arXiv subjects

Ribamar R. R. Reis

Publications and source records attributed to Ribamar R. R. Reis.

At least 19 recordsLinked to original sources

arXiv2026

Modeling the probability distribution for cosmological analysis with photometrically classified samples

In this work we investigated methods for the accurate and efficient incorporation of photometrically classified supernovae into cosmological analyses, and to assess the impact of the additional uncertainty associated with this procedure on the ability of Type Ia supernovae (SNeIa) tests to place constraints on cosmological models. We proposed a simplified likelihood, in which the contamination is described as a redshift dependent change in the mean of the usually assumed Gaussian distribution, and we tested this hypothesis against the usual two-component approach, based on the BEAMS framework. Using the latest version of the DES supernova sample, dubbed DES-Dovekie, we compared the results when using type probabilities from different classifiers, such as SNIRF and SCONE, and applying different cuts on these probabilities. We show that the new model is strongly favored by the Bayes factor, when compared with the current one, for all configurations, allowing an improvement on the constraining power of photometric supernova data.

astro-ph.CO↗

arXiv2026

Influence of photometric galaxies redshift distribution in BAO estimation

In the present study, we use the DES Y3 catalog of LRG to incorporate the realistic galaxies' redshift Probability Distribution Function(PDF) into the correlation function cosmological model. We used four different photo-z estimators ANNz2, BPZ, ENF, and DNF to compare how they affect the BAO feature constraint. Moreover, each algorithm included two sample selections based on distinct PDF shapes; one where the PDFs are nearly Gaussian and another opting for the least noisy PDFs with a pronounced peak. Following a parametrised model, we estimated the shift parameter $α$ for the ANNz2 three cuts and the DNF full samples. We found that the BAO from ANNz2 Gaussian sample selection is closer to the fiducial Planck 18 $Λ$CDM model. Later, we computed the correlation function $ξ_perp(z_p)$ by getting the bin pairs transversal to each other using CAMB. The kernel window function is the $f(z|z_p)$ which is the selection of the PDF value when the photometric redshift is nearly the same as the spectroscopic redshift estimated by the matched spectroscopic sample. For compatible z eff, we concluded that the shape of the galaxy redshift PDF could shift the BAO feature position either by including the PDF in the model or not. We also learnt that, given the same spectroscopic sample, ANNz2 estimator with its respective selection samples outperforms other estimators for most parameters examined. When the dark energy equation of state parameter, $w_0$, is considered, DNF emerges as the optimal algorithm, provided it has sufficient statistical data. Our analysis recommends that upcoming photo-z survey collaborations incorporate multiple photo-z estimation algorithms in their cosmological inference process; this approach will facilitate comprehension of systematic effects on various parameters.

astro-ph.CO↗

arXiv2025

Baryon Acoustic Oscillations in tomographic Angular Density and Redshift Fluctuations

In this work we examine the baryon acoustic oscillations (BAO) in 2D angular and redshift space $\{θ, Δz\}$, with $Δz$ denoting the redshift difference between two given angular shells. We thus work in the context of tomographic analyses of the large scale structure (LSS) where data are sliced in different redshift shells and constraints on Cosmology are extracted from the auto and cross-angular spectra of two different probes, namely the standard galaxy angular density fluctuations (ADF, or 2D clustering), and the galaxy angular redshift fluctuations (ARF). For these two observables we study by first time how the BAO peak arises in the $\{θ, Δz\}$ plane. Despite being a weak feature (particularly for $Δz \neq 0$), a Fisher forecast analysis shows that, a priori, most of the information on cosmological and galaxy bias parameters is carried by the BAO features in shell auto- and cross-angular power spectra. The same study shows that a joint probe analysis (ADF+ARF) increases the Fisher determinant associated to cosmological parameters such as $H_0$ or the Dark Energy Chevallier-Polarski-Linder (CPL) parameters $\{w_0,w_a\}$ by at least an order of magnitude. We also study how the Fisher information on cosmological and galaxy bias-related parameters behaves under different redshift shell configurations: including cross-correlations to neighbour shells extending up to $(Δz)^{\rm tot}\sim 0.6$ ($(Δz)^{\rm tot}\sim 0.4$) for ADF (ARF) is required for Fisher information to converge. At the same time, configurations using narrow shell widths ($σ_z \leq 0.02$) preserve the cosmological information associated to peculiar velocities and typically yield Fisher determinants that are about two orders of magnitudes larger than for wider shell ($σ_z>0.02$) configurations.

astro-ph.CO↗

arXiv2025

Pz Cats: Photometric redshift catalogs based on DES Y3 BAO sample

Over the years, photometric redshift estimation (photo-z) has advanced through various methods. This study evaluates four distinct photo-z estimators-ANNz2, BPZ, ENF, and DNF-using the Dark Energy Survey Y3 BAO Sample. Unlike most studies, we explore selecting optimal galaxies based on their redshift Probability Distribution Function (PDF) by either reducing noise or identifying those approximating a Gaussian distribution. We cross-matched 25,760 galaxies drawn from four spectroscopic surveys with the photo-z sample to comprehend redshift bias and its 68th percentile $σ_{68}$. The lowest $σ$ for all estimators was found in the range $0.79<z_p<0.85$. Among the estimators, DMF exhibited the greatest bias, while ENF, ANNz2, and BPZ showed decreased precision outside 0.7 to 0.9 redshift range. To select galaxies with minimal bias, ANNz2 emerged as the most reliable algorithm across all criteria. PDFs selection significantly improves colour representation over the spectroscopic sample, underscoring the role of magnitude space in selection. While ANNz2 achieved superior precision, ENF poorly selected Gaussian PDFs, leaving few galaxies for LSS evaluation. Despite smooth PDFs, catastrophic redshift errors were present. Though DNF had the poorest precision, it offered enough galaxies for cosmological use. Subsampling galaxies with secondary peaks less than 30\% of the main peak height, termed Small Peaks, showed ANNz2 excelled. The catalogs produced have been published as Pz Cats within the ZENODO repository.

astro-ph.GA↗

arXiv2024

Baryon Acoustic Oscillations from galaxy surveys

We conducted a review of the fundamental aspects of describing and detecting the Baryon Acoustic Oscillation (BAO) feature in galaxy surveys, emphasizing the optimal tools for constraining this probe based on the type of observation. Additionally, we included new results with two spectroscopic datasets to determine the best-fit model for the power spectrum, $P(k)$. Using the framework described in a previous analysis, we applied this to a different sub-sample of the BOSS survey, specifically galaxies with redshifts $0.3<z<0.65$. We also examined the eBOSS dataset with redshifts $0.6<z<1.0$, adjusting the number of parameters in the traditional polynomial fit to account for the higher redshift range. Our results showed that the dilation scale parameter $α$ derived from the BOSS dataset had smaller error bars compared to the eBOSS dataset, attributable to the larger number of luminous red galaxies (LRGs) in the BOSS sample. We also compared our findings with other surveys such as WiggleZ, DES Y6, and DESI III, noting that photometric surveys typically yield larger error bars due to their lower precision. The DESI III results were in good agreement with ours within $1σ$, with most bins close to unity. The variation of $α$ with respect to the redshift is an unresolved issue in the field, appearing in both three-dimensional and angular tomographic analyses.

astro-ph.CO↗

arXiv2024

Addressing type Ia supernova color variability with a linear spectral template

Type Ia Supernovae (SNeIa) provided the first evidence of an accelerated expansion of the universe and remain a valuable probe to cosmology. They are deemed standardizable candles due to the observed correlations between its luminosity and photometric quantities. This characteristic can be exploited to estimate cosmological distances after accounting for the observed variations. There is however a remaining dispersion unaccounted for in the current state-of-the-art standardization methods. In an attempt to explore this issue, we propose a simple linear 3-component rest-frame flux description for a light-curve fitter. Since SNIa intrinsic color index variations are expected to be time-dependent, our description builds-up upon the mathematical expression of the well known SALT2 for rest-frame flux, whilst we drop the exponential factor and add an extra model component with time and wavelength dependencies. The model components are obtained by performing either Principal Component Analysis (PCA) or Factor Analysis (FA) onto a representative training set. The constraining power of the Pure Expansion Template for Supernovae (PETS), is evaluated and we found compatible results with SALT2 for $Ω_{m0}$ and $Ω_{Λ0}$ within 68% uncertainty between the two models, with PETS' fit parameters exhibiting non negligible linear correlations with SALT2' parameters. For both model versions we verified that the first component describes mainly color index variations, as a dominant effect on SNIa spectra. The model nuisance parameter which multiplies the color index variation-like fit parameter shows evolution with redshift in an initial binned cosmology analysis. This behavior can be due to selection effects. Overall, our model shows promise, as there are still a few aspects to be refined; however, it still falls short in reducing the unaccounted dispersion.

astro-ph.CO↗

arXiv2024

A unified approach to coupled homogeneous linear wave propagation in generic gravity

Wave propagation is a common occurrence in all of physics. A linear approximation provides a simpler way to describe various fields related to observable phenomena in laboratory physics as well as astronomy and cosmology, allowing us to probe gravitation through its effect on the trajectories of particles associated with those fields. This paper proposes a unified framework to describe the wave propagation of a set of interacting tensor fields that obey coupled homogeneous linear second-order partial differential equations for arbitrary curved spacetimes, both Lorentzian and metric-affine. We use JWKB Ansätze for all fields, written in terms of a perturbation parameter proportional to a representative wavelength among them, deriving a set of hierarchical algebraic and differential equations that link the fields' phases and different order amplitudes. This allows us to reobtain the well-known laws of geometrical optics and beyond geometrical optics in a generalized form, showing that these laws are independent of the rank of the fields involved. This is true as long as what we refer to as the kinetic tensor of a given field satisfies a set of diagonality conditions, which further imply a handful of simplifications on the transport equations obtained in the subleading orders of the JWKB Ansätze. We explore these results in several notable examples in Lorentzian and metric-affine spacetimes, illustrating the reach of our derivations in general relativity, reduced Horndeski theories, spacetimes with completely antisymmetric torsion and Weyl spacetimes. The formalism presented herein lays the groundwork for the study of rays associated with different types of waves in curved spacetimes and provides the tools to compute modifications to their brightness evolution laws, consequential distance duality relations, and beyond geometrical optics phenomena.

gr-qc↗

arXiv2024

Angular Correlation Function from sample covariance with BOSS and eBOSS LRG

The Baryon Acoustic Oscillations (BAO) are one of the most used probes to understand the accelerated expansion of the Universe. Traditional methods rely on fiducial model information within their statistical analysis, which may be a problem when constraining different families of models. This work aims to provide a method that constrains $θ_{BAO}$ through a model-independent approach using the covariance matrix from the galaxy sample from thin redshift bins, later validated with a mock sample covariance matrix. We used widths of $δz = 0.002$ separation for all samples as the basis for a sample covariance matrix weighted by the statistical importance of the redshift bin. Each sample belongs to the Sloan Digital Sky Survey: BOSS1, BOSS2, and eBOSS, with effective redshift $z_{eff}$: 0.35, 0.51, 0.71, and different numbers of bins with 50, 100, and 200. To get $θ_{BAO}$, we correct the angular separation from the polynomial fit ($θ_{fit}$) by comparing each bin correlation function with the correlation function of the whole set, a parameter named $\tildeα$. We also tested such correction by choosing the bin at $z_{eff}$ and found that for eBOSS $θ_{BAO}$ is in $1 σ$ agreement with the Planck 18 model. Finally, we found that the sample covariances are noisy compared to the mocks for lower $z$ samples, something expected due to nonlinear effects. Such noise impact can be seen in the parameter constraints but does not affect the eBOSS covariance sample. It is shown that mocks' results do tend to its chosen fiducial cosmology $θ_{BAO}$. BOSS1 and BOSS2 showed agreement with Planck 18 and an agreement with Pantheon + S$H_0$ES when $\tildeα$ is based on the bin $z=z_{eff}$.

astro-ph.CO↗

arXiv2023

Compact stars in scalar-tensor theories with a single-well potential and the corresponding $f(R)$ theory

The macroscopic properties of compact stars in modified gravity theories can be significantly different from the general relativistic (GR) predictions. Within the gravitational context of scalar-tensor theories, with a scalar field $ϕ$ and coupling function $Φ(ϕ)= \exp[2ϕ/\sqrt{3}]$, we investigate the hydrostatic equilibrium structure of neutron stars for the simple potential $V(ϕ)= ωϕ^2/2$ defined in the Einstein frame (EF). From the scalar field in the EF, we also interpret such theories as $f(R)$ gravity in the corresponding Jordan frame (JF). The mass-radius relations, proper mass, and binding energy are obtained for a polytropic equation of state (EoS) in the JF. Our results reveal that the maximum-mass values increase substantially as $ω$ gets smaller, while the radius and mass decrease in the low-central-density region as we move further away from the pure GR scenario. Furthermore, a cusp is formed when the binding energy is plotted as a function of the proper mass, which indicates the appearance of instability. Specifically, we find that the central-density value where the binding energy is a minimum corresponds precisely to $dM/dρ_c^J = 0$ on the $M(ρ_c^J)$-curve.

gr-qc↗

arXiv2022

Charged quark stars in metric $f(R)$ gravity

We provide the modified TOV equations for the hydrostatic equilibrium of charged compact stars within the metric $f(R)$ gravitational background. We adopt the MIT bag model EoS for the dense matter and assume a charge distribution where the electric charge density $ρ_{\rm ch}$ is proportional to the standard energy density $ρ$. Using the Starobinsky model, we explore the role of the $αR^2$ term, where $α$ is a free constant and $R$ the Ricci scalar, on the global properties of charged stars such as radius, mass and total charge. We present the dependence of the structure of the star for several values of $α$ and for different values of the constant parameter $β\equiv ρ_{\rm ch}/ρ$. Remarkably, we find that the radius decreases with respect to its GR value for low central densities, while the opposite occurs in the high-central-density region. The mass measured at the surface always decreases and the maximum-total charge undergoes a substantial increase as the parameter $α$ increases. We also illustrate the variations of the asymptotic mass as a consequence of the electric charge and the extra quadratic term.

gr-qc↗

arXiv2022

$R^2$-gravity quark stars from perturbative QCD

We investigate the structure of quark stars in the framework of $f(R)= R+ αR^2$ gravity using an equation of state for cold quark matter obtained from perturbative QCD, parametrized only by the renormalization scale. We show that a considerably large range of the free parameter $α$, within and even beyond the constraints previously reported in the literature, yield non-negligible modifications in the mass and radius of stars with large central mass densities. Besides, their stability against baryon evaporation is analyzed through the behavior of the associated total binding energies for which we show that these energies are slightly affected by the modified gravity term in the regime of high proper (baryon) masses.

gr-qc↗

arXiv2021

Neutron stars in $f(R,T)$ gravity with conserved energy-momentum tensor: Hydrostatic equilibrium and asteroseismology

We investigate the equilibrium and radial stability of spherically symmetric relativistic stars, considering a polytropic equation of state (EoS), within the framework of $f(R,T)$ gravity with a conservative energy-momentum tensor. Both modified stellar structure equations and Chandrasekhar's pulsation equations are derived for the $f(R,T)= R+ h(T)$ gravity model, where the function $h(T)$ assumes a specific form in order to safeguard the conservation equation for the energy-momentum tensor. The neutron star properties, such as radius, mass, binding energy and oscillation spectrum are studied in detail. Our results show that a cusp -- which signals the appearance of instability -- is formed when the binding energy is plotted as a function of the compact star proper mass. We find that the squared frequency of the fundamental vibration mode passes through zero at the central-density value corresponding to such a cusp where the binding energy is a minimum.

gr-qc↗

arXiv2021

Influence of gravitational waves upon light in the Minkowski background: from null geodesics to interferometry

We have recently derived a manifestly covariant evolution law, under the geometrical optics approximation of the vacuum Maxwell's equations, for the electric field along null geodesics in a general spacetime, relative to an arbitrary set of instantaneous observers [arXiv:2004.03496]. As one of its applications, we derive here the final detected intensity signal arising from a prototypical laser interferometric gravitational wave (GW) Michelson-Morley detector, comoving with transverse traceless (TT) observers, valid for both long and short GW wavelengths. One of our main results is the presentation of the integrated null geodesic parametric equations exchanged between two TT observers in terms of explicitly observable quantities and the profile of the plane GW packet. This allows us to revisit the derivation of the consequential radar distance and Doppler shift, taking the opportunity to discuss some related subtle conceptual issues and how they might affect the interferometric process. Another achievement is the calculation of the electric field in each arm up to the detection event, for any relative orientations of the arms and the GW direction. The main quantitative result is the new expression for the final interference pattern, for normal GW incidence, which turns out to have three contributions: (i) the well-known traditional one due to the difference in optical paths, and two new ones due to (ii) the Doppler effect, and (iii) the divergence of the laser beams. The quantitative relevance of the last two contributions is compared to the traditional one and shown to be negligible within the geometrical optics regime of light. Although in general further contributions from the non-parallel transport of the polarization vector are expected, again in the case of GW normal incidence, such a vector is indeed parallel transported, and those contributions are absent.

gr-qc↗

arXiv2021

Radial oscillations and stability of compact stars in $f(R, T) = R+ 2βT$ gravity

We examine the static structure configurations and radial stability of compact stars within the context of $f(R, T)$ gravity, with $R$ and $T$ standing for the Ricci scalar and trace of the energy-momentum tensor, respectively. Considering the $f(R, T)=R+2βT$ functional form, with $β$ being a constant, we derive the corresponding hydrostatic equilibrium equation and the modified Chandrasekhar's pulsation equation. The mass-radius relations and radial mode frequencies are obtained for some realistic equations of state. Our results show that the traditional stellar stability criteria, namely, the necessary condition $dM/dρ_c >0$ and sufficient condition $ω^2 >0$, still hold in this theory of gravity.

gr-qc↗

arXiv2021

Influence of gravitational waves upon light. Part I. Null geodesics, radar distance and frequency shift

We explore different facets of the action of linearized gravitational waves in Minkowski spacetime background upon light, under the electromagnetic geometrical optics limit, covering the main aspects: light trajectory perturbations, radar distance and light frequency shift. For this purpose, we consider observers comoving with the transverse traceless gauge coordinates. We compute the parametrized null geodesics exchanged between two of these observers, presenting explicitly the constants of motion as functions of observables, determining therefrom both the radar distance between the observers and the electromagnetic round-trip frequency shift caused by the gravitational wave. Also, a comparison is made between these results and what one would obtain by using a frequently adopted hybrid model in which the spatial trajectory of light is unchanged. Finally, we revisit and provide an explanation, resorting to the constancy of the phase along a light ray, to the fundamental puzzling question of how one is able to detect gravitational waves by means of interferometry if both light wavelength and detector arms are stretched.

gr-qc↗

arXiv2021

Influence of gravitational waves upon light. Part II. Electric field propagation and interference pattern in a gravitational wave detector

In this second article of the series, we apply our recently derived equation for the electric field propagation along light rays [arXiv:2004.03496], valid on the electromagnetic geometrical optics limit, to the special case of a toy interferometer used to detect gravitational waves in a flat background. Such an equation shows that, assuming the detector is in the transverse-traceless frame, which has a local shearing relative motion due to the gravitational wave perturbations, the electric field does not propagate as in an inertial reference frame in Minkowski spacetime. We present the electric field at the end of the interferometric process, for arbitrary arm configurations with respect to the plane gravitational wave packet propagation direction. Then, for normal incidence, we compute the interference pattern and, in addition to the usual term associated with the difference in path traveled by light in the arms, we deduce two new contributions to the final intensity, arising from: (i) the round-trip electromagnetic frequency shift and (ii) the divergence of the light beam. Their quantitative relevance is compared to the traditional contribution and shown to be typically negligible due to the geometrical optics regime of light. Moreover, a non-parallel transport of the polarization vector takes place, in general, because of the gravitational wave, a feature which could generate further contributions. However, we conclude that for the normal incidence case such vector is parallel transported, preventing this kind of correction.

gr-qc↗

arXiv2020

Strongest constraint in $f(R) = R+ αR^2$ gravity: stellar stability

In the metric approach of $f(R)$ theories of gravity, the fourth-order field equations are often recast as effective Einstein equations in the presence of standard matter and a curvature fluid (which gathers all the extra terms), always in the Jordan frame. In this picture, we investigate the strong gravity regime of the $f(R) = R+ αR^2$ model. In particular, we focus on the stability of a compact star composed by a mixture of ordinary matter -- described by a polytropic equation of state -- and an effective curvature fluid in an otherwise standard Einstein gravity, so that we are able to apply the usual equations that govern the radial adiabatic oscillations of relativistic stars. Our new restriction on the free parameter is $α\lesssim 2.4 \times 10^8\ \text{cm}^2$ in order to guarantee stellar stability, about $100$ times more restrictive than previous results (based on mass-radius relations alone) in the literature.

gr-qc↗

arXiv2020

On the cosmological performance of photometrically classified supernovae with machine learning

The efficient classification of different types of supernova is one of the most important problems for observational cosmology. However, spectroscopic confirmation of most objects in upcoming photometric surveys, such as the The Rubin Observatory Legacy Survey of Space and Time (LSST), will be unfeasible. The development of automated classification processes based on photometry has thus become crucial. In this paper we investigate the performance of machine learning (ML) classification on the final cosmological constraints using simulated lightcurves from The Supernova Photometric Classification Challenge, released in 2010. We study the use of different feature sets for the lightcurves and many different ML pipelines based on either decision tree ensembles or automated search processes. To construct the final catalogs we propose a threshold selection method, by employing a \emph{Bias-Variance tradeoff}. This is a very robust and efficient way to minimize the Mean Squared Error. With this method we were able to get very strong cosmological constraints, which allowed us to keep $\sim 75\%$ of the total information in the type Ia SNe when using the SALT2 feature set and $\sim 33\%$ for the other cases (based on either the Newling model or on standard wavelet decomposition).

astro-ph.CO↗