arXiv subjects

Samuel Lepe

Publications and source records attributed to Samuel Lepe.

At least 37 records · Page 2Linked to original sources

arXiv2024

Generalized second law of thermodynamics for the matter creation scenario and emergence of phantom regime

This work is focused on the exploration of the thermodynamics foundations of the matter creation scenario when a generalized form of the second law of thermodynamics for this scheme is implemented. In this scenario we consider an expanding cosmology in which the created matter is trapped by the apparent horizon. The scheme leads to phantom evolution but at first glance it lacks of physical consistency. However, the inclusion of chemical potential into the description solves the thermodynamics issues of the model and determines the behavior of the cosmic fluid, in other words, the cosmic fluid now can behave as phantom dark energy or as quintessence one.

gr-qc↗

arXiv2023

Thermodynamics of a transient phantom scenario

This work is devoted to the thermodynamics description of a phantom scenario proposed previously by the authors. The presence of negative chemical potential is unavoidable if we allege for a well defined thermodynamics framework since the cosmological model passages from phantom stage at present time to a future de Sitter evolution. As noted earlier in other works, we find that the negativity of the chemical potential is necessary to save phantom dark energy from thermodynamics inconsistencies.

gr-qc↗

arXiv2023

Holographic dark energy in curved spacetime for interacting fluids

In this work we explore the scope of two holographic dark energy models within the interacting scenario for the dark sector and spatial curvature. For one holographic model we consider the usual formula for the dark energy density with the Hubble scale and the second model is given in terms of a function instead of a constant parameter as in the usual holographic formula. In this description both holographic models admit a future singularity. The use of recent cosmological data is considered only to support the physical results of our proposal. The interaction term for each holographic model, $Q$, keeps positive along the cosmic evolution and are not given a priori, they are reconstructed from the dynamics of the model. The temperatures for the components of the dark sector are computed and exhibit a growing behavior in both scenarios. The cosmic evolution in this context it is not adiabatic and the second law it is fulfilled only under certain well-established conditions for the temperatures of the cosmic components and positive $Q$-terms.

gr-qc↗

arXiv2022

Phantom cosmologies from QCD ghost dark energy

We review a dynamical dark energy model scarcely studied in the literature and we introduce two possible generalizations. We discuss separately the behavior of the original model and a minimal extension of it by exploring some early and late times limits, we find that the cosmic components are related by their parameters state. In order to have access to the phantom regime we present two dark energy densities inspired from the holographic approach and from the emergent entropic forces model in the early universe. For the first case we obtain a Type III singularity and in the second proposal we have a transition from decelerated to accelerated cosmic expansion that starts as phantom type. However, the final stage of the universe is a de Sitter state.

gr-qc↗

arXiv2022

Holography and matter creation revisited

The consideration of the holographic dark energy approach and matter creation effects in a single cosmological model is carried out in this work accordingly, we discuss some cases of interest. We test this cosmological proposal against recent observations. Considering the best fit values obtained for the free cosmological parameters, the model exhibits a transition from a decelerated cosmic expansion to an accelerated one. The deceleration-acceleration transition can not be determined consistently from the coincidence parameter for this kind of model. At effective level the parameter state associated to the whole energy density contribution describes a quintessence scenario in the fluid analogy at present time.

gr-qc↗

arXiv2021

Unified dissipative dark matter model in a non linear Israel-Stewart theory

In this paper we study the Statefinder and $Om(x)$ diagnoses of an exact solution of a unified dissipative dark matter model in the framework the Israel-Stewart theory and its non linear extension, considering the condition of positive entropy production. The implementation of two diagnoses in the pair of solutions reveal that both solutions can emulate the $Λ$CDM model at some stage of the cosmic evolution and behave as quintessence dark energy. The stability of both solutions is discussed from the point of view of the Liapunov's method, showing that they are asymptotic stable. Additionally we show that the solutions are also stable under small perturbations. A simple analysis of the solutions with the use of observational data suggests that the non linear regime could lead to a inconsistencies-free scenario, i.e., in agreement with the bounds obtained from the perturbative analysis for the velocity of bulk viscous perturbations.

gr-qc↗

arXiv2021

Averaging generalized scalar field cosmologies III: Kantowski--Sachs and closed Friedmann--Lemaître--Robertson--Walker models

Scalar field cosmologies with a generalized harmonic potential and matter with energy density $ρ_m$, pressure $p_m$, and barotropic equation of state (EoS) $p_m=(γ-1)ρ_m, \; γ\in[0,2]$ in Kantowski-Sachs (KS) and closed Friedmann--Lemaître--Robertson--Walker (FLRW) metrics are investigated. We use methods from non--linear dynamical systems theory and averaging theory considering a time--dependent perturbation function $D$. We define a regular dynamical system over a compact phase space, obtaining global results. That is, for KS metric the global late--time attractors of full and time--averaged systems are two anisotropic contracting solutions, which are non--flat locally rotationally symmetric (LRS) Kasner and Taub (flat LRS Kasner) for $0\leq γ\leq 2$, and flat FLRW matter--dominated universe if $0\leq γ\leq \frac{2}{3}$. For closed FLRW metric late--time attractors of full and averaged systems are a flat matter--dominated FLRW universe for $0\leq γ\leq \frac{2}{3}$ as in KS and Einstein-de Sitter solution for $0\leqγ<1$. Therefore, time--averaged system determines future asymptotics of full system. Also, oscillations entering the system through Klein-Gordon (KG) equation can be controlled and smoothed out when $D$ goes monotonically to zero, and incidentally for the whole $D$-range for KS and for closed FLRW (if $0\leq γ< 1$) too. However, for $γ\geq 1$ closed FLRW solutions of the full system depart from the solutions of the averaged system as $D$ is large. Our results are supported by numerical simulations.

gr-qc↗

arXiv2021

Adiabatic analysis of the rotating BTZ black hole

In this paper we analyze some interesting features of the thermodynamics of the rotating BTZ black hole from the Carathéodory axiomatic postulate, for which, we exploit the appropriate Pfaffian form. The allowed adiabatic transformations are then obtained by solving the corresponding Cauchy problem, and are studied accordingly. Furthermore, we discuss the implications of our approach, regarding the the second and third laws of black hole thermodynamics. In particular, the merging of two extremal black holes is studied in detail.

gr-qc↗

arXiv2021

Matter production effects and interacting scenario within a reconstructed mimetic cosmology for late times

In this work we explore two possible scenarios that can be considered to extend a recent proposed model by the authors known as reconstructed mimetic cosmology. This study is complemented with an statistical analysis for each case. The first scenario considers the inclusion of matter production as a possible source of cosmic expansion in the reconstructed mimetic model, at effective level was found that this construction can cross the phantom divide, the model evolves from quintessence to phantom dark energy. The second scenario corresponds to a construction of an interacting scheme for the dark sector which is described by the unified mimetic model. The resulting interaction term (not imposed by an Ansatz), $Q$, exhibits changes of sign leading to the violation of the second law along the cosmic evolution and non adiabaticity; the temperatures for the components of the dark sector are computed and such components are shown to be out of thermal equilibrium.

gr-qc↗

arXiv2021

Averaging Generalized Scalar Field Cosmologies II: Locally Rotationally Symmetric Bianchi I and flat Friedmann-Lemaître-Robertson-Walker models

Scalar field cosmologies with a generalized harmonic potential and a matter fluid with a barotropic Equation of State (EoS) with barotropic index $γ$ for the Locally Rotationally Symmetric (LRS) Bianchi I and flat Friedmann-Lemaître-Robertson-Walker (FLRW) metrics are investigated. Methods from the theory of averaging of nonlinear dynamical systems are used to prove that time-dependent systems and their corresponding time-averaged versions have the same late-time dynamics. Therefore, the simplest time-averaged system determines the future asymptotic behavior. Depending on the values of $γ$, the late-time attractors of physical interests are flat quintessence dominated FLRW universe and Einstein-de Sitter solution. With this approach, the oscillations entering the system through the Klein-Gordon (KG) equation can be controlled and smoothed out as the Hubble parameter $H$ - acting as time-dependent perturbation parameter - tends monotonically to zero. Numerical simulations are presented as evidence of such behavior.

gr-qc↗

arXiv2021

Five-dimensional Einstein-Chern-Simons cosmology

We consider a five-dimensional Einstein-Chern-Simons action which is composed of a gravitational sector and a sector of matter, where the gravitational sector is given by a Chern-Simons gravity action instead of the Einstein-Hilbert action, and where the matter sector is given by a perfect fluid. The gravitational lagrangian is obtained gauging some Lie-algebras, which in turn, were obtained by S-expansion procedure of Anti-de Sitter and de Sitter algebras. On the cosmological plane, we discuss the field equations resulting from the Anti-de Sitter and de Sitter frameworks and we show analogies with four-dimensional cosmological schemes.

gr-qc↗

arXiv2021

Averaging Generalized Scalar Field Cosmologies I: Locally Rotationally Symmetric Bianchi III and open Friedmann-Lemaître-Robertson-Walker models

Scalar field cosmologies with a generalized harmonic potential and a matter fluid with a barotropic Equation of State (EoS) with barotropic index $γ$ for Locally Rotationally Symmetric (LRS) Bianchi III metric and open Friedmann-Lemaître-Robertson-Walker (FLRW) metric are investigated. Methods from the theory of averaging of nonlinear dynamical systems are used to prove that time-dependent systems and their corresponding time-averaged versions have the same late-time dynamics. Therefore, simple time-averaged systems determine the future asymptotic behavior. Depending on values of barotropic index $γ$ late-time attractors of physical interests for LRS Bianchi III metric are Bianchi III flat spacetime, matter dominated FLRW universe (mimicking de Sitter, quintessence or zero acceleration solutions) and matter-curvature scaling solution. For open FLRW metric late-time attractors are a matter dominated FLRW universe and Milne solution. With this approach, oscillations entering nonlinear system through Klein-Gordon (KG) equation can be controlled and smoothed out as the Hubble factor $H$ - acting as a time-dependent perturbation parameter - tends monotonically to zero. Numerical simulations are presented as evidence of such behaviour.

gr-qc↗

arXiv2021

Reconstructing Mimetic Cosmology

We explore the mimetic gravity formulation with the inclusion of a scalar field potential namely, $V(ϕ)$. However, we are not considering any {\it a priori} specific form this term. By means of the Chevallier-Polarski-Linder parametrization for the parameter state of the fluid we can construct an explicit function for such potential in terms of the cosmological redshift and obtain analytical solutions in the mimetic gravity approach. We revise some cosmological implications of these results and additionally we perform a numerical reconstruction for the potential $V$ as a function of the mimetic scalar field, $ϕ$.

gr-qc↗

arXiv2020

Cosmic expansion with matter creation and bulk viscosity

We explore the cosmological implications at effective level of matter creation effects in a dissipative fluid for a FLRW geometry; we also perform a statistical analysis for this kind of model. By considering an inhomogeneous Ansatz for the particle production rate we obtain that for created matter of dark matter type we can have a quintessence scenario or a future singularity known as little rip; in dependence of the value of a constant parameter, $η$, which characterizes the matter production effects. The dimensionless age of this kind of Universe is computed, showing that this number is greater than the standard cosmology value, this is typical of universes with presence of dark energy. The inclusion of baryonic matter is studied. By implementing the construction of the particle production rate for a dissipative fluid by considering two approaches for the expression of the bulk viscous pressure; we find that in Eckart model we have a big rip singularity leading to a catastrophic matter production and in the truncated version of the Israel-Stewart model such rate remains bounded leading to a quintessence scenario. For a non adiabatic dissipative fluid, we obtain a positive temperature and the cosmic expansion obeys the second law of thermodynamics.

gr-qc↗

arXiv2020

Interacting dark matter and cosmic acceleration

We study the effect of an explicit interaction between two scalar fields components describing dark matter in the context of a recent proposal framework for interaction. We find that, even assuming a very small coupling, it is sufficient to explain the observational effects of a cosmological constant, and also overcome the problems of the $Λ$CDM model without assuming an exotic dark energy.

astro-ph.CO↗

arXiv2020

The Spin Tensor of Dark Matter and the Hubble Parameter Tension

Allowing for a nonvanishing spin tensor for cold dark matter ($ω_{\mathrm{DM}}=0$) has the consequence of giving rise to an effective $\mathrm{FLRW}$ dynamics with a small negative barotropic constant for an effective dark matter density ($-1/3<ω_{\mathrm{eff}}\leq0$). This turns out to solve the Hubble parameter tension in a straightforward way.

gr-qc↗

arXiv2020

Non-zero torsion and late cosmology

In this work, we study some thermodynamical aspects associated with torsion in a flat FLRW spacetime cosmic evolution. By implementing two Ansatze for the torsion term, we find that the model admits a phantom regime or a quintessence behavior. This scheme differs from the $Λ$CDM model at the thermodynamical level. The resulting cosmic expansion is not adiabatic, the fulfillment of the second law of thermodynamics requires a positive torsion term, and the temperature of the cosmic fluid is always positive. The entropy of the torsion phantom scenario is negative, but introducing chemical potential solves this issue. For a Dirac-Milne type Universe, the torsion leads to a growing behavior for the temperature of the fluid but has no incidence on the rate of expansion.

gr-qc↗

arXiv2020

Cosmological Dark Matter Amplification through Dark Torsion

A cosmological approach based on considering a cosmic background with non-zero torsion is shown in order to give an option of explaining a possible phantom evolution, not ruled out according to the current observational data. We revise some aspects of the formal schemes on torsion and, according them, we develop a formalism which can be an interesting alternative for exploring Cosmology.

gr-qc↗