http://arxiv.org/abs/0812.2720 Chandra Cluster Cosmology Project III: Cosmological Parameter Constraints A.Vikhlinin, A.V.Kravtsov, R.A.Burenin, H.Ebeling, W.R.Forman, A.Hornstrup, C.Jones, S.S.Murray, D.Nagai, H.Quintana, A.Voevodkin ApJ, in press: Feb 10, 2009 issue (Submitted on 15 Dec 2008) "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 400deg2 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 OmegaLambda>0 with a ~5 sigma 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 mnu<0.33 eV at 95% CL. We also present updated measurements of OmegaM*h and sigma8 from the low-redshift cluster mass function." Sean Carroll has a good post on this at Cosmic Variance. The data plots he shows in a couple of figures tell the story clearly. The dark energy fraction is a key cosmological parameter and this gives new way to constrain it, another way to measure. http://blogs.discovermagazine.com/cosmicvariance/2008/12/16/dark-energy-no-longer-a-surprise/ The third figure that Carroll shows (taken from the paper cited above) is a good illustration of how observational cosmology works. It shows overlapping confidence ovals constraining two important cosmological model parameters---OmegaLambda the dark energy fraction, and w the dark energy equation of state---using various batches of data. The new data is the distribution of clusters by mass, in two redshift ranges (roughly speaking near range and medium range). Earlier data are from measurements of SN supernovae BAO baryon acoustic oscillations WMAP cosmic microwave background In the paper they use OmegaX for the dark energy fraction, which is less common notation but sometimes easier to write. Here is Carroll's explanation: ==quote== Alexey Vikhlinin and collaborators have used observations from the Chandra X-ray satellite to uncover new evidence for dark energy. ... In particular, they simply count the number of galaxy clusters with various masses at various redshifts, and compare with the predictions of models with and without dark energy. If there were no dark energy, matter would keep clustering on larger and larger scales as the universe expanded, making new clusters all the way. But if dark energy eventually takes over, the creation of new clusters begins to turn off, as the dark energy provides an extra push of expansion beneath the feet of the particles that would like to cluster together, preventing them from doing so. ==endquote== As figures 1 and 2 show, there are fewer massive clusters than would be predicted in the absence of accelerated expansion (if Lambda were zero---if dark energy had not kicked in a few billion years ago) The effect of the cosmological constant, or dark energy, has apparently been to disperse galaxies which would otherwise have gathered into clusters---making for fewer of the more massive clusters.