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)
2. Flat universe (CMB anisotropies)
+Low matter density (several independent
measurements)
= Missing energy component
0.7 = 1.0 – 0.3
Tests for Dark Energy
•Evidence for acceleration (SNIa, SZ)
• CMB anisotropies and W0 = 1
PLUS
Matter density estimates: Wm ~ 0.3, LSS
• Weak lensing, strong lensing, galaxy counts,
angular diameter (Alcock-Paczynski) tests
• Direct measure of the expansion rate
Dark Energy (Wx)
• characterize by equation of state
w = P(z) / r(z)
• w = -1 for a cosmological constant
• can be time dependent
need observations over a
range of redshifts
Evidence for Acceleration
•Riess et al. 1998
Perlmutter et al. 1999
Type Ia supernovae
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
Wm = 0.3, WL=0.7
Evidence for Acceleration (cont’d)
Perlmutter et al. 1999
Evidence for Acceleration (cont’d)
• Riess et al. (2001)
• NICMOS serendipitous
SN 1997ff
z = 1.7
Wm
Current evidence:
Galaxy kinematics
Wm ~ 0.3
Cluster baryons
• fb ~ 10-20%
• Wb h2 = 0.02
• Wm ~ 0.3-0.4
X-ray gas
Lensing
Boomerang: Netterfield et al. (2001)
W0 = 1.03
0.06
W0
DASI: Pryke et al. (2001)
W0 = 1.04
0.06
•For same matter content, very different geometry allowed
•CMB measurements give no information w(z)
To break degeneracies: H0, galaxy power spectrum,
weak lensing (Hu, Huterer, Turner)
CMB and Supernovae
• de Bernardis et al (2001)
• Boomerang + SNIa
• orthogonal constraints
Wm = 0.31
WL = 0.71
0.13
0.11
Combining Constraints
CMB &
LSS
SNIa
Combined
constraints
Perlmutter, Turner & White
Phys. Rev. Lett. (1999)
• LSS & CMB constraints are
orthogonal to supernova
constraints
• sample of ~ 50 supernovae
• Peacock & Dodds power
spectrum
Huterer & Turner (2001)
Constraining Quintessence
Solid line: wq = -0.8
Dashed line: w = -1
A Challenge!!!
Baccigalupi et al. 2001
Best fit: wq = -0.8
Wq = 0.72
Combining Constraints
Combined maximum likelihood
analysis:
Wang et al. (2000)
-1 < w < -0.6
Gravitational Lens Statistics
Challenges:
• Mass distribution of lenses (SIS)
• Evolution dependence (merger rates not
well constrained)
• Extinction due to dust
• Small number statistics
Dev et al. (2001):
•
w < -0.04, Wm < 0.9 at 68%CL
•
If w = -1, Wm = 0.3 at 68%CL
•
w = -0.33, Wm = 0.0 BEST FIT
Gravitational Lenses
Wm=1.0 Wm=0.3,open Wm=0.3,flat
N(z) versus z
Predicted & observed
Flat universe, Wm = 0.2
Fundamental plane for lens
galaxies
Cheng & Krauss (1998)
Kochanek et al. (1999)
Age Constraints
•
•
H0t0
consistency check on
acceleration
not probe of w(z)
0.25
0.35
Wm
Huterer
& Turner
(2001)
H0r/H0t0
• H0 = 72
• t0 = 13
8 km/sec/Mpc (Freedman et al. 2001)
1.5 Gyr (Chaboyer 2001, Krauss 2000)
H0 t0 = 0.93
0.15
w < -0.5 (Huterer & Turner 2001)
The Future
Direct Measure of the
Expansion Rate
Loeb (1998) : Lyman alpha clouds
•~2 m/s/CENTURY!
• not yet feasible
Freedman (2001)
No one said this would be easy…
Challenges:
Supernovae:
• Evolution
• Dust
• Metallicity
• Calibration
• Environment
• K-corrections
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
CMB anisotropies:
•Many parameters
•Strong degeneracies
•No w(z) constraint
No one said this would be easy…
Challenges:
Angular Diameters:
(correlation functions)
• Geometry
• Small effect
• Peculiar Velocities
Age comparison:
•Limits to H0 t0
•Model uncertainties
(stellar evolution)
•Zero point calibrations
•Dust, metallicity
•Cosmic variance
•No w(z) information
Number counts:
•Counting statistics
•Galaxy evolution
•Infall
•Velocity errors
•Incompleteness
•Modeling (N-body)
•Cosmic variance
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 W0 = 1 strong
indication of missing energy component
• Consistency checks from numerical simulations,
galaxy power spectrum, age
• w(z) not yet observationally constrained