arXiv subjects

L. G. Medeiros

Publications and source records attributed to L. G. Medeiros.

At least 19 recordsLinked to original sources

arXiv2026

Brans-Dicke-like field for co-varying $G$ and $c$: observational constraints

Ref. [Symmetry 15 (2023) 709] introduced a Brans-Dicke-like framework wherein the scalar field $ϕ$ is composed of both $G$ and $c$ which, for this reason, co-vary according to $c^{3}/G=\text{constant}$. In this paper, we use observational data to constrain the supposed co-varying $G$ and $c$, under the hypothesis of the validity of the standard Lemaitre formula $1+z\sim a^{-1}$. The datasets include SN Ia, BAO and the value of $θ$ extracted from CMB data. A proxy function is demanded for the varying $c$ since the framework does not provide a closed set of equations for computing the functional form of either $G$ or $c$ uniquely. Accordingly, we choose three separate parameterizations for $c\left(z\right)$ inspired both by desirable properties of the varying speed of light (VSL) and by successful phenomenological models from the literature -- including the one by Gupta (CCC framework in e.g. Ref. [Mon. Not. R. Astron. Soc., 498 (2020) 4481-4491]. When combined with DESI, Pantheon+ data strongly favor a variable speed of light with more than $3σ$ confidence level for all parameterizations considered in this paper, whereas Union2.1 suggests no variation of the speed of light. As we shall demonstrate, this apparent discrepancy is due to a strong correlation that emerges between $H_0$ and VSL.

astro-ph.CO↗

arXiv2026

Gravitational waveforms from inspiraling compact binaries in quadratic gravity and their parameterized post-Einstein characterization

We investigate gravitational waveforms from the inspiral phase of compact binary systems within the framework of quadratic gravity and map their deviations from general relativity into the parameterized post-Einstein (PPE) formalism to constrain the theory parameters. Quadratic gravity generically includes a massive spin-2 ghost, which leads to ill-defined energy and angular momentum fluxes. Following earlier proposals, we remove these unphysical features by imposing a constraint on the massive spin-2 mode, restricting it to propagate only the same polarizations of general relativity. Within the quadrupole approximation, we derive the radiative degrees of freedom, including massless and massive tensor modes, as well as a massive scalar field. Using the stationary phase approximation, we compute the Fourier-domain waveform of the massless tensor modes and extract the phase corrections. For small deviations from general relativity, we show that both the scalar and massive tensor modes can be consistently embedded into the PPE framework, extending previous results that considered only scalar fields. We derive updated constraints on the parameters of quadratic gravity, finding bounds improved by several orders of magnitude compared to existing limits. Finally, we present forecasts for the sensitivity of the Einstein Telescope to these deviations.

gr-qc↗

arXiv2025

Starobinsky Inflation and the Latest CMB Data: A Subtle Tension?

We analyze the Starobinsky inflation model and the impact of curvature corrections, particularly a cubic $R^3$ term, to assess their behavior in light of the latest observational results from the Atacama Cosmology Telescope (ACT). With the recent sixth data release (DR6), the scalar spectral index was measured to be $n_s=0.9743 \pm 0.0034$, which appears to exclude the pure Starobinsky model at approximately the $2σ$ level. In this paper, we implement the Starobinsky inflationary potential directly into the CLASS code, without relying on the slow-roll approximation, and we constrain the number of e-folds of inflation $N_k$ using a theoretically motivated range derived from reheating considerations and standard couplings between matter fields and gravity. We show that it is still possible to identify a significant region of parameter space where the Starobinsky model remains highly consistent with the latest observational data. While the pure Starobinsky model remains a compelling candidate for cosmic inflation, we explore how including a cubic $R^3$ term can shift its predictions to better align with the Planck and ACT measurements.

astro-ph.CO↗

arXiv2025

Critical masses and numerical computation of massive scalar quasinormal modes in Schwarzschild black holes

We present a comprehensive analysis of the quasinormal modes (QNMs) of a massive scalar field in Schwarzschild spacetime using two complementary numerical techniques: the Hill-determinant method and Leaver continued-fraction method. Our study systematically compares the performance, convergence, and consistency of the two approaches across a wide range of field masses and angular momenta. We identify three critical mass thresholds, $m_{\rm lim}$, $m_{\rm max}$, and $m_{zd}$, which govern qualitative changes in the QNM spectrum. In particular, long-lived modes emerge at $m_{zd}$, where the imaginary part of the frequency vanishes and the mode becomes essentially non-decaying. This phenomenon is robust across multipoles and may have important implications for the phenomenology of massive fields around black holes. Our results provide a detailed numerical characterization of massive scalar QNMs and highlight the complementary strengths of the Hill-determinant and continued-fraction methods, paving the way for future studies of rotating or charged black holes and quasi-bound states.

gr-qc↗

arXiv2025

Higher-order gravity models: corrections up to cubic curvature invariants and inflation

We construct a higher-order gravity model including all corrections up to mass dimension six. Starting from the Jordan frame, we derive the field equations and specialize to the FLRW background, where the dynamics take the form of a four-dimensional autonomous system. Focusing on the $R+R^{2}+RR_{μν}R^{μν}$ case, we obtain linearized equations in the parameter $γ_{0}$ and analyze the resulting phase space. The model exhibits the main desirable features of an inflationary regime, with a slow-roll attractor and a stable critical point corresponding to the end of inflation. Analytical expressions for the scalar spectral index $n_{s}$ and the tensor-to-scalar ratio $r$ show that the model is consistent with Planck, BICEP/Keck, and BAO data if $|γ_{0}|\lesssim 10^{-3}$. Moreover, negative values of $γ_{0}$ restore compatibility with recent ACT, Planck, and DESI results, suggesting that higher-order corrections may be relevant in refining inflationary cosmology.

gr-qc↗

arXiv2025

Gauges for quadratic gravity: the extended transverse gauge and the energy-momentum tensor of the massive spin-2 field

We study the 4D Einstein-Hilbert action extension based on the square of the curvature tensors. Analyses of gauge and perturbation modes are often done considering the Teyssandier gauge condition. Although this approach can be useful for other higher-order extensions of quadratic gravity, a generalized transverse (or Lorentz) gauge is clear and sufficient for the present case, as explained here. We provide a detailed analysis of the generalized transverse gauge condition, its residual gauge symmetry, the physical modes, the induced energy-momentum tensor (EMT) of the massive spin-2 mode (which is gauge-dependent), and a comparison with the Teyssandier gauge. The derivation is valid for any EMT. We also compare the induced EMT in quadratic gravity with the massive spin-2 Fierz-Pauli EMT, which is different from the previous cases. The comparison is further developed by considering a spherical isothermal sphere, which works as an approximation to virialized spherical systems.

gr-qc↗

arXiv2025

Gravitational Waves Emission in Quadratic Gravity: longitudinal modes, angular momentum emission, and positivity of the radiated power

In this paper, the emission of gravitational waves in quadratic gravity theory is examined. The wave equations for massless and massive perturbations are derived, followed by the calculation of the energy and angular momentum radiated. In the quadrupole approximation, and taking into account only the transverse-traceless modes, it is shown that the theory avoids the issues generated by the Ostrogradsky instabilities and achieves positive energy and angular momentum emissions. As an example, a rotating ellipsoid with free precession is analyzed, and the effects of the massive perturbations on its rotation are highlighted.

gr-qc↗

arXiv2024

Inflationary dynamics in modified gravity models

Higher-order theories of gravity are a branch of modified gravity wherein the geometrodynamics of the four-dimensional Riemannian manifold is determined by field equations involving derivatives of the metric tensor of order higher than two. This paper considers a general action built with the Einstein-Hilbert term plus additional curvature-based invariants, viz. the Starobinsky $R^{2}$-type term, a term scaling with $R^{3}$, and a correction of the type $R\square R$. The focus is on the background inflationary regime accommodated by these three models. For that, the higher-order field equations are built and specified for the FLRW line element. The dynanical analysis in the phase space is carried in each case. This analysis shows that the Starobinsky-plus-$R^{3}$ model keeps the good features exhibited by the pure Starobinsky inflationary model, although the set of initial conditions for the inflaton field $χ$ leading to a graceful exit scenario is more contrived; the coupling constant $α_{0}$ of the $R^{3}$ invariant is also constrained by the dynamical analysis. The Starobinsky-plus-$R\square R$ model turns out being a double-field inflation model; it consistently enables an almost-exponential primordial acceleration followed by a radiation dominated universe if its coupling $β_{0}$ takes values in the interval $0\leqβ_{0}\leq3/4$. The models introducing higher-order correction to Starobinsky inflation are interesting due to the possibility of a running spectral index $n_{s}$, something that is allowed by current CMB observations.

gr-qc↗

arXiv2023

Second-order corrections to Starobinsky inflation

Higher-order theories of gravity are extensions to general relativity (GR) motivated mainly by high-energy physics searching for GR ultraviolet completeness. They are characterized by the inclusion of correction terms in the Einstein-Hilbert action that leads to higher-order field equations. In this paper, we propose investigating inflation due to the GR extension built with all correction terms up to the second-order involving only the scalar curvature $R$, namely, $R^{2}$, $R^{3}$, $R\square R$. We investigate inflation within the Friedmann cosmological background, where we study the phase space of the model, as well as explore inflation in slow-roll leading-order. Furthermore, we describe the evolution of scalar perturbations and properly establish the curvature perturbation. Finally, we confront the proposed model with recent observations from Planck, BICEP3/Keck, and BAO data.

astro-ph.CO↗

arXiv2023

Viable wormhole solution in Bopp-Podolsky electrodynamics

Following a recent approach in which the gravitational field equations in curved spacetimes were presented in the Bopp--Podolsky electrodynamics, we obtained an approximate and spherically symmetric wormhole solution in this context. The calculations were carried out up to the linear approximation in both the spacetime geometry and the radial electric field. The solution presents a new parameter that comes from the Lagrangian of the model. Such a parameter was constrained by using the shadow radius of Sagittarius A*, recently revealed by the Event Horizon Telescope Collaboration. Remarkably, the wormhole presented here is viable when its shadow is compared to the Sagittarius A* shadow.

gr-qc↗

arXiv2023

Gravitational waves from inspiraling black holes in quadratic gravity

We peform a study of gravitational waves emitted by inspiraling black holes in the context of quadratic gravity. By linearizing the field equations around a flat background, we demonstrate that all degrees of freedom satisfy wave-like equations. These degrees of freedom split into three modes: a massive spin-$2$ mode, a massive spin-$0$ mode, and the expected massless spin-$2$ mode. We construct the energy-momentum tensor of gravitational waves and show that, due to the massive spin-$2$ mode, it presents the Ostrogradski instability. We also show how to deal with this possible pathology and obtain consistent physical interpretations for the system. Using the energy-momentum tensor, we study the influence of each massive mode in the orbital dynamics and compare it with the standard result of General Relativity. Moreover, we present two methods to constrain the parameter $α$ associated with the massive spin-$2$ contribution. From the first method, using the combined waveform for the spin-$2$ modes, we obtain the constraint $ α\lesssim 1.1 \times 10^{21} m^{2}$. In the second method, using the coalescence time, we get the constraint $ α\lesssim 1.1 \times 10^{13} m^{2}$.

gr-qc↗

arXiv2023

Modified Starobinsky inflation by the $R\ln\left( \square\right) R$ term

In the context of effective theories of gravity, a minimalist bottom-up approach which takes into account $1$-loop quantum corrections leads to modifications in the Einstein-Hilbert action through the inclusion of four extra terms: $R^{2}$, $C_{κραβ}C^{κραβ}$, $R\ln\left( \square\right) R$ and $C_{κραβ}\ln\left( \square\right) C^{κραβ}$. The first two terms are necessary to guarantee the renormalizability of the gravitational theory, and the last two terms (nonlocal terms) arise from the integration of massless/light matter fields. This work aims to analyze how one of the nonlocal terms, namely $R\ln\left( \square\right) R$, affects the Starobinsky inflation. We consider the nonlocal term as a small correction to the $R^{2}$ term, and we demonstrate that the model behaves like a local model in this context. In addition, we show that the approximate model in the Einstein frame is described by a canonical scalar field minimally coupled to general relativity. Finally, we study the inflationary regime of this model and constrain its free parameters through observations of CMB anisotropies.

gr-qc↗

arXiv2022

Gravitational waves in higher-order $R^{2}$ gravity

We perform a comprehensive study of gravitational waves in the context of the higher-order quadratic scalar curvature gravity, which encompasses the ordinary Einstein-Hilbert term in the action plus an $R^{2}$ contribution and a term of the type $R\square R$. The main focus is on gravitational waves emitted by binary systems such as binary black holes and binary pulsars in the approximation of circular orbits and nonrelativistic motion. The waveform of higher-order gravitational waves from binary black holes is constructed and compared with the waveform predicted by standard general relativity; we conclude that the merger occurs earlier in our model than what would be expected from GR. The decreasing rate of the orbital period in binary pulsars is used to constrain the coupling parameters of our higher-order $R^{2}$ gravity; this is done with Hulse-Taylor binary pulsar data leading to $κ_{0}^{-1}\lesssim1.1\times10^{16}\,\text{m}^{2}$, where $κ_{0}^{-1}$ is the coupling constant for the $R^{2}$ contribution.

gr-qc↗

arXiv2022

Higher-order extension of Starobinsky inflation: initial conditions, slow-roll regime, and reheating phase

The most current observational data corroborate the Starobinsky model as one of the strongest candidates in the description of an inflationary regime. Motivated by such success, extensions of the Starobinsky model have been increasingly recurrent in the literature. The theoretical justification for this is well grounded: higher-order gravities arise in high-energy physics in the search for the ultraviolet completeness of general relativity. In this paper, we propose to investigate the inflation due to the extension of the Starobinsky model characterized by the inclusion of the $R^{3}$ term. We make a complete analysis of the potential and phase space of the model, where we observe the existence of three regions with distinct dynamics for the scalar field. We can establish restrictive limits for the number of $e$-folds through a study of the reheating and by considering the usual couplings of the standard matter fields and gravity. Thereby, we duly confront our model with the observational data from Planck, BICEP3/Keck, and BAO. Finally, we discuss how the inclusion of the cubic term restricts the initial conditions necessary for the occurrence of a physical inflation.

astro-ph.CO↗

arXiv2022

Analytical warm dark matter power spectrum on small scales

Using the Reduced Relativistic Gas (RRG) model, we analytically determine the matter power spectrum for Warm Dark Matter (WDM) on small scales, $k>1\ h\text{/Mpc}$. The RRG is a simplified model for the ideal relativistic gas, but very accurate in the cosmological context. In another work, we have shown that, for typical allowed masses for dark matter particles, $m>5\ \text{keV}$, the higher order multipoles, $\ell>2$, in the Einstein-Boltzmann system of equations are negligible on scales $k<10\ h\text{/Mpc}$. Hence, we can follow the perturbations of WDM using the ideal fluid framework, with equation of state and sound speed of perturbations given by the RRG model. We derive a Mészáros like equation for WDM and solve it analytically in radiation, matter and dark energy dominated eras. Joining these solutions, we get an expression that determines the value of WDM perturbations as a function of redshift and wavenumber. Then we construct the matter power spectrum and transfer function of WDM on small scales and compare it to some results coming from Lyman-$α$ forest observations. Besides being a clear and pedagogical analytical development to understand the evolution of WDM perturbations, our power spectrum results are consistent with the observations considered and the other determinations of the degree of warmness of dark matter particles.

astro-ph.CO↗

arXiv2020

Spherically Symmetric Solutions in Higher-Derivative Theories of Gravity

Higher-order theories of gravity have received much attention from several areas including quantum gravity, string theory and cosmology. This paper proposes a higher-order gravity whose action includes all curvature scalar terms up to the second-order corrections of general relativity, namely, $R^{2}$, $R^{3}$ and $R\square R$. Then, we explore spherically symmetric and static solutions in the weak-field regime and black holes context. All solutions in the weak-field regime due to a point mass are deduced, and by making a stability analysis of these solutions, we restrict them to Yukawa-type solutions. In regard to black hole solutions, we use the Lichnerovicz method to investigate the possibility of existence of non-Schwarzschild black holes. The results obtained show that non-Schwarzschild solutions might exist. However, for reasonable values of the parameters of the theory, its horizon radii are extremely small making macroscopic black holes different from Schwarzschild unfeasible.

gr-qc↗

arXiv2019

Constraining nonlinear corrections to Maxwell electrodynamics using $γγ$ scattering

The recent light-by-light scattering cross section measurement made by the ATLAS\ Collaboration is used to constrain nonlinear corrections to Maxwell electrodynamics parametrized by the Lagrangian $L=F+4αF^{2}+4βG^{2}+4δFG$. The ion's radiation is described using the equivalent photon approximation, and the influence of four different nuclear charge distributions is evaluated. Special attention is given to the interference term between the Standard Model and the nonlinear corrections amplitudes. By virtue of the quadratic dependence on $α$, $β$ and $δ$, the nonlinear contribution to the Standard Model $γγ$ cross section is able to delimit a finite region of the parameter's phase space. The upper values for $α$, $β$ in this region are of order $10^{-10}$GeV, a constraint of at least $12$ orders of magnitude more precise when compared to low-energy experiments. An upper value of the same order for $δ$ is obtained for the first time in the LHC energy regime. We also give our predictions for the Standard Model cross section measured at ATLAS for each distribution and analyze the impact of the absorption factor. We finally give predictions for the future measurements to be done with upgraded tracking acceptance $\left\vert η\right\vert <4$ by the ATLAS Collaboration.

hep-ph↗

arXiv2019

Theoretical foundations of the reduced relativistic gas in the cosmological perturbed context

The Reduced Relativistic Gas (RRG) is a simplified version of the ideal relativistic gas, which assumes that all particles have the same momentum magnitude. Although this is a very idealized situation, the resulting model preserves the phenomenology of Maxwell-Boltzmann distribution and, in some situations, can be described as a perfect fluid, without introducing large errors in both cosmological background and first-order perturbations. The perfect fluid description of RRG model was already used to study the warmness of dark matter, massive neutrinos and interaction of baryons and photons before recombination, showing very good agreement with previous works based on the full Einstein-Boltzmann system of equations. In order to understand these results and construct a more general and formal framework for RRG, we develop a theoretical description of first-order cosmological perturbations of RRG, based on a distribution function which encodes the simplifying assumption that all particles have the same momentum magnitude. The full set of Einstein-Boltzmann equations for RRG distribution are derived and quantities beyond the perfect fluid approximation are studied. Using RRG to describe warm dark matter, we show that, for particles with $m \sim \text{keV}$, the perfect fluid approximation is valid on scales $k < 10\, \text{h}/\text{Mpc}$, for most of the universe evolution. We also determine initial conditions for RRG in the early universe and study the evolution of potential in a toy model of universe composed only by RRG.

astro-ph.CO↗