(From a talk given at the Kavli-CERCA conference on the Future of Cosmology)

Probes of Dark Energy

The linear theory matter power spectrum at z=3, as predicted from the WMAP 1st year data. The error bars indicate the standard deviation computed from the distribution of 6 parameters in a model fit to the WMAP data using the MCMC chains available at http://cosmologist.info/cosmomc. Note that once the major degeneracy with the optical depth (tau) is removed the amplitude and shape of the linear theory power spectrum, in physical units, at z=3 is very well predicted from the WMAP data. The evolution to z=0 depends on the constituents of the universe below z=3, e.g. on the dark energy equation of state, and the comparison with local surveys requires a translation from 1/Mpc to h/Mpc units. The constraint on the power spectrum is tightest in the acoustic peak regime, near k=0.02/Mpc.

Currently the best constraint on dark energy from the CMB comes from the distance to the last scattering surface (z~1100). This is known to about 3% from the 1st year WMAP data, with the uncertainty being dominated not from the measurement of the peak positions (known to 1%) but from the uncertainty in the physical size of the sound horizon at last scattering. This latter uncertainty is dominated by our uncertainty in the physical matter density, which is currently about 8%. Once we measure the higher peaks and damping tail (e.g. with Planck) we should reduce the uncertainty on the matter density to 0.9% and the distance to last scattering to 0.2%. At that point the CMB will provide a strong constraint on dark energy which is complementary to supernovae or other low redshift probes.

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