Constraining Dark Energy with X-ray Galaxy Clusters, Supernovae and the Cosmic Microwave Background

We present new constraints on the evolution of dark energy from an analysis of Cosmic Microwave Background, supernova and X-ray galaxy cluster data. Our analysis employs a minimum of priors and exploits the complementary nature of these data sets. We examine a series of dark energy models with up to three free parameters: the current dark energy equation of state w_0, the early time equation of state w_et and the scale factor at transition, a_t. From a combined analysis of all three data sets, assuming a constant equation of state and that the Universe is flat, we measure w_0=-1.05+0.10-0.12. Including w_et as a free parameter and allowing a_t to vary over the range 0.5<a_t<0.95 where the data sets have discriminating power, we measure w_0=-1.27+0.33-0.39 and w_et=-0.66+0.44-0.62. We find no significant evidence for evolution in the dark energy equation of state parameter with redshift. Marginal hints of evolution in the supernovae data become less significant when the cluster constraints are also included in the analysis. The complementary nature of the data sets leads to a tight constraint on the mean matter density, Omega_m and alleviates a number of other parameter degeneracies, including that between the scalar spectral index n_s, the physical baryon density Omega_bh^2 and the optical depth tau. This complementary nature also allows us to examine models in which we drop the prior on the curvature. For non-flat models with a constant equation of state, we measure w_0=-1.09+0.12-0.15 and Omega_de=0.70+-0.03. Our analysis includes spatial perturbations in the dark energy fluid, assuming a sound speed c_s^2 =1. For our most general dark energy model, not including such perturbations would lead to spurious constraints on w_et which would be tighter by approximately a factor two with the current data. (abridged)Comment: 11 pages, 13 figures, 2 tables. Accepted for publication in MNRAS. Two new figures added: Fig.9 shows the effects of including dark energy perturbations and Fig.10 compares X-ray cluster data with 2dF dat

The structure and assembly history of cluster-size haloes in Self-Interacting Dark Matter

We perform dark-matter-only simulations of 28 relaxed massive cluster-sized haloes for Cold Dark Matter (CDM) and Self-Interacting Dark Matter (SIDM) models, to study structural differences between the models at large radii, where the impact of baryonic physics is expected to be very limited. We find that the distributions for the radial profiles of the density, ellipsoidal axis ratios, and velocity anisotropies ($\beta$) of the haloes differ considerably between the models (at the $\sim1\sigma$ level), even at $\gtrsim10\%$ of the virial radius, if the self-scattering cross section is $\sigma/m_\chi=1$ cm$^2$ gr$^{-1}$. Direct comparison with observationally inferred density profiles disfavours SIDM for $\sigma/m_\chi=1$ cm$^2$ gr$^{-1}$, but in an intermediate radial range ($\sim3\%$ of the virial radius), where the impact of baryonic physics is uncertain. At this level of the cross section, we find a narrower $\beta$ distribution in SIDM, clearly skewed towards isotropic orbits, with no SIDM (90\% of CDM) haloes having $\beta>0.12$ at $7\%$ of the virial radius. We estimate that with an observational sample of $\sim30$ ($\sim10^{15}$ M$_\odot$) relaxed clusters, $\beta$ can potentially be used to put competitive constraints on SIDM, once observational uncertainties improve by a factor of a few. We study the suppression of the memory of halo assembly history in SIDM clusters. For $\sigma/m_\chi=1$ cm$^2$ gr$^{-1}$, we find that this happens only in the central halo regions ($\sim1/4$ of the scale radius of the halo), and only for haloes that assembled their mass within this region earlier than a formation redshift $z_f\sim2$. Otherwise, the memory of assembly remains and is reflected in ways similar to CDM, albeit with weaker trends.Comment: 15 pages, 15 figures. Submitted to MNRAS. Revisions: added new figure with an observational comparison of density profiles, improvements and corrections to the section on velocity anisotropie