arXiv subjects

Manuel Gonzalez-Espinoza

Publications and source records attributed to Manuel Gonzalez-Espinoza.

28 records · Page 2Linked to original sources

arXiv2021

Parametrized post-Newtonian formalism in higher-order Teleparallel Gravity

We study the parametrized post-Newtonian (PPN) limit of higher-derivative-torsion Modified Teleparallel Gravity. We start from the covariant formulation of modified Teleparallel Gravity by restoring the spin connection of the theory. Then, we perform the post-Newtonian expansion of the tetrad field around the Minkowski background and find the perturbed field equations. We compute the PPN metric for the higher-order Teleparallel Gravity theories which allows us to show that at the post-Newtonian limit this more general class of theories are fully conservative and indistinguishable from General Relativity . In this way, we extend the results that were already found for $F(T)$ gravity in previous works. Furthermore, our calculations reveal the importance of considering a second post-Newtonian (2PN) order approximation or a parametrized post-Newtonian cosmology (PPNC) framework where additional perturbative modes coming from general modifications of Teleparallel Gravity could lead to new observable imprints.

gr-qc↗

arXiv2021

Stability of scalar perturbations in scalar-torsion $f(T,ϕ)$ gravity theories in the presence of a matter fluid

We study the viability conditions for the absence of ghost, gradient and tachyonic instabilities, in scalar-torsion $f(T,ϕ)$ gravity theories in the presence of a general barotropic perfect fluid. To describe the matter sector, we use the Sorkin-Schutz action and then calculate the second order action for scalar perturbations. For the study of ghost and gradient instabilities, we found that the gravity sector keeps decoupled from the matter sector and then applied the viability conditions for each one separately. Particularly, we verified that this theory is free from ghost and gradient instabilities, obtaining the standard results for matter, and for the gravity sector we checked that the corresponding speed of propagation satisfies $c_{s,g}^2=1$. On the other hand, in the case of tachyonic instability, we obtained the general expressions for the mass eigenvalues and then evaluated them in the scaling matter fixed points of a concrete model of dark energy. Thus, we found a space of parameters where it is possible to have a stable configuration respecting the constraints from the CMB measurements and the BBN constraints for early dark energy. Finally, we have numerically corroborated these results by solving the cosmological equations for a realistic cosmological evolution with phase space trajectories undergoing scaling matter regimes, and then showing that the system presents a stable configuration throughout cosmic evolution.

gr-qc↗

arXiv2021

Reconstructing inflation in scalar-torsion $f(T,ϕ)$ gravity

It is investigated the reconstruction during the slow-roll inflation in the most general class of scalar-torsion theories whose Lagrangian density is an arbitrary function $f(T,ϕ)$ of the torsion scalar $T$ of teleparallel gravity and the inflaton $ϕ$. For the class of theories with Lagrangian density $f(T,ϕ)=-M_{pl}^{2} T/2 - G(T) F(ϕ) - V(ϕ)$, with $G(T)\sim T^{s+1}$ and the power $s$ as constant, we consider a reconstruction scheme for determining both the non-minimal coupling function $F(ϕ)$ and the scalar potential $V(ϕ)$ through the parametrization (or attractor) of the scalar spectral index $n_{s}(N)$ and the tensor-to-scalar ratio $r(N)$ as functions of the number of $e-$folds $N$. As specific examples, we analyze the attractors $n_{s}-1 \propto 1/N$ and $r\propto 1/N$, as well as the case $r\propto 1/N (N+γ)$ with $γ$ a dimensionless constant. In this sense and depending on the attractors considered, we obtain different expressions for the function $F(ϕ)$ and the potential $V(ϕ)$, as also the constraints on the parameters present in our model and its reconstruction.

gr-qc↗

arXiv2021

Cosmological dynamics of dark energy in scalar-torsion $f(T,ϕ)$ gravity

It is investigated the cosmological dynamics of scalar-torsion $f(T,ϕ)$ gravity as a dark energy model, where $T$ is the torsion scalar of teleparallel gravity and $ϕ$ is a canonical scalar field. In this context, we are concerned with the phenomenology of the class of models with non-linear coupling to gravity and exponential potential. We obtain the critical points of the autonomous system, along with the stability conditions of each one of them and their cosmological properties. Particularly, we show the existence of new attractors with accelerated expansion, as well as, new scaling solutions in which the energy density of dark energy scales as the background fluid density, thus, defining the so-called scaling radiation and scaling matter epochs. The scaling solutions are saddle points, and therefore, the system exits these solutions to the current epoch of cosmic acceleration, towards an attractor point describing the dark energy-dominated era.

gr-qc↗

arXiv2020

Generating primordial fluctuations from modified teleparallel gravity with local Lorentz-symmetry breaking

In the context of modified teleparallel gravity, we study the generation of primordial density fluctuations in a general scalar-torsion theory whose Lagrangian density is an arbitrary function $f(T,ϕ)$ of the torsion scalar $T$ and a scalar field $ϕ$, plus the kinetic term of this latter. It is well known that generic modifications of teleparallel gravity are not invariant under six-parameter local Lorentz transformations. In order to restore the local Lorentz symmetry, we have incorporated six additional degrees of freedom in the form of Goldstone modes of the symmetry breaking through a Lorentz rotation of the tetrad field. After integrating out all the auxiliary modes, we obtain a second order action for the scalar and tensor propagating modes and their power spectrum generated during inflation. It is found that an explicit mass term emerges in the second order action for curvature perturbation, describing the imprints of local Lorentz violation at first-order of slow-roll. We show that only inflationary models with nonminimal coupling functions $f(T,ϕ)$ which are non-linear in $T$, including the case of $f(T)$ gravity with minimally coupled scalar field, can generate primordial fluctuations. For a concrete model of inflation, we study the power-law potential by using the latest Planck data.

gr-qc↗

arXiv2019

Slow-roll inflation in generalized scalar-torsion gravity

We study the dynamics of inflation in a generalized scalar-torsion gravity scenario by assuming a canonical scalar field non-minimally coupled to torsion with a Galileon-type self-interaction. After obtaining the field equations for a flat FRW background, we derive the second order action for both scalar and tensor perturbations to compute the power spectra of primordial fluctuations. As particular models, we studied at first, a power-law form of coupling function $F(x)=1+ξx^{2}/2$, with $x\equiv ϕ/M_{pl}$, and a monomial scalar field potential $V(x)=λx^{n}/n$ which is ruled out at $2σ$ level by current observational data for $n\geq 2$. Under slow-roll approximation we obtain analytical expressions for the background as well as perturbative dynamics, and we show that the predictions of the model are consistent with current Planck 2018 constraints on the spectral index $n_{s}$ and the tensor-to-scalar ratio $r$ through the $n_s-r$ plane. Accordingly, this model is in agreement with current observational bounds only within the $95\%$ C.L. region in the case of chaotic quadratic inflation ($n=2$), whereas that for the other monomial potentials such as $n=4/3$, $n=1$ and $n=2/3$, it is found that they are even more favoured, overlapping their results with the $68\%$ C.L. region from last Planck data. Secondly, we studied a model in which the presence of both non-minimal coupling to gravity and the Galileon non-linear self-interaction $γ(\partial ϕ)^2 \Boxϕ$ leads to a suppression of the tensor-to-scalar ratio compared to those predicted in the standard scenario, then predicting $0.024\lesssim r\lesssim 0.069$. This result allows us to reconcile chaotic quadratic inflation with current Planck data up to the $68\%$ C.L. region.

gr-qc↗

arXiv2019

Does the second law hold at cosmic scales?

The second law of thermodynamics is known to hold at small scales also when gravity plays a leading role, as in the case of black holes and self-gravitating radiation spheres. It has been suggested that it should as well at large scales. Here, by a purely kinematic analysis \textemdash based on the history of the Hubble factor and independent of any cosmological model \textemdash , we explore if this law is fulfilled in the case of homogeneous and isotropic universes regardless of the sign of the spatial curvature.

astro-ph.CO↗

arXiv2018

Growth of matter overdensities in non-minimal torsion-matter coupling theories

We study the evolution of cosmological perturbations around a homogeneous and isotropic background in the framework of the non-minimal torsion-matter coupling extension of $f(T)$ gravity. We are concerned with the effects of the non-minimal coupling term on the growth of matter overdensities. Under the quasi-static approximation, we derive the equation which governs the evolution of matter density perturbations, and it is shown that the effective gravitational coupling 'constant' acquires an additional contribution due to the non-minimal matter-torsion coupling term. In this way, this result generalizes those previously obtained for the growth of matter overdensities in the case of minimal $f(T)$ gravity. In order to get a feeling of our results we apply them to the important case of a power-law coupling function, which we assume to be the responsible for the late-time accelerated expansion in the dark energy regime. Thereby, analytic solutions for the matter density perturbation equation in the regime of dark matter dominance and the dark energy epoch are obtained, along with a complete numerical integration of this equation. In particular, we show that this model predicts a growth index larger than those obtained for $Λ$CDM model, indicating therefore a smaller growth rate. Concomitantly, we show that the model at hand is potentially capable in alleviating the existing $σ_{8}$-tension, being that it can provide us a $fσ_{8}$ prediction which is $\sim 4-5$ per cent below the respective prediction of concordance model.

gr-qc↗

arXiv2017

Black hole stability under odd-parity perturbations in Horndeski gravity

We study the stability under linear odd-parity perturbations of static spherically symmetric black holes in Horndeski gravity. We derive the master equation for these perturbations and obtain the conditions of no-ghost and Laplacian instability. In order for the black hole solutions to be stable, we study their generalized "Regge-Wheeler potential". It turns out that the problem is reduced to an algebraic problem where three functions characterizing the black hole should be positive outside the horizon to prove the stability. We found that these conditions are similar to the no-ghost and Laplacian instability conditions. We apply our results to various known solutions.

gr-qc↗

arXiv2016

Scalar Perturbations of two-dimensional Horava-Lifshitz Black Holes

In this article, we study the stability of black hole solutions found in the context of dilatonic Horava-Lifshitz gravity in $1+1$ dimensions by means of the quasinormal modes approach. In order to find the corresponding quasinormal modes, we consider the perturbations of massive and massless scalar fields minimally coupled to gravity. In both cases, we found that the quasinormal modes have a discrete spectrum and are completely imaginary, which leads to damping modes. For a massive scalar field and a non-vanishing cosmological constant, our results suggest unstable behaviour for large values of the scalar field mass.

gr-qc↗