S/N
6 at least 2 magnitudes below peak brightness [9, 10].
SNAP SCIENCE
SNAP will conduct two primary surveys, a 15 square degree ultra-deep ( m
AB
30 for
point sources) supernova survey, and a
300 square degree deep ( m
AB
27 8 for point
sources) weak lensing survey. 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 ?rst goal is to provide precision measurements
of the cosmological parameters: the matter density,?
M
, will be measured to 0.02,
while ?
?
, and the curvature parameter, ?
k
, will both be determined to an accuracy of
0.04. The SNAP measurements will be largely orthogonal to the CMB measurements
in the ?
M
?
?
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 v ariation w .
To complement its supernova cosmology observations, SNAP will conduct a wide-
area weak lensing survey. These weak lensing observations provide important indepen-
dent measurements and complementary determinations of the dark matter and dark en-
ergy content of the Universe. They will substantially enhance SNAP’s ability to constrain
the nature of dark energy[11]. SNAP weak lensing observations bene?t enormously from
the high spatial resolution, the accurate photometric redshifts, and the very high surface
density of resolved galaxies available in these deep observations.
Although the SNAP mission is tailored for supernova and weak lensing observ ations,
with the large survey ?eld, depth, spatial resolution, temporal sampling and wavelength
coverage into the infrared the resulting data sets will provide a rich program of aux-
iliary science. Here we highlight a few selected areas where the large area deep-?eld
observations are expected to signi?cantly impact our understanding of the Universe:
• Galaxies – Within the ultra-deep 15 square degree survey area, SNAP will obtain
accurate photometric redshift measurements for at least 5
10
7
galaxies to z =3.5.
This will provide a unique opportunity to study the evolution of galaxies through
more than 90% of the age of the Univ erse.
• Galaxy clusters – Galaxy clusters, the most massive bound objects and probably
largest structures in the Universe, pro vide important probes of our understanding
of structure formation. The SNAP surveys will provide detailed information on
roughly 15,000 galaxy clusters with masses above 5
10
13
M
.
• Quasars – NIR photometry extends the redshift range for quasar discovery using
colors and dropout surveys. SNAP will be able to detect quasars beyond redshift 10,
and to probe the quasar luminosity function to 100 times fainter than the brightest
quasars.
• GRBs – Gamma-ray bursts continue to pose a great mystery. Recent observations
point to GRBs as the product of core-collapse of super-massive short lived stars. If
so, then GRBs may trace the star formation rate and thus GRBs coincident with the
epoch of ?rst stars formation are e xpected. The most distant GRB currently known
occurred at redshift of 3.4. SNAP will be able to identify GRB afterglows to z
10.
• Reionisation – Most likely the reionisation of the Universe did not occur as a single
instant in time, but rather as a complex process happening at slightly different
epochs in different parts of the Univ erse. By identifying many quasars and galaxies
to z
10, SNAP will map the epoch of reionization in unprecedented detail.
• Gravitational lensing – The high spatial resolution of SNAP NIR observations will
enable the discovery of a large number of new strong lenses. In weak lensing
measurements, SNAP spatial resolution and NIR sensitivity will allow the use of
a huge number of faint, high-redshift background galaxies. With these galaxies, it
will be possible to extend weak lensing studies to lower mass objects, and to study
lens objects beyond z
1 – measurements which are impossible from the ground.
CONCLUSION
SNAP presents a unique opportunity to probe the dark energy and advance our under-
standing of the Universe. It will discov er and precisely measure thousands of supernovae
of Type Ia and will provide a combination of depth, solid angle and angular resolution
heretofore unachieved. From the data collected, it will be possible to precisely measure
the equation of state of the Universe, measure the history of its accelerations and decel-
erations and to study the nature of dark energy, which is causing the current acceleration
of the expansion of the Universe. SNAP will be able to measure the equation of state,
w , of the Universe as well as its variation over time, w
. This detailed knowledge will
allow to distinguish between different models for the nature of the dark energy and lead
to deeper understanding of the Universe.
ACKNOWLEDGMENTS
This work was supported by the U.S. Department of Energy.
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