813 research outputs found
Dissipation of dark matter
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
Pas, 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, Pas.Comment: 10 pages, 3 figures, published in PR
Matter power spectrum for the generalized Chaplygin gas model: The relativistic case
The generalized Chaplygin gas (GCG) model is the prototype of a unified model
of dark energy (DE) and dark matter (DM). It is characterized by
equation-of-state (EoS) parameters and . We use a statistical
analysis of the 2dFGRS data to constrain these parameters. In particular, we
find that very small (close to zero) and very large values () of
the equation-of-state parameter are preferred. To test the validity of
this type of unification of the dark sector we admit the existence of a
separate DM component in addition to the Chaplygin gas and calculate the
probability distribution for the fractional contributions of both components to
the total energy density. This analysis favors a model for which the Universe
is nearly entirely made up of the separate DM component with an almost
negligible Chaplygin gas part. This confirms the results of a previous
Newtonian analysis.Comment: Latex file, 8 pages, 15 figures in eps forma
A note on acoustic black holes in neo-Newtonian theory
Newtonian fluid dynamics allows the construction of acoustic metrics from
which black hole configurations can be studied. However, relativistic pressure
effects are neglected within Newtonian theory. We study acoustic black holes in
the framework of neo-Newtonian hydrodynamics, which is designed to take into
account relativistic inertial effects of the pressure . Within this new
hydrodynamical context we show how can influence the formation of the
acoustic horizons.Comment: Latex file, 10 pages. Some discussions extended. Accepted for
publication in MPL
Newtonian View of General Relativistic Stars
Although general relativistic cosmological solutions, even in the presence of
pressure, can be mimicked by using neo-Newtonian hydrodynamics, it is not clear
whether there exists the same Newtonian correspondence for spherical static
configurations. General relativity solutions for stars are known as the
Tolman-Oppenheimer-Volkoff (TOV) equations. On the other hand, the Newtonian
description does not take into account the total pressure effects and therefore
can not be used in strong field regimes. We discuss how to incorporate pressure
in the stellar equilibrium equations within the neo-Newtonian framework. We
compare the Newtonian, neo-Newtonian and the full relativistic theory by
solving the equilibrium equations for both three approaches and calculating the
mass-radius diagrams for some simple neutron stars equation of state.Comment: 6 pages, 3 figures. v2 matches accepted version (EPJC
Constraints on dissipative unified dark matter
Modern cosmology suggests that the Universe contains two dark components --
dark 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 for the coefficient of bulk viscosity
( denotes the mass/energy density), which for mimics the
CDM 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 and neglecting
baryons) are strongly challenged.Comment: 17 pages, 12 figures, JCAP publishe
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