161 research outputs found
Inflation in Terms of a Viscous van der Waals Coupled Fluid
We propose to describe the acceleration of the universe by introducing a
model of two coupled fluids. We focus on the accelerated expansion at the early
stages. The inflationary expansion is described in terms of a van der Waals
equation of state for the cosmic fluid, when account is taken of bulk
viscosity. We assume that there is a weak interaction between the van der Waals
fluid and the second component (matter). The gravitational equations for the
energy densities of the two components are solved for a homogeneous and
isotropic Friedmann-Robertson-Walker universe, and analytic expressions for the
Hubble parameter are obtained. The slow-roll parameters, the spectral index,
and the tensor-to-scalar ratio are calculated and compared with the most recent
astronomical data from the Planck satellite. Given reasonable restriction on
the parameters, the agreement with observations is favorable.Comment: 7 pages, no figures. To appear in Int. J. Geom. Meth. Mod. Phy
Little Rip and Pseudo Rip Phenomena from Coupled Dark Energy
We consider Little Rip (LR) and Pseudo Rip (PR) cosmological models with two
interacting ideal fluids, corresponding to dark energy and dark matter. The
interaction between the dark energy and the dark matter fluid components is
described in terms of the parameters in the equations of state for the LR and
PR universes. In contrast to a model containing only a pure dark energy, the
presence of the interaction term between the fluid components in the
gravitational equations leads to a modification of the equation of state
parameters. The properties of the early universe in this formalism are pointed
out.Comment: 11 pages, Latex2e, no figures. To appear in Modern Physics Letters
Holographic description of the dissipative unified dark fluid model with axion field
In this article we extend an axion F(R) gravity model, and apply the
holographic principle to describe in a unifying manner the early and the
late-time universe when the general equation of state (EoS) contains a bulk
viscosity. We assume a spatially flat Friedmann-Robertson-Walker (FRW) universe
model. We use a description based on the generalized infrared-cutoff
holographic dark energy proposed by Nojiri and Odintsov (2006, 2017), and
explore the evolution of the universe when the EoS describes the asymptotic
behavior between the dust in the early universe and the late universe. We
explore various forms of the bulk viscosity, and calculate analytical
expressions for the infrared cutoffs in terms of the particle horizon. In this
way we obtain a unifying description of the early and the late-time universe in
the presence of axion matter, via a viscous holographic fluid model.Comment: 7 pages, latex 2e, to appear in Int. J. Mod. Phys.
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