arXiv subjects

Emilio Elizalde

Publications and source records attributed to Emilio Elizalde.

At least 37 records · Page 2Linked to original sources

arXiv2020

Cosmological dynamics in $R^2$ gravity with logarithmic trace term

A novel function for modified gravity is proposed, $f(R, T)=R+λR^2+2β\ln(T)$, with constants $λ$ and $β$, scalar curvature $R$, and the trace of stress energy tensor $T$, satisfying $T=ρ-3p>0$. Subsequently, two equations of state (EoS) parameters, namely $ω$ and a parametric form of the Hubble parameter $H$, are employed in order to study the accelerated expansion and initial cosmological bounce of the corresponding universe. Hubble telescope experimental data for redshift $z$ within the range $0.07\leq z \leq 2.34$ are used to compare the theoretical and observational values of the Hubble parameter. Moreover, it is observed that all the energy conditions are fulfilled within a neighborhood of the bouncing point $t=0$, what shows that the necessary condition for violation of the null energy condition, within a neighborhood of the bouncing point in general relativity, could be avoided by modifying the theory in a reasonable way. Furthermore, a large amount of negative pressure is found, which helps to understand the late time accelerated expansion phase of the universe.

physics.gen-ph↗

arXiv2020

An analysis of the $H_{0}$ tension problem in a universe with a viscous dark fluid

In this paper, two inhomogeneous single fluid models for the Universe, which are able to naturally solve the $H_{0}$ tension problem, are discussed. The analysis is based on a Bayesian Machine Learning approach that uses a generative process. The adopted method allows to constrain the free parameters of each model by using the model itself, only. The observable is taken to be the Hubble parameter, obtained from the generative process. Using the full advantages of our method, the models are constrained for two redshift ranges. Namely, first this is done with mock $H(z)$ data over $z\in [0,2.5]$, thus covering known $H(z)$ observational data, which are most helpful to validate the fit results. Then, aiming to extend to redshift ranges to be covered by the most recent ongoing and future planned missions, the models are constrained for the range $z\in[0,5]$, too. Full validation of the results for this extended redshift range will have to wait for the near future, when higher redshift $H(z)$ data become available. This makes our models fully falsifiable. Finally, our second model here is able to explain the BOSS reported value for $H(z)$ at $z=2.34$.

gr-qc↗

arXiv2019

Viable non-singular cosmic bounce in holonomy improved F(R) gravity endowed with a Lagrange multiplier

Matter and quasi-matter bounce scenarios are studied for an F(R) gravity model with holonomy corrections and a Lagrange multiplier, with a scale factor $a(t) = \left(a_0t^2 + 1 \right)^n$, where the Hubble parameter squared has a linear and a quadratic dependence on the effective energy density. Provided $n < 1/2$, it is shown that the primordial curvature perturbations are generated deeply into the contracting era, at large negative time, which makes the low-curvature limit a good approximation for calculating the perturbation power spectrum. Moreover, it is shown that, for $n$ within this range, the obtained cosmological quantities are fully compatible with the Planck constraints, and that the "low curvature limit" comes as a viable approximation to calculate the power spectra of both scalar and tensor perturbations. Using reconstruction techniques for F(R) gravity with the Lagrange multiplier, the precise form of the effective F(R) gravity is found, from which one determines the power spectra of scalar and tensor perturbations in such bouncing scenario. Correspondingly, the spectral index for curvature perturbations and the tensor to scalar ratio are obtained, and these values are successfully confronted with the latest Planck observations. Further, it is shown that both the weak and the null energy conditions are satisfied, thanks to the holonomy corrections performed in the theory--which are then proven to be necessary for achieving this goal. In fact, when approaching the bouncing era, the holonomy corrections become significant and play a crucial role in order to restore the energy conditions. Summing up, a cosmological bouncing scenario with the scale factor above and fulfilling the energy conditions can be adequately described by the F(R) model with a Lagrange multiplier and holonomy corrections, which prove to be very important.

gr-qc↗

arXiv2019

Wormhole models in $f(\textit{R}, \textit{T})$ gravity

Models of static wormholes within the $f(\textit{R}, \textit{T})$ extended theory of gravity are investigated and, in particular, the family $f(\textit{R}, \textit{T}) = R + λT$, with $T = ρ+ P_{r} + 2P_{l}$ being the trace of the energy-momentum tensor. Models corresponding to different relations for the pressure components (radial and lateral), and several equations of state (EoS), reflecting different matter content, are worked out explicitly. The solutions obtained for the shape functions of the generated wormholes obey the necessary metric conditions, as manifested in other studies in the literature. The respective energy conditions reveal the physical nature of the wormhole models thus constructed. It is found, in particular, that for each of those considered, the parameter space can be divided into different regions, in which the exact wormhole solutions fulfill the null~(NEC) and the weak energy conditions~(WEC), respectively, in terms of the lateral pressure. Moreover, the dominant energy condition~(DEC) in terms of both pressures is also valid, while $ρ+ P_{r} + 2P_{l} = 0$. A similar solution for the theory $P_{r} = ω_{1} ρ+ ω_{2} ρ^{2}$ is found numerically, where $ω_{1}$ and $ω_{2}$ are either constant or functions of $r$, leading to the result that the NEC in terms of the radial pressure is also valid. For non-constant $ω_{i}$ models, attention is focused on the behavior $ω_{i} \propto r^{m}$. To finish, the question is addressed, how $f(R) = R+αR^{2}$ will affect the wormhole solutions corresponding to fluids of the form $P_{r} = ω_{1} ρ+ ω_{2} ρ^{2}$, in the three cases mentioned above. Issues concerning the nonconservation of the matter energy-momentum tensor, the stability of the solutions obtained, and the observational possibilities for testing these models are discussed in the last section.

gr-qc↗

arXiv2019

Reconstruction Procedure for Nonlocal Gauss-Bonnet Models

We investigate the cosmological dynamics of nonlocally corrected gravity involving a function of the inverse d'Alembertian acting on the Gauss-Bonnet term. Casting the dynamical equations in local form, we derive the reconstruction procedure. We find conditions on the model parameters that are sufficient for the existence of de Sitter solutions and obtain these solutions explicitly.

gr-qc↗

arXiv2019

The inflationary universe in F(R) gravity with antisymmetric tensor fields and their suppression during the universe evolution

The intriguing question, why the present scale of the universe is free from any perceptible footprints of rank-2 antisymmetric tensor fields? (generally known as Kalb-Ramond fields) is addressed. A quite natural explanation of this issue is given from the angle of higher-curvature gravity, both in four- and in five-dimensional spacetime. The results here obtained reveal that the amplitude of the Kalb-Ramond field may be actually large and play a significant role during the early universe, while the presence of higher-order gravity suppresses this field during the cosmological evolution, so that it eventually becomes negligible in the current universe. Besides the suppression of the Kalb-Ramond field, the extra degree of freedom in $F(R)$ gravity, usually known as scalaron, also turns out to be responsible for inflation. Such F(R) gravity with Kalb-Ramond fields may govern the early universe to undergo an inflationary stage at early times (with the subsequent graceful exit) for wider range of F(R) gravity than without antisymmetric fields.. Furthermore, the models---in four- and five-dimensional spacetimes---are linked to observational constraints, with the conclusion that the corresponding values of the spectral index and tensor-to-scalar ratio closely match the values provided by the Planck survey 2018 data.

gr-qc↗

arXiv2018

Reasons in favor of a Hubble-Lemaitre-Slipher's (HLS) law

Based on historical facts, revisited from a present-day perspective, and on the documented opinions of the scientists involved in the discovery themselves, strong arguments are given in favor of a proposal to include prominent astronomer Vesto Slipher to the suggested addition of Georges Lemaître's name to Hubble's law on the expansion of the Universe, and thus eventually call it Hubble-Lemaître-Slipher's (HLS) law.

physics.hist-ph↗

arXiv2018

Wormholes with $ρ(R,R^{\prime})$ matter in $f(\textit{R}, \textit{T})$ gravity

Models of static wormholes are investigated in the framework of $f(\textit{R}, \textit{T})$ gravity ($\textit{R}$ being the curvature scalar, and $\textit{T}$ the trace of the energy momentum tensor). An attempt to link the energy density of the matter component to the Ricci scalar is made, which for the Morris and Thorne wormhole metric, with constant redshift function, yields $R(r) = 2b^{\prime}(r)/r^{2}$. Exact wormhole solutions are obtained for three particular cases: $ρ(r) = αR(r) + βR^{\prime}(r)$, $ρ(r) = αR^{2}(r) + βR^{\prime}(r)$, and $ρ(r) = αR(r) + βR^{2}(r)$, when $f(\textit{R}, \textit{T}) = R + 2 λT$. For the two first ones, traversable wormhole models are obtained. When the matter energy density is of the 3rd type, only solutions with constant shape may correspond to traversable wormholes. Exact wormhole solutions with same properties can be constructed for $ρ= αR(r) + βR^{-2}(r)$, $ρ= αR(r) + βr R^{2}(r)$, $ρ= αR(r) + βr^{-1} R^{2}(r)$, $ρ= αR(r) + βr ^{2} R^{2}(r)$, $ρ= αR(r) + βr^{3} R^{2}(r)$ and $ρ= αr^m R(r) \log (βR(r))$, as well. On the other hand, for $f(\textit{R}, \textit{T}) = R + γR^{2} + 2 λT$ gravity, two wormhole models are constructed, assuming that the energy density of the wormhole matter is $ρ(r) = αR(r) + βR^{2}(r)$ and $ρ(r) = αR(r) + βr^{3} R^{2}(r)$, respectively. In this case, the functional form of the shape function is taken to be $b(r) = \sqrt{\hat{r}_{0} r}$ and possible existence of appropriate static traversable wormhole configurations is proven. These results can be viewed as an initial step towards using specific properties of the new exact wormhole solutions, in order to propose new functional forms for describing the matter content of the wormhole.

gr-qc↗

arXiv2018

Linking little rip cosmologies with regular early universes

Cosmological fluids with a Generalized Equation of State (GEoS) are here considered, whose corresponding EoS parameter $ω$ describes a fluid with phantom behavior, namely $ω<-1$, 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.

gr-qc↗

arXiv2018

On wormhole formation in $f(\textit{R}, \textit{T})$ gravity: varying Chaplygin gas and barotropic fluid

Formation of specific static wormhole models is discussed, by assuming an $f(\textit{R}, \textit{T}) = R + 2 λT$ extended theory of gravity, $T =- ρ+ P_{r} + 2P_{l}$ being the trace of the energy momentum tensor. In the first part, wormhole solutions are constructed imposing that the radial pressure admits an equation of state corresponding to a varying Chaplygin gas. Two forms for the varying Chaplygin gas are considered, namely $P_{r} = - B b(r)^{u}/ρ^α$ and $P_{r} = -B R(r)^{m}/ ρ^{α}$, respectively. In the second part, the wormhole models are constructed assuming that the radial pressure can be described by a varying barotropic fluid. In particular, $P_{r} = ωb(r)^{v} ρ$ and $P_{r} =\hat{ ω} r^{k} R(r)^η ρ$ are considered, respectively, leading to two additional, traversable wormhole models. In all cases, $b(r)$ is the shape function, and $R(r)$ the Ricci scalar obtained from the wormhole metric for a redshift function equal to $1$. With the help of specific examples, it is demonstrated that the shape functions of the exact wormhole models previously constructed do obey the necessary metric conditions. The same energy conditions help reveal the physical properties of these models. A general feature is the violation of the NEC~($ρ+ P_{i} \geq 0$) in terms of the radial pressure $P_{r}$ at the throat of the wormhole. For some of the models, one can satisfy the NEC at the throat while a violation of the DEC~($ρ- P_{i} \geq 0$) occurs. To summarize, exact wormhole models can be constructed with a possible violation of the NEC and DEC at the throat of the wormhole, while being $ρ\geq 0 $. Thus, the interesting feature appears that one has a violation of the WEC~($ρ\geq 0$ and $ρ+ P_{i} \geq 0$) not related to the energy density behavior (the index i, being r resp. l, indicates radial resp. lateral pressure).

gr-qc↗

arXiv2018

Swampland criteria for a dark-energy dominated universe, ensuing from Gaussian process and H(z) data analysis

Implications of the string Swampland criteria for a dark energy dominated universe, obtained by using Gaussian processes and $H(z)$ data analysis, are discussed. In particular, the Swampland criteria for a scalar-field dark energy, without assuming any specific form for the potential. By allowing the Gaussian process to reconstruct the form of the potential from $H(z)$ data, upper bounds on the second Swampland criterion (involving $|V^{\prime}|/V$) for two different kernel functions (the squared exponential and Matern~($ν= 9/2$) kernels) are estimated . The approach here differs from previous studies, since the upper bound of the second Swampland criterion is derived in a thoroughly model-independent way, without resorting to a model-to-model comparison strategy. The analysis is performed using the latest values of $H_{0}$ reported by the Planck and Hubble missions. Results for the estimation of the constant of $SC2$ hint towards the possibility of getting upper bounds well behind the estimations for the dark energy dominated universe as reported in previous studies, which involved model-to-model comparison. Estimations from this new approach turn to be quite sensitive and just depend on the quality of the data and on the kernel employed. This study is a first attempt towards the exploitation of the Swampland criteria in a model-independent way and may be extended to involve other datasets and, also, in trying to understand what is the impact of higher-redshift data on the upper bounds. In the analysis, $40$-point $H(z)$ data have been used, consisting of a $30$-point sample deduced from a differential age method and an additional $10$-point sample obtained from the radial BAO method. Hints towards the possibility of eventually disproving the Swampland conjecture are noted.

astro-ph.CO↗

arXiv2018

Cosmological singularities in interacting dark energy models with an $ω(q)$ parametrization

Future singularities arising in a family of models for the expanding Universe, characterized by sharing a convenient parametrization of the energy budget in terms of the deceleration parameter, are classified. Finite-time future singularities are known to appear in many cosmological scenarios, in particular, in the presence of viscosity or non-gravitational interactions, the last being known to be able to suppress or just change in some cases the type of the cosmological singularity. Here, a family of models with a parametrization of the energy budget in terms of the deceleration parameter are studied in the light of Gaussian processes using reconstructed data from $40$-value $H(z)$ datasets. Eventually, the form of the possible non-gravitational interaction between dark energy and dark matter is constructed from these smoothed $H(z)$ data. Using phase space analysis, it is shown that a non-interacting model with dark energy $ω_\mathrm{de} = ω_{0} + ω_{1}q$ ($q$ being the deceleration parameter) may evolve, after starting from a matter dominated unstable state, into a de Sitter Universe (the solution being in fact a stable node). Moreover, for a model with interaction term $Q = 3 H b ρ_\mathrm{dm}$ ($b$ is a parameter and $H$ the Hubble constant) three stable critical points are obtained, what may have important astrophysical implications. In addition, part of the paper is devoted to a general discussion of the finite-time future singularities obtained from direct numerical integration of the field equations, since they appear in many cosmological scenarios and could be useful for future extended studies of the models here introduced. Numerical solutions for the new models, produce finite-time future singularities of Type I or Type III, or an $ω$-singularity, provided general relativity describes the background dynamics.

gr-qc↗

arXiv2018

"All that matter ... in one Big Bang ...," & other cosmological singularities

The first part of this paper contains a brief description of the beginnings of modern cosmology, which, the author will argue, was most likely born in the Year 1912. Some of the pieces of evidence presented here have emerged from recent research in the history of science, and are not usually shared with the general audiences in popular science books. In special, the issue of the correct formulation of the original Big Bang concept, according to the precise words of Fred Hoyle, is discussed. Too often, this point is very deficiently explained (when not just misleadingly) in most of the available generalist literature. Other frequent uses of the same words, Big Bang, as to name the initial singularity of the cosmos, and also whole cosmological models, are then addressed, as evolutions of its original meaning. Quantum and inflationary additions to the celebrated singularity theorems by Penrose, Geroch, Hawking and others led to subsequent results by Borde, Guth and Vilenkin. And corresponding corrections to the Einstein field equations have originated, in particular, $R^2$, $f(R)$, and scalar-tensor gravities, giving rise to a plethora of new singularities. For completeness, an updated table with a classification of the same is given.

physics.hist-ph↗

arXiv2017

On how the Cyberspace arose to fulfill theoretical physicists' needs and eventually changed the World: Personal recallings and a practitioner's perspective

The very humble origins of the Cyberspace, and all the related developments that smoothly conspired and converged towards this concept, making its emergence possible, as the personal computer, TEX and LATEX, the Fax, the internet, the cellphone, and the World Wide Web, are discussed, always from a personal perspective. A separate, comprehensive explanation of the reasons for the appearance and subsequent evolution of each of these different phenomena, with explicit assessments and a future prospective of the main subject, is presented.

physics.pop-ph↗

arXiv2016

Inhomogeneous imperfect fluid inflation

A generalized equation of state corresponding to a model that includes a Chaplygin gas and a viscous term is investigated, in the context of the reconstruction program in scalar field cosmology. The corresponding inflationary model parameters can be conveniently adjusted in order to reproduce the most recent PLANCK data. The influence of the Chaplygin gas term contribution, in relation with previous models, is discussed. Exit from inflation is shown to occur quite naturally in the new model.

gr-qc↗

arXiv2016

De Sitter and power-law solutions in some models of modified gravity

Inspired by some recent works of Lovelock Brans-Dicke gravity and mimetic gravity, cosmology solutions in extensions of these two modified gravities are investigated. A non-local term is added to the Lovelock Brans-Dicke action and Gauss-Bonnet terms to the mimetic action,correspondingly. De Sitter and power scale factor solutions are then obtained in both theories. They can provide natural new approaches to a more accurate description of the unverse evolution.

gr-qc↗

arXiv2016

Singular inflation from Born-Infeld-f(R) gravity

Accelerating dynamics from Born-Infeld-$f(R)$ gravity are studied in a simplified conformal approach without matter. Explicit unification of inflation with late-time acceleration is realized within this singular inflation approach, which is similar to Odintsov-Oikonomou singular $f(R)$ inflation. Our model turns out to be consistent with the latest release of Planck data.

gr-qc↗

arXiv2016

Spotting deviations from R^2 inflation

We discuss the soundness of inflationary scenarios in theories beyond the Starobinsky model, namely a class of theories described by arbitrary functions of the Ricci scalar and the K-essence field. We discuss the pathologies associated with higher-order equations of motion which will be shown to constrain the stability of this class of theories. We provide a general framework to calculate the slow-roll parameters and the corresponding mappings to the theory parameters. For paradigmatic gravitational models within the class of theories under consideration we illustrate the power of the Planck/Bicep2 latest results to constrain such gravitational Lagrangians. Finally, bounds for potential deviations from Starobinsky-like inflation are derived.

gr-qc↗