arXiv subjects

Giovanni Otalora

Publications and source records attributed to Giovanni Otalora.

29 records · Page 2Linked to original sources

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.

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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.

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arXiv2020

Dynamical systems methods and statefinder diagnostic of interacting vacuum energy models

We study three interacting dark energy models within the framework of four-dimensional General Relativity and a spatially flat Universe. In particular, we first consider two vacuum models where dark energy interacts with dark matter, while relativistic matter as well as baryons are treated as non-interacting fluid components. Secondly, we investigate a third model where the gravitational coupling is assumed to be a slowly-varying function of the Hubble rate and dark energy and dark matter interact as well. We compute the statefinders parameters versus red-shift as well as the critical points and their nature applying dynamical systems methods. In the case of only an interaction term, our main findings indicate that i) significant differences between the models are observed as we increase the strength of the interaction term, and ii) all the models present an unique attractor corresponding to acceleration. On the other hand, when we allow for a variable gravitational coupling, we find that i) the deviation from the concordance model depends of both the strength of gravitational coupling parameter and the interaction term, and ii) there is an unique attractor corresponding to acceleration.

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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.

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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.

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arXiv2018

Inflation from a nonlinear magnetic monopole field nonminimally coupled to curvature

In the context of nonminimally coupled $f(R)$ gravity theories, we study early inflation driven by a nonlinear monopole magnetic field which is nonminimally coupled to curvature. In order to isolate the effects of the nonminimal coupling between matter and curvature we assume the pure gravitational sector to have the Einstein-Hilbert form. Thus, we study the most simple model with a nonminimal coupling function which is linear in the Ricci scalar. From an effective fluid description, we show the existence of an early exponential expansion regime of the Universe, followed by a transition to a radiation-dominated era. In particular, by applying the most recent results of the Planck collaboration we set the limits on the parameter of the nonminimal coupling, and the quotient of the nonminimal coupling and the nonlinear monopole magnetic scales. We found that these parameters must take large values in order to satisfy the observational constraints. Furthermore, by obtaining the relation for the graviton mass, we show the consistency of our results with the recent gravitational wave data GW$170817$ of LIGO and Virgo.

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arXiv2018

Gauss-Bonnet dark energy on Hořava-Lifshitz cosmology

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

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arXiv2017

Dynamics of viscous cosmologies in the full Israel-Stewart formalism

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

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arXiv2016

Modified teleparallel gravity with higher-derivative torsion terms

We construct $F(T,\left(\nabla{T}\right)^2,\Box {T})$ gravitational modifications, which are novel classes of modified theories arising from higher-derivative torsional terms in the action, and are different than their curvature analogue. Applying them in a cosmological framework we obtain an effective dark energy sector that comprises of the novel torsional contributions. We perform a detailed dynamical analysis for two specific examples, extracting the stable late-time solutions and calculating the corresponding observables. We show that the thermal history of the universe can be reproduced, and it can result in a dark-energy dominated, accelerating universe, where the dark-energy equation-of-state parameter lies in the quintessence regime, or may exhibit the phantom-divide crossing during the cosmological evolution. Finally, the scale factor behaves asymptotically either as a power-law or as an exponential, in agreement with observations.

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arXiv2015

Dynamical system analysis for nonminimal torsion-matter coupled gravity

In this work, we perform a detailed dynamical analysis for the cosmological applications of a nonminimal torsion-matter coupled gravity. Two alternative formalisms are proposed, which enable one to choose between the easier approach for a given problem, and furthermore, we analyze six specific models. In general, we extract fixed points corresponding either to dark-matter dominated, scaling decelerated solutions, or to dark-energy dominated accelerated solutions. Additionally, we find that there is a small parameter region in which the model can experience the transition from the matter epoch to a dark-energy era. These features are in agreement with the observed universe evolution, and make the theory a successful candidate for the description of Nature.

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arXiv2015

A novel teleparallel dark energy model

Although equivalent to general relativity, teleparallel gravity is conceptually speaking a completely different theory. In this theory, the gravitational field is described by torsion, not by curvature. By working in this context, a new model is proposed in which the four-derivative of a canonical scalar field representing dark energy is nonminimally coupled to the "vector torsion". This type of coupling is motivated by the fact that a scalar field couples to torsion through its four-derivative, which is consistent with local spacetime kinematics regulated by the de Sitter group $SO(1,4)$. It is found that the current state of accelerated expansion of the Universe corresponds to a late-time attractor that can be (i) a dark-energy-dominated de Sitter solution ($ω_ϕ=-1$), (ii) a quintessence-type solution with $ω_ϕ\geq-1$, or (iii) a phantom-type $ω_ϕ<-1$ dark energy.

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