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A. F. Santos

Publications and source records attributed to A. F. Santos.

At least 91 records · Page 5Linked to original sources
arXiv2019

Ricci dark energy in bumblebee gravity model

The Ricci dark energy is a model inspired by the holographic dark energy models with the dark energy density being proportional to Ricci scalar curvature. Here this model is studied in the bumblebee gravity theory. It is a gravitational theory that exhibit spontaneous Lorentz symmetry breaking. Then the modified Friedmann equation is solved for two cases. In the first case the coupling constant $ξ$ is equal to zero. And in the second case a solution in the vacuum, where the bumblebee field becomes a constant that minimizes the potential, is considered. The coupling constant controls the interaction gravity-bumblebee.

gr-qc↗
arXiv2019

On Lorentz violation in $e^{-}\!\!+\!e^{+}\!\rightarrow\!μ^{-}\!\!+\!μ^{+}$ scattering at finite temperature

Small violation of Lorentz and CPT symmetries may emerge in models unifying gravity with other forces of nature. An extension of the standard model with all possible terms that violate Lorentz and CPT symmetries are included. Here a CPT-even non-minimal coupling term is added to the covariant derivative. This leads to a new interaction term that breaks the Lorentz symmetry. Our main objective is to calculate the cross section for the $e^{-}\!\!+\!e^{+}\!\rightarrow\!μ^{-}\!\!+\!μ^{+}$ scattering in order to investigate any violation of Lorentz and/or CPT symmetry at finite temperature. Thermo Field Dynamics formalism is used to consider finite temperature effects.

hep-th↗
arXiv2019

Lorentz violation and Gravitoelectromagnetism: Casimir effect and Stefan-Boltzmann law at Finite temperature

The standard model and general relativity are local Lorentz invariants. However it is possible that at Planck scale there may be a breakdown of Lorentz symmetry. Models with Lorentz violation are constructed using Standard Model Extension (SME). Here gravitational sector of the SME is considered to analyze the Lorentz violation in Gravitoelectromagnetism (GEM). Using the energy-momentum tensor, the Stefan-Boltzmann law and Casimir effect are calculated at finite temperature to ascertain the level of local Lorentz violation. Thermo Field Dynamics (TFD) formalism is used to introduce temperature effects.

hep-th↗
arXiv2018

On Scalar Electromagnetism in Phase Space

In this paper the interaction of a scalar field and the electromagnetic field in phase space is analyzed. The scattering process is calculated up to first order in the Planck constant which is obtained by an expansion of the Moyal product in phase space. The transition amplitude is calculated in the same context.

hep-th↗
arXiv2018

Lorentz violation, Möller scattering and finite temperature

Lorentz and CPT symmetries may be violated in new physics that emerges at very high energy scale, i.e., at the Planck scale. The differential cross section of the Möller scattering, due to Lorentz violation at finite temperature is calculated. Lorentz-violating effects emerge from an interaction vertex due to a CPT-odd nonminimal coupling in the covariant derivative. The finite temperature effects are determined using the Thermo Field Dynamics (TFD) formalism.

hep-th↗
arXiv2018

On causality violation in Lyra Geometry

In this paper the causality issues are discussed in a non-Riemannian geometry, called Lyra geometry. It is a non-Riemannian geometry originated from Weyl geometry. In order to compare this geometry with the Riemannian geometry, the Einstein field equations are considered. It is verified that the Gödel and Gödel-type metric are consistent with this non-Riemannian geometry. A non-trivial solution for Gödel universe in the absence of matter sources is determined without analogue in general relativity. Different sources are considered and then different conditions for causal and non-causal solutions are discussed.

gr-qc↗
arXiv2018

Lorentz violation, Gravitoelectromagnetism and Bhabha Scattering at finite temperature

Gravitoelectromagnetism (GEM) is an approach for the gravitation field that is described using the formulation and terminology similar to that of electromagnetism. The Lorentz violation is considered in the formulation of GEM that is covariant in its form. In practice such a small violation of the Lorentz symmetry may be expected in a unified theory at very high energy. In this paper a non-minimal coupling term, which exhibits Lorentz violation, is added as a new term in the covariant form. The differential cross section for Bhabha scattering in the GEM framework at finite temperature is calculated that includes Lorentz violation. The Thermo Field Dynamics (TFD) formalism is used to calculate the total differential cross section at finite temperature. The contribution due to Lorentz violation is isolated from the total cross section. It is found to be small in magnitude.

hep-th↗
arXiv2018

Lorentz Violation, Gravitoelectromagnetic Field and Bhabha Scattering

Lorentz symmetry is a fundamental symmetry in the Standard Model (SM) and in General Relativity (GR). This symmetry holds true for all models at low energies. However at energies near the Planck scale, it is conjectured that there may be a very small violation of Lorentz symmetry. The Standard Model Extension (SME) is a quantum field theory that includes a systematic description of Lorentz symmetry violations in all sectors of particle physics and gravity. In this paper SME is considered to study the physical process of Bhabha Scattering in the Gravitoelectromagnetism (GEM) theory. GEM is an important formalism that is valid in a suitable approximation of general relativity. A new non-minimal coupling term that violates Lorentz symmetry is used in this paper. Differential cross section for gravitational Bhabha scattering is calculated. The Lorentz violation contributions to this GEM scattering cross section are small and are similar in magnitude to the case of the electromagnetic field.

hep-th↗
arXiv2017

On Fermion-Graviton Scattering at Finite Temperature

The early universe is dominated by phenomena at high temperatures. The thermal effects decrease during evolution of the universe. However there are some phenomena, such as processes inside stars and black holes, where the role of the temperature is important. Study of processes involving gravitons at finite temperature is a viable means to understand gravitational processes in the universe. Using the Thermo Field Dynamics formalism transition amplitudes involving gravitons and fermions at finite temperature are calculated. In this approach our results consist of two parts: one corresponding to the zero temperature part and the other corresponding to a temperature dependent part.

gr-qc↗
arXiv2017

Non-relativistic Casimir effect at finite temperature

The Galilean covariance, formulated in 5-dimensions space, describes the non-relativistic physics in a way similar to quantum field theory. Using a non-relativistic approach the Stefan-Boltzmann law and the Casimir effect at finite temperature for a particle with spin zero and $1/2$ are calculated. The Thermo Field Dynamics is used to calculate the temperature effect.

hep-th↗
arXiv2017

Gödel-type solutions within the f(R,Q) gravity

In this paper, we deal with the $f(R,Q)$ gravity whose action depends, besides of the scalar curvature $R$, on the higher-derivative invariant $Q=R_{μν}R^{μν}$. In order to compare this theory with the usual General Relativity (GR), we verify the consistency of Gödel-type solutions within the $f(R,Q)$ gravity and discuss the related causality issues. Explicitly, we show that in the $f(R,Q)$ gravity there are new Gödel-type completely causal solutions having no analogue in the general relativity. In particular, a remarkable Gödel-type solution corresponding to the conformally flat space and maximally symmetric for physically well-motivated matter sources, with no necessity of cosmological constant, has been considered. We demonstrate that, in contrast to GR framework, $f(R,Q)$ gravity supports new vacuum solutions with the requirement for the cosmological constant to be non-zero. Finally, causal solutions are obtained for a particular choice $f(R,Q)=R+αR^2+ βQ$.

hep-th↗
arXiv2016

Gödel and Gödel-type universes in Brans-Dicke theory

In this paper, conditions for existence of Gödel and Gödel-type solutions in Brans-Dicke (BD) scalar-tensor theory and their main features are studied. The consistency of equations of motion, causality violation and existence of CTCs (closed time-like curves) are investigated. The role which cosmological constant and Mach principle play to achieve the consistency of this model is studied.

hep-th↗
arXiv2016

Gravitational Casimir effect at finite temperature

The energy-momentum tensor for the gravitoelectromagnetism (GEM) theory in the real-time finite temperature field theory formalism is presented. Expressions for the Casimir energy and pressure at zero and finite temperature are obtained. An analysis of the Casimir effect for the GEM field is developed.

hep-th↗
arXiv2016

Chern-Simons modified gravity and closed time-like curves

We verify the consistency of the Gödel-type solutions within the four-dimensional Chern-Simons modified gravity with the non-dynamical Chern-Simons coefficient, for different forms of matter including dust, fluid, scalar field and electromagnetic field, and the related causality issues. Unlike the general relativity, the vacuum solution turns out to be possible in our theory. Another essentially new result of our theory having no analogue in the general relativity consists in the existence of the hyperbolic causal solutions for the physically well-motivated matter.

hep-th↗
arXiv2016

Scattering of Fermions by Gravitons

The interaction between gravitons and fermions is investigated in the teleparallel gravity. The scattering of fermions and gravitons in the weak field approximation is analyzed. The transition amplitudes of M$\varnothing$ller, Compton and new gravitational scattering are calculated.

gr-qc↗
arXiv2016

Quantized gravitoelectromagnetism theory at finite temperature

The Gravitoelectromagnetism (GEM) theory is considered in a lagrangian formulation using the Weyl tensor components. A perturbative approach to calculate processes at zero temperature has been used. Here the GEM at finite temperature is analyzed using Thermo Field Dynamics, real time finite temperature quantum field theory. Transition amplitudes involving gravitons, fermions and photons are calculated for various processes. These amplitudes are likely of interest in astrophysics.

hep-th↗
arXiv2016

On FRW Model in Conformal Teleparallel Gravity

In this paper we use the conformal teleparallel gravity to study an isotropic and homogeneous Universe which is settled by the FRW metric. We solve the field equations and we obtain the behavior of some cosmological parameters such as scale factor, deceleration parameter and the energy density of the perfect fluid which is the matter field of our model. The field equations, that we called modified Friedmann equations, allow us to define a dark fluid, with dark energy density and dark pressure, responsible for the acceleration in the Universe.

gr-qc↗