Physics Division Colloquium - 27 February 2009

I will describe cosmological constraints derived using sample of 400
deg^{2} galaxy cluster survey derived from ROSAT satellite X-ray
data with high-resolution deep follow-up with the Chandra X-ray satellite.
The sample allows robust determination of the cluster mass functions at
redshifts (0<z<0.8).
Evolution of the mass function between these redshifts provides the first
confirmation of accelerated expansion of the universe, independent of
supernovae Type Ia measurements.
Quantitatively, the measured evolution requires
Ω_{Λ}>0 with a ~5σ significance, and
constrains the dark energy equation of state parameter to
w_{0}=-1.14±0.21, assuming constant w and flat universe.
Cluster information also significantly improves constraints when combined
with other methods.
Fitting our cluster data jointly with the latest supernovae, WMAP, and
baryonic acoustic oscillations measurements, we obtain
w_{0} = -0.991 ± 0.045 (stat) ± 0.039 (sys), a factor
of 1.5 reduction in statistical uncertainties, and nearly a factor of 2
improvement in systematics compared to constraints that can be obtained
without clusters.
The joint analysis of these four datasets puts a conservative upper limit on
the masses of light neutrinos, Σ m_{ν}<0.33 eV at 95% CL.
I will also present updated measurements of Ω_{M}h and
σ_{8} from the low-redshift cluster mass function.

The information about the 400d cluster survey can be found at:

hea-www.harvard.edu/400d/cosm/

The paper describing the main results can be found here:

xxx.lanl.gov/abs/0812.2720

ANL Physics Division Colloquium Schedule