Dark Side of the Universe
Yun Wang
STScI, January 21, 2008
beware of the dark side …
Master Yoda
Yun Wang, 1/21/2008
Outline
• Dark energy: introduction and
current constraints
• Observational methods for dark
energy search
• Future prospects
Yun Wang, 1/21/2008
How do we know there is
dark energy?
We infer its existence via its
influence on the expansion
history of the universe.
Yun Wang, 1/21/2008
First Evidence for Dark Energy
in the Hubble Diagrams of Supernovae [dL(z)]
(Riess et al. 1998, Schmidt et al. 1998, Perlmutter et al. 1999)
Yun Wang, 1/21/2008
Alternative Analysis of First Evidence
Flux-averaged and combined data of 92 SNe Ia from Riess/Schmidt et al.
(1998) and Perlmutter et al. (1999).
[Wang (2000)]
Deceleration parameter
q0 =m/2-
Data favor q0 <0:
cosmic acceleration
Yun Wang, 1/21/2008
Wang & Tegmark 2005
Yun Wang, 1/21/2008
w(z) = w0+wa(1-a)
1+z = 1/a
z: cosmological redshift
a: cosmic scale factor
WMAP3
+182 SNe Ia (Riess et al.
2007, inc SNLS and
nearby SNe)
+SDSS BAO
(Wang & Mukherjee 2007)
Modelindependent
constraints on
dark energy
(as proposed by
Wang & Garnavich 2001)
Wang & Mukherjee (2007)
Yun Wang, 1/21/2008
Yun Wang, 1/21/2008
Wang & Mukherjee (2007)
[See Wang & Tegmark
(2005) for the method to
derive uncorrelated estimate
of H(z) using SNe.]
H(z) = [da/dt]/a
What is dark energy?
Two Possibilities:
(1) Unknown energy component
(2) Modification of Einstein’s theory of general
relativity (a.k.a. Modified Gravity)
Yun Wang, 1/21/2008
Some Candidates for Dark Energy
cosmological constant (Einstein 1917)
quintessence (Freese, Adams, Frieman, Mottola 1987; Linde
1987; Peebles & Ratra 1988; Frieman et al. 1995; Caldwell, Dave, & Steinhardt
1998; Dodelson, Kaplinghat, & Stewart 2000)
k-essence: (Armendariz-Picon, Mukhanov, & Steinhardt 2000)
Modified Gravity
Vacuum Metamorphosis (Parker & Raval 1999)
Modified Friedmann Equation (Freese & Lewis 2002)
Phantom DE from Quantum Effects (Onemli & Woodard 2004)
Backreaction of Cosmo. Perturbations (Kolb, Matarrese, &
Riotto 2005)
Yun Wang, 1/21/2008
How We Probe Dark Energy
• Cosmic expansion history H(z) or DE density X(z):
tells us whether DE is a cosmological constant
H2(z) = 8 G[m(z) + r(z) +X(z)]/3  k(1+z)2
• Cosmic large scale structure growth rate function fg(z),
or growth history G(z):
tells us whether general relativity is modified
fg(z)=dln/dlna, G(z)=(z)/(0)
=[m-m]/m
Yun Wang, 1/21/2008
Observational Methods for
Dark Energy Search
• SNe Ia (Standard Candles):
method through which DE has been discovered;
independent of clustering of matter, probes H(z)
• Baryon Acoustic Oscillations (Standard Ruler):
calibrated by CMB, probes H(z). [The same observations,
if optimized, probe growth rate fg(z) as well.]
• Weak Lensing Tomography and CrossCorrelation Cosmography:
probes growth factor G(z), and H(z)
• Galaxy Cluster Statistics:
probes H(z)
Yun Wang, 1/21/2008
Supernovae as Standard Candles
Lightcurves of 22 SNe Ia
(left, Riess et al. 1999):
very different from that
of SNe II (below).
Measuring the apparent peak
brightness and the redshift of SNe Ia
gives dL(z), hence H(z)
Yun Wang, 1/21/2008
Spectral Signature of SNe Ia
Primary feature: Si II 6355 at
rest=6150Å
Secondary feature: Si II 4130 dip
blueshfted to 4000Å
SN Ia 1999ff (z=0.455):
a: Ca II H and K absorption
b: Si II 4130 dip blueshfted to 4000Å
c: blueward shoulder of Fe II 4555
d: Fe II 4555 and/or Mg II 4481
e: Si III 4560
i: Si II 5051
SN IIb 1993J: double peak centered just
blueward of 4000Å, due to Ca II H and K
absorption at 3980Å due to blueshufted
H, but not similar to Ia redward of
4100Å.
[Coil et al. 2000, ApJ, 544, L111]
Yun Wang, 1/21/2008
Theoretical understanding of SNe Ia
Binary  C/O white dwarf at the Chandrasekher limit (~ 1.4 MSun)
 explosion
 radioactive decay of 56Ni and 56Co: observed brightness
• explosion: carbon burning begins as a turbulent deflagration,
then makes a transition to a supersonic detonation
• earlier transition:
cooler explosion  less 56Ni produced: dimmer SN Ia
lower opacity  faster decline of the SN brightness
Wheeler 2002 (resource letter)
Yun Wang, 1/21/2008
Calibration of SNe Ia
Phillips 1993
Riess, Press, & Kirshner 1995
Brighter SNe Ia
decline more slowly
 make a correction
to the brightness based
on the decline rate.
26 SNe Ia with
Bmax-Vmax  0.20 from
the Calan/Tololo sample
[Hamuy et al. 1996,
AJ, 112, 2398]
Yun Wang, 1/21/2008
Getting the most distant SNe Ia:
critical for measuring the evolution in dark energy density:
Wang & Lovelave (2001)
Yun Wang, 1/21/2008
Ultra Deep Supernova Survey
To determine whether SNe Ia are good cosmological
standard candles, we need to nail the systematic
uncertainties (luminosity evolution, gravitational
lensing, dust). This will require at least hundreds of
SNe Ia at z>1. This can be easily accomplished by doing
an ultra deep supernova survey using a dedicated
telescope, which can be used for other things
simultaneously (weak lensing, gamma ray burst
afterglows, etc).
Wang 2000a, ApJ (astro-ph/9806185)
Yun Wang, 1/21/2008
SNe Ia as Cosmological Standard Candles
Systematic effects:
dust: can be constrained using multi-color data.
(Riess et al. 1998; Perlmutter et al. 1999)
gray dust: constrained by the cosmic far infrared
background.
(Aguirre & Haiman 2000)
gravitational lensing: its effects can be reduced by
flux-averaging.
(Wang 2000; Wang, Holz, & Munshi 2002)
SN Ia evolution (progenitor population drift):
*compare like with like at low z and high z
*observe SNe Ia at 1.5<z<3 to probe evolution
(Branch et al. 2001; Riess & Livio 2006)
Yun Wang, 1/21/2008
Weak Lensing of SNe Ia
Kantowski, Vaughan, & Branch 1995
Frieman 1997
Wambsganss et al. 1997
Holz & Wald 1998
Metcalf & Silk 1999
Wang 1999
WL of SNe Ia can be modeled by a Universal Probability Distribution for Weak
Lensing Magnification (Wang, Holz, & Munshi 2002)
The WL systematic of SNe Ia can be removed by flux averaging
(Wang 2000; Wang & Mukherjee 2003)
Yun Wang, 1/21/2008
Baryon acoustic oscillations as
Blake & Glazebrook 2003
a standard
ruler
Δr = Δr = 148 Mpc
= standard ruler
||
┴
Δr|| = (c/H)Δz
BAO “wavelength” in transverse
direction in slices of z : DA(z)
Seo & Eisenstein 2003
Δr┴ = DAΔθ
BAO “wavelength” in radial
direction in slices of z : H(z)
Detection of BAO in the SDSS data [Eisenstein et al. 2005]
Yun Wang, 1/21/2008
DE eq. of state
w(z)=w0+wz
Wang 2006
Yun Wang, 1/21/2008
Wang 2006
Yun Wang, 1/21/2008
BAO systematic effects
• Galaxy clustering bias (how light traces mass)
• Redshift space distortions (artifacts not present in
real space)
• Nonlinear gravitational clustering
– small scale information in P(k) is destroyed by cosmic
evolution due to mode-coupling (nonlinear modes)
– Intermediate scale P(k) significantly altered in shape
(shape is measured cleanly only at k < 0.1h/Mpc at
z=0)
(e.g., White 2005; Jeong & Komatsu 2006;
Koehler, Schuecker, & Gebhardt 2007)
Yun Wang, 1/21/2008
Weak Lensing Tomography and
Cross-Correlation Cosmography
Yun Wang, 1/21/2008
• Weak Lensing Tomography:
compare observed cosmic shear correlations
with theoretical/numerical predictions to
measure cosmic large scale structure growth
history G(z) and H(z) [Wittman et al. 2000]
• WL Cross-Correlation Cosmography
measure the relative shear signals of galaxies at
different distances for the same foreground
mass distribution: gives distance ratios
dA(zi)/dA(zj) that can be used to obtain cosmic
expansion history H(z) [Jain & Taylor 2003]
Yun Wang, 1/21/2008
Measurements of cosmic shear
(WL image distortions of a few percent)
left:top-hat shear variance; right: total shear correlation function. 8=1 (upper); 0.7 (lower). zm=1.
[Heymans et al. 2005]
First conclusive detection of cosmic shear was made in 2000.
Yun Wang, 1/21/2008
Cosmological parameter constraints from WL
L: 8 from analysis of clusters of galaxies (red) and WL (other). [Hetterscheidt et al.
(2006)]
R: DE constraints from CFHTLS Deep and Wide WL survey. [Hoekstra et al. (2006)]
Yun Wang, 1/21/2008
WL systematics effects
• Bias in photometric redshift distribution (< 0.1%
required to avoid significant degradation of DE
constraints)
• PSF correction (errors in calibration of the PSF
isotropic smearing and correction of PSF anisotropy)
• Biased selection of the galaxy sample
• Intrinsic distortion signal (intrinsic alignment of
galaxies)
(e.g., Casertano 2002; King & Schneider 2003; Hirata & Seljak2004;
Heymans et. Al. 2006; Huterer et al. 2006)
Yun Wang, 1/21/2008
Clusters as DE probe
1) Use the cluster number density and its redshift
distribution, as well as cluster distribution on large
scales.
2) Use clusters as standard candles by assuming a
constant cluster baryon fraction, or use combined
X-ray and SZ measurements for absolute distance
measurements.
• Large, well-defined and statistically complete
samples of galaxy clusters are prerequisites.
(e.g. Haiman, Mohr, Holder 2001; Vikhlinin et al. 2003;
Schuecker et al. 2003; Allen et al. 2004; Molnar et al. 2004)
Yun Wang, 1/21/2008
Clusters as DE probe
• Requirements for future surveys:
– selecting clusters using data from X-ray satellite with
high resolution and wide sky coverage
– Multi-band optical and near-IR surveys to obtain photoz’s for clusters.
• Systematic uncertainties: uncertainty in the
cluster mass estimates that are derived from
observed properties, such as X-ray or optical
luminosities and temperature.
(e.g. Majumdar & Mohr 2003, 2004; Lima & Hu 2004)
Yun Wang, 1/21/2008
Future Prospects
Yun Wang, 1/21/2008
DETF recommendations
• Aggressive program to explore DE as fully as possible.
• DE program with multiple techniques at every stage, at least one
of these is a probe sensitive to the growth of cosmic structure in
the form of galaxies and clusters of galaxies.
• DE program in Stage III (near-term) designed to achieve at least
a factor of 3 gain over Stage II (ongoing) in the figure of merit.
• DE program in Stage IV (long-term) designed to achieve at least
a factor of 10 gain over Stage II in the figure of merit.
• Continued research and development investment to optimize
JDEM, LST, and SKA (Stage IV) to address remaining technical
questions and systematic-error risks.
• High priority for near-term projects to improve understanding of
dominant systematic effects in DE measurements, and wherever
possible, reduce them.
• A coherent program of experiments designed to meet the above
coals and criteria.
Yun Wang, 1/21/2008
NRC BEPAC
Recommendation 1
NASA and DOE should proceed immediately
with a competition to select a Joint Dark Energy
Mission for a 2009 new start. The broad mission
goals in the Request for Proposal should be (1) to
determine the properties of dark energy with high
precision and (2) to enable a broad range of
astronomical investigations. The committee
encourages the Agencies to seek as wide a variety of
mission concepts and partnerships as possible.
Yun Wang, 1/21/2008
Future Dark Energy Surveys
•
•
•
•
•
•
(an incomplete list)
Essence (2002-2007): 200 SNe Ia, 0.2 < z < 0.7, 3 bands, Dt ~ 2d
Supernova Legacy Survey (2003-2008): 2000 SNe Ia to z=1
CFHT Legacy (2003-2008): 2000 SNe Ia, 100’s high z SNe, 3 bands, Dt ~ 15d
ESO VISTA (2005?-?): few hundred SNe, z < 0.5
Pan-STARRS (2006-?): all sky WL, 100’s SNe y1, z < 0.3, 6 bands, Dt = 10d
WiggleZ on AAT using AAOmega (2006-2009): 1000 deg2 BAO, 0.5< z < 1
• ALPACA (?): 50,000 SNe Ia per yr to z=0.8, Dt = 1d , 800 sq deg WL & BAO with
photo-z
• Dark Energy Survey (?): cluster at 0.1<z<1.3, 5000 sq deg WL, 2000 SNe at
0.3<z<0.8
• HETDEX (2010): 200 sq deg BAO, 1.8 < z < 3.
• WFMOS on Subaru (?): 2000 sq deg BAO, 0.5<z<1.3 and 2.5<z<3.5
• LSST (2012?): 0.5-1 million SNe Ia y1, z < 0.8, > 2 bands, Dt = 4-7d; 20,000 sq
deg WL & BAO with photo-z
• JDEM (2017?): several competing mission concepts [ADEPT, DESTINY, JEDI,
SNAP]
• EDEM (2017?): two competing mission concepts [DUNE and SPACE]
Yun Wang, 1/21/2008
Future Dark Energy Space Missions
• JDEM (2017?): several mission concepts
–
–
–
–
ADEPT: BAO (spec-z) and SNe
DESTINY: SNe, WL, and BAO (photo-z)
JEDI: SNe, WL, and BAO (spec-z),
SNAP: SNe, WL, and BAO (photo-z)
• EDEM (2017?): two mission concepts
– DUNE: WL, BAO (photo-z)
– SPACE: BAO (spec-z) and SNe
Yun Wang, 1/21/2008
How many methods should we use?
• The challenge to solving the DE mystery will
not be the statistics of the data obtained, but
the tight control of systematic effects inherent
in the data.
• A combination of three most promising
methods (SNe, BAO, WL), each optimized
by having its systematics minimized by
design, provides the tightest control of
systematics.
Yun Wang, 1/21/2008
Joint Efficient Dark-energy
Investigation (JEDI):
a candidate implementation of JDEM
http://jedi.nhn.ou.edu/
Yun Wang, 1/21/2008
JEDI Collaboration
PI: Yun Wang (U. of Oklahoma)
Deputy PI: Edward Cheng (Conceptual Analytics)
Scientific Steering Committee:
Arlin Crotts (Columbia), Tom Roellig (NASA Ames), Ned Wright (UCLA)
SN Lead: Peter Garnavich (Notre Dame), Mark Phillips (Carnegie Observatory)
WL Lead: Ian Dell’Antonio (Brown)
BAO Lead: Leonidas Moustakas (JPL)
Eddie Baron (U. of Oklahoma)
Stefano Casertano (STScI)
Salman Habib (LANL)
Katrin Heitmann (LANL)
John MacKenty (STScI)
Judy Pipher (U. of Rochester)
Robert Silverberg (NASA GSFC)
Gordon Squires (Caltech)
Max Tegmark (MIT)
Yun Wang, 1/21/2008
David Branch (U. of Oklahoma)
Bill Forrest (U. of Rochester)
Mario Hamuy (U. of Chile)
Alexander Kutyrev (NASA GSFC)
Craig McMurtry (U. of Rochester)
William Priedhorsky (LANL)
Volker Springel (Max Planck Insti.)
Jason Surace (Caltech)
Craig Wheeler (UT Austin)
JEDI: exploiting 0.8-4 µm “sweet spot”
- lowest sky background region within ~0.3-100 µm wavelengths
- rest wavelengths in red/near-IR for redshifts 0 < z < 4
Background sky spectrum: Leinert 1998, A&AS, 127, 1
JEDI: the Power of Three
Independent Methods
Supernovae as standard candles:
luminosity distances dL(zi)
Baryon acoustic oscillations as a
standard ruler:
cosmic expansion rate H(zi)
angular diameter distance dA(zi)
(cosmic structure growth rate fg(z)
from the same data)
Weak lensing tomography and
cosmography:
cosmic structure growth history
G(z); ratios of dA(zi)/dA(zj)
The three independent methods to probe H(z) [and two independent methods to
probe gravity] will provide a powerful cross check, and allow JEDI to place
accurate and precise constraints on dark energy.
Yun Wang, 1/21/2008
JEDI Measures H(z) to ≤ 2% accuracy using
supernovae and baryon acoustic oscillations
Note that the errors
go opposite ways in
the two methods.
Wang et al.,
in preparation
(2008)
Yun Wang, 1/21/2008
SPectroscopic All-sky Cosmic Explorer
Andrea Cimatti (UniBO), Massimo Robberto (STScI) & the SPACE Team
http://urania.bo.astro.it/cimatti/space/
PI: A. Cimatti (University of Bologna, Italy) + co-PI: M. Robberto (STScI, USA)
Co-Is (in boldface : coordinator of SPACE Working Groups):
Austria :W. Zeilinger (U.Wien); France: E. Daddi (CEA Saclay,), M. Lehnert, F. Hammer (Meudon), O. Le Fevre, J.-P.
Kneib, J. G. Cuby, L. Tresse, R. Grange, M. Saisse (LAM); Germany: S. White, G. Kauffmann, B. Ciardi, G. De Lucia,
J. Blaizot (MPA Garching), F. Bertoldi (U. Bonn), E. Schinnerer, A. Martinez-Sansigre, F. Walter, J. Kurk, J. Steinacker
(MPIA Heidelberg); International: P. Rosati, P. Padovani (ESO); D. Macchetto (ESA); Italy: A. Ferrara (SISSA), A.
Franceschini (U. Padova), A. Renzini (INAF OAPD), S. Cristiani, M. Magliocchetti, E. Pian, F. Pasian, A. Zacchei (INAF
OATS), G. Zamorani, M. Mignoli, L. Pozzetti, C. Gruppioni, A. Comastri (INAF OABO), N. Mandolesi, R. C.Butler, C.
Burigana, L. Nicastro, F. Finelli, L. Valenziano, G. Morgante, L. Stringhetti, F. Villa, F. Cuttaia, E. Palazzi, A. De Rosa,
A. Gruppuso, A. Bulgarelli, F. Gianotti, M. Trifoglio, F. Paresce (INAF IASFBO), L. Guzzo, F. Zerbi, E. Molinari, P.
Spanó (INAF Milano), R. Salvaterra (U. Milano), M. Bersanelli (U. Milano), D. Maccagni, B. Garilli, M.
Scodeggio, D. Bottini, P. Franzetti (INAF IASFMI), T. Oliva (Arcetri, TNG); Netherlands: M. Franx, H. Roettgering, M.
Kriek (U. Leiden); Romania: L. Popa (U. Bucharest); Spain: R. Rebolo, M. Zapatero Osorio, M. Balcells (IAC), A. Perez
Garrido, A. Díaz Sánchez, I. Villó Pérez (UPCT, U. Politecnica de Cartagena); Switzerland: H. Shea (École Polytechnique
Lausanne); United Kingdom: C. Frenk, C. Baugh, I. Smail, S. Cole, R. Bower, T. Shanks, M. Ward (U. Durham , Inst.
Comp. Cosmology), R. Content, R. Sharples, S. Morris (U. Durham, Centre for Advanced Instrumentation), J. Silk (U.
Oxford), J. Dunlop, R. McLure, M. Cirasuolo (ROE), R. Kennicutt (IoA, Cambridge), M. Jarvis (U. Hertfordshire);
USA: Y. Wang (U. Oklahoma), X. Fan (U. Arizona), P. Madau (UCSC), M. Stiavelli , I. N. Reid, M. Postman, R. White,
S. Casertano, S. Beckwith (STScI), J. Gardner, M. Clampin, R. Kimble (GSFC), A. Szalay, R. Wyse (JHU), A. Shapley
(Princeton), N. Wright (UCLA), M. Strauss (Princeton), M. Urry (Yale), A. Burgasser (MIT), J. Rayner (Hawaii), B.
Mobasher (UC Riverside), M. Di Capua (UMD), L. Hillenbrand (Caltech), M. Meyer (Steward).
Yun Wang, 1/21/2008
The power of spectroscopic redshifts
spectroscopic z
Yun Wang, 1/21/2008
photometric z with
optimistic σz=0.02(1+z)
SPACE MISSION SUMMARY
Yun Wang, 1/21/2008
Telescope diameter
1.5m
Optical configuration
Ritchey-Chrétien
Wavelength range
0.6 - 1.8 mm
Optical quality
Diffraction limited
>0.65mm
Pointing stability
0.1” rms/ 30min
Overall mass
1486 kg
Data rate
1.5Mbit/s
Orbit/Launcher
L2/Soyuz
Launch date
Mid 2017
Mission Duration
5 years
Partners
ESA-NASAEuropean Agencies
Left: 0.2% systematic assumed in each z bin.
Right: 1% systematic assumed in each z bin
Growth rate function & galaxy clusters provide
additional improvements + breaking H(z) degeneracies + test on gravitational theories
Yun Wang, 1/21/2008
ALPACA
•
•
•
•
8m liquid mirror telescope
FOV: 2.5 deg diameter
Imaging =0.3-1mm
50,000 SNe Ia per yr to
z=0.8, 5 bands, Dt = 1d
• ~1000 (deg)2 WL & BAO
with photo-z
Project Scientist:
Arlin
Crotts
Observatory Scientist:
Paul Hickson
Science Advisory Council Chair: Yun
Wang
Yun Wang, 1/21/2008
• 8.4m (6.5m clear aperture) telescope; FOV: 3.5 deg diameter; 0.3-1mm
• 106 SNe Ia y1, z < 0.8, 6 bands, Dt = 7d
• 20,000 (deg)2 WL & BAO with photo-z
Yun Wang, 1/21/2008
Differentiating
dark energy
and
modified
gravity
fg = dln/dlna
 = (m-m)/m
Wang (2007)
Yun Wang, 1/21/2008
Conclusions
 Unraveling the nature of DE is one of the most important
problems in cosmology. Current data (SNe Ia, CMB, and LSS)
are consistent with a constant X(z) at 68% confidence. However,
the reconstructed X(z) still has large uncertainties at z > 0.5.
 DE search methods’ checklist:
1) Supernovae
2) Baryon acoustic oscillations (galaxy redshift survey)
3) Weak lensing
4) Clusters
 A combination of different methods is required to achieve
accurate and precise constraints on the time dependence of X(z)
and to probe gravity . This will have a fundamental impact on
particle physics and cosmology.
Yun Wang, 1/21/2008
What is the fate of the universe?
Wang & Tegmark, PRL (2004)
Yun Wang, 1/21/2008
DETF Definitions
• DETF figure of merit
= 1/[area of 95% C.L. w0-wa error ellipse]
• DETF stages for DE probes:
–
–
–
–
Stage I: Current knowledge
Stage II: Ongoing projects
Stage III: Near-term, medium-cost projects,
Stage IV: Long-term, high-cost projects (JDEM,
LST, SKA)
Yun Wang, 1/21/2008
Growth history of structure from WL
Cosmic shear signal
on fixed angular
scales as a function
of redshift.
[Massey et al. (2007)]
Yun Wang, 1/21/2008
Forecasting of DE constraints from WL
DUNE: 20,000 sq deg WL survey with zm=1, 1 broad red band,
photo-z from ground surveys [Refregier et al. (2006)]
Yun Wang, 1/21/2008
DE constraints from WL depend on the
accuracy of photometric redshifts
Huterer et al. (2006)
Yun Wang, 1/21/2008
Baryon acoustic
oscillations as
a standard ruler
Blake & Glazebrook 2003
Seo & Eisenstein 2003
Comparing observed
acoustic scales with
expected values
(calibrated by CMB)
gives us H(z) [radial
direction] and DA(z)
[transverse direction]
Yun Wang, 1/21/2008
Redshift space distortions
Large scale compression
due to linear motions
gives the Kaiser factor
=fg/b,
fg =dlnG/dlna~ (a)0.55
G(z)=(z)/(0)
(a)=m/.
Yun Wang, 1/21/2008
f(z) traces how structure grows inside the box
 gravitation theory. H(z) measures how the
box expands with time  equation of state w(z)
z=6
z=2
z=0
Models with the same expansion history
H(z) but different gravitational theory will
have a different growth rate function f(z).
Discrepancy between f(z) and H(z) from GR:
smoking gun for New Physics. Need both
H(z) and f(z) to break possible degeneracies.
Yun Wang, 1/21/2008
Image credit: V. Springel
SPACE INSTRUMENT PERFORMANCE
Total field of view
51’ x 27’ (0.4 sq. degrees)
Nr. and type of DMDs
4 CINEMA chip (2048x1050)
Total nr. of mirrors
8.8 million
Mirror field of view
0.75” x 0.75”
Number of spectra
~ 6,000 simultaneous
Detector Pixel size
0.375” x 0.375”
Dispersing element
Prism R~400; 0.8-1.8mm
Imaging filters
z, J, H, narrow band
Detector
HgCdTe 0.4-1.8µm, 2k x 2k
Nr. of detectors
16 (4 mosaics of 2x2 chips)
Detector Temperature
~145 K
QE
>75% average
Readout noise
5e-/multiple read
Observing modes
Broad- and narrow-band imaging, multi-slit, slitless
spectroscopy
Yun Wang, 1/21/2008
Combining ALPACA Dark Energy
Constraints
The simplest dark energy investigation
method sensitivities to estimate are SN Ia
standard candles, weak lensing shear and
baryon acoustic oscillations. To express
dark energy dynamics, we use w = w0 + wa
a = w0 + wa /(1+z), where wa describes the
redshift change in w. A few points:
If SN Ia method systematics ~ 10%,
baryon oscillations are more useful. If ~
2%, SN are more useful, comparable to
weak lensing constraints.
Current limits combining CMB
anisotropies, LSS and SN Ia constrain w at
the 10% level. ALPACA could improve
this 5x. Limit on wa would be vital in
distinguishing dark energy models.
Corasaniti et al. (2006)
Yun Wang, 1/21/2008
Dataset
error on: m
SNe (2% syst.+WMAP)
0.03
SNe+BAO
0.02
WL
0.02
SNe+BAO+WL
0.01
SNe+BAO+WL+Planck
0.003
Planck only
0.013
w0
0.15
0.11
0.20
0.04
0.02
0.19
wa
1.0
0.65
0.57
0.16
0.04
0.94
ESA-ESO WG recommendations
• Wide-field optical and near-IR imaging survey [WL/CL]
– ESA: satellite with high resolution wide-field optical and near-IR imaging
– ESO: optical multi-color photometry from the ground
– ESO: large spectroscopic survey (>100,000 redshifts over ~10,000 sq deg
to calibration of photo-z’s)
• Secure access to an instrument with capability for massive
multiplexed deep spectroscopy (several thousand simultaneous
spectra over one sq deg) [BAO]
• A supernova survey with multi-color imaging to extend existing
samples of z=0.5-1 SNe by an order of magnitude, and improve
the local sample of SNe. [SNe]
• Use a European Extremely Large Telescope (ELT) to study SNe
at z >1. [SNe]
Yun Wang, 1/21/2008
Understanding SN Ia Spectra
Solid: Type Ia SN 1994D, 3 days before maximum brightness
Dashed: a PHOENIX synthetic spectrum (Lentz, Baron, Branch, Hauschildt
2001, ApJ 557, 266)
Yun Wang, 1/21/2008
Evidence for Dark Energy
Speeding up of cosmic expansion increases the distance
between two galaxies (Milky Way and supernova host
galaxy), which would lead to fainter than expected
observed supernovae.
Observed supernovae are fainter than expected, so the
expansion of the universe must have accelerated.
For convenience, the unknown cause for the
observed acceleration of the cosmic expansion
is dubbed dark energy.
Yun Wang, 1/21/2008
Model Selection Using Bayesian Evidence
Bayes theorem: P(M|D)=P(D|M)P(M)/P(D)
Bayesian edidence: E=L()Pr()d
:likelihood of the model given the data.
Jeffreys interpretational scale of DlnE between two models:
DlnE<1: Not worth more than a bare mention.
1<DlnE<2.5: Significant.
2.5<DlnE<5: Strong to very strong.
5<DlnE: Decisive.
SNLS (SNe)+WMAP3+SDSS(BAO):
Compared to , DlnE=-1.5 for constant wX model
DlnE=-2.6 for wX(a)=w0+wa(1-a) model
Relative prob. of three models: 77%, 18%, 5%
Liddle, Mukherjee, Parkinson, & Wang (2006)
Yun Wang, 1/21/2008
Need for a space-based mission in the near-infrared
- Sample selected in the near-IR to mAB ≈ 22-23 : 0<z<2, weak k-corrections,
all galaxy types (including E/S0), stellar mass–selected, less affected by dust extinction
- Sky background is 500-1000 times lower in space
- No OH emission lines, no telluric absorptions
- Near-IR spectroscopy : rest-frame optical strongest features visible at
all redshifts, E/S0 galaxies, Lyα up to z ≈ 10+
- Moderate spectral resolution (spec-z efficiency, resolve Hα and [N II])
- Digital Micro Mirrors (DMDs)
Yun Wang, 1/21/2008
Requirements for a cosmological mission
To address the key questions of cosmology (not only Dark Energy !)
Observation of a huge volume of the Universe (> 10,000 deg2 , 0<z<2)
Spectroscopic approach (powerful vs photometric SEDs and photo-z)
Wide-field, high “multiplexing”, high survey speed
Slit spectroscopy (vs slitless) : SNR ≈ 65 times higher
Yun Wang, 1/21/2008
SPACE survey programs
“All-sky” near-infrared imaging & spectroscopic survey of ¾
of the sky (3π sr). Sample selected in H-band (AB<23.0).
Random sampling rate of 1/3  ≈ Half-billion galaxies at
0 < z < 2 with spectroscopic redshifts, plus quasars up to z ≈ 12
Deep near-infrared imaging and spectroscopy of 10 deg2 down
to H(AB) < 26. About 2 million galaxies and AGN at 2 < z < 10.
(90% random sampling rate) + Type Ia Supernovae to z ≈ 2.
Galactic plane survey
Open time for Guest Observer programs
Yun Wang, 1/21/2008
SNe with SPACE
• 4 deg Deep Field (H<26) (e.g. within the SPACE Deep Field, 10 deg )
• Repeated visits of the same field every 7 - 10 days (1 visit = 4 days to cover 4 deg )
• Advantage to obtain spectra of all SNe in the field
• Near-IR is crucial : spectra of high-z SNe, less dust extinction
• N ≈ 2300 SNe to z ≈ 2 in about 5 – 7 months spread over 1 year (faster than SNAP)
• Synergy with SN “finder” (e.g. SNAP) would be extremely powerful
• Inclusion of SN program in SPACE will depend on the developments of other
2
2
2
projects in space and on the ground (e.g. SNAP, CFHT, Pan-STARRS, LSST, …)
Yun Wang, 1/21/2008