The observational fact that the present values of the densities of dark energy and dark matter are of the same order of magnitude, ρ de0 /ρ dm0 ∼ O(1), seems to indicate that we are currently living in a very special period of the cosmic history. Within the standard model, a density ratio of the order of one just at the present epoch can be seen as coincidental since it requires very special initial conditions in the early Universe. The corresponding "why now" question constitutes the cosmological "coincidence problem". According to the standard model the equality ρ de = ρ dm took place "recently" at a redshift z ≈ 0.55. The meaning of "recently" is, however, parameter dependent. In terms of the cosmic time the situation looks different. We discuss several aspects of the "coincidence problem", also in its relation to the cosmological constant problem, to issues of structure formation and to cosmic age considerations.

By using observations of the Hulse-Taylor pulsar we constrain the gravitational wave (GW) speed to the level of 10 −2 . We apply this result to scalar-tensor theories that generalize Galileon 4 and 5 models, which display anomalous propagation speed and coupling to matter for GWs. We argue that this effect survives conventional screening due to the persistence of a scalar field gradient inside virialized overdensities, which effectively "pierces" the Vainshtein screening. In specific branches of solutions, our result allows to directly constrain the cosmological couplings in the effective field theory of dark energy formalism.

We calculate static and spherically symmetric solutions for the Rastall modification of gravity to describe Neutron Stars (NS). The key feature of the Rastall gravity is the non-conservation of the energy-momentum tensor proportionally to the space-time curvature. Using realistic equations of state for the NS interior we place a conservative bound on the non-GR behaviour of the Rastall theory which should be 1% level. This work presents the more stringent constraints on the deviations of GR caused by the Rastall proposal.

Fluids often display dissipative properties. We explore dissipation in the form of bulk viscosity in the cold dark matter fluid. We constrain this model using current data from supernovae, baryon acoustic oscillations and the cosmic microwave background. Considering the isotropic and homogeneous background only, viscous dark matter is allowed to have a bulk viscosity 10 7 Pa·s, also consistent with the expected integrated Sachs-Wolfe effect (which plagues some models with bulk viscosity). We further investigate the small-scale formation of viscous dark matter halos, which turns out to place significantly stronger constraints on the dark matter viscosity. The existence of dwarf galaxies is guaranteed only for much smaller values of the dark matter viscosity, 10 −3 Pa·s. PACS numbers: 98.80-k, 95.36+x

We model the dark sector of the cosmic substratum by a viscous fluid with an equation of state p = −ζΘ, where Θ is the fluid-expansion scalar and ζ is the coefficient of bulk viscosity for which we assume a dependence ζ ∝ ρ ν on the energy density ρ. The homogeneous and isotropic background dynamics coincides with that of a generalized Chaplygin gas with equation of state p = −A/ρ α .The perturbation dynamics of the viscous model, however, is intrinsically non-adiabatic and qualitatively different from the Chaplygin-gas case. In particular, it avoids short-scale instabilities and/or oscillations which apparently have ruled out unified models of the Chaplygin-gas type. We calculate the matter power spectrum and demonstrate that the non-adiabatic model is compatible with the data from the 2dFGRS and the SDSS surveys. A χ 2 -analysis shows, that for certain parameter combinations the viscous-dark-fluid (VDF) model is well competitive with the ΛCDM model. These results indicate that non-adiabatic unified models can be seen as potential contenders for a General-Relativity-based description of the cosmic substratum. *

Modern cosmology suggests that the Universe contains two dark componentsdark matter and dark energy -both unkown in laboratory physics and both lacking direct evidence. Alternatively, a unified dark sector, described by a single fluid, has been proposed. Dissipation is a common phenomenon in nature and it thus seems natural to consider models dominated by a viscous dark fluid. We focus on the study of bulk viscosity, as isotropy and homogeneity at large scales implies the suppression of shear viscosity, heat flow and diffusion. The generic ansatz ξ ∝ ρ ν for the coefficient of bulk viscosity (ρ denotes the mass/energy density), which for ν = −1/2 mimics the ΛCDM background evolution, offers excellent fits to supernova and H(z) data. We show that viscous dark fluids suffer from large contributions to the integrated Sachs-Wolfe effect (generalising a previous study by Li & Barrow) and a suppression of structure growth at small-scales (as seen from a generalized Meszaros equation). Based on recent observations, we conclude that viscous dark fluid models (with ξ ∝ ρ ν and neglecting baryons) are strongly challenged.

In a homogeneous and isotropic universe bulk viscosity is the unique viscous effect capable to modify the background dynamics. Effects like shear viscosity or heat conduction can only change the evolution of the perturbations. The existence of a bulk viscous pressure in a fluid, which in order to obey to the second law of thermodynamics is negative, reduces its effective pressure. We discuss in this study the degeneracy in bulk viscous cosmologies and address the possibility that phantom dark energy cosmology could be caused by the existence of non-equilibrium pressure in any cosmic component. We establish the conditions under which either matter or radiation viscous cosmologies can be mapped into the phantom dark energy scenario with constraints from multiple observational data-setsComment: 10 pages, 2 figures, accepted for publication in PR

We investigate the cosmological perturbation dynamics for a universe consisting of pressureless baryonic matter and a viscous fluid, the latter representing a unified model of the dark sector. In the homogeneous and isotropic background the \textit{total} energy density of this mixture behaves as a generalized Chaplygin gas. The perturbations of this energy density are intrinsically non-adiabatic and source relative entropy perturbations. The resulting baryonic matter power spectrum is shown to be compatible with the 2dFGRS and SDSS (DR7) data. A joint statistical analysis, using also Hubble-function and supernovae Ia data, shows that, different from other studies, there exists a maximum in the probability distribution for a negative present value $q_0 \approx - 0.53$ of the deceleration parameter. Moreover, while previous descriptions on the basis of generalized Chaplygin gas models were incompatible with the matter power spectrum data since they required a much too large amount of pressureless matter, the unified model presented here favors a matter content that is of the order of the baryonic matter abundance suggested by big-bang nucleosynthesis.Comment: 19 pages, 6 figure