arXiv subjects

S. H. Pereira

Publications and source records attributed to S. H. Pereira.

At least 19 recordsLinked to original sources

arXiv2026

From Hubble to Snap Parameters: A Gaussian Process Reconstruction

By using recent $H(z)$ and SNe Ia data, we reconstruct the evolution of kinematic parameters $H(z)$, $q(z)$, jerk and snap, using a model-independent, non-parametric method, namely, the Gaussian Processes. Throughout the present analysis, we have allowed for a spatial curvature prior, based on Planck 18 constraints. In the case of SNe Ia, we modify a python package (GaPP) in order to obtain the reconstruction of the fourth derivative of a function, thereby allowing us to obtain the snap from comoving distances. Furthermore, using a method of importance sampling, we combine $H(z)$ and SNe Ia reconstructions in order to find joint constraints for the kinematic parameters. We find for the current values of the parameters: $H_0 =67.2 \pm 6.2$ km/s/Mpc, $q_0 = -0.54^{+0.06}_{-0.05}$, $j_0=0.94^{+0.20}_{-0.18}$, $s_0=-0.62^{+0.26}_{-0.25}$ at 1$σ$ c.l. We find that these reconstructions are compatible with the predictions from flat $Λ$CDM model, at least for 2$σ$ confidence intervals.

astro-ph.CO↗

arXiv2026

Revisiting the Constancy of the Speed of Light: Galaxy Cluster Mass Bias Implications

In recent years, improvements in galaxy cluster observations have enabled a variety of tests of fundamental physics using these systems. In this work, we test the constancy of the speed of light, $c$, by combining X-ray gas mass fraction measurements from galaxy clusters with SNe Ia luminosity distance measurements from Pantheon+. We adopt the SH0ES prior on $H_0$ and the $Ω_b/Ω_m$ ratio from galaxy clustering observations, thereby minimizing the dependence of our analysis on any specific cosmological model. We explore different assumptions for the cluster mass calibration (mass bias), including \textsc{CLASH}, \textsc{CCCP}, and Planck-based estimates. We find no deviation from a constant $c$ when adopting \textsc{CLASH} or \textsc{CCCP} priors, while Planck-based calibration yields a mild tension, with the hypothesis of constant $c$ being only marginally consistent at the $2σ$ level, indicating a non-negligible sensitivity of the results to the adopted calibration scheme.

astro-ph.CO↗

arXiv2026

Dynamical dark energy from Kretschmann scalar at low redshifts

In this work, we present a cosmological model in which the cosmological constant term is replaced by the Kretschmann scalar at the level of the action. In this way, it becomes possible to implement a model of dynamical dark energy. After constraining the free parameters using observational data from supernovae and cosmic chronometers, we show that the model provides a good fit to the observational data. In particular, we show that, at least at low redshifts, the behavior of the equation-of-state parameter $w(z)$ closely reproduces that obtained in phenomenological models that have been recently studied based on the latest observational data from the DESI collaboration. Likewise, the present model also indicates the occurrence of a phantom-crossing regime.

astro-ph.CO↗

arXiv2025

Kinematic reconstruction of torsion as dark energy in Friedmann cosmology

In this paper we study the effects of torsion of space-time in the expansion of the Universe as a candidate to dark energy. The analysis is done by reconstructing the torsion function along cosmic evolution by using observational data of Supernovae type Ia, Hubble parameter {and Baryon Acoustic Oscillation} measurements. We have used a kinematic model for the parameterization of the comoving distance and the Hubble parameter, then the free parameters of the models are constrained by observational data. The reconstruction of the torsion function is obtained directly from the data, using the kinematic parameterizations.

gr-qc↗

arXiv2025

Can decaying vacuum solve the H_0 Tension?

In the present work we analyze two different models of interaction between dark energy and dark matter, also known as vacuum decay models or $Λ(t)$CDM models. In both models, when the $H_0$ parameter is constrained by the Planck distance priors, its value is compatible with a higher value of $H_0$, in agreement with SH0ES data, while simultaneously reducing the values of $Ω_m$ and $Ω_b$. In both models, we find $H_0=73.1\pm0.86$ at 68\% c.l. by combining Planck+SH0ES data. We also find the decay parameter to be $\varepsilon=0.0197^{+0.0032}_{-0.0027}$ for one model and $\varepsilon=0.0203\pm0.0034$ for the other one. From these analyses, a noninteracting model is excluded at least at $6σ$ c.l.! This shows that these types of models are promising in solving or at least alleviating the $H_0$ tension problem. Our analysis also shows a preference for the decay of vacuum into dark matter, in agreement to thermodynamic analyses.

astro-ph.CO↗

arXiv2023

Can the Universe decelerate in the future?

The possibility of an expanding decelerating Universe in the distant future is investigated in the context of a quintessence scalar field cosmology. Such a conceivable evolution is tested against SNe Ia and $H(z)$ cosmic chronometers data, and also through a model independent method based on Gaussian Processes. The scalar field model is an extension of the exponential Ratra-Peebles (RP) quintessential cosmology whose potential now depends on a pair of parameters ($α, λ)$ and predicts a decelerated expansion in the future. Different from RP approach the $α$ parameter allows for a decelerating cosmology in the future thereby frustrating the inevitable evolution for a de Sitter Cosmology as predicted by the cosmic concordance model ($Λ$CDM). The statistical model analysis is updated with the most recent SNe Ia and $H(z)$ data thereby obtaining $H_0 = 68.6\pm3.7$ km/s/Mpc, $Ω_{\Phi0} = 0.735^{+0.083}_{-0.069} $, $α< 6.56$ and $λ< 0.879 $ (at $2σ$ c.l.). It is also found that the extended RP model allows for a future deceleration both for $H(z)$ and SNe Ia data. In the (model-independent) Gaussian Processes analysis, however, future deceleration is allowed only in the case of $H(z)$ data.

astro-ph.CO↗

arXiv2023

A Method for Obtaining Cosmological Models Consistency Relations and Gaussian Processes Testing

In the present work, we apply consistency relation tests to several cosmological models, including the flat and non-flat $Λ$CDM models, as well as the flat XCDM model. The analysis uses a non-parametric Gaussian Processes method to reconstruct various cosmological quantities of interest, such as the Hubble parameter $H(z)$ and its derivatives from $H(z)$ data, as well as the comoving distance and its derivatives from SNe Ia data. We construct consistency relations from these quantities which should be valid only in the context of each model and test them with the current data. We were able to find a general method of constructing such consistency relations in the context of $H(z)$ reconstruction. In the case of comoving distance reconstruction, there were not a general method of constructing such relations and this work had to write an specific consistency relation for each model. From $H(z)$ data, we have analyzed consistency relations for all the three above mentioned models, while for SNe Ia data we have analyzed consistency relations only for flat and non-flat $Λ$CDM models. Concerning the flat $Λ$CDM model, some inconsistency was found, at more than $2σ$ c.l., with the $H(z)$ data in the interval $1.8\lesssim z\lesssim2.4$, while the other models were all consistent at this c.l. Concerning the SNe Ia data, the flat $Λ$CDM model was consistent in the $0<z<2.5$ interval, at $1σ$ c.l., while the nonflat $Λ$CDM model was consistent in the same interval, at 2$σ$ c.l.

astro-ph.CO↗

arXiv2022

Observational constraints on varying fundamental constants in a minimal CPC model

A minimal model based on the Co-varying Physical Couplings (CPC) framework for gravity is proposed. The CPC framework is based on the assumptions of a metric-compatible four-dimensional Riemannian manifold where a covariantly conserved stress-energy tensor acts as source of the field equations which are formally the same as Einstein field equations, but where the couplings $\{ G, c,Λ\}$ are allowed to vary simultaneously. The minimal CPC model takes $Λ$ as a genuine constant while $c$ and $G$ vary in an entangled way that is consistent with Bianchi identity and the aforementioned assumptions. The model is constrained using the most recent galaxy cluster gas mass fraction observational data. Our result indicates that the functions $c(z)$ and $G\left(z\right)=G_{0}\left(c/c_{0}\right)^{4}$ are compatible with constant couplings for the three different parameterizations of $c=c(z)$ adopted here.

gr-qc↗

arXiv2022

Determination of the Kinematic Parameters from SNe Ia and Cosmic Chronometers

In this work, by assuming a spatially flat Universe, we have tested 8 kinematic parametrization models with $H(z)$ data from Cosmic Chronometers and SNe Ia from Pantheon compilation. Our aim is obtain the current values for the Hubble constant ($H_0$), deceleration parameter ($q_0$), jerk ($j_0$) and snap ($s_0$) parameters independently from a dynamical model. By using a Bayesian model comparison, three models are favoured: a model with the deceleration parameter ($q$) linearly dependent on the redshift, $q$ linearly dependent on the scale factor and a model with a constant jerk. The model with constant jerk is slightly favoured by this analysis, furnishing $H_0=68.8^{+3.7}_{-3.6}$ km/s/Mpc, $q_0=-0.58\pm0.13$, $j_0=1.15^{+0.56}_{-0.53}$ and $s_0=-0.25^{+0.40}_{-0.30}$. The other models are compatible with the constant jerk model, except for the snap parameter, where we have found $s_0=4.0^{+3.4}_{-3.0}$ for the model with $q$ linearly dependent on the scale factor. (All uncertainties in the Abstract correspond to 95\% c.l.).

astro-ph.CO↗

arXiv2022

Gaussian Processes Reconstruction of the Dark Energy Potential

Scalar Fields (SF) have emerged as natural candidates for dark energy as quintessential or phantom fields, as they are the main ingredient of inflation theories. Instead of assuming some form for the scalar field potential, however, this work reconstructs the SF potential directly from observational data, namely, \textbf{Hubble and SNe Ia data}. We show that two popular forms for the SF potentials, namely, the power-law and the quadratic free-field, are compatible with the reconstructions thus obtained, at least for some choices of the priors of the matter density and curvature parameters and for some redshift intervals.

astro-ph.CO↗

arXiv2022

A pilot study on canonical gravity with mass dimension one fermions

The mass dimension one (MDO) fermionic field is built on a complete set of dual-helicity eigenspinors of the charge conjugation operator, which obeys the statistic of Fermi-Dirac. These spinors are a potential candidate for the description of dark matter. The redefinition of the dual structure of this object holds a local adjacent theory and satisfies complete Lorentz invariance. In this work, we investigate important aspects of the interaction of this fermion with gravity in the light of a canonical formulation in ADM formalism. We construct an action via tetrad fields using a manifold on a family of space-like surfaces $Σ_t$ that carries the MDO matter field, and additionally we propose a condition for the action to have a term associated with dark energy. We found the Hamiltonian and diffeomorphism constraints at the classical level for gravitational dynamics with the immersion of this material content in space-time, which leads us to the interpretation of the contribution of dark matter energy density, parallel to the lapse function of foliation and its directional flux of energy density in the hypersurface of manifold.

gr-qc↗

arXiv2022

Fermionic wave functions and Grassmann fields as possible sources of dark energy

We study a cosmological model with a fermionic field which can be interpreted as a source of dark energy in the universe. Two different approaches were considered, the first one with a massless fermionic field represented by a standard wave-function and the second one where a massive field is a Grassmann variable. {The first case naturally reduces to a XCDM model with a constant equation of state parameter, while the last case reproduces a $w(z)$CDM model for a massive field}, and in the massless limit, the intrinsic grassmannian property of the field leads always to a vacuum equation of state parameter, irrespective the specific form of the potential. Both cases leads to a dark energy contribution of the fermionic sector. The models are totally compatible with recent cosmological data from Supernovae, BAO and Hubble parameter measurements. A brief study of linear evolution of density perturbations shows that some of the small scale problems related to standard model can be at least alleviated.

physics.gen-ph↗

arXiv2022

Testing a varying-$Λ$ model for dark energy within Co-varying Physical Couplings framework

The Co-varying Physical Couplings (CPC) framework is a modified gravity set up assuming Einstein Field Equations wherein the quantities $\{G,c,Λ\}$ are promoted to space-time functions. Bianchi identity and the requirement of stress-energy tensor conservation entangle the possible variations of the couplings $\{G,c,Λ\}$, which are forced to co-vary as dictated by the General Constraint (GC). In this paper we explore a cosmological model wherein $G$, $c$ and $Λ$ are functions of the redshift respecting the GC of the CPC framework. We assume a linear parametrization of $Λ$ in terms of the scale factor $a$. We use the ansatz $\dot{G}/G = σ\left( \dot{c}/c \right)$ with $σ=$ constant to deduce the functional forms of $c=c(z)$ and $G=G(z)$. We show that this varying-$\{G,c,Λ\}$ model fits SNe Ia data and $H(z)$ data with $σ= 3$. The model parameters can be constrained to describe dark energy at the background level.

gr-qc↗

arXiv2022

Degeneracy pressure of mass dimension one fermionic fields and the dark matter halo of galaxies

In this paper, the recently proposed mass dimension one fermionic field is supposed to be responsible for the dark matter halo around galactic nuclei, through the quantum degeneracy pressure effect of the field. It will be showed that the mass-ratio relation for dwarf galaxies can be well explained for a particle dark matter mass of about $100 - 200$eV. For a large galaxy, as Milky Way, the observational data for rotation curve can be well reproduced for a particle mass of about 23eV, with the addition of other substructures.

astro-ph.CO↗

arXiv2022

Can dark matter-dark energy interaction alleviate the Cosmic Coincidence Problem?

In this paper we study a model of interacting dark energy - dark matter where the ratio between these components is not constant, changing from early to late times in such a way that the model can solve or alleviate the cosmic coincidence problem (CP). The interaction arises from an assumed relation of the form $ρ_x \propto ρ_d^α$, where $ρ_x$ and $ρ_d$ are the energy densities of dark energy and dark matter components, respectively, and $α$ is a free parameter. For a dark energy equation of state parameter $w=-1$ we found that, if $α=0$, the standard $Λ$CDM model is recovered, where the coincidence problem is unsolved. For $0<α<1$, the CP would be alleviated and for $α\sim 1$, the CP would be solved. The dark energy component is analyzed with both $w=-1$ and $w \neq -1$. Using Supernovae type Ia and Hubble parameter data constraints, in the case $w=-1$ we find $α=0.109^{+0.062}_{-0.072}$ at 68% C.L., and the CP is alleviated. For $w\neq -1$, a degeneracy arises on the $w$ - $α$ plane. In order to break such degeneracy we add cosmic microwave background distance priors and baryonic acoustic oscillations data to the constraints, yielding $α=-0.075\pm 0.046$ at 68% C.L.. In this case we find that the CP is not alleviated even for 2$σ$ interval for $α$. Furthermore, this last model is discarded against flat $Λ$CDM according to BIC analysis.

astro-ph.CO↗

arXiv2022

Dark matter from torsion in Friedmann cosmology

A cosmological model in an Einstein-Cartan framework endowed with torsion is studied. For a torsion function assumed to be proportional to Hubble expansion function, namely $ϕ=-αH$, the contribution of torsion function as a dark matter component is studied in two different approaches. In the first one, the total matter energy density is altered by torsion coupling $α$, giving rise to an effective dark matter and cosmological constant terms that reproduce quite well the flat cosmic concordance model. In the second approach, starting with just standard baryonic matter plus a cosmological constant term, it is obtained that the coupling of torsion with baryons and cosmological constant term naturally gives rise to a dark matter contribution, together a modified cosmological term. In this model the dark matter sector can be interpreted as an effective coupling of the torsion function with the ordinary baryonic matter and cosmological constant. Finally, it is shown that both models are totally compatible with recent cosmological data from Supernovae and Hubble parameter measurements.

gr-qc↗

arXiv2022

A test of the standard dark matter density evolution law using galaxy clusters and cosmic chronometers

In this letter, we implement a test of the standard law for the dark matter density evolution. For this purpose, only a flat universe and the validity of the FRW metric are assumed. A deformed dark matter density evolution law is considered, given by $ρ_c(z) \propto (1+z)^{3+ε}$, and constraints on $ε$ are obtained by using galaxy cluster gas mass fractions, and cosmic chronometers measurements. We find that $ε=0$ within 2$σ$ c.l., in full agreement with other recent analyses.

astro-ph.CO↗

arXiv2021

A search for the variation of speed of light using galaxy cluster gas mass fraction measurements

In this paper, we implement a new method to test the invariance of the speed of light ($c$) as a function of redshift, by combining the measurements of galaxy cluster gas mass fraction, $H(z)$ from cosmic chronometers, and Type-Ia supernovae (SNe Ia). In our analyses, we consider both a constant depletion factor (which corresponds to the ratio by which the cluster gas mass fraction is depleted with respect to the universal baryonic mean) and one varying with redshift. We also consider the influence of different $H_0$ estimates on our results. We look for a variation of $c$, given by $c(z)=c_0(1+c_1z)$. We find a degeneracy between our final results on $c$ variation and the assumptions on the gas mass fraction depletion factor. Most of our analyses indicate negligible variation of the speed of light.

astro-ph.CO↗