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R. Valentim

Publications and source records attributed to R. Valentim.

13 recordsLinked to original sources
arXiv2026

Kinematic Reconstruction of $Λ(t)$CDM Models

In this work, we have \textbf{analysed} two kinematic parametrizations for $Λ(t)$CDM models, namely, the linear expansions $Λ(z)=Λ_0+Λ_1z$ and $Q(z)=Q_0+Q_1z$, where $Q$ is the interaction term. In the case of the $Q(z)$ parametrization, we have also tested the particular case of a constant interaction term, $Q(z)=Q_0$. In order to constrain the free parameters of these models, we have used Cosmic Chronometers (CC), SNe Ia data (Pantheon+\&SH0ES) and BAO data. As a general result, we have found weak constraints over the free parameters of the analysed models. In the case of $Λ(z)$, we have found for the $Λ$ variation parameter, $Ω_{\Lambda1}\equiv\frac{Λ_1}{3H_0^2}=0.02\pm0.14$. In the case of the $Q(z)$ parametrization, we have worked with the dimensionless interaction term $\gQ(z)\equiv\frac{8πGQ(z)}{3H_0^3}$, from which we have found $\gQ_0 = 2.2 \pm 2.7$ and $\gQ_1 = -6.2 \pm 7.6$. In the particular case of a constant interaction term, we have found $\gQ_0 = 0.18 \pm 0.7$. All these constraints are at 68\% c.l. The constraints we have obtained are compatible with the standard $Λ$CDM model, although still providing a large margin for $Λ$ variation.

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

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↗
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

Thermodynamic constraints on matter creation models

Entropy is a fundamental concept from Thermodynamics and it can be used to study models on context of Creation Cold Dark Matter (CCDM). From conditions on the first ($\dot{S}\geq0$)\footnote{Throughout the present work we will use dots to indicate time derivatives and dashes to indicate derivatives with respect to scale factor.} and second order ($\ddot{S}<0$) time derivatives of total entropy in the initial expansion of Sitter through the radiation and matter eras until the end of Sitter expansion, it is possible to estimate the intervals of parameters. The total entropy ($S_{t}$) is calculated as sum of the entropy at all eras ($S_γ$ and $S_{m}$) plus the entropy of the event horizon ($S_h$). This term derives from the Holographic Principle where it suggests that all information is contained on the observable horizon. The main feature of this method for these models are that thermodynamic equilibrium is reached in a final de Sitter era. Total entropy of the universe is calculated with three terms: apparent horizon ($S_{h}$), entropy of matter ($S_{m}$) and entropy of radiation ($S_γ$). This analysis allows to estimate intervals of parameters of CCDM models.

gr-qc↗
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

Gaussian Process Estimation of Transition Redshift

This paper aims to put constraints on the transition redshift $z_t$, which determines the onset of cosmic acceleration, in cosmological-model independent frameworks. In order to do that, we use the non-parametric Gaussian Process method with $H(z)$ and SNe Ia data. The deceleration parameter reconstruction from $H(z)$ data yields $z_t=0.59^{+0.12}_{-0.11}$. The reconstruction from SNe Ia data assumes spatial flatness and yields $z_t=0.683^{+0.11}_{-0.082}$. These results were found with a Gaussian kernel and we show that they are consistent with two other kernel choices.

astro-ph.CO↗
arXiv2017

Bayesian correction of $H(z)$ data uncertainties

We compile 41 $H(z)$ data from literature and use them to constrain O$Λ$CDM and flat $Λ$CDM parameters. We show that the available $H(z)$ suffers from uncertainties overestimation and propose a Bayesian method to reduce them. As a result of this method, using $H(z)$ only, we find, in the context of O$Λ$CDM, $H_0=69.5\pm2.5\mathrm{\,km\,s^{-1}Mpc^{-1}}$, $Ω_m=0.242\pm0.036$ and $Ω_Λ=0.68\pm0.14$. In the context of flat $Λ$CDM model, we have found $H_0=70.4\pm1.2\mathrm{\,km\,s^{-1}Mpc^{-1}}$ and $Ω_m=0.256\pm0.014$. This corresponds to an uncertainty reduction of up to 30\% when compared to the uncorrected analysis in both cases.

astro-ph.CO↗
arXiv2017

Evidence for two neutrinos bursts from SN1987A

The SN1987A in the Giant Magellanic Cloud was an amazing and extraordinary event because it was detected in real time for different neutrinos experiments ($ν$s) around the world. Approximate $\sim25$ events were observed in three different experiments: Kamiokande II (KII) $\sim 12$, Irvine-Michigan-Brookhaven (IMB) $\sim 8$ e Baksan $\sim 5$, plus a contrived burst at Mont Blanc (Liquid Scintillator Detector - LSD) later dismissed because of energetic requirements (Aglietta et al. 1988). The neutrinos have an important play role into the neutron star newborn: at the moment when the supernova explodes the compact object remnant is freezing by neutrinos ($\sim99\%$ energy is lost in the few seconds of the explosion). The work is motivated by neutrinos' event in relation arrival times where there is a temporal gap between set of events ($\sim6\mbox{s}$). The first part of dataset came from the ordinary mechanism of freezing and the second part suggests different mechanism of neutrinos production. We tested two models of cooling for neutrinos from SN1987A: 1st an exponential cooling is an ordinary model of cooling and 2nd a two-step temperature model that it considers two bursts separated with temporal gap. Our analysis was done with Bayesian tools ({\it Bayesian Information Criterion} - BIC) The result showed strong evidence in favor of a two-step model against one single exponential cooling ($\ln\mbox{B}_{ij} > 5.0$), and suggests the existence of two neutrino bursts at the moment the neutron star was born.

astro-ph.HE↗
arXiv2017

Bayesian analysis of CCDM Models

Creation of Cold Dark Matter (CCDM), in the context of Einstein Field Equations, leads to negative creation pressure, which can be used to explain the accelerated expansion of the Universe. In this work we tested six different spatially flat models for matter creation using statistical tools, at light of SN Ia data: Akaike Information Criterion (AIC), Bayesian Information Criterion (BIC) and Bayesian Evidence (BE). These approaches allow to compare models considering goodness of fit and number of free parameters, penalizing excess of complexity. We find that JO model is slightly favoured over LJO/$Λ$CDM model, however, neither of these, nor $Γ=3αH_0$ model can be discarded from the current analysis. Three other scenarios are discarded either from poor fitting, either from excess of free parameters.

astro-ph.CO↗
arXiv2016

The masses of neutron stars

We present in this article an overview of the problem of neutron star masses. After a brief appraisal of the methods employed to determine the masses of neutron stars in binary systems, the existing sample of measured masses is presented, with a highlight on some very well-determined cases. We discuss the analysis made to uncover the underlying distribution and a few robust results that stand out from them. The issues related to some particular groups of neutron stars originated from different channels of stellar evolution are shown. Our conclusions are that last century's paradigm that there a single, $1.4 M_{\odot}$ scale is too simple. A bimodal or even more complex distribution is actually present. It is confirmed that some neutron stars have masses of $\sim 2 M_{\odot}$, and, while there is still no firm conclusion on the maximum and minimum values produced in nature, the field has entered a mature stage in which all these and related questions can soon be given an answer.

astro-ph.HE↗
arXiv2011

On the mass distribution of neutron stars

The distribution of masses for neutron stars is analyzed using the Bayesian statistical inference, evaluating the likelihood of proposed gaussian peaks by using fifty-four measured points obtained in a variety of systems. The results strongly suggest the existence of a bimodal distribution of the masses, with the first peak around $1.37 {M_{\odot}}$, and a much wider second peak at $1.73 {M_{\odot}}$. The results support earlier views related to the different evolutionary histories of the members for the first two peaks, which produces a natural separation (even if no attempt to "label" the systems has been made here), and argues against the single-mass scale viewpoint. The bimodal distribution can also accommodate the recent findings of $\sim M_{\odot}$ masses quite naturally. Finally, we explore the existence of a subgroup around $1.25 {M_{\odot}}$, finding weak, if any, evidence for it. This recently claimed low-mass subgroup, possibly related to $O-Mg-Ne$ core collapse events, has a monotonically decreasing likelihood and does not stand out clearly from the rest of the sample.

astro-ph.SR↗