arXiv subjects

Joel Saavedra

Publications and source records attributed to Joel Saavedra.

At least 55 records · Page 3Linked to original sources

arXiv2018

Quasinormal Modes of a Schwarzschild Black Hole Immersed in an Electromagnetic Universe

We study the quasinormal modes (QNMs) of the Schwarzschild black hole immersed in an electromagnetic (em) universe. The immersed Schwarzschild black hole (ISBH) is originated from the metric of colliding em waves with double polarization [Class. Quantum Grav. 12, 3013 (1995)]. The perturbation equations of the scalar fields for the ISBH geometry are written in the form of separable equations. We show that these equations can be transformed to the confluent Heun's equations, for which we are able to use the known techniques to perform the analytical quasinormal (QNM) analysis of the solutions. Furthermore, we employ numerical methods [Mashhoon and $6^{th}$-order Wentzel-Kramers-Brillouin (WKB)] to derive the QNMs. The results obtained are discussed and depicted with the appropriate plots.

physics.gen-ph↗

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.

gr-qc↗

arXiv2018

Deflection of light by rotating regular black holes using the Gauss-Bonnet theorem

In this paper, we study the weak gravitational lensing in the spacetime of rotating regular black hole geometries such as Ayon-Beato-García (ABG), Bardeen, and Hayward black holes. We calculate the deflection angle of light using the Gauss-Bonnet theorem (GBT) and show that the deflection of light can be viewed as a partially topological effect in which the deflection angle can be calculated by considering a domain outside of the light ray applied to the black hole optical geometries. Then, we demonstrate also the deflection angle via the geodesics formalism for these black holes to verify our results and explore the differences with the Kerr solution. These black holes have in addition to the total mass and rotation parameter, different parameters as electric charge, magnetic charge, and deviation parameter. Newsworthy, we find that the deflection of light has correction terms coming from these parameters which generalizes the Kerr deflection angle.

gr-qc↗

arXiv2018

Hawking radiation and propagation of massive charged scalar field on a three-dimensional Gödel black hole

In this paper we consider the three-dimensional Gödel black hole as a background and we study the vector particle tunneling from this background in order to obtain the Hawking temperature. Then, we study the propagation of a massive charged scalar field and we find the quasinormal modes analytically, which turns out be unstable as a consequence of the existence of closed time-like curves. Also, we consider the flux at the horizon and at infinity, and we compute the reflection and transmission coefficients as well as the absorption cross section. Mainly, we show that massive charged scalar waves can be superradiantly amplified by the three-dimensional Gödel black hole and that the coefficients have an oscillatory behavior. Moreover, the absorption cross section is null at the high frequency limit and for certain values of the frequency.

gr-qc↗

arXiv2018

On the phantom barrier crossing and the bounds on the speed of sound in non-minimal derivative coupling theories

In this paper we investigate the so called "phantom barrier crossing" issue in a cosmological model based in the scalar-tensor theory with non-minimal derivative coupling to the Einstein's tensor. Special attention will be paid to the physical bounds on the squared sound speed. The numeric results are geometrically illustrated by means of a qualitative procedure of analysis that is based on the mapping of the orbits in the phase plane onto the surfaces that represent physical quantities in the extended phase space, that is: the phase plane complemented with an additional dimension relative to the given physical parameter. We find that the cosmological model based in the non-minimal derivative coupling theory -- this includes both the quintessence and the pure derivative coupling cases -- has serious causality problems related with superluminal propagation of the scalar and tensor perturbations. Even more disturbing is the finding that, despite that the underlying theory is free of the Ostrogradsky instability, the corresponding cosmological model is plagued by the Laplacian (classical) instability related with negative squared sound speed. This instability leads to an uncontrollable growth of the energy density of the perturbations that is inversely proportional to their wavelength. We show that independent of the self-interaction potential, for the positive coupling the tensor perturbations propagate superluminally, while for the negative coupling a Laplacian instability arises. This latter instability invalidates the possibility for the model to describe the primordial inflation.

gr-qc↗

arXiv2018

Constraints on the radiation temperature before inflation

We consider the short period of cosmic expansion ranging from the end of the Planck era to the beginning of inflation and set upper and lower limits on the temperature of the radiation at the commencement of the inflationary phase.

gr-qc↗

arXiv2017

The Effect of the Gauss-Bonnet term to Hawking Radiation from arbitrary dimensional Black Brane

We investigate the probabilities of the tunneling and the radiation spectrums of the massive spin-1 particles from arbitrary dimensional Gauss-Bonnet-Axions (GBA) Anti-de Sitter (AdS) black brane, via using the WKB approximation to the Proca spin-1 field equation. The tunneling probabilities and Hawking temperature of the arbitrary dimensional GBA AdS black brane is calculated via the Hamilton-Jacobi approach. We also compute the Hawking temperature via Parikh-Wilczek tunneling approach. The results obtained from the two methods are consistent. In our setup, the Gauss-Bonnet (GB) coupling affects the Hawking temperature if and only if the momentum of axion fields is non-vanishing.

gr-qc↗

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.

gr-qc↗

arXiv2017

Thermodynamical and dynamical properties of Charged BTZ Black Holes

We investigate the spacetime properties of BTZ black holes in the presence of the Maxwell field and Born-Infeld field and find rich properties in the spacetime structures when the model parameters are varied. Employing the Landau-Lifshitz theory, we examine the thermodynamical phase transition in the charged BTZ black holes. We further study the dynamical perturbation in the background of the charged BTZ black holes and find different properties in the dynamics when the thermodynamical phase transition occurs.

hep-th↗

arXiv2017

Dynamical Analysis of an Integrable Cubic Galileon Cosmological Model

Recently a cubic Galileon cosmological model was derived by the assumption that the field equations are invariant under the action of point transformations. The cubic Galileon model admits a second conservation law which means that the field equations form an integrable system. The analysis of the critical points for this integrable model is the main subject of this work. To perform the analysis, we work on dimensionless variables different from that of the Hubble normalization. New critical points are derived while the gravitational effects which follow from the cubic term are studied.

gr-qc↗

arXiv2017

Motion of magnetically charged particles in a magnetically charged stringy black hole spacetime

We study the motion of massive particles with electric and magnetic charges in the background of a magnetically charged Garfinkle-Horowitz-Strominger stringy black hole. We solve analytically the equations of motion of the test particles and we describe the orbital motion in terms of the Weierstrass elliptic functions. We find that there are critical values of the magnetic charge of the black hole and the magnetic charge of the test particle which characterize the bound and unbound orbits and we study two observables, the perihelion shift and the Lense-Thirring effect. The trajectories depend on the electric and magnetic charges of the test particle. While the angular-motion depends on the electric charge of the test particle, the $r$ and $t$-motion depends on the mass and the magnetic charge of the test particle.

gr-qc↗

arXiv2017

Superradiant Instability of Near Extremal and Extremal Four-Dimensional Charged Hairy Black Hole in anti-de Sitter Spacetime

We study the instability of near extremal and extremal four-dimensional AdS charged hairy black hole to radial neutral massive and charged massless scalar field perturbations. We solve the scalar field equation by using the improved asymptotic iteration method and the time domain analysis and we find the quasinormal frequencies. For the charged scalar perturbations, we find the superradiance condition by computing the reflection coefficient in the low-frequency limit and we show that in the superradiance regime, which depends on the scalar hair charge, all modes of radial charged massless perturbations are unstable, indicating that the charged hairy black hole is superradiantly unstable. On the other hand, calculating the quasinormal frequencies of radial neutral scalar perturbations in this background, we found stability of the charged hairy black hole.

gr-qc↗

arXiv2016

Stealths on $(1+1)$-dimensional dilatonic gravity

We study gravitational stealth configurations emerging on a charged dilatonic $(1+1)$-D black hole spacetime. We accomplish this by considering the coupling of a non-minimally scalar field $ϕ$ and a self-interacting scalar field $Ψ$ living in a $(1+1)$-D charged black hole background. In addition, the self-interacting potential for $Ψ$ is obtained which exhibits transitions for some specific values of the non-minimal parameter. Atypically, we found that the solutions for these stealth scalar fields do not have a dependence on the temporal coordinate.

gr-qc↗

arXiv2016

Curvaton reheating in non-minimal derivative coupling to gravity: NO models

The curvaton reheating mechanism in a non-minimal derivative coupling to gravity for any non-oscillating (NO) model is studied. In this framework, we analyze the energy density during the kinetic epoch and we find that this energy has a complicated dependencies of the scale factor. Considering this mechanism, we study the decay of the curvaton in two different scenarios and also we determine the reheating temperatures. As an example the NO model, we consider an exponential potential and we obtain the reheating temperature indirectly from the inflation through of the number of e-folds.

gr-qc↗

arXiv2016

Brans-Dicke Galileon and the Variational Principle

This paper is aimed at a (mostly) pedagogical exposition of the derivation of the motion equations of certain modifications of general relativity. Here we derive in all detail the motion equations in the Brans-Dicke theory with the cubic self-interaction. This is a modification of the Brans-dicke theory by the addition of a term in the Lagrangian which is non-linear in the derivatives of the scalar field: it contains second-order derivatives. This is the basis of the so-called Brans-Dicke Galileon. We pay special attention to the variational principle and to the algebraic details of the derivation. It is shown how higher order derivatives of the fields appearing in the intermediate computations cancel out leading to second order motion equations. The reader will find useful tips for the derivation of the field equations of modifications of general relativity such as the scalar-tensor theories and $f(R)$ theories, by means of the (stationary action) variational principle. The content of this paper is specially recommended to those graduate and postgraduate students who are interested in the study of the mentioned modifications of general relativity.

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↗

arXiv2015

Nonminimal derivative coupling scalar-tensor theories: odd-parity perturbations and black hole stability

We derive the odd parity perturbation equation in scalar-tensor theories with a non minimal kinetic coupling sector of the general Horndeski theory, where the kinetic term is coupled to the metric and the Einstein tensor. We derive the potential of the perturbation, by identifying a master function and switching to tortoise coordinates. We then prove the mode stability under linear odd- parity perturbations of hairy black holes in this sector of Horndeski theory, when a cosmological constant term in the action is included. Finally, we comment on the existence of slowly rotating black hole solutions in this setup and discuss their implications on the physics of compact objects configurations, such as neutron stars.

gr-qc↗

arXiv2015

Higher Order Lagrangians inspired by the Pais-Uhlenbeck Oscillator and their cosmological applications

We study higher derivative terms associated with scalar field cosmology. We consider a coupling between the scalar field and the geometry inspired by the Pais-Uhlenbeck oscillator, given by $α\partial_μ\partial^μϕ\partial_ν\partial^νϕ.$ We investigate the cosmological dynamics in a phase space. For $α>0$, we provide conditions for the stability of de Sitter solutions. In this case the crossing of the phantom divide $w_{DE}=-1$ occurs once; thereafter, the equation of state parameter remains under this line, asymptotically reaching towards the de Sitter solution from below. For $α<0,$ which is the portion of the parameter space where in addition to crossing the phantom divide, cyclic behavior is possible, we present regions in the parameter space where, according to Smilga's classification the ghost has benign or malicious behavior.

gr-qc↗