Current Observational Constraints on Dark Energy Chicago, December 2001 Wendy Freedman Carnegie Observatories, Pasadena CA

Current Observational / Experimental Questions What is the nature of dark matter? Is the universe accelerating? What is the nature of dark energy?

Current Evidence for Dark Energy 1. Two independent teams studying type Ia supernovae at high z: Riess et al. (1998); Perlmutter et al. (1999) 0.7 = 1.0 – Flat universe (CMB anisotropies) +Low matter density (several independent measurements) = Missing energy component

Tests for Dark Energy CMB anisotropies and     PLUS   Matter density estimates:  m ~ 0.3, LSS Evidence for acceleration (SNIa, SZ) Direct measure of the expansion rate Weak lensing, strong lensing, galaxy counts, angular diameter (Alcock-Paczynski) tests

Dark Energy (  x ) characterize by equation of state w = P(z) /  z) w = -1 for a cosmological constant can be time dependent need observations over a range of redshifts

Evidence for Acceleration  m = 0.3,   =0.7 Riess et al Perlmutter et al Advantages: small dispersion single objects (simpler than galaxies) can be observed over wide z range Challenges: dust (grey dust) chemical composition evolution photometric calibration environmental differences Type Ia supernovae

Evidence for Acceleration (cont’d) Perlmutter et al. 1999

Evidence for Acceleration (cont’d) Riess et al. (2001) SN 1997ff NICMOS serendipitous z = 1.7

mm Current evidence: Galaxy kinematicsCluster baryons f b ~ 10-20%  b h 2 = 0.02  m ~ X-ray gas Lensing  m ~ 0.3

 Boomerang: Netterfield et al. (2001)    DASI: Pryke et al. (2001)    For same matter content, very different geometry allowed CMB measurements give no information w(z) To break degeneracies: H 0, galaxy power spectrum, weak lensing ( Hu, Huterer, Turner )

CMB and Supernovae  m =   = de Bernardis et al (2001) Boomerang + SNIa orthogonal constraints

Combining Constraints Perlmutter, Turner & White Phys. Rev. Lett. (1999) Huterer & Turner (2001) LSS & CMB constraints are orthogonal to supernova constraints sample of ~ 50 supernovae Peacock & Dodds power spectrum SNIa CMB & LSS Combined constraints

Constraining Quintessence Solid line: w q = -0.8 Dashed line: w = -1 A Challenge!!! Best fit: w q = -0.8  q = 0.72 Baccigalupi et al. 2001

Combining Constraints Wang et al. (2000) Combined maximum likelihood analysis: -1 < w < -0.6

Gravitational Lens Statistics Dev et al. (2001): w < -0.04,  m < 0.9 at 68%CL If w = -1,  m = 0.3 at 68%CL w = -0.33,  m = 0.0 BEST FIT Challenges: Mass distribution of lenses (SIS) Evolution dependence (merger rates not well constrained) Extinction due to dust Small number statistics

Gravitational Lenses Kochanek et al. (1999) Cheng & Krauss (1998) N(z) versus z Predicted & observed Flat universe,  m = 0.2 Fundamental plane for lens galaxies  m =1.0  m =0.3,open  m =0.3,flat

Age Constraints consistency check on acceleration not probe of w(z) H 0 = 72 8 km/sec/Mpc (Freedman et al. 2001) t 0 = Gyr (Chaboyer 2001, Krauss 2000) H 0 t 0 = w < -0.5 (Huterer & Turner 2001) Huterer & Turner (2001) H0t0H0t mm H 0 r/H 0 t 0

The Future

Direct Measure of the Expansion Rate Loeb (1998) : Lyman alpha clouds ~2 m/s/CENTURY! not yet feasible Freedman (2001)

CMB anisotropies: Many parameters Strong degeneracies No w(z) constraint No one said this would be easy… Supernovae: Evolution Dust Metallicity Calibration Environment K-corrections Challenges: Lensing Statistics: Evolution (merging) Dust extinction Velocity dispersions Model dependence Numbers small Weak Lensing: Seeing effects Shear signal small Intrinsic alignment Instrumental noise Crowding of galaxies PSF anisotropy Cosmic variance

No one said this would be easy… Angular Diameters: (correlation functions) Geometry Small effect Peculiar Velocities Challenges: Number counts: Counting statistics Galaxy evolution Infall Velocity errors Incompleteness Modeling (N-body) Cosmic variance Age comparison: Limits to H 0 t 0 Model uncertainties (stellar evolution) Zero point calibrations Dust, metallicity Cosmic variance No w(z) information

Summary of Current Observational Constraints Tantalizing evidence of acceleration in redshift range 0.5 < z < 1.0 Perhaps first evidence of deceleration at z~1.7 CMB anisotropies and    strong indication of missing energy component Consistency checks from numerical simulations, galaxy power spectrum, age w(z) not yet observationally constrained