arXiv subjects

Lázaro L. Sales

Publications and source records attributed to Lázaro L. Sales.

7 recordsLinked to original sources

arXiv2026

Saha's ionization in the Schwarzschild spacetime

We investigate the Saha ionization equilibrium in the exterior Schwarzschild spacetime. Starting from the mass-shell condition for a massive particle, we derive the conserved energy associated with the timelike Killing symmetry and obtain its nonrelativistic limit in terms of the Schwarzschild lapse function. This energy is then incorporated into the Maxwell--Boltzmann distribution to construct the ionization balance as seen by an asymptotic observer. We show that the gravitational field modifies the Saha relation through the redshift of both the particle energies and the thermodynamic variables. By imposing the Tolman--Ehrenfest and Klein equilibrium conditions, the formulation expressed entirely in terms of locally measured quantities reduces to the standard flat-spacetime Saha equation. We further analyze the gravitational redshift of the ionization energy, the relative departure from the flat-spacetime ionization ratio, and the corresponding shift of the ionization fraction as a function of the asymptotic temperature. We also present the relativistic classical counterpart based on the Maxwell--Jüttner distribution and show explicitly that its leading correction is negligible in the temperature range where neutral hydrogen is appreciably abundant. These results provide a consistent description of ionization equilibrium in a static gravitational field and clarify the distinction between gravitational redshift effects and intrinsic modifications of local atomic physics.

gr-qc↗

arXiv2026

Testing Statistical Isotropy in the FRB Sky Distribution: A Selection-Function-Aware Framework

We perform a test of statistical isotropy in the Universe using the sky distribution of fast radio bursts (FRBs), based on a compilation of $4066$ events detected by multiple surveys. Our method is based on the two-point angular correlation function $w(θ)$ as in the Landy--Szalay estimator, together with a tomographic absolute-anisotropy statistic, and estimates their observational uncertainties from complementary jackknife and bootstrap resampling. Both estimators are confronted with hierarchical ensembles of isotropic mock catalogs that propagate the uncertainties of empirically reconstructed survey selection functions, as well as the Poisson fluctuations of the isotropic realizations. The significances are obtained from a covariance-aware, SVD-regularized $χ^2$ statistic calibrated empirically against the mock ensemble, and we evaluate four nested scenarios that progressively incorporate a Galactic-plane mask and the survey selection functions. As for our results, we find that the raw FRB sky is strongly inconsistent with isotropy; Galactic masking alone reduces the tension by only a factor of $\sim 3$, whereas the selection functions reduce it by nearly four orders of magnitude, showing that the apparent anisotropy is driven by the highly non-uniform sky coverage of the contributing surveys, overwhelmingly dominated by CHIME. Only when both effects are combined we obtain that the observed distribution is fully consistent with statistical isotropy. This result is independently corroborated by the absolute-anisotropy estimator, and is stable under variations of the analysis parameters. Therefore, we find that the FRB sky distribution is consistent with statistical isotropy, helping confirm one of the main predictions of the standard model scenario.

astro-ph.CO↗

arXiv2026

Constraints on Dark Energy and Modified Gravity Models from Fast Radio Bursts and Late-Time Geometric Probes

We investigate the impact of 104 localized FRBs on cosmological parameter estimation when combined with three established late-time probes: Cosmic Chronometers (CC), Type Ia Supernovae (SNe), and Baryon Acoustic Oscillations (BAO). By performing a Bayesian analysis of three dark energy models ($Λ$CDM, $w$CDM, and CPL) and three viable $f(R)$ gravity scenarios -- the Appleby-Battye (AB), Hu-Sawicki (HS), and Starobinsky (ST) models -- , we find that FRBs substantially improve the constraints on the baryon density $Ω_{\rm b}$ by $25\%$--$43\%$, the Hubble constant $H_0$ by $12\%$--$35\%$, and the SNe absolute magnitude $M_B$ by $10\%$--$32\%$. Constraints on dark energy parameters show more modest improvements, with $w$ improving by $\sim 9\%$ in $w$CDM and $(w_0,w_a)$ improving by $\sim(8,22)\%$ in the CPL parametrization. Modified gravity parameters remain weakly constrained, with improvements of only $6\%$--$15\%$, indicating the limited sensitivity of current datasets to departures from $Λ$CDM. The Figure of Merit analysis shows overall improvements ranging from $\sim 48\%$ ($Λ$CDM) to $\sim 91\%$ (CPL), driven by enhanced precision in the $(H_0, Ω_{\rm b})$ plane. Model comparison reveals moderate statistical preference for extensions beyond $Λ$CDM: AIC strongly favors $w$CDM, CPL, HS, and ST with $Δ\mathrm{AIC} < -7$, and LRT yields $p \leq 0.004$, while BIC returns to positive evidence ($-3.2 < Δ\mathrm{BIC} < -2.7$). These results show that FRBs may be useful as a complementary probe, particularly for constraining $Ω_{\rm b}$ and alleviating key late-time degeneracies.

astro-ph.CO↗

arXiv2026

Constraints on the baryon density from fast radio bursts using a non-parametric reconstruction of the Hubble parameter

In this study, we use a sample of 130 well-localized fast radio bursts (FRBs) to constrain the physical baryon density $Ω_{\rm b}h^2$, and the astrophysical contribution from host galaxies. The cosmological dependence entering the intergalactic dispersion measure is described through a non-parametric reconstruction of the Hubble parameter $H(z)$ obtained from cosmic chronometer data using the \texttt{ReFANN} neural-network framework, independently of the FRB sample. Within a Bayesian analysis, we jointly infer $Ω_{\rm b}h^2$ and the parameters of a log-normal host-galaxy distribution, namely its median $e^μ$ and logarithmic scatter $σ_{\rm host}$, using both real FRB data and a mock catalog. For the real sample, we obtain $Ω_{\rm b}h^2=0.02236\pm0.00090$, $e^μ=178.15^{+16.51}_{-16.97}~\mathrm{pc}\,\mathrm{cm}^{-3}$, and $σ_{\rm host}=0.794^{+0.064}_{-0.067}$. For the mock catalog, we find $Ω_{\rm b}h^2=0.02248\pm0.00018$, $e^μ=182.36^{+6.83}_{-6.48}~\mathrm{pc}\,\mathrm{cm}^{-3}$, and $σ_{\rm host}=0.711^{+0.024}_{-0.025}$. The baryon density constraint from the real FRB sample is in excellent agreement with both Big Bang Nucleosynthesis and Planck CMB determinations, differing from their central values by only $\simeq 0.05\%$. The mock analysis further illustrates the potential of future FRB samples, reducing the uncertainty on $Ω_{\rm b}h^2$ to the sub-percent level while remaining statistically consistent with early-Universe constraints. Our findings show that combining FRB dispersion measures with a non-parametric reconstruction of the expansion history provides a robust pathway to constrain both cosmological and astrophysical parameters, establishing FRBs as a complementary low-redshift probe of the baryon density.

astro-ph.CO↗

arXiv2026

Cosmographic constraints from late-time probes including fast radio bursts

In this study, we use late-time probes, such as well-localized fast radio bursts (FRBs), baryon acoustic oscillations (BAO), supernovae (SNe), and cosmic chronometers (CC) to constrain cosmological parameters through a model-independent cosmographic approach. By integrating FRB data with BAO from DESI DR2, SNe, and CC, we derive constraints on the Hubble constant ($H_0$), the deceleration parameter ($q_0$), and the jerk parameter ($j_0$), using Markov Chain Monte Carlo (MCMC) analysis for parameter estimation. The cosmographic approach with FRBs alone provides $H_0 = 66.35^{+4.13}_{-5.04} \, \text{km} \, \text{s}^{-1} \, \text{Mpc}^{-1}$, $q_0 = -0.33^{+0.21}_{-0.15}$, and $j_0 = 0.83^{+0.57}_{-0.67}$, corresponding to a precision of $\sim 6\%$ for the Hubble constant and showing consistency with the $Λ$CDM expectation. The DESI+CMB dataset yields $H_0 = 65.59^{+1.25}_{-1.24} \, \text{km} \, \text{s}^{-1} \, \text{Mpc}^{-1}$, $q_0 = -0.29^{+0.07}_{-0.08}$, and $j_0 = 0.58^{+0.03}_{-0.04}$, providing a $\sim 2\%$ precision on $H_0$ and may suggest a possible tension in the late-time kinematic sector relative to the $Λ$CDM expectation when BAO measurements are calibrated with a Planck-inferred sound horizon. Combining the FRB, SNe, DESI+CMB, and CC datasets further tightens the constraints to $H_0 = 68.03^{+0.53}_{-0.52} \, \text{km} \, \text{s}^{-1} \, \text{Mpc}^{-1}$, $q_0 = -0.41 \pm 0.02$, and $j_0 = 0.55 \pm 0.02$, with the jerk parameter remaining lower than $j_0 = 1$ at the $1σ$ confidence level. These findings hint at a possible late-time kinematic tension, as indicated by the inferred value of the jerk parameter, which is primarily driven by the DESI+CMB dataset under standard early-Universe assumptions for the sound horizon.

astro-ph.CO↗

arXiv2025

Stefan-Boltzmann Law and Thermal Casimir Effect in Neutron Star Spacetime via Thermo Field Dynamics

We investigate the thermal Casimir effect for a massless scalar field in the curved spacetime of a neutron star within the Thermo Field Dynamics (TFD) formalism. Starting from the renormalized energy-momentum tensor, we generalize the Stefan-Boltzmann law to include gravitational redshift and curvature corrections governed by the Tolman-Oppenheimer-Volkoff (TOV) metric. Finite temperature and spatial compactification are introduced simultaneously, allowing a unified and consistent treatment of both vacuum and thermal contributions inside and outside the star. Analytical expressions are derived for the high- and low-temperature limits, showing explicitly how curvature and redshift modify the characteristic $T^4$ dependence of thermal radiation. The results reveal that strong gravity significantly alters the local energy density and pressure, demonstrating the nontrivial interplay between quantum vacuum fluctuations and compact astrophysical geometries. A polytropic model is considered to perform numerical analyses, highlighting the influence of the spacetime background on vacuum fluctuations.

gr-qc↗

arXiv2020

An alternative method for solving the Gaussian integral

In this paper, we have proposed a new method for solving the Gaussian integral. Introducing a parameter that depends on a $n$ index, we have found a general solution for this type of integral inspired by Taylor series of a simple function. We have demonstrated that this parameter represents the Taylor series coefficients of this function, a result very newsworthy. We have also introduced some Theorems that are proved by mathematical induction. The proposed method in this work has shown more practical and accessible than some methods found in the literature. As a test for the method, we have investigated a non-extensive version for the particle number density in Tsallis framework, which enabled us to evaluate the functionality of the method. Besides, solutions for a certain class of the gamma and factorial functions are derived. Moreover, we have presented a simple application in fractional calculus. In conclusion, we believe in the relevance of this work because it presents a new form of solving the Gaussian integral having the differential calculus as a tool.

math.GM↗