arXiv subjects

Janilo Santos

Publications and source records attributed to Janilo Santos.

5 recordsLinked to original sources

arXiv2018

Homogeneous Gödel-type solutions in hybrid metric-Palatini gravity

[Abridged] If gravitation is to be described by a hybrid metric-Palatini $f(\mathcal{R})$ gravity theory there are a number of issues that ought to be examined in its context, including the question as to whether its equations allow homogeneous Gödel-type solutions, which necessarily leads to violation of causality. Here, to look further into the potentialities and difficulties of $f(\mathcal{R})$ theories, we examine whether they admit Gödel-type solutions for well-motivated matter source. We first show that under certain conditions on the matter sources the problem of finding out space-time homogeneous solutions in $f(\mathcal{R})$ theories reduces to the problem of determining solutions of Einstein's field equations with a cosmological constant. Employing this far-reaching result, we determine a general Gödel-type whose matter source is a combination of a scalar with an electromagnetic field plus a perfect fluid. This general Gödel-type solution contains special solutions in which the essential parameter $m^2$ can be $m^{2} > 0$, $m=0$, and $m^{2} < 0$, covering thus all classes of homogeneous Gödel-type spacetimes. This general solution also contains all previously known solution as special cases. The bare existence of these Gödel-type solutions makes apparent that hybrid metric-Palatini gravity does not remedy causal anomaly in the form of closed timelike curves that are permitted in general relativity.

gr-qc↗

arXiv2017

Strong energy condition and the repulsive character of f(R) gravity

The Raychaudhuri equation enables to examine the whole spacetime structure without specific solutions of Einstein's equations, playing a central role for the understanding of the gravitational interaction in Cosmology. In General Relativity, without considering a cosmological constant, a non-positive contribution in the Raychaudhuri equation is usually interpreted as the manifestation of the attractive character of gravity. In this case, particular energy conditions -- indeed the strong energy condition -- must be assumed in order to guarantee the attractive character. In the context of f(R) gravity, however, even assuming the standard energy conditions one may have a positive contribution to the Raychaudhuri equation. Besides providing a simple way to explain the observed cosmic acceleration, this fact opens the possibility of a repulsive character of this kind of gravity. In order to discuss physical bounds on f(R) models, we address the attractive/non-attractive character of f(R) gravity considering the Raychaudhuri equation and assuming the strong energy condition along with recent estimates of the cosmographic parameters.

gr-qc↗

arXiv2015

The virial theorem in Eddington-Born-Infeld gravity

We consider the possibility that the Eddington-Born-Infeld (EBI) modified gravity provides an alternative explanation for the mass discrepancy in clusters of galaxies. For this purpose we derive the modified Einstein field equations, finding an additional "geometrical mass" term which provides an effective contribution to the gravitational binding energy. Using some approximations and assumptions for weak gravitational fields, and taking into account the collisionless relativistic Boltzmann equation, we derive a generalized version of the virial theorem in the framework of EBI gravity. We show that the "geometrical mass" term may account for the well known virial mass discrepancy in clusters of galaxies. We also derive the velocity dispersion relation for galaxies in the clusters, which could provide an efficient method for testing EBI gravity from astrophysical observations.

gr-qc↗

arXiv2012

Impedance of rigid bodies in one-dimensional elastic collisions

In this work we study the problem of one-dimensional elastic collisions of billiard balls, considered as rigid bodies, in a framework very different from the classical one presented in text books. Implementing the notion of impedance matching as a way to understand eficiency of energy transmission in elastic collisions, we find a solution which frames the problem in terms of this conception. We show that the mass of the ball can be seen as a measure of its impedance and verify that the problem of maximum energy transfer in elastic collisions can be thought of as a problem of impedance matching between different media. This approach extends the concept of impedance, usually associated with oscillatory systems, to system of rigid bodies.

physics.class-ph↗

arXiv2012

Palatini approach to modified f(R) gravity and its bi-metric structure

f(R) gravity theories in the Palatini formalism has been recently used as an alternative way to explain the observed late-time cosmic acceleration with no need of invoking either dark energy or extra spatial dimension. However, its applications have shown that some subtleties of these theories need a more profound examination. Here we are interested in the conformal aspects of the Palatini approach in extended theories of gravity. As is well known, extremization of the gravitational action a la Palatini, naturally "selects" a new metric h related to the metric g of the subjacent manifold by a conformal transformation. The related conformal function is given by the derivative of f(R). In this work we examine the conformal symmetries of the flat (k=0) FLRW spacetime and find that its Conformal Killing Vectors are directly linked to the new metric h and also that each vector yields a different conformal function.

gr-qc↗