arXiv subjects

M. J. S. Houndjo

Publications and source records attributed to M. J. S. Houndjo.

66 records · Page 4Linked to original sources

arXiv2012

Reconstructing $f(R,T)$ gravity from holographic dark energy

We consider cosmological scenarios based on $f(R,T)$ theories of gravity ($R$ is the Ricci scalar and $T$ is the trace of the energy-momentum tensor) and numerically reconstruct the function $f(R,T)$ which is able to reproduce the same expansion history generated, in the standard General Relativity theory, by dark matter and holographic dark energy. We consider two special $f(R,T)$ models: in the first instance, we investigate the modification $R + 2f(T)$, i.e. the usual Einstein-Hilbert term plus a $f(T)$ correction. In the second instance, we consider a $f(R)+λT$ theory, i.e. a $T$ correction to the renown $f(R)$ theory of gravity.

gr-qc↗

arXiv2012

Quantum effects near future singularities

General relativity allows a variety of future singularities to occur in the evolution of the universe. At these future singularities, the universe will end in a singular state after a finite proper time and geometrical invariants of the space time will diverge. One question that naturally arises with respect to these cosmological scenarios is the following: can quantum effects lead to the avoidance of these future singularities? We analyze this problem considering massless and conformally coupled scalar fields in an isotropic and homogeneous background leading to future singularities. It is shown that near strong, big rip-type singularities, with violation of the energy conditions, the quantum effects are very important, while near some milder classes of singularity like the sudden singularity, which preserve the energy conditions, quantum effects are irrelevant.

gr-qc↗

arXiv2012

Reconstruction of f(T) gravity according to holographic dark energy

We develop the reconstruction of a model of $f(T)$ gravity according to the holographic dark energy. $T$ is the torsion scalar and its initial value from the Teleparallel gravity is imposed for fitting the initial value of the function $f(T)$. The result shows a polynomial function for $f(T)$, and we also observe that, when $Ω_V\rightarrow 1$ at the future time, $ω_V$ may cross -1 for some values of the input parameter $b$. Another interesting aspect of the obtained model is that it provides the unification of the dark matter with the dark energy.

gr-qc↗

arXiv2011

New Black Holes Solutions in a Modified Gravity

We present a theory of modified gravity, inspired by the gauge theories, where the commutator algebra of covariant derivative gives us an added term with respect to the General Relativity, which represents the interaction of gravity with a substratum. New spherically symmetric solutions of this theory are obtained and can be view as solutions that reproduce, the mass, the charge, the cosmological constant and the Rindler acceleration, without coupling with the matter content, i.e., in the vacuum.

gr-qc↗

arXiv2011

New Static Solutions in f(T) Theory

We consider the equations of motion of an anisotropic space-time in $f(T)$ theory, where $T$ is the torsion. New spherically symmetric solutions of black holes and wormholes are obtained with a constant torsion and the cases for which the radial pressure is proportional to a real constant, to some algebraic functions $f(T)$ and their derivatives $f_T(T)$, or vanish identically.

astro-ph.CO↗

arXiv2011

Quantum effects from particle production on background evolution and Cardy-Verlinde formula in f(R) gravity

We investigate particle production in an expanding universe under the assumption that the Lagrangian contains the Einstein term $R$ plus a modified gravity term of the form $R^α$, where $α$ is a constant. Dark fluid is considered as the main content of the universe and the big rip singularity appears. Quantum effects due to particle creation is analysed near the singularity and we find that for $α\in ]1/2, 1[$, quantum effects are dominant and the big rip may be avoided whereas for $α\in J$ the dark fluid is dominant and the singularity remains. The Cardy-Verlinde formula is also introduced and its equivalence with the total entropy of the universe is checked. It is found that this can always occur in Einstein gravity while in f(R) gravity, it holds only for $α=\frac{n+1}{2n+6}$, $n$ being the space dimension, corresponding to the situation in which the big rip cannot be avoided.

astro-ph.CO↗

arXiv2011

Reconstruction of f(R, T) gravity describing matter dominated and accelerated phases

We investigate the cosmological reconstruction in modified f(R,T) gravity, where R is the Ricci scalar and T the trace of the stress-energy tensor. Special attention is attached to the case in which the function f is given by f (R, T) = f1 (R) + f2 (T). The use of auxiliary scalar field is considered with two known examples for the scale factor corresponding to an expanding universe. In the first example, where ordinary matter is usually neglected for obtaining the unification of matter dominated and accelerated phases with f(R)gravity, it is shown in this paper that this unification can be obtained without neglect ordinary matter. In the second example, as in f(R)gravity, model of f(R, T) gravity with transition of matter dominated phase to the acceleration phase is obtained. In both cases, linear function of the trace is assumed for f2(T) and it is obtained that f1(R) is proportional to a power of R with exponents depending on the input parameters.

astro-ph.CO↗

arXiv2011

Transition between phantom and non-phantom phases with time dependent cosmological constant and Cardy-Verlinde formula

We investigate the transition phenomenon of the universe between a phantom and a non-phantom phases. Particular attention is devoted to the case in which the cosmological constant depends on time and is proportional to the square of the Hubble parameter. Inhomogeneous equations of state are used and the equation of motion is solved. We find that, depending on the choice of the input parameters, the universe can transit from the non-phantom to the phantom phase leading to the appearance of singularities. In particular, we find that the phantom universe ends in the singularity of type III, unlike the case without variable cosmological constant in which the phantom phase ends exclusively in the big rip (singularity of type I). The Cardy-Verlinde formula is also introduced for inhomogeneous equation of state and we find that its equivalence with the total entropy of the universe, coming from the Friedmann equations, occurs only for special choice of the input parameter $m$ at the present time.

astro-ph.CO↗

arXiv2010

Conformal anomaly around the sudden singularity

Quantum effects due to particle creation on a classical sudden singularity have been investigated in a previous work. The conclusion was that quantum effects do not lead to the avoidance nor the modification of the sudden future singularity. In this paper, we investigate quantum corrections coming from conformal anomaly near the sudden future singularity. We conclude that when the equation of state is chosen to be $p=-ρ-Aρ^α$, the conformal anomaly can transform the sudden singularity in the singularity of type III for any $α> 1/2$ and in the singularity of the type I (the big rip) or the big crunch for $1/2<α<3/2$.

hep-th↗

arXiv2009

Quantum particle production near the big rip revisited

The effect of quantum particle production near the big rip singularity has been investigated previously, with the conclusion that the energy of the produced particle decreases as the future singularity is approached. Hence, the effect of particle production would not be effective to avoid the big rip singularity. That calculation was performed by introducing an ultra-violet cut-off. In the present work we consider a renormalization of the energy-momentum tensor, obtainning a different expression for the particle production. The new expression seems to indicate that the effect of particle production may be dominant as the singularity is approached.

gr-qc↗

arXiv2008

Quantum Particle Production at Sudden Singularities

We investigate the effects of quantum particle production on a classical sudden singularity occurring at fine time in a Friedmann universe. We use an exact solution to describe an initially radiation-dominated universe that evolves into a sudden singularity at finite time. We calculate the density of created particles exactly and find that it is generally much smaller than the classical background density and pressure which produce the sudden singularity. We conclude that, in the example studied, the quantum particle production does not lead to the avoidance or modification to the sudden future singularity. We argue that the effects of small residual anisotropies in the expansion will not change these results and show how they can be related to studies of classical particle production using a bulk viscosity. We conclude that we do not expect to see significant observable effects from local sudden singularities on our past light cone.

gr-qc↗

arXiv2007

Particle Production in an expanding universe dominated by dark energy fluid

We investigate the fate of particle production in an expanding universe dominated by a perfect fluid with equation of state $p = αρ$. The rate of particle production, using the Bogolioubov coefficients, are determined exactly for any value of $α$ in the case of a flat universe. When the strong energy condition is satisfied, the rate of particle production decreases as time goes on, in agreement to the fact that the four-dimensional curvature decreases with the expansion; the opposite occurs when the strong energy condition is violated. In the phantomic case, the rate of particle production diverges in a finite time. This may lead to a backreaction effect, leading to the avoidance of the big rip singularity, specially if $- 1 > α> - {5/3}$.

gr-qc↗