arXiv subjects

Javier E. Gonzalez

Publications and source records attributed to Javier E. Gonzalez.

9 recordsLinked to original sources

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↗

arXiv2025

Observational constraints on the product of dark energy chemical potential and number density in out-of-equilibrium models

In this work, we impose observational limits on the product of dark energy chemical potential, $μ$, and number density, $n$, at the present time in out-of-equilibrium models, considering that particles can be created or destroyed in the fluid at a rate $Γ=3αH(a)$, where $α$ is a constant and $H(a)\equiv\dot{a}/a$ is the Hubble parameter. We combine the bounds derived from the positivity of entropy and the second law of thermodynamics with observational constraints on the Chevallier-Polarski-Linder (CPL) and Barboza-Alcaniz (BA) parameterizations of the equation of state (EoS) of the component. We use Type Ia supernovae (SN Ia) data from Pantheon+; baryon acoustic oscillation (BAO) data from DESI DR2; and cosmic microwave background (CMB) measurements from Planck. For $α>0$ (particle creation), the thermodynamic restrictions yield only upper limits for the $μ_{0}n_{0}$ product, while in the case of $α<0$ (particle destruction) they establish both upper and lower limits, allowing for a range of values to be obtained. In both scenarios, however, we find that the chemical potential of dark energy must be negative, $μ<0$, which indicates a preference for the phantom regime. In particular, when $α<0$, it is noted that the thermodynamic bounds are simultaneously compatible only for very small absolute values of $α$, with $α=-0.0002$ being the limiting case and resulting in $μ_{0}n_{0}(α=-0.0002)=-2.2_{-0.7}^{+1.0}\,\,GeV/m^{3}$.

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arXiv2025

An investigation of a varying G through Strong Lensing and SNe Ia observations

In this paper, we analyze the potential variation of the gravitational constant $G$ using data from strong gravitational lensing systems and Type Ia supernovae. Testing $G(z)$ parameterizations where $G(z) = G_0(1 + G_1z)$ and $G(z) = G_0(1 + z)^{G_1}$, we also account for the influence of $G$ on the luminosity of SNe Ia through the Chandrasekhar mass-luminosity relation. Only the flat universe hypothesis is considered. Constraints from 158 lensing systems and the Pantheon+ sample show no significant evidence of $G$ variation. However, although the results are compatible with no variation, the errors are not yet sufficiently restrictive to rule out any variation of $G$ with high statistical confidence. This study highlights the viability of using combined astrophysical data to probe variations in fundamental constants, suggesting that future surveys could refine these constraints.

astro-ph.CO↗

arXiv2025

Evidence for dynamical dark energy from DESI-DR2 and SN data? A symbolic regression analysis

Recent measurements of Baryon Acoustic Oscillations (BAO) from the Dark Energy Spectroscopic Survey (DESI DR2), combined with data from the cosmic microwave background (CMB) and Type Ia supernovae (SNe), challenge the $Λ$-Cold Dark Matter ($Λ$CDM) paradigm. They indicate a potential evolution in the dark energy equation of state (EoS), $w(z)$, as suggested by analyses that employ parametric models. In this paper, we use a model-independent approach known as high performance symbolic regression (PySR) to reconstruct $w(z)$ directly from observational data, allowing us to bypass prior assumptions about the underlying cosmological model. Our findings confirm that the DESI DR2 data alone agree with the $Λ$CDM model ($w(z) = -1$) at the redshift range considered. Additionally, when combining DESI data with existing compilations of SN distance measurements, such as Patheon+ and DESY5, we observe no deviation from the $Λ$CDM model within $3σ$ (C.L.) for the interval of values of present-day matter density parameter $Ω_m$ and the sound horizon at the drag epoch $r_d$ currently constrained by observational data. Therefore, similarly to the DESI DR1 case, these results suggest that it is still premature to claim statistically significant evidence for a dynamical EoS or deviations from the $Λ$CDM model based on the current DESI data in combination with supernova measurements.

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arXiv2024

Non-parametric reconstruction of the fine structure constant with galaxy clusters

Testing possible variations in fundamental constants of nature is a crucial endeavor in observational cosmology. This paper investigates potential cosmological variations in the fine structure constant ($α$) through a non-parametric approach, using galaxy cluster observations as the primary cosmological probe. We employ two methodologies based on galaxy cluster gas mass fraction measurements derived from X-ray and Sunyaev-Zeldovich observations, along with luminosity distances from type Ia supernovae. We also explore how different values of the Hubble constant ($H_0$) impact the variation of $α$ across cosmic history. When using the Planck satellite's $H_0$ observations, a constant $α$ is ruled out at approximately the 3$σ$ confidence level for $z \lesssim 0.5$. Conversely, employing local estimates of $H_0$ restores agreement with a constant $α$.

astro-ph.CO↗

arXiv2024

Unveiling the Hubble Constant through Galaxy Cluster Gas Mass Fractions

In this work, we obtain Hubble constant ($H_0$) estimates by using two galaxy cluster gas mass fraction measurement samples, Type Ia supernovae luminosity distances, and the validity of the cosmic distance duality relation. Notably, the angular diameter distance (ADD) to each galaxy cluster in the samples is determined by combining its gas mass fraction measurement with galaxy clustering observations, more precisely, the $Ω_b/Ω_m$ ratio. Such a combination results in a $H_0$ estimate that is independent of a specific cosmological framework. In one of the samples, the gas fraction measurements were calculated in spherical shells at radii near $r_{\rm 2500}$ (44 data points), while in the other (103 data points) the measurements were calculated within $ r_{\rm 500}$. We find $H_0=72.7^{+6.3}_{-5.6}$ km/s/Mpc at 68\% CL for the joint analysis of these data sets. We also investigate the impact on the $H_0$ determination by exploring the precision and number of gas mass fraction data by performing a data Monte Carlo simulation. Our simulations show that future measurements could achieve a precision of up to 5\% for $H_0$.

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arXiv2021

A model-independent reconstruction of dark sector interactions

Relaxing the conventional assumption of a minimal coupling between the dark matter (DM) and dark energy (DE) fields introduces significant changes in the predicted evolution of the Universe. Therefore, testing such a possibility constitutes an essential task not only for cosmology but also for fundamental physics. In a previous communication [Phys. Rev. D99, 043521, 2019], we proposed a new null test for the $Λ$CDM model based on the time dependence of the ratio between the DM and DE energy densities which is also able to detect potential signatures of interaction between the dark components. In this work, we extend that analysis avoiding the $ Λ$CDM assumption and reconstruct the interaction in the dark sector in a fully model-independent way using data from type Ia supernovae, cosmic chronometers and baryonic acoustic oscillations. According to our analysis, the $Λ$CDM model is consistent with our model-independent approach at least at $3σ$ CL over the entire range of redshift studied. On the other hand, our analysis shows that the current background data do not allow us to rule out the existence of an interaction in the dark sector. Finally, we present a forecast for next-generation LSS surveys. In particular, we show that Euclid and SKA will be able to distinguish interacting models with about 4\% of precision at $z\approx 1$.

astro-ph.CO↗

arXiv2021

Testing the consistency between cosmological data: the impact of spatial curvature and the dark energy EoS

The results of joint analyses of available cosmological data have motivated an important debate about a possible detection of a non-zero spatial curvature. If confirmed, such a result would imply a change in our present understanding of cosmic evolution with important theoretical and observational consequences. In this paper we discuss the legitimacy of carrying out joint analyses with the currently available data sets and explore their implications for a non-flat universe and extensions of the standard cosmological model. We use a robust tension estimator to perform a quantitative analysis of the physical consistency between the latest data of Cosmic Microwave Background, type Ia supernovae, Baryonic Acoustic Oscillations and Cosmic Chronometers. We consider the flat and non-flat cases of the $Λ$CDM cosmology and of two dark energy models with a constant and varying dark energy EoS parameter. The present study allows us to better understand if possible inconsistencies between these data sets are significant enough to make the results of their joint analyses misleading, as well as the actual dependence of such results with the spatial curvature and dark energy parameterizations.

astro-ph.CO↗

arXiv2020

Is there evidence for a hotter Universe?

The measurement of present-day temperature of the Cosmic Microwave Background (CMB), $T_0 = 2.72548 \pm 0.00057$ K (1$σ$), made by the Far-InfraRed Absolute Spectrophotometer (FIRAS), is one of the most precise measurements ever made in Cosmology. On the other hand, estimates of the Hubble Constant, $H_0$, obtained from measurements of the CMB temperature fluctuations assuming the standard $Λ$CDM model exhibit a large ($4.1σ$) tension when compared with low-redshift, model-independent observations. Recently, some authors argued that a slightly change in $T_0$ could alleviate or solve the $H_0$-tension problem. Here, we investigate evidence for a hotter or colder universe by performing an independent analysis from currently available temperature-redshift $T(z)$ measurements. Our analysis (parametric and non-parametric) shows a good agreement with the FIRAS measurement and a discrepancy of $\gtrsim 1.9σ$ from the $T_0$ values required to solve the $H_0$ tension. This result reinforces the idea that a solution of the $H_0$-tension problem in fact requires either a better understanding of the systematic errors on the $H_0$ measurements or new physics.

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