### New contraints on dark energy from X-ray galaxy cluster studies

David Rapetti

SLAC

**Abstract:**

Most of the energy density of the Universe appears to be in the form of
dark matter and dark energy, and yet these two components are the most
intriguing mysteries in current cosmology. Using two complementary X-ray
galaxy cluster studies we present new constraints on the mean matter
density of the Universe, dark energy density, normalization of the density
fluctuation power spectrum, and dark energy equation of state. First,
using Chandra measurements of the X-ray gas mass fraction in 42 hot, X-ray
luminous, dynamically relaxed galaxy clusters spanning the redshift range
0.05<z<1.1, and employing standard priors on the Hubble constant, H_0, and
the mean baryon density, w_bh2, we obtain tight constraint on the mean
matter density and a detection of the effects of dark energy on the
distances to the clusters comparable in significance to recent type Ia
supernovae (SNIa) studies. Second, using the X-ray luminosity function of
the Massive Cluster Survey (MACS) in combination with the local BCS and
REFLEX galaxy cluster samples, the mass function predictions of Jenkins et
al. (2001), a mass-luminosity relation calibrated using the data of
Reiprich and Bohringer (2002), and standard priors on H_0 and w_bh2, we
obtain the first precise determination of the dark energy equation of
state from measurements of the growth of cosmic structure in galaxy
clusters. Combining our results with independent constraints from cosmic
microwave background and SNIa studies removes the need for priors on H_0
and w_bh2 and leads to tighter constraints. Both of our results represent
strong, independent evidences for cosmic acceleration.