Overview of the SuperNova/Acceleration Probe (SNAP)

TL;DR

SNAP tackles the problem of precisely characterizing cosmic acceleration by building a dedicated space-based survey of Type Ia supernovae from $z\approx0.1$ to $1.7$. Its approach combines a $2$-m wide-field, three-mirror telescope with a $0.7$ deg$^2$ imager and a low-resolution IFU spectrograph to deliver high-quality light curves and spectra for about $2000$ SNe Ia, while aggressively controlling systematic uncertainties. The paper demonstrates that, with this data, SNAP can constrain $\Omega_M$, $\Omega_\Lambda$, and the dark-energy equation of state $w$ to precisions such as $\sigma_{\Omega_M} \approx 0.02$, $\sigma_{\Omega_\Lambda} \approx 0.05$, and $\sigma_w \approx 0.05$ for a constant $w$, and can probe potential redshift evolution $w(z)$, thus distinguishing a cosmological constant from dynamical dark-energy models. The mission also emphasizes a uniform, complete dataset and complementary measurements (weak lensing, Type II SNe), promising broad scientific utility beyond SN cosmology and advancing empirical cosmology in the era of precision measurements.

Abstract

The SuperNova / Acceleration Probe (SNAP) is a space-based experiment to measure the expansion history of the Universe and study both its dark energy and the dark matter. The experiment is motivated by the startling discovery that the expansion of the Universe is accelerating. A 0.7 square-degree imager comprised of 36 large format fully-depleted n-type CCD's sharing a focal plane with 36 HgCdTe detectors forms the heart of SNAP, allowing discovery and lightcurve measurements simultaneously for many supernovae. The imager and a high-efficiency low-resolution integral field spectrograph are coupled to a 2-m three mirror anastigmat wide-field telescope, which will be placed in a high-earth orbit. The SNAP mission can obtain high-signal-to-noise calibrated light-curves and spectra for over 2000 Type Ia supernovae at redshifts between z=0.1 and 1.7. The resulting data set can not only determine the amount of dark energy with high precision, but test the nature of the dark energy by examining its equation of state. In particular, dark energy due to a cosmological constant can be differentiated from alternatives such as "quintessence", by measuring the dark energy's equation of state to an accuracy of +/-0.05, and by studying its time dependence.

Figures (6)

• Figure 1: There is strong evidence for the existence of a cosmological vacuum energy density. Plotted are $\Omega_M$---$\Omega_\Lambda$ confidence regions for current SN p99, galaxy cluster, and CMB results. These results rule out a simple flat, [$\Omega_M=1$, $\Omega_\Lambda=0$] cosmology. Their consistent overlap is a strong indicator for dark energy. Also shown is the expected confidence region from the SNAP satellite for an $\Omega_M=0.28$ flat Universe.
• Figure 2: Best-fit 68%, 90%, 95%, and 99% confidence regions in the $\Omega_{\rm M}$--$w$ plane for an additional energy density component, $\Omega_w$, characterized by an equation-of-state $w = p/\rho$. (For Einstein's cosmological constant, $\Lambda$, $w = -1$.) The fit is constrained to a flat cosmology ($\Omega_{\rm M} + \Omega_w =1$). Also shown is the expected confidence region allowed by SNAP assuming $w=-1$ and $\Omega_M=0.28$.
• Figure 3: Accuracy in estimating the equation of state parameter, $w$, as a function of maximum redshift probed in SN Ia surveyslinder02. The cases where $w$ is assumed constant in time are labeled as $\sigma(w)$, while the cases where $w$ is allowed to vary with time as $w=w_0+w' z$ are labeled as $\sigma(w_0)$. The lower two curves assume that the experiment is free of any systematic errors, while the upper two curves are for the case where systematic errors are present at the 2% level. The top, heavy curve corresponds to the most realistic case. It is clear that even with modest systematic errors good accuracy requires probing to high redshift. In all cases a flat universe is assumed; a prior is also placed on $\Omega_M$, with a less constrained prior of $\sigma_{\Omega_M}=0.03$ for the cases where systematics are taken into account.
• Figure 4: A cross-sectional view of the SNAP satellite. The principal assembly components are the telescope, optical bench, instruments, propulsion deck, bus, stray light baffles, thermal shielding and entrance door.
• Figure 5: Side view of our baseline optical configuration, with a 2-m primary mirror, a 0.45 meter secondary mirror, a folding flat, and a 0.7 meter tertiary mirror. An optional auxiliary focus using the center of the field is possible.