I will describe cosmological constraints derived using sample of 400 deg2 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 ΩΛ>0 with a ~5σ significance, and constrains the dark energy equation of state parameter to w0=-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 w0 = -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 mν<0.33 eV at 95% CL. I will also present updated measurements of ΩMh and σ8 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
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