arXiv subjects

Samuel Lepe

Publications and source records attributed to Samuel Lepe.

At least 55 records · Page 3Linked to original sources

arXiv2020

Modeling holographic dark energy with particle and future horizons

In this work we explore some cosmological properties coming from the particle and future horizons when are considered as candidates to model the dark energy sector within a holographic context in a flat Friedmann-Lemaitre-Robertson-Walker universe; we focus on some thermodynamics characteristics of the resulting dark energy scenario. Within the interacting scheme for cosmological fluids we obtain that in the dark sector, the dark energy fluid will always have negative entropy production and additionally, the positivity for the entropy and temperature can not be guaranteed simultaneously; this result holds for both horizons. However, this last issue can be solved if chemical potential is introduced in the thermodynamics description. For the non interacting approach, we obtain similar results as those of the single fluid description for the entropy behavior. We also find that the model admits a genuine big rip singularity when the dark energy density is sketched by the future horizon, in consequence the resulting parameter state can cross to the phantom regime. For the particle horizon case the cosmological fluid can emulate ordinary matter and the coincidence parameter has a decreasing behavior when the future horizon is elected.

gr-qc↗

arXiv2020

Challenging matter creation models in the phantom divide

We perform a study both statistical and theoretical for cosmological models of matter creation and their ability to describe effective phantom models of dark energy. Such models are beyond the $Λ$CDM model since the resulting cosmic expansion is not adiabatic. In fact, we show that this approach exhibits transient phantom/quintessence scenarios at present time and tends to the standard cosmological model at some stage of the cosmic evolution. We discuss some generalities of the thermodynamics properties for this type of cosmological model; we emphasize on the behavior of the temperature associated to dark matter, which keeps positive along cosmic evolution together with the entropy. The enrichment of this type of model by means of the incorporation of cosmological constant and dissipative effects in the fluid description to explore their cosmological consequences in the expansion of the universe is considered. Finally, a generalization for the matter production rate as an inhomogeneous expression of the Hubble parameter and its derivatives is discussed; as in all the cases examined, such election leads to an effective phantom/quintessence behavior.

gr-qc↗

arXiv2019

A thermodynamics revision of Rastall gravity

In this work we study some aspects of the Rastall gravity, being the thermodynamics consistency of the model the core of this paper, for this purpose we will consider the dynamical equations of Rastall model in a flat FLRW geometry. Under a holographic description can be seen that this scenario for gravity contributes to the energy density of the fluid with an extra term that can be related to the deceleration parameter, providing a way to estimate the value of the Rastall parameter, termed as $ξλ$, at present time. By adopting a specific Ansatz for the $ξλ$ term it is possible to determine that the behaviour of the Hubble parameter in Rastall gravity has a similar aspect to the $Λ$CDM model at late times, but at thermodynamics level differs from the standard cosmology since the adiabatic behaviour for the entropy depends on the value of the parameter state, $ω$. However, the entropy has a positive growth and simultaneously its convexity condition can be guaranteed; when other contributions are considered such as matter production and chemical potential, the adiabatic expansion can not be achieved, but the theory keeps its thermodynamics consistency. The chemical potential seems to have an interesting role since at effective level we could have a cosmological constant or phantom expansion in the model.

gr-qc↗

arXiv2019

Hairy black hole stability under odd parity perturbations in the Einstein-Gauss-Bonnet model

Supported by the use of a regular scalar field we find a black hole solution in the Einstein-Gauss-Bonnet model. From the obtained solution we can recover the Schwarzschild black hole as in other works. Later, by implementing the odd parity perturbations method we study the stability of the linearized equations of motion of the model, we find the explicit form of the Regge-Wheeler potential and we explore the condition of vanishing perturbations at the horizon of the black hole. We test the stability of the obtained solution by checking the positivity condition of the Regge-Wheeler potential. Finally, we show that the stability of model depends on the value of a parameter introduced in the profile for the scalar field.

gr-qc↗

arXiv2019

Qualitative description of the universe in the interacting fluids scheme

In this work we present a qualitative description of the evolution of a curved universe when we consider the interacting scheme for the constituents of the dark sector. The resulting dynamics can be modeled by a set of Lotka-Volterra type equations. For this model a future singularity is allowed, therefore the cyclic behavior for the energy interchange between the components of the universe is present only at some stage of the cosmic evolution. Due to the presence of the future singularity, the model exhibits global instability.

gr-qc↗

arXiv2018

Dark energy and Dark matter interaction in light of the second law of thermodynamics

In the context of thermodynamics we discuss the way inevitable emerge an interaction between dark components, and in this way, provide a mechanism to understand the limits of the LCDM model and the class of interaction models between dark components. Using observational data we have tested two models of explicit interaction between dark components and reconstructed the evolution of temperatures for both components. We found that observations suggest the interaction exist with energy flowing from dark energy to dark matter. The best fit also suggest a phantom equation of state parameter for dark energy. We discuss the results having in mind the constraints imposed by thermodynamics.

astro-ph.CO↗

arXiv2018

A holographic cut-off inspired in the apparent horizon

In this work we consider a general function for the $c^{2}$ term that appears in the conventional expression for the holographic dark energy in a FLRW curved spacetime. The cut-off prospective is inspired in the apparent horizon length. By exploring the slowly varying condition for the $c^{2}$ term, we obtain a range of validity for this holographic proposal. Under these considerations can be found that this holographic cut-off is adequate to describe the late cosmic evolution. Additionally, by considering some values constrained with the use of observational data for some cosmological parameters in the context of dark energy models, such as the deceleration parameter or the parameter state, can be shown that this holographic model remains {\it close} to the $Λ$CDM model.

gr-qc↗

arXiv2018

Analysing dissipative effects in the $Λ$CDM model

In the present paper, the effects of viscous dark matter are analysed within the $Λ$CDM model. Here we consider bulk viscosity through the Israel-Stewart theory approach, leading to an effective pressure term in the continuity equation that accomplishes for the dissipative effects of the dark matter fluid. Then, the corresponding equation for viscosity is solved and a general equation for the Hubble parameter is obtained with the presence of a cosmological constant. The existence of de Sitter solutions is discussed, where a wider range of solutions is found in comparison to the $Λ$CDM model. Also the conditions for the near thermodynamical equilibrium of the fluid is analysed. Finally, a qualitative analysis provides some constraints on the model by using Supernovae Ia data which reveals the possible importance of causal thermodynamics in cosmology.

gr-qc↗

arXiv2018

Thermodynamically allowed phantom cosmology with viscous fluid

In this work we present an analysis of the phantom zone in a causal viscous cosmology from a thermodynamic point of view. In this description we consider a chemical potential and the approach of irreversible processes. We assume a flat universe filled with a single dissipative fluid described by a barotropic equation of state, $p = ωρ$. This model allows to construct a negative chemical potential for the phantom regime but also this construction allows us to have positive definite temperature and entropy.

gr-qc↗

arXiv2018

Holographic approach for dark energy - dark matter interaction in curved FLRW spacetime

In this work we explore under a holographic approach the dark energy - dark matter interaction in a non-flat Friedmann-Lemaitre-Robertson-Walker spacetime, with a cut-off for the dark energy component given in terms of the Hubble scale. Based on results coming from the use of observational data, we consider a positive interaction $Q$-term together with a Chevallier-Polarsky-Linder type parametrization for the coincidence parameter, we realize that the model admits a Type III future singularity, in this singular universe we have that for a null value of the curvature parameter its scale factor is a constant value. With the use of some cosmological parameters constrained with observational data, we obtain that the crossing of the phantom divide is possible. Once the singular nature is obtained for the model, its Statefinder diagnosis reveals a behavior that is {\it far} from the $Λ$-CDM model.

gr-qc↗

arXiv2018

Gauss-Bonnet dark energy on Hořava-Lifshitz cosmology

We investigate the Gauss-Bonnet dark energy model and its deformed version on Hořava-Lifshitz cosmology, which belongs to the class of cosmologies obtained from the so-called projectable version of Hořava-Lifshitz gravity. In particular, we investigate the bulk/boundary interaction in this scenario through the $Q$ function, which we interpret as a measure of the energy transference between the bulk and the spacetime boundary. Then we discuss whether the thermal equilibrium will be stable or not, once it is reached, and the validity of the generalized second law. We show that the $Q$ function can exhibit sign changes along the cosmic evolution and the Universe reaches the thermal equilibrium as a transient phenomenon.

gr-qc↗

arXiv2017

Accelerated and decelerated expansion in a causal dissipative cosmology

In this work we explore a new cosmological solution for an universe filled with one dissipative fluid, described by a barotropic EoS $p = ωρ$, in the framework of the full Israel-Stewart theory. The form of the bulk viscosity has been assumed of the form $ξ= ξ_{0}ρ^{1/2}$. The relaxation time is taken to be a function of the EoS, the bulk viscosity and the speed of bulk viscous perturbations, $c_{b}$. The solution presents an initial singularity, where the curvature scalar diverges as the scale factor goes to zero. Depending on the values for $ω$, $ξ_{0}$, $c_{b}$ accelerated and decelerated cosmic expansion can be obtained. In the case of accelerated expansion, the viscosity drives the effective EoS to be of quintessence type, for the single fluid with positive pressure. Nevertheless, we show that only the solution with decelerated expansion satisfies the thermodynamics conditions $dS/dt > 0$ (growth of the entropy) and $d^{2}S/dt^{2} < 0$ (convexity condition). We show that an exact stiff matter EoS is not allowed in the framework of the full causal thermodynamic approach; and in the case of a EoS very close to the stiff matter regime, we found that dissipative effects becomes negligible so the entropy remains constant. Finally, we show numerically that the solution is stable under small perturbations.

gr-qc↗

arXiv2017

Dynamics of viscous cosmologies in the full Israel-Stewart formalism

It is performed a detailed dynamical analysis for a bulk viscosity model in the full Israel-Stewart formalism for a spatially flat Friedmann-Robertson-Walker Universe. In our study we have considered the total cosmic fluid constituted by radiation, dark matter and dark energy. The dark matter fluid is treated as an imperfect fluid which has a bulk viscosity that depends on its energy density in the usual form $ξ(ρ_{m})=ξ_{0} ρ_{m}^{1/2}$, whereas the other components are assumed to behave as perfect fluids with constant EoS parameter. We show that the thermal history of the Universe is reproduced provided that the viscous coefficient satisfies the condition $ξ_{0}\ll 1$, either for a zero or a suitable nonzero coupling between dark energy and viscous dark matter. In this case, the final attractor is a dark-energy-dominated, accelerating Universe, with effective EoS parameter in the quintessence-like, cosmological constant-like or phantom-like regime, in agreement with observations. As our main result, we show that in order to obtain a viable cosmological evolution and at the same time alleviating the cosmological coincidence problem via the mechanism of scaling solution, an explicit interaction between dark energy and viscous dark matter seems inevitable. This result is consistent with the well-known fact that models where dark matter and dark energy interact with each other have been proposed to solve the coincidence problem. Furthermore, by insisting in above, we show that in the present context a phantom nature of this interacting dark energy fluid is also favored.

gr-qc↗

arXiv2017

Phantom solution in a non-linear Israel-Stewart theory

In this paper we present a phantom solution with a big rip singularity in a non-linear regime of the Israel-Stewart formalism. In this framework it is possible to extend this causal formalism in order to describe accelerated expansion, where assumption of near equilibrium is no longer valid. We assume a flat universe filled with a single viscous fluid ruled by a barotropic EoS, $p=ωρ$, which can represent a late time accelerated phase of the cosmic evolution. The solution allows to cross the phantom divide without evoking an exotic matter fluid and the effective EoS parameter is always lesser than $-1$ and constant in time.

gr-qc↗

arXiv2016

Crossing the phantom divide with dissipative normal matter in the Israel-Stewart-Hiscock formalism

A phantom solution in the framework of the causal Israel-Stewart (IS) formalism is discussed. We assume a late time behavior of the cosmic evolution by considering only one dominant matter fluid with viscosity. In the model it is assumed a bulk viscosity of the form $ξ= ξ_{0}ρ^{1/2}$, where $ρ$ is the energy density of the fluid. We evaluate and discuss the behavior of the thermodynamical parameters associated to this solution, like the temperature, rate of entropy, entropy, relaxation time, effective pressure and effective EoS. A discussion about the assumption of near equilibrium of the formalism and the accelerated expansion of the solution is presented. The solution allows to cross the phantom divide without evoking an exotic matter fluid and the effective EoS parameter is always lesser than $-1$ and time independent. A future singularity (big rip) occurs, but different from the Type I (big rip) solution classified in S. Nojiri, S. D. Odintsov and S. Tsujikawa, Phys. Rev. D 71, 063004 (2005), if we consider others thermodynamics parameters like, for example, the effective pressure in the presence of viscosity or the relaxation time.

gr-qc↗

arXiv2015

Bulk-Boundary interaction and the second law in Hořava-Lifshitz cosmology

By defining $Q$ as a function which realizes the energy transference between the bulk and the boundary of spacetime, as we interprete it here in the framework of Hořava-Lifshitz (HL) cosmology, flat case, we discuss the validity of the second law of thermodynamics in the light of the sign changes of $Q$ (changes in the direction of energy transference) and its consequences through the cosmic evolution, in particular, whether the thermal equilibrium between bulk and boundary is reached or not. Additionally, we discuss possible phase transitions experienced by the bulk and the boundary (seen as sign changes in their heat capacities) through the cosmic evolution. The energy density in the bulk is modeled under an holographic perspective. As far as we know, currently there is not observational data on bulk-boundary interaction.

gr-qc↗

arXiv2015

Interacting cosmic fluids and phase transitions under a holographic modeling for dark energy

We discuss the consequences of possible sign changes of the $Q$-function which measures the energy transference between dark energy and dark matter. We investigate this scenario from a holographic perspective to modeling the dark energy by a linear-parametrization of the equation of state parameter denoted by $ω$. By imposing the strong constraint of the second law of thermodynamics, we show that sign changes of $Q$ due to the cosmic evolution imply changes in the temperatures of dark energy and dark matter, respectively. We also discuss the phase transitions, in the past and future, experienced by dark energy and dark matter (or, equivalently, the sign changes of their heat capacities).

gr-qc↗

arXiv2015

Holographic energy density on Hořava-Lifshitz cosmology

In Hořava-Lifshitz cosmology we use the holographic Ricci-like cut-off for the energy density proposed by L. N. Granda and A. Oliveros and under this framework we study, through the cosmic evolution at late times, the sign change in the amount of non-conservation energy ($Q$) present in this cosmology. We revise the early stage (curvature-dependent) of this cosmology, where a term reminiscent of stiff matter is the dominant, and in this stage we find a power-law solution for the cosmic scale factor although $ω=-1$. Late and early phantom schemes are obtained without requiring $ω<-1$. Nevertheless, these schemes are not feasible according to what is shown in this paper. We also show that $ ω=-1$ alone does not imply a de Sitter phase in the present cosmology. Thermal aspects are revised by considering the energy interchange between the bulk and the spacetime boundary and we conclude that there is no thermal equilibrium between them. Finally, a ghost scalar graviton (extra degree of freedom in HL gravity) is required by the observational data.

hep-th↗