10 research outputs found
Viscous Fluids and Gauss-Bonnet Modified Gravity
We study effects of cosmic fluids on finite-time future singularities in
modified -gravity, where and are the Ricci scalar and the
Gauss-Bonnet invariant, respectively. We consider the fluid equation of state
in the general form, , and we suppose the existence of a
bulk viscosity. We investigate quintessence region () and phantom
region () and the possibility to change or avoid the singularities
in -gravity. Finally, we study the inclusion of quantum effects in
large curvatures regime.Comment: 14 page
Interacting Kasner-type cosmologies
It is well known that Kasner-type cosmologies provide a useful framework for
analyzing the three-dimensional anisotropic expansion because of the
simplification of the anisotropic dynamics. In this paper relativistic
multi-fluid Kasner-type scenarios are studied. We first consider the general
case of a superposition of two ideal cosmic fluids, as well as the particular
cases of non-interacting and interacting ones, by introducing a
phenomenological coupling function . For two-fluid cosmological scenarios
there exist only cosmological scaling solutions, while for three-fluid
configurations there exist not only cosmological scaling ones, but also more
general solutions. In the case of triply interacting cosmic fluids we can have
energy transfer from two fluids to a third one, or energy transfer from one
cosmic fluid to the other two. It is shown that by requiring the positivity of
energy densities there always is a matter component which violates the dominant
energy condition in this kind of anisotropic cosmological scenarios.Comment: Accepted for publication in Astrophysics &Space Science, 8 page
Cosmological scaling solutions in generalised Gauss-Bonnet gravity theories
The conditions for the existence and stability of cosmological power-law
scaling solutions are established when the Einstein-Hilbert action is modified
by the inclusion of a function of the Gauss-Bonnet curvature invariant. The
general form of the action that leads to such solutions is determined for the
case where the universe is sourced by a barotropic perfect fluid. It is shown
by employing an equivalence between the Gauss-Bonnet action and a scalar-tensor
theory of gravity that the cosmological field equations can be written as a
plane autonomous system. It is found that stable scaling solutions exist when
the parameters of the model take appropriate values.Comment: 10 pages and 5 figure
Parameterization and Reconstruction of Quasi Static Universe
We study a possibility of the fate of universe, in which there is neither the
rip singularity, which results in the disintegration of bound systems, nor the
endless expansion, instead the universe will be quasi static. We discuss the
parameterization of the corresponding evolution and the reconstruction of the
scalar field model. We find, with the parameterization consistent with the
current observation, that the current universe might arrive at a quasi static
phase after less than 20Gyr.Comment: minor changes and Refs. added, publish in EPJ