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

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

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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 – 0.3 2. Flat universe (CMB anisotropies) +Low matter density (several independent measurements) = Missing energy component

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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

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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

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Evidence for Acceleration m = 0.3, =0.7 Riess et al. 1998 Perlmutter et al. 1999 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

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

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

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mm Current evidence: Galaxy kinematicsCluster baryons f b ~ 10-20% b h 2 = 0.02 m ~ 0.3-0.4 X-ray gas Lensing m ~ 0.3

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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 )

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CMB and Supernovae m = 0.31 0.13 = 0.71 0.11 de Bernardis et al (2001) Boomerang + SNIa orthogonal constraints

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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

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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

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

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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

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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

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Age Constraints consistency check on acceleration not probe of w(z) H 0 = 72 8 km/sec/Mpc (Freedman et al. 2001) t 0 = 13 1.5 Gyr (Chaboyer 2001, Krauss 2000) H 0 t 0 = 0.93 0.15 w < -0.5 (Huterer & Turner 2001) Huterer & Turner (2001) H0t0H0t0 0.25 0.35 mm H 0 r/H 0 t 0

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The Future

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

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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

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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

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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

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