85 research outputs found
Consequences of energy conservation violation: Late time solutions of subclass of gravity using dynamical system approach
Very recently, the authors of [PRL {\bf 118} (2017) 021102] have shown that
violation of energy-momentum tensor (EMT) could result in an accelerated
expansion state via appearing an effective cosmological constant, in the
context of unimodular gravity. Inspired by this outcome, in this paper we
investigate cosmological consequences of violation of the EMT conservation in a
particular class of gravity when only the pressure-less fluid is
present. In this respect, we focus on the late time solutions of models of the
type . As the first task, we study the solutions
when the conservation of EMT is respected and then we proceed with those in
which violation occurs. We have found, provided that the EMT conservation is
violated, there generally exist two accelerated expansion solutions which their
stability properties depend on the underlying model. More exactly, we obtain a
dark energy solution for which the effective equation of state (EoS) depend on
model parameters and a de Sitter solution. We present a method to parametrize
function which is useful in dynamical system approach and has
been employed in the herein model. Also, we discuss the cosmological solutions
for models with in the presence of the ultra
relativistic matter.Comment: 27 pages, 4 figure
Late-time cosmological evolution of a general class of f(R, T) gravity with minimal curvature-matter coupling
In this work, we study the late-time cosmological solutions of
f(R,T)=g(R)+h(-T) models assuming that the conservation of the energy-momentum
tensor (EMT) is violated. We perform our analysis through constructing an
autonomous dynamical system for the equations of motion. We study the stability
properties of solutions via considering linear perturbations about the related
equilibrium points. These parameters which are constructed out of the functions
g(R) and h(-T) play the main role in finding the late time behavior of the
solutions.Comment: 28 pages, 4 figure
Stability of the Einstein static Universe in Einstein-Cartan-Brans-Dicke gravity
In the present work we consider the existence and stability of Einstein
static {\sf ES} Universe in Brans-Dicke ({\sf BD}) theory with non-vanishing
spacetime torsion. In this theory, torsion field can be generated by the {\sf
BD} scalar field as well as the intrinsic angular momentum (spin) of matter.
Assuming the matter content of the Universe to be a Weyssenhoff fluid, which is
a generalization of perfect fluid in general relativity ({\sf GR}) in order to
include the spin effects, we find that there exists a stable {\sf ES} state for
a suitable choice of the model parameters. We analyze the stability of the
solution by considering linear homogeneous perturbations and discuss the
conditions under which the solution can be stable against these type of
perturbations. Moreover, using dynamical system techniques and numerical
analysis, the stability regions of the {\sf ES} Universe are parametrized by
the {\sf BD} coupling parameter and first and second derivatives of the {\sf
BD} scalar field potential, and it is explicitly shown that a large class of
stable solutions exists within the respective parameter space. This allows for
non-singular emergent cosmological scenarios where the Universe oscillates
indefinitely about an initial {\sf ES} solution and is thus past eternal.Comment: 24 pages, 9 figures and 2 table
Bouncing cosmological solutions from f(R,T) gravity
In this work we study classical bouncing solutions in the context of gravity in a flat {\sf FLRW} background using a
perfect fluid as the only matter content. Our investigation is based on
introducing an effective fluid through defining effective energy density and
pressure; we call this reformulation as the "effective picture". These
definitions have been already introduced to study the energy conditions in
gravity. We examine various models to which different
effective equations of state, corresponding to different
functions, can be attributed. It is also discussed that one can link between an
assumed model in the effective picture and the theories
with generalized equation of state ({\sf EoS}). We obtain cosmological
scenarios exhibiting a nonsingular bounce before and after which the Universe
lives within a de-Sitter phase. We then proceed to find general solutions for
matter bounce and investigate their properties. We show that the properties of
bouncing solution in the effective picture of gravity are
as follows: for a specific form of the function, these solutions
are without any future singularities. Moreover, stability analysis of the
nonsingular solutions through matter density perturbations revealed that except
two of the models, the parameters of scalar-type perturbations for the other
ones have a slight transient fluctuation around the bounce point and damp to
zero or a finite value at late times. Hence these bouncing solutions are stable
against scalar-type perturbations. It is possible that all energy conditions be
respected by the real perfect fluid, however, the null and the strong energy
conditions can be violated by the effective fluid near the bounce event.Comment: 49 pages, 11 figures, one tabl
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