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In this work we study the thermodynamics of a cosmological unimodular gravity model at late times by considering two different energy diffusion functions that emerges in these scenarios, and that encodes the current for the non-conservation of the energy-momentum tensor, usually termed as $Q(t)$. Specifically, we discuss the barotropic and the continuous spontaneous localization models as energy diffusion functions. The consistency conditions demanded for the entropy of the system in terms of the cosmological parameters of the model: positive production ($dS/dt>0$) and convexity condition ($d^{2}S/dt^{2} <0$), are investigated. We show that these conditions strongly constraint the viavility of these models. Additionally, we comment about our results and compare with those obtained in recent works where the restriction of the parameters for these two diffusion models was implemented with the use of cosmological data., Comment: 8 pages, no figures

Viscous unified dark matter models aim to describe the dark sector of the Universe, as the dark matter fluid itself gives rise to an accelerated expansion due to a negative bulk viscous pressure. However, in most studies, radiation is often disregarded as a minor factor in dynamical system analyses, overlooking whether radiation domination is achievable. In this paper, we rigorously examine this critical aspect for common parameterizations of bulk viscosity, denoted as $\xi$, within two general classes of viscous unified models. Our findings reveal significant inconsistencies in models where $\xi \propto H^{1-2s} \rho_{m}^{s}$ with $s\leq 0$, and surprisingly, in models where $\xi \propto \rho_{m}^{s}$ with exponents $s<0$, as they both fail to produce a radiation-dominated era. Moreover, the exponent $s$ must lye within the interval $0 \leq s < 1/2$ for the latter model to correctly describes the cosmological evolution. These results underscore the need of including these constraints as a prior in statistical analyses of observational data, with implications for current statistical inferences for the second model, where both prior and best-fit values of $s$ often fall outside the acceptable range., Comment: 20 page

In this paper, we investigate the behavior of a massive scalar field dark matter scenarios in the large mass limit around a central Reissner-Nordstr\"{o}m black hole. This study is motivated by observations from the Event Horizon Telescope collaboration, which does not exclude the possibility of the existence of such black holes. Through these inquiries, we uncover that the electric charge may significantly impact the scalar field profile and the density profile in the vecinty of the black hole. For the maximum electric charge allowed by the constraints of the Event Horizon Telescope, the maximum accretion rate decreases by $\thicksim$ 50 \% compared to the Schwarszchild case for marginally bound orbits. The maximum accretion rate of the massive scalar field is approximately $\dot M_{\text{SFDM}} \thicksim 10^{-8} M_{\odot} \;\text{yr}^{-1}$, which is significantly lower than the typical baryonic accretion rate commonly found in the literature. This implies that the scalar cloud located at the center of galaxies may have survived untill present times., Comment: 13 pages, 5 figures, updated references, corrections suggested by the referees, and version accepted for publication in PRD

Considering the condition on conservation of energy momentum tensor (EMT), we study late time cosmological solutions in the context of $f(R,T)=R+\alpha T^{n}$ gravity (where $\alpha$ and $n$ are constants) in a flat FLRW spacetime. The present model discusses the case of a barotropic perfect fluid as the matter content of the Universe, along with the case when dissipative effects are taken into account. Briefly, assuming a single perfect fluid we find that the mentioned model is not capable of presenting an observationally consistent picture of the late time accelerated expansion of the Universe; nevertheless, the model leads to admissible solutions when the bulk viscosity is included. In this regard, a consistent setting is found considering the Eckart, Truncated and Full Israel-Stewart theories which determine the behavior of bulk viscosity. In the presence of bulk viscosity, the behavior of the deceleration parameter (DP) shows that the underlying model can describe an acceptable evolution even if the isotropic pressure of matter content of the Universe is negligible., Comment: 18 pages and 7 figures

In this paper, we explore the behavior of light ray trajectories in the exterior geometry of a rotating black hole within the Rastall theory of gravity, which is surrounded by an inhomogeneous anisotropic electronic cold plasma. By specifying the plasma's frequency profile, we derive fully analytical solutions for the temporal evolution of spacetime coordinates using elliptic integrals and Jacobi elliptic functions. These solutions illustrate various possible orbits. Throughout the study, we compare the results with those in the vacuum case, emphasizing the influence of plasma. Additionally, we utilize the analytical solutions to establish the lens equation for the considered spacetime. The investigation also addresses the significance of spherical photon orbits on critical trajectories, by presenting several examples., Comment: 33 pages, 37 figures

We study the black hole spacetime structure of a model consisting of the standard Maxwell theory and a $p$-power-Yang-Mills term. This non-linear contribution introduces a non-Abelian charge into the global solution, resulting in a modified structure of the standard Reissner-Nordstr\"{o}m black hole. Specifically, we focus on the model with $p=1/2$, which gives rise to a new type of modified Reissner-Nordstr\"{o}m black hole. For this class of black holes, we compute the event horizon, the innermost stable circular orbit, and the conditions to preserve the weak cosmic censorship conjecture. The latter condition sets a well-established relation between the electric and the Yang-Mills charges. As a first astrophysical implication, the accretion properties of spherical steady flows are investigated in detail. Extensive numerical examples of how the Yang-Mills charge affects the accretion process of an isothermal fluid in comparison to the standard Reissner-Nordstr\"{o}m and Schwarzschild black holes are displayed. Finally, analytical solutions in the fully relativistic regime, along with numerical computations, of the mass accretion rate for a polytropic fluid in terms of the electric and Yang-Mills charges are obtained. As a main result, the mass accretion rate efficiency is considerably improved, with respect to the standard Reissner-Nordstr\"{o}m and Schwarzschild solutions, for negative values of the Yang-Mills charge., Comment: 19 pages, 8 figures. References updated, some comments included and figure 1 added to match published version

In this paper, we present a rotating de Rham-Gabadadze-Tolley black hole with a positive cosmological constant in massive gravity, achieved by applying a modified Newman-Janis algorithm. The black hole exhibits stable orbits of constant radii, prompting a numerical study on the behavior of the solutions to a nonic equation governing the radii of planar orbits around the black hole. Additionally, we investigate the stability of orbits near the event horizon and provide a comprehensive analytical examination of the solutions to the angular equations of motion. This is followed by simulating some spherical particle orbits around the black hole., Comment: 20 pages, 19 figures, 2 tables

Self-interacting scalar field dark matter can be seen as an extension of the free case known as Fuzzy dark matter. The interactive case is capable of reproducing the positive features of the free case at both astrophysical and cosmological scales. On the other hand, current imaging black holes (BHs) observations provided by the Event Horizon Telescope (EHT) collaboration cannot rule out the possibility that BHs can carry some amount of charge. Motivated by these aspects, and by the possibility of detecting dark matter through its gravitational imprints on BH observations, in this paper, we extend previous studies of accretion of self-interacting scalar field dark matter to the charged BH case. Our analysis is based on the assumption on spherically symmetric flow and employs a test fluid approximation. All analytical expressions are derived from the ground up in Schwarzschild coordinates. Concretely, we implement analytical and numerical approaches to investigate the impact of the charge on the energy flux. From this analysis, we notice that the mass accretion rate efficiency is reduced up to $\sim 20\%$ for the maximum allowed charge. Additionally, considering the mass accretion rate of M87$^{\star}$ inferred from Polarization data of the EHT, we infer the conservative bound $ \lambda_4 > (1.49-10.2)( m / 1 \rm {eV} )^4$ based on the simple criterion that ensures the mass accretion rate caused by DM remains subdominant compared to the baryonic component., Comment: 14 pages, 7 figures, typos corrected, references updated, adjusted colors in figure 3. Matches PRD version

Running vacuum models and viscous dark matter scenarios beyond perfect fluid idealization are two appealing theoretical strategies that have been separately studied as alternatives to solve some problems rooted in the $\Lambda$CDM cosmological model. In this paper, we combine these two notions in a single cosmological setting and investigate their cosmological implications, paying particular attention in the interplay between these two constituents in different cosmological periods. Specifically, we consider a well-studied running vacuum model inspired by renormalization group, and a recently proposed general parameterization for the bulk viscosity $\xi$. By employing dynamical system analysis, we explore the physical aspects of the new phase space that emerges from the combined models and derive stability conditions that ensure complete cosmological dynamics. We identify four distinct classes of models and find that the critical points of the phase space are non-trivially renewed compared to the single scenarios. We then proceed, in a joint and complementary way to the dynamical system analysis, with a detailed numerical exploration to quantify the impact of both the running parameter and the bulk viscosity coefficient on the cosmological evolution. Thus, for some values of the model parameters, numerical solutions show qualitative differences from the $\Lambda$CDM model, which is phenomenologically appealing in light of cosmological observations., Comment: 26 pages and 13 figures

In this study, we focus on a static spherically symmetric $f(R)$ black hole spacetime characterized by a linear dark matter-related parameter. Our investigation delves into understanding the influence of different assumed values of this parameter on the observable characteristics of the black hole. To fulfill this task, we investigate the light deflection angles, which are inferred from direct analytical calculations of null geodesics.} To examine the black hole's properties further, we assume an optically thin accretion disk and explore various emission profiles. Additionally, we investigate the shadow cast by the illuminated black hole when affected by the disk. Furthermore, we simulate the brightness of an infalling spherical accretion in the context of silhouette imaging for the black hole. Our findings indicate that, except for some specific cases, the observed brightness of the accretion disk predominantly arises from direct emission, rather than lensing and photon rings. Moreover, we reveal that the linear dark parameter of the black hole significantly influences the shadow size and brightness. Our discussion covers both analytical and numerical approaches, and we utilize ray-tracing methods to produce accurate visualizations., Comment: 28 pages, 87 figures

In the present work, we investigate the consequences of running gravitational coupling on the properties of the three-dimensional BTZ black hole. We take as starting point the functional form of gravitational coupling obtained in the context of asymptotic safe gravity theory. By using the standard scale setting relation where $k\sim \xi/r^n$, we compute the solution of the Einstein field equations. We get and analyze the horizon and the thermodynamic properties of this new class of black hole solutions. The impact of the scale--dependent parameter $\xi$ on the cosmological "constant" and metric functions are briefly discussed. We find that the null energy condition is also violated in this setup when scale-dependent gravity and Newton's coupling (coming from the asymptotic safety scenario of gravity) are simultaneously taken into account., Comment: 12 pages, 6 figures. Accepted for publication in EPJ C

Bulk viscosity in cold dark matter is an appealing feature that introduces distinctive phenomenological effects in the cosmological setting as compared to the $\Lambda$CDM model. Under this view, we propose a general parametrization of the bulk viscosity of the form $\xi\sim H^{1-2s} \rho_{m}^{s}$, that covers intriguingly some well-known cases in the Eckart's theory. Some advantages of this novel parametrization are: first, it allows to write the resulting equations of cosmological evolution in the form of an autonomous system for any value of $s$, so a general treatment of the fixed points and stability can be done, and second, the bulk viscosity effect is consistently handled so that it naturally turns off when matter density vanishes. As a main result we find, based on detailed dynamical system analysis, one-parameter family of de-Sitter-like asymptotic solutions with non-vanishing bulk viscosity coefficient during different cosmological periods. Numerical computations are performed jointly along with analytical phase space analysis in order to assess more quantitatively the bulk viscosity effect on the cosmological background evolution. Finally, as a first contact with observation we derive constraints on the free parameters of some bulk viscosity models with specific $s$-exponents from Supernovae Ia and observations of the Hubble parameter, by performing a Bayesian statistical analysis thought the Markov Chain Monte Carlo method., Comment: 26 pages, 6 figures. Typos fixed, phase space diagrams included (figure 1). Accepted for publication in the European Physical Journal Plus

In this paper, we study the thermodynamical and mathematical consistencies for a non-singular early-time viscous cosmological model known as soft-Big Bang, which was previously found in [N. Cruz, E. Gonz\'alez, and J. Jovel, Phys. Rev. D \textbf{105}, 024047 (2022)]. This model represents a flat homogeneous and isotropic universe filled with a dissipative radiation fluid and a cosmological constant $\Lambda$, which is small but not negligible, in the framework of Eckart's theory. In particular, we discuss the capability of the solution in the fulfillment of the three following conditions: (i) the near equilibrium condition, which is assumed in Eckart's theory of non-perfect fluids, (ii) the mathematical stability of the solution under small perturbations, and (iii) the positiveness of the entropy production. We have found that this viscous model can describe the radiation domination era of the $\Lambda$CDM model and, at the same time, fulfill the three conditions mentioned by the fulfillment of a single constraint on the bulk viscous coefficient $\xi_{0}$, finding also that this non-singular model has a positive energy density in the infinity past which is infinity hotter with a constant entropy., Comment: 12 pages, 5 figures. arXiv admin note: text overlap with arXiv:2202.02362, some typos corrected

Extensions to a $\Lambda$DM model have been explored to face current tensions that occur within its framework, which encompasses broadening the nature of the dark matter (DM) component to include warmness and a non-perfect fluid description. In this paper, we investigated the late-time cosmological evolution of an exact solution recently found in [N. Cruz, E. Gonz\'alez, and J. Jovel, Phys. Rev. D \textbf{105}, 024047 (2022)], which describe a viscous warm $\Lambda$DM model ($\Lambda$WDM) with a DM component that obeys a polytropic equation of state (EoS), which experience dissipative effects with a bulk viscosity proportional to its energy density, with proportionality constant $\xi_{0}$. This solution has the particularity of having a very similar behavior to the $\Lambda$CDM model for small values of $\xi_{0}$, evolving also to a de Sitter type expansion in the very far future. We explore firstly the thermodynamic consistences of this solution in the framework of the Eckart's theory of non-perfect fluids, focusing on the fulfillment of the two following conditions: (i) the near equilibrium condition and (ii) the positiveness of the entropy production. We explore the range of parameters of the model that allow to fulfilling these two conditions at the same time, finding that a viscous WDM component is compatible with both ones, being in this sense, a viable model from the thermodynamic point of view. Besides, we constraint the free parameters of the model with the observational data coming form supernovae Ia (SNe Ia) and the observational Hubble parameter data (OHD).(Abridged), Comment: 16 pages, 6 figures

In this paper we explore the different types of singularities that arise in the $\Lambda$CDM model when dissipative processes are considered, in the framework of the Eckart's theory. In particular, we study the late-time behavior of $\Lambda$CDM model with viscous cold dark matter (CDM) and an early-time viscous radiation domination era with cosmological constant (CC). The fluids are described by the barotropic equation of state (EoS) $p=(\gamma-1)\rho$, where $p$ is the equilibrium pressure of the fluid, $\rho$ their energy density, and $\gamma$ is the barotropic index. We explore two particular cases for the bulk viscosity $\xi$, a constant bulk viscosity $\xi=\xi_{0}$, and a bulk viscosity proportional to the energy density of the fluid $\xi=\xi_{0}\rho$. Due to some previous investigations that have explored to describe the behavior of the universe with a negative CC, we extend our analysis to this case. We found that future singularities like Big-Rip are allowed but without having a phantom EoS associated to the DE fluid. Big-Crunch singularities also appears when a negative CC is present, but also de Sitter and even Big-Rip types are allowed due to the negative pressure of the viscosity, which opens the possibility of an accelerated expansion in AdS cosmologies. We also discuss a very particular solution without Big Bang singularity that arises in the early-time radiation dominant era of our model known as Soft-Big Bang., Comment: 17 pages, 13 figures

In this work, we test the capability of an exact solution found in the framework of a nonlinear extension of the Israel-Stewart theory to fit the supernovae Ia, gravitational lensing, and black hole shadow data. This exact solution is a generalization of one previously found for a dissipative unified dark matter model in the context of the near-equilibrium description of dissipative processes, where we do not have the full regime of the nonlinear picture. This generalized solution is restricted to the case where a positive entropy production is guaranteed and is tested under the condition that ensures its causality, local existence, and uniqueness. From the observational constraints, we found that this generalized solution is a good candidate in the description of the observational late-time data used in this work, with best-fit values $H_{0}=73.2_{-0.9}^{+0.8}\,\frac{km/s}{Mpc}$, $q_{0}=-0.41_{-0.03}^{+0.03}$, $\hat{\xi}_{0}=0.88_{-0.17}^{+0.09}$, $\epsilon=0.34_{-0.04}^{+0.03}$, and $k=0.27_{-0.20}^{+0.37}$. Therefore, we show that the nonlinear regime of the Israel-Stewart theory consistently describes the recent accelerated expansion of the universe without the inclusion of some kind of dark energy component and also provides a more realistic description of the fluids that make up the late Universe., Comment: 13 pages, 4 figures. Improved version, updated data analysis in the "Observational Constraints" section

We study the impact of a non-vanishing (positive) cosmological constant on the innermost and outermost stable circular orbits (ISCOs and OSCOs, respectively) within massive gravity in four dimensions. The gravitational field generated by a point-like object within this theory is known, generalizing the usual Schwarzschild--de Sitter geometry of General Relativity. In the non-relativistic limit, the gravitational potential differs by the one corresponding to the Schwarzschild--de Sitter geometry by a term that is linear in the radial coordinate with some prefactor $\gamma$, which is the only free parameter. Starting from the geodesic equations for massive test particles and the corresponding effective potential, we obtain a polynomial of fifth order that allows us to compute the innermost and outermost stable circular orbits. Next, we numerically compute the real and positive roots of the polynomial for several different structures (from the hydrogen atom to stars and globular clusters to galaxies and galaxy clusters) considering three distinct values of the parameter $\gamma$, determined using physical considerations, such as galaxy rotation curves and orbital precession. Similarly to the Kottler spacetime, both ISCOs and OSCOs appear. Their astrophysical relevance as well as the comparison with the Kottler spacetime are briefly discussed., Comment: 8 pages, 1 figure, 2 tables. Accepted for publication in Universe

Unimodular gravity is an appealing approach to address the cosmological constant problem. In this scenario, the vacuum energy density of quantum fields does not gravitate and the cosmological constant appears merely as an integration constant. Recently, it has been shown that energy diffusion that may arise in quantum gravity and in theories with spontaneous collapse is compatible with this framework by virtue of its restricted diffeomorphism invariance. New studies suggest that this phenomenon could lead to higher-order equations in the context of homogeneous and isotropic Universe, affecting the well-posedness of their Cauchy initial-value problem. In this work, we show that this issue can be circumvented by assuming an equation of state that relates the energy density to the function that characterizes the diffusion. As an application, we solve the field equations analytically for an isotropic and homogeneous Universes in a barotropic model and in the mass-proportional continuous spontaneous localization (CSL) scenario, assuming that only dark matter develops energy diffusion. Different solutions possessing phase transition from decelerated to accelerated expansion are found. We use cosmological data of type Ia Supernovae and observational Hubble data to constrain the free parameters of both models. It is found that very small but nontrivial energy nonconservation is compatible with the barotropic model. However, for the CSL model, we find that the best-fit values are not compatible with previous laboratory experiments. We comment on this fact and propose future directions to explore energy diffusion in cosmology., Comment: v1. 14 pages, 10 figures; v2. Typos corrected, references added, and more explanations provided. Accepted for publication in Phys. Rev. D

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 $\Lambda$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., Comment: New results were found and the core of the paper changed completely. Now we contrast the fitting of the exact solution in the non linear approach against LCDM model using supernovae type Ia data

In this paper a cosmological solution of polynomial type $H \approx ( t + const.)^{-1}$ for the causal thermodynamical approach of Isarel-Stewart, found in \cite{MCruz:2017, Cruz2017}, is constrained using the joint of the latest measurements of the Hubble parameter (OHD) and Type Ia Supernovae (SNIa). Since the expansion described by this solution does not present a transition from a decelerated phase to an accelerated one, both phases can be well modeled connecting both phases by requiring the continuity of the Hubble parameter at $z=z_{t}$, the accelerated-decelerated transition redshift. Our best fit constrains the main free parameters of the model to be $A_1= 1.58^{+0.08}_{-0.07}$ ($A_2=0.84^{+0.02}_{-0.02}$) for the accelerated (decelerated) phase. For both phases we obtain $q=-0.37^{+0.03}_{-0.03}$ ($0.19^{+0.03}_{-0.03}$) and $\omega_{eff} = -0.58^{+0.02}_{-0.02}$ ($-0.21^{+0.02}_{-0.02}$) for the deceleration parameter and the effective equation of state, respectively. Comparing our model and LCDM statistically through the Akaike information criterion and the Bayesian information criterion we obtain that the LCDM model is preferred by the OHD+SNIa data. Finally, it is shown that the constrained parameters values satisfy the criterion for a consistent fluid description of a dissipative dark matter component, but with a high value of the speed of sound within the fluid, which is a drawback for a consistent description of the structure formation. We briefly discuss the possibilities to overcome this problem with a non-linear generalization of the causal linear thermodynamics of bulk viscosity and also with the inclusion of some form of dark energy., Comment: Accepted for publication in PRD

In this paper we study the consistency of a cosmological model representing a universe filled with a one-component dissipative dark matter fluid, in the framework of the causal Israel-Stewart theory, where a general expression arising from perturbation analysis for the relaxation time $\tau$ is used. This model is described by an exact analytic solution recently found in [N. Cruz, E. Gonz\'alez and G. Palma, Gen. Rel. Grav. \textbf{52}, 62 (2020), which depends on several model parameters as well as integration constants, allowing the use of Type Ia Supernovae and Observational Hubble data to perform an astringent observational test. The constraint regions found for the parameters of the solution allow the existence of an accelerated expansion of the universe at late times, after the domination era of the viscous pressure, which holds without the need of including a cosmological constant. Nevertheless, the fitted parameter values lead to drawbacks as a very large non-adiabatic contribution to the speed of sound, and some inconsistencies, not totally conclusive, with the description of the dissipative dark matter as a fluid, which is nevertheless a common feature of these kind of models., Comment: 11 pages, 3 figures, 2 Tables. Second Revised version, including new references, new SNe Ia data sample + the Observational Hubble data and slightly modified concluding remarks. Note: For the sake of completeness, the essential equations and basic descriptions to set the Israel-Stewart framework are quoted from our slightly older reference arXiv:1812.05009

In this article we report a novel analytic solution for a cosmological model with a matter content described by a one component dissipative fluid, in the framework of the causal Israel-Stewart theory. Some physically well motivated analytical relations for the bulk viscous coefficient, the relaxation time and a bariotropic equation of state are postulated. We study within the parameter space, which label the solution, a suited region compatible with an accelerated expansion of the universe for late times, as well as stability properties of the solution at the critical parameter values $ \gamma = 1$ and for $ s = 1/2 $. We study as well the consequences that arise from the positiveness of the entropy production along the time evolution. In general, the accelerated expansion at late times is only possible when $\epsilon\geq 1/18$, which implies a very large non-adiabatic contribution the speed of sound., Comment: 12 pages, 3 figures. We have partially rewritten the article emphasizing the mathematical aspects related to the new analytical solution found in the frame of a causal Israel-Stewart cosmology. The title as well as the abstract were slightly modified according to the former resubmission of the paper

Cosmological fluids with a Generalized Equation of State (GEoS) are here considered, whose corresponding EoS parameter $\omega$ describes a fluid with phantom behavior, namely $\omega<-1$, but leading to universes free of singularities at any past or future, finite time. Thus avoiding, in particular, the Big Bang and the Big Rip singularities, the last one considered to be typical in phantom fluid models. More specifically, such GEoS fluid cosmologies lead to regular Little Rip universes. A remarkable new property of these solutions is proven here, namely that they avoid the initial singularity at early times; therefore, they are able to describe emergent universes. Solutions of this kind had been studied previously, but only either as late time or as early time solutions; never as solutions covering both epochs simultaneously. Appropriate conditions are proposed here that relate the Little Rip cosmologies with the initial regular universe, for the future and past regimes, respectively. This is done by taking as starting point the conditions under which a given scale factor corresponds to a Little Rip universe., Comment: 18 pages, 17 figures, analysis extended

In the present paper, the effects of viscous dark matter are analysed within the $\Lambda$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 $\Lambda$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., Comment: 11 pages, 4 figures, analysis extended

It is well known that for spatially flat FRW cosmologies, the holographic dark energy disfavours the Hubble parameter as a candidate for the IR cutoff. For overcoming this problem, we explore the use of this cutoff in holographic ellipsoidal cosmological models, and derive the general ellipsoidal metric induced by a such holographic energy density. Despite the drawbacks that this cutoff presents in homogeneous and isotropic universes, based on this general metric, we developed a suitable ellipsoidal holographic cosmological model, filled with a dark matter and a dark energy components. At late time stages, the cosmic evolution is dominated by a holographic anisotropic dark energy with barotropic equations of state. The cosmologies expand in all directions in accelerated manner. Since the ellipsoidal cosmologies given here are not asymptotically FRW, the deviation from homogeneity and isotropy of the universe on large cosmological scales remains constant during all cosmic evolution. This feature allows studied holographic ellipsoidal cosmologies to be ruled by an equation of state $\omega=p/\rho$, whose range belongs to quintessence or even phantom matter., Comment: To be published in EPJC, 10 pages, 2 figures

In this work we explore a new cosmological solution for an universe filled with one dissipative fluid, described by a barotropic EoS $p = \omega \rho$, in the framework of the full Israel-Stewart theory. The form of the bulk viscosity has been assumed of the form $\xi = \xi_{0}\rho^{1/2}$. 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 $\omega$, $\xi_{0}$, $c_{b}$ 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., Comment: 13 pages, 5 figures. Improved version accepted for publication in PRD

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, $p=\omega \rho$, 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., Comment: 18 pages and 6 figures. Published version in PLB

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 $\xi= \xi_{0}\rho^{1/2}$, where $\rho$ 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., Comment: 9 pages, 2 figures

We found non singular solutions for universes filled with a fluid which obey a Generalized Equation of State of the form $P(\rho)=-A\rho+\gamma\rho^{\lambda}$. An emergent universe is obtained if $A=1$ and $\lambda =1/2$. If the matter source is reinterpret as that of a scalar matter field with some potential, the corresponding potential is derived. For a closed universe, an exact bounce solution is found for $A=1/3$ and the same $\lambda $. We also explore how the composition of theses universes can be interpreted in terms of known fluids. It is of interest to note that accelerated solutions previously found for the late time evolution also represent regular solutions at early times., Comment: This work has been send to Journal of Physics: Conference Series (IOP), in the context of "XIX Simposio de la Sociedad Chilena de F\'isica 2014"

We investigate the thermodynamics of general fluids that have the constriction that their pressure is constant. For example, this happens in the case of pure dust matter, for which the pressure vanishes and also in the case of standard dark matter phenomenology. Assuming a finite non-zero pressure, the corresponding dynamics is richer than one naively would expect. In particular, it can be considered as a unified description of dark energy and dark matter. We first consider the more general thermodynamic properties of this class of fluids finding the important result that for them adiabatic and isothermal processes should coincide. We therefore study their behaviors in curved space-times where local thermal equilibrium can be appealed. Thus, we show that this dark fluid degenerates with the dark sector of the LCDM model only in the case of adiabatic evolution. We demonstrate that, adding dissipative processes, a phantom behavior can occur and finally we further highlight that an arbitrary decomposition of the dark sector, into ad hoc dark matter and dark energy terms, may give rise to phantom dark energy, whereas the whole dark sector remains non-phantom., Comment: 20 pages, 6 figures

We investigate the phase space dynamics of a bulk viscosity model in the Eckart approach for a spatially flat Friedmann-Robertson-Walker universe. We have included two barotropic fluids and a dark energy component. One of the barotropic fluids is treated as an imperfect fluid having bulk viscosity, whereas the other components are assumed to behave as perfect fluids. Both barotropic fluids are identified as either radiation or dark matter. Considering that the bulk viscosity acts on either radiation or dark matter, we find that viscous phantom solutions with stable behavior are not allowed in the framework of complete cosmological dynamics. Only an almost zero value of the bulk viscosity allows a transition from a radiation-dominated to a matter-dominated epoch, which then evolves to an accelerated late time expansion, dominated by dark energy., Comment: 10 pages, 5 figures. Misprints corrected to match published version

The braneworld model proposed by Dvali, Gabadadze and Porrati (DGP) leads to an accelerated universe without cosmological constant or other form of dark energy for the positive branch $(\epsilon =+1)$. For the negative branch $(\epsilon =-1)$ we have investigated the behavior of a model with an holographic Ricci-like dark energy and dark matter, where the IR cutoff takes the form $\alpha H^2 + \beta \dot{H}$, being $H$ the Hubble parameter and $\alpha$, $\beta$ positive constants of the model. We perform an analytical study of the model in the late-time dark energy dominated epoch, where we obtain a solution for $r_cH(z)$, where $r_c$ is the leakage scale of gravity into the bulk, and conditions for the negative branch on the holographic parameters $\alpha$ and $\beta$, in order to hold the conditions of weak energy and accelerated universe. On the other hand, we compare the model versus the late-time cosmological data using the latest type Ia supernova sample of the Joint Light-curve Analysis (JLA), in order to constraint the holographic parameters in the negative branch, as well as $r_cH_0$ in the positive branch, where $H_0$ is the Hubble constant. We find that the model has a good fit to the data and that the most likely values for $(r_cH_0, \alpha, \beta)$ lie in the permitted region found from an analytical solution in a dark energy dominated universe. We give a justification to use holographic cut-off in 4D for the dark energy in the 5 dimensional DGP model. Finally, using the Bayesian Information Criterion we find that this model it is disfavored compared with the flat $\Lambda$CDM model., Comment: 9 pages, 3 figures

We present a study of the geodesic equations of a black hole space-time which is a solution of the three-dimensional NMG theory and is asymptotically Lifshitz with $z=3$ and $d=1$ as found in [Ayon-Beato E., Garbarz A., Giribet G. and Hassaine M., Phys. Rev. {\bf D} 80, 104029 (2009)]. By means of the corresponding effective potentials for massive particles and photons we find the allowed motions by the energy levels. Exact solutions for radial and non-radial geodesics are given in terms of the Weierstrass elliptic $\wp$, $\sigma$, and $\zeta$ functions., Comment: 10 pages, 6 figures, accepted for publication in Eur. Phys. J. C

We explore a cosmological model composed by a dark matter fluid interacting with a dark energy fluid. The interaction term has the non-linear lambda*(rho_m)^alpha * (rho_e)^beta form, where rho_m and rho_e are the energy densities of the dark matter and dark energy, respectively. The parameters alpha and beta are in principle not constrained to take any particular values, and were estimated from observations. We perform an analytical study of the evolution equations, finding the fixed points and their stability properties in order to characterize suitable physical regions in the phase space of the dark matter and dark energy densities. The constants (lambda, alpha, beta) as well as w_m and w_e of the EoS of dark matter and dark energy respectively, were estimated using the cosmological observations of the type Ia supernovae and the Hubble expansion rate H(z) data sets. We find that the best estimated values for the free parameters of the model correspond to a warm dark matter interacting with a phantom dark energy component, with a well goodness-of-fit to data. However, using the Bayesian Information Criterion (BIC) we find that this model is overcame by a warm dark matter -- phantom dark energy model without interaction, as well as by the LCDM model. We find also a large dispersion on the best estimated values of the (lambda, alpha, beta) parameters, so even if we are not able to set strong constraints on their values, given the goodness-of-fit to data of the model, we find that a large variety of theirs values are well compatible with the observational data used., Comment: 22 pages, 7 figures, 2 tables. Replacement with the published version

We explore the cosmological constraints on the parameter w_dm of the dark matter barotropic equation of state (EoS) to investigate the "warmness" of the dark matter fluid. The model is composed by the dark matter and dark energy fluids in addition to the radiation and baryon components. We constrain the values of w_dm using the latest cosmological observations that measure the expansion history of the Universe. When w_dm is estimated together with the parameter w_de of the barotropic EoS of dark energy we found that the cosmological data favor a value of w_dm = 0.006 +- 0.001, suggesting a -warm- dark matter, and w_de= -1.11 +- 0.03$ that corresponds to a phantom dark energy, instead of favoring a cold dark matter and a cosmological constant (w_dm = 0, w_de = -1). When w_dm is estimated alone but assuming w_de = -1, -1.1, -0.9, we found w_dm = 0.009 +- 0.002, 0.006 +- 0.002, 0.012 +- 0.002 respectively, where the errors are at 3 sigma (99.73%), i.e., w_dm > 0 with at least 99.73% of confidence level. When (w_dm, \Omega_dm0) are constrained together, the best fit to data corresponds to (w_dm=0.005 +- 0.001, \Omega_dm0 = 0.223 +- 0.008) and with the assumption of w_de = -1.1 instead of a cosmological constant (i.e., w_de = -1). With these results we found evidence of w_dm > 0 suggesting a -warm- dark matter, independent of the assumed value for w_{\rm de}, but where values w_de < -1 are preferred by the observations instead of the cosmological constant. These constraints on w_dm are consistent with perturbative analyses done in previous works., Comment: 21 pages, 11 figures, 4 tables

In this work we address the study of null geodesics in the background of Reissner-Nordstr\"om Anti de Sitter black holes. We compute the exact trajectories in terms of elliptic functions of Weierstrass, obtaining a detailed description of the orbits in terms of charge, mass and the cosmological constant. The trajectories of the photon are classified using the impact parameter., Comment: 13 pages, 10 figures

The braneworld model proposed by Dvali, Gabadadze and Porrati leads to an accelerated universe without cosmological constant or other form of dark energy. Nevertheless, we have investigated the consequences of this model when an holo- graphic dark energy is included, taken the Hubble scale as IR cutoff. We have found that the holographic dark energy leads to an accelerated universe flat (de Sitter like expansion) for the two branch: {\ko} = \pm1 of the DGP model. Nevertheless, in universes with no null curvature the dark energy presents an EoS corresponding to a phantom fluid during the present era and evolving to a de Sitter like phase for future cosmic time. In the special case in which the holographic parameter c is equal to one we have found a sudden singularity in closed universes. In this case the expansion is decelerating. Manuscript, Comment: Latex, 12 pages, 4 figures; Submitted to Phys. Lett. B

We propose a holographic tachyon model of dark energy with interaction between the components of the dark sector. The correspondence between the tachyon field and the holographic dark energy densities allows the reconstruction of the potential and the dynamics of the tachyon scalar field in a flat Friedmann-Robertson-Walker universe. We show that this model can describe the observed accelerated expansion of our universe with a parameter space given by the most recent observational results., Comment: 7 pages, 8 figures, accepted for publication in IJMPD

A cosmological model of dark energy interacting with dark matter and another general component of the universe is considered. The evolution equations for coincidence parameters r and s, which represent the ratios between the dark energy and the matter and the other cosmic fluid, respectively, are analyzed in terms of the stability of stationary solutions. The obtained general results allow to shed some light on the coincidence problem and in the equations of state of the three interacting fluids, due to the constraints imposes by the stability of the solutions. We found that for an interaction proportional to the sum of the DE density and the third fluid density, the hypothetical fluid must have positive pressure, which leads naturally to a cosmological scenario with radiation, unparticle or even some form of warm DM as the third interacting fluid., Comment: 14 pages

An effective fluid description, for a brane world model in five dimensions, is discussed for both signs of the brane tension. We found several cosmological scenarios where the effective equation differs widely from the bare equation of state. For universes with negative brane tension, with a bare fluid satisfying the strong energy condition, the effective fluid can cross the barrier $\omega _{eff} =-1$., Comment: 6 pages

We consider a connection between the holographic dark energy density and the kinetic k-essence energy density in a flat FRW universe. With the choice $c\geq1$, the holographic dark energy can be described by a kinetic k-essence scalar field in a certain way. In this paper we show this kinetic k-essential description of the holographic dark energy with $c\geq1$ and reconstruct the kinetic k-essence function F(X)., Comment: 6 pages, 4 figures, revised version, accepted for publication in Phys.Lett.B

A cosmological model of an holographic dark energy interacting with dark matter throughout a decaying term of the form $Q=3(\lambda_1\rho_{DE} + \lambda_2\rho_m) H$ is investigated. General constraint on the parameters of the model are found when accelerated expansion is imposed and we found a phantom scenarios, without any reference to a specific equation of state for the dark energy. The behavior of equation of stated for dark energy is also discussed., Comment: 9 pages

A cosmological model of dark energy interacting with dark matter and another general component of the universe is investigated. We found general constraints on these models imposing an accelerated expansion. The same is also studied in the case for holographic dark energy.