Searcharxiv⌕ Search

arXiv subjects

P. H. R. S. Moraes

Publications and source records attributed to P. H. R. S. Moraes.

At least 37 records · Page 2Linked to original sources
arXiv2021

The maximum mass of neutron stars may be higher than expected: an inference from binary systems

We have analyzed in this work the updated sample of neutron star masses derived from the study of a variety of 96 binary systems containing at least one neutron star using Bayesian methods. After updating the multimodality of the distributions found in previous works, we determined the maximum mass implied by the sample using a robust truncation technique, with the result $m_{max} \sim 2.5-2.6 \, M_{\odot}$. We have checked that this mass is actually consistent by generating synthetic data and employing a Posterior Predictive Check. A comparison with seven published $m_{max}$ values inferred from the remnant of the NS-NS merger GW170817 was performed and the tension between the latter and the obtained $m_{max}$ value quantified. Finally, we performed a Local Outlier Factor test and verified that the result for $m_{max}$ encompasses the highest individual mass determinations with the possible exception of PSR J1748-2021B. The conclusion is that the whole distribution already points toward a high value of $m_{max}$, while several lower values derived from the NS-NS merger event are disfavored and incompatible with the higher binary system masses. A large $m_{max}$ naturally accommodates the lower mass component of the event GW190814 as a neutron star.

astro-ph.HE↗
arXiv2021

Traversable wormholes in the traceless $f(R,T)$ gravity

We present a traversable wormhole solution using the traceless $f(R,T)$ theory of gravity. In the $f(R,T)$ gravity, the Ricci scalar $R$ in the Einstein-Hilbert action is replaced by a function of $R$ and trace of the energy momentum tensor $T$. The traceless version of the $f(R,T)$ gravity gives rise to a possible wormhole geometry without need for "exotic matter", which violates the principle of causality. Using a physically plausible ansatz for the wormhole's shape function, the traceless field equations lead to compliance with the weak energy condition at very well defined intervals of the coupling constant $λ$ in the $f(R,T)=R+2λT$ form. Our solution leads to other well-behaved energy conditions considering some possible values of the parameter $ω$ in the equation of state $p_r=ωρ$, with $p_r$ being the radial pressure and $ρ$ the density. The energy conditions are obeyed in the ranges $λ< -4π$ and $ω> -1$. Through the calculation of the Volume Integral Quantifier, one sees that this wormholes can be traversable and respect the causality, since the amount of exotic matter in its interior can be arbitrarily small.

gr-qc↗
arXiv2020

Neutron stars in $f(\mathcal{R,T})$ gravity using realistic equations of state in the light of massive pulsars and GW170817

In this work we investigate neutron stars (NS) in $f(\mathcal{R,T})$ gravity for the case $R+2λ\mathcal{T}$, $\mathcal{R}$ is the Ricci scalar and $\mathcal{T}$ the trace of the energy-momentum tensor. The hydrostatic equilibrium equations are solved considering realistic equations of state (EsoS). The NS masses and radii obtained are subject to a joint constrain from massive pulsars and the event GW170817. The parameter $λ$ needs to be negative as in previous NS studies, however we found a minimum value for it. The value should be $|λ|\lesssim0.02$ and the reason for so small value in comparison with previous ones obtained with simpler EsoS is due to the existence of the NS crust. The pressure in theory of gravity depends on the inverse of the sound velocity $v_s$. Since, $v_s$ is low in the crust, $|λ|$ need to be very small. We found that the increment in the star mass is less than $1\%$, much smaller than previous ones obtained not considering the realistic stellar structure, and the star radius cannot become larger, its changes compared to GR is less than $3.6\%$ in all cases. The finding that using several relativistic and non-relativistic models the variation on the NS mass and radius are almost the same for all the EsoS, manifests that our results are insensitive to the high density part of the EsoS. It confirms that stellar mass and radii changes depend only on crust, where the EoS is essentially the same for all the models. The NS crust effect implying very small values of $|λ|$ does not depend on the theory's function chosen, since for any other one the hydrostatic equilibrium equation would always have the dependence $1/v_s$. Finally, we highlight that our results indicate that conclusions obtained from NS studies done in modified theories of gravity without using realistic EsoS that describe correctly the NS interior can be unreliable.

astro-ph.HE↗
arXiv2020

Modelling a $2.5 \, M_{\odot}$ Compact Star with Quark Matter

The detection of an unexpected $\sim 2.5 M_{\odot}$ component in the gravitational wave event GW190814 has puzzled the community of High-Energy astrophysicists, since in the absence of further information it is not clear whether this is the heaviest "neutron star" ever detected or either the lightest black hole known, of a kind absent in the local neighbourhood. We show in this work a few possibilities for a model of the former, in the framework of three different quark matter models with and without anisotropy in the interior pressure. As representatives of classes of "exotic" solutions, we show that even though the stellar sequences may reach this ballpark, it is difficult to fulfill simultaneously the constraint of the radius as measured by the NICER team for the pulsar PSR J0030+0451. Thus, and assuming both measurements stand, compact neutron stars can not be all made of self-bound quark matter, even within anisotropic solutions which boost the maximum mass well above the $\sim 2.5 M_{\odot}$ figure. We also point out that a very massive compact star will limit the absolute maximum matter density in the present Universe to be less than 6 times the nuclear saturation value.

astro-ph.HE↗
arXiv2020

Fast-roll solutions from two scalar fields inflation

One common approach for cosmic inflation consists in couple Einstein's gravity with a scalar field, often referred to inflaton field. In order to derive analytic simple scenarios, we usually work in the {\it slow-roll} regime. In such an approximation one considers the scalar field potentials to be nearly flat. It is possible to directly generalize such an approach for hybrid inflationary models, where the inflaton sector is composed of two or more scalar fields. However, the Friedmann equations and the equations of motion for such hybrid models are often hard to be analytically solved. Another recent path to describe cosmic inflation is through the so-called {\it fast-roll} regime, where one considers exactly flat potentials. Our purpose in this work is to obtain solutions for a hybrid inflaton model in the fast-roll regime. Cosmological interpretations through the behavior of Hubble, slow-roll, and equation of state parameters are also presented.

gr-qc↗
arXiv2020

Kinematic Constraints on Spatial Curvature from Supernovae Ia and Cosmic Chronometers

An approach to estimate the spatial curvature $Ω_k$ from data independently of dynamical models is suggested, through kinematic parameterizations of the comoving distance ($D_{C}(z)$) with third degree polynomial, of the Hubble parameter ($H(z)$) with a second degree polynomial and of the deceleration parameter ($q(z)$) with first order polynomial. All these parameterizations were done as function of redshift $z$. We used SNe Ia dataset from Pantheon compilation with 1048 distance moduli estimated in the range $0.01<z<2.3$ with systematic and statistical errors and a compilation of 31 $H(z)$ data estimated from cosmic chronometers. The spatial curvature found for $D_C(z)$ parametrization was $Ω_{k}=-0.03^{+0.24+0.56}_{-0.30-0.53}$. The parametrization for deceleration parameter $q(z)$ resulted in $Ω_{k}=-0.08^{+0.21+0.54}_{-0.27-0.45}$. The $H(z)$ parametrization has shown incompatibilities between $H(z)$ and SNe Ia data constraints, so these analyses were not combined. The $D_C(z)$ and $q(z)$ parametrizations are compatible with the spatially flat Universe as predicted by many inflation models and data from CMB. This type of analysis is very appealing as it avoids any bias because it does not depend on assumptions about the matter content of the Universe for estimating $Ω_k$.

astro-ph.CO↗
arXiv2020

Modeling Dark Matter Halos with Nonlinear Field Theories

In the present work, we adopt a nonlinear scalar field theory coupled to the gravity sector to model galactic dark matter. We found analytical solutions for the scalar field coupled to gravity in the Newtonian limit, assuming an isotropic spacetime and a field potential, with a position dependent form of the superpotential, which entails the nonlinear dynamics of the model with self-interactions. The model introduces a position dependent enhancement of the self-interaction of the scalar fields towards the galaxy center, and while going towards the galaxy border the interaction tends to vanish building a non self-interacting DM scenario. The developed approach is able to provide a reasonable analytical description of the rotation curves in both dwarf and low surface brightness late-type galaxies, with parameters associated with the dynamics of the scalar field.

gr-qc↗
arXiv2020

Inflation in $f(R,T)$ Gravity

The article presents modeling of inflationary scenarios for the first time in the $f(R,T)$ theory of gravity. We assume the $f(R,T)$ functional from to be $R + ηT$, where $R$ denotes the Ricci scalar, $T$ the trace of the energy-momentum tensor and $η$ the model parameter (constant). We first investigated an inflationary scenario where the inflation is driven purely due to geometric effects outside of GR. We found the inflation observables to be independent of the number of e-foldings in this setup. The computed value of the spectral index is consistent with latest Planck 2018 dataset while the scalar to tensor ratio is a bit higher. We then proceeded to analyze the behavior of an inflation driven by $f(R,T)$ gravity coupled with a real scalar field. By taking the slow-roll approximation, we generated interesting scenarios where a Klein Gordon potential leads to observationally consistent inflation observables. Our results makes it clear-cut that in addition to the Ricci scalar and scalar fields, the trace of energy momentum tensor also play a major role in driving inflationary scenarios.

gr-qc↗
arXiv2020

Hydrostatic equilibrium configurations of neutron stars in a non-minimal geometry-matter coupling theory of gravity

In this work we analyze hydrostatic equilibrium configurations of neutron stars in a non-minimal geometry-matter coupling (GMC) theory of gravity. We begin with the derivation of the hydrostatic equilibrium equations for the $f(R,L) $ gravity theory, where $R$ and $L$ are the Ricci scalar and Lagrangian of matter, respectively. We assume $f(R,L)=R/2+[1+σR]L$, with $σ$ constant. To describe matter inside neutron stars we assume the polytropic equation of state $p=K ρ^γ$, with $K$ and $γ= 5/3 $ being constants. We show that in this theory it is possible to reach the mass of massive pulsars such as PSR J2215+5135. As a feature of the GMC theory, very compact neutron stars with radius $\sim8$km and $M\sim 2.6M_\odot$ are stable, thus surpassing the Buchdal and Schwarzschild radius limits. Moreover, the referred stellar diameter is obtained within the range of observational data.

gr-qc↗
arXiv2020

Viability of the $R+e^T$ cosmology

We propose a new theoretical approach for a cosmological model, which starts from an exponential of the trace of the energy-momentum tensor-dependence on the gravitational action, to be summed to the Ricci scalar. We derive the referred field equations and Friedmann-like equations. We derive the scale factor, Hubble parameter and deceleration parameter, in terms of both time and redshift. In possession of those parameters in terms of the redshift, we confront their predictions with the observational Hubble dataset and the outcomes are pretty satisfactory so that the model can be seen as a new alternative to the cosmological constant problem. We also present the statefinder diagnostic and discuss profoundly the dynamical behaviour of the model.

gr-qc↗
arXiv2019

Cosmology from a running vacuum model driven by a scalar field

The possibility that the vacuum energy density $ρ_Λ$ is, indeed, varying in time, has been investigated lately from the running vacuum models perspective. Motivated by such models, in the present work, we relate the decaying vacuum energy $ρ_Λ(t)$ with a scalar field $ϕ$. We derive the equations of motion from such a premise and implement the first-order formalism in order to obtain analytical solutions to the cosmological parameters. We show that those are in agreement with recent Planck observational data. We discuss the physical consequences of having $ρ_Λ(t)$ related to $ϕ$.

gr-qc↗
arXiv2019

Cosmology from an exponential dependence on the trace of the energy-momentum tensor -- Numerical approach and cosmological tests

In this paper, we present the cosmological scenario obtained from $f(R,T)$ gravity by using an exponential dependence on the trace of the energy-momentum tensor. With a numerical approach applied to the equations of motion, we show several precise fits and the respective cosmological consequences. As a matter of completeness, we also analyzed cosmological scenarios where this new version of $f(R,T)$ is coupled with a real scalar field. In order to find analytical cosmological parameters, we used a slow-roll approximation for the evolution of the scalar field. This approximation allowed us to derived the Hubble and the deceleration parameters whose time evolutions describe the actual phase of accelerated expansion, and corroborate with our numerical investigations. Therefore, the analytical parameters unveil the viability of this proposal for $f(R,T)$ in the presence of an inflaton field.

gr-qc↗
arXiv2019

Strange stars in energy-momentum-conserved $f(R,T)$ gravity

For the accurate understanding of compact objects such as neutron stars and strange stars, the Tolmann-Openheimer-Volkof (TOV) equation has proved to be of great use. Hence, in this work, we obtain the TOV equation for the energy-momentum-conserved $f(R,T)$ theory of gravity to study strange quark stars. The $f(R,T)$ theory is important, especially in cosmology, because it solves certain incompleteness of the standard model. In general, there is no intrinsic conservation of the energy-momentum tensor in the $f(R,T)$ gravity. Since this conservation is important in the astrophysical context, we impose the condition $\nabla T_{μν}=0$, so that we obtain a function $f(R,T)$ that implies conservation. This choice of a function $f(R,T)$ that conserves the momentum-energy tensor gives rise to a strong link between gravity and the microphysics of the compact object. We obtain the TOV by taking into account a linear equation of state to describe the matter inside strange stars, such as $p=ωρ$ and the MIT bag model $p=ω(ρ-4B)$. With these assumptions it was possible to derive macroscopic properties of these objects.

gr-qc↗
arXiv2019

An exponential shape function for wormholes in modified gravity

Here, we propose a new exponential shape function in wormhole geometry within modified gravity. The energy conditions and equation of state parameter are obtained. The radial and tangential null energy conditions are validated, as well as the weak energy condition, which indicates the absence of exotic matter due to modified gravity allied with such a new proposal.

physics.gen-ph↗
arXiv2019

A unique equation of state for the universe evolution from AdS$_5$ space-time

We apply the Induced Matter Model to a five-dimensional metric. For the case with null cosmological constant, we obtain a solution able to describe the radiation-dominated era of the universe. The positive $Λ$ case yields a bounce cosmological model. In the negative five-dimensional cosmological constant case, the scale factor is obtained as $a(t)\sim\sqrt[]{\sinh t}$, which is able to describe not only the late-time cosmic acceleration but also the non-accelerated stages of the cosmic expansion in a continuous form. This solution together with the extra-dimensional scale factor solution yields the material content of the model to be remarkably related through an equation of state analogous to the renowned MIT bag model equation of state for quark matter $p=(ρ-4B)/3$. In our case, $ρ=ρ_m+B$, with $ρ_m$ being the energy density of relativistic and non-relativistic matter and $B=\ Λ /16π$ represents the bag energy constant, which plays the role of the dark energy in the four-dimensional universe, with $Λ$ being the cosmological constant of the AdS$_5$ space-time. Our model satisfactorily fits the observational data for the low redshift sample of the experimental measurement of the Hubble parameter, which resulted in $H_0=72.2^{+5.3}_{-5.5}$km s$^{-1}$ Mpc$^{-1}$.

gr-qc↗
arXiv2019

Wormholes in exponential $f(R,T)$ gravity

Alternative gravity is nowadays an extremely important tool to address some persistent observational issues, such as the dark sector of the universe. They can also be applied to stellar astrophysics, leading to outcomes one step ahead of those obtained through General Relativity. In the present article we test a novel $f(R,T)$ gravity model within the physics and geometry of wormholes. The $f(R,T)$ gravity is a reputed alternative gravity theory in which the Ricci scalar $R$ in the Einstein-Hilbert gravitational lagrangian is replaced by a general function of $R$ and $T$, namely $f(R,T)$, with $T$ representing the trace of the energy-momentum tensor. We propose, for the first time in the literature, an exponential form for the dependence of the theory on $T$. We derive the field equations as well as the non-continuity equation and solve those to wormhole metric and energy-momentum tensor. The importance of applying alternative gravity to wormholes is that through these theories it might be possible to obtain wormhole solutions satisfying the energy conditions, departing from General Relativity well-known outcomes. In this article, we indeed show that it is possible to obtain wormhole solutions satisfying the energy conditions in the exponential $f(R,T)$ gravity. Naturally, there is still a lot to do with this model, as cosmological, galactical and stellar astrophysics applications, and the reader is strongly encouraged to do so, but, anyhow, one can see the present outcomes as a good indicative for the theory.

gr-qc↗
arXiv2019

The trace of the trace of the energy-momentum tensor-dependent Einstein's field equations

The $f(R,T)$ gravity field equations depend generically on both the Ricci scalar $R$ and trace of the energy-momentum tensor $T$. Within the assumption of perfect fluids, the theory carries an arbitrariness regarding the choice of the matter lagrangian density $\mathcal{L}$, not uniquely defined. Such an arbitrariness can be evaded by working with the trace of the theory field equations. From such an equation, one can obtain a form for $\mathcal{L}$, which does not carry the arbitrariness. The obtained form for $\mathcal{L}$ shows that the $f(R,T)$ gravity is unimodular. A new version of the theory is, therefore, presented and forthcoming applications are expected.

gr-qc↗