  Chandra observations of large samples of galaxy clusters detected in X-rays
by ROSAT provide a new, robust determination of the cluster mass functions at
low and high redshifts. Statistical and systematic errors are now sufficiently
small, and the redshift leverage sufficiently large for the mass function
evolution to be used as a useful growth of structure based dark energy probe.
In this paper, we present cosmological parameter constraints obtained from
Chandra observations of 36 clusters with <z>=0.55 derived from 400deg^2 ROSAT
serendipitous survey and 49 brightest z=~0.05 clusters detected in the All-Sky
Survey. Evolution of the mass function between these redshifts requires
Omega_Lambda>0 with a ~5sigma 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,
Sum m_nu<0.33 eV at 95% CL. We also present updated measurements of Omega_M*h
and sigma_8 from the low-redshift cluster mass function.

Astrophysics