arXiv subjects

Ronaldo C. Batista

Publications and source records attributed to Ronaldo C. Batista.

9 recordsLinked to original sources

arXiv2026

Implications of a high growth index on the variation of $G$

A recent determination of the growth index indicates a value significantly higher than the $Λ$CDM prediction, suggesting that alternative scenarios to $Λ$CDM may be required. In this work, we investigate whether a time-varying Newton's constant, $G_N$, can account for such a high growth index, $γ=0.063\pm0.025$. Adopting a phenomenological approach, we study two parameterizations of the effective gravitational coupling, $G_{\rm eff}$, one based on a Taylor expansion and another linked to the energy density parameter of Dark Energy. We constrain the models with Cosmic Chronometers (CC), Dark Energy Spectroscopic Instrument baryon acoustic oscillations (data release 2), CMB priors, and a gaussian likelihood for the growth index. We show that the constant $γ$ approximation is accurate for the parametrization linked to the energy density parameter of dark energy, but presents a non-negligible error for the other case, which we treat as a systematic error in the analysis. We find a $2.4σ-3.4σ$ tension level with constant $G_{\rm eff}$, depending on the parametrization. The results indicate that $G_{\rm eff}<G_N$ around the period of accelerated expansion, corresponding to a weaker effective gravitational interaction on cosmological scales, which leads to a suppression of the growth of cosmological structures.

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arXiv2025

Can dark energy explain a high growth index?

A promising way to test the physics of the accelerated expansion of the Universe is by studying the growth rate of matter fluctuations, which can be parametrized by the matter energy density parameter to the power $γ$, the so-called growth index. It is well-known that the $Λ$CDM cosmology predicts $γ=0.55$. However, using observational data, Nguyen et al. (2023) measured a much higher $γ=0.633^{+0.025}_{-0.024}$, excluding the $Λ$CDM value within $3.7σ$. In this work, we analyze whether Dark Energy (DE) with the Equation of State (EoS) parameter described by the CPL parametrization can significantly modify $γ$ with respect to the predicted $Λ$CDM one. Besides the usual Smooth DE (SDE) scenario, where DE perturbations are neglected on small scales, we also consider the case of Clustering Dark Energy (CDE), which has more potential to impact the growth of matter perturbations. In order to minimally constrain the background evolution and assess the largest meaningful $γ$ distribution, we use data from $32$ Cosmic Chronometers, $H(z$), data points. In this context, we found that both SDE and CDE models described by the CPL parametrization have almost negligible probability of providing $γ>0.6$. However, given that the measured $γ$ value assumes the $Λ$CDM background, a direct statistical measure of the incompatibility between theory and the measured value can not be done for other backgrounds. Thus, we devise a method in order to make a quick estimation of the $γ$ constraints for CPL background. This method indicates that, when using DESI DR2 BAO data to constrain background parameters, no significant changes in the $γ$ central value and uncertainty is observed. (Abridged)

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arXiv2025

On the virialization threshold for halo mass functions

In a recent study by Euclid collaboration, the halo mass function (HMF) has been fitted with accuracy better than $1\%$ for the $Λ$CDM model. Several parameters were introduced and fitted against N-body simulations, assuming the usual linearly extrapolated matter density contrast at the collapse time, $δ_c$, as a basic threshold for halo formation. As a result, a new function that multiplies $δ_c$ was introduced, producing an effective threshold that varies both with redshift and mass scale. We show that the redshift evolution of this effective threshold is similar to the one of the linear extrapolated matter density contrast at the virialization time, $δ_{\rm v}$. Assuming the Euclid HMF as a fiducial model, we refit the Sheth-Tormen (ST) HMF using $δ_{\rm v}$ as a threshold. This new fit improves the agreement between ST-HMF and the Euclid one with respect to Despali et al. (2016) fit, specially at high masses. Interestingly, the parameters $a$ and $p$ in this refit have values closer to the Press-Schechter limit of the ST-HMF, showing that the use of $δ_{\rm v}$ can provide semi-analytical HMF less dependent on extra parameters. Moreover, we analyze the consistency of the ST-HMF fitted with $δ_{\rm v}$ in smooth dark energy models with time-varying equation of state, finding an overall good agreement with the evolution of halo abundances expected from the linear evolution of perturbations and the Euclid HMF extrapolated to these scenarios. These findings suggest that the use $δ_{\rm v}$ as a basic function to describe the threshold for halo formation can be a good guide when considering extrapolations for models beyond $Λ$CDM, which are typically harder to study in simulations.

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arXiv2022

A short review on clustering dark energy

We review dark energy models which can present non-negligible fluctuations on scales smaller than Hubble radius. Both linear and nonlinear evolutions of dark energy fluctuations are discussed. The linear evolution has a well-established framework, based on linear perturbation theory in General Relativity, and is well studied and implemented in numerical codes. We highlight the main results from linear theory to explain how dark energy perturbations become important on the scales of interest for structure formation. Next, we review some attempts to understand the impact of clustering dark energy models in the nonlinear regime, usually based on generalizations of the Spherical Collapse Model. We critically discuss the proposed generalizations of the Spherical Collapse Model that can treat clustering dark energy models and their shortcomings. Proposed implementations of clustering dark energy models in halo mass functions are reviewed. We also discuss some recent numerical simulations capable of treating dark energy fluctuations. Finally, we make an overview of the observational predictions based on these models.

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arXiv2022

Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies

In this paper we will list a few important goals that need to be addressed in the next decade, also taking into account the current discordances between the different cosmological probes, such as the disagreement in the value of the Hubble constant $H_0$, the $σ_8$--$S_8$ tension, and other less statistically significant anomalies. While these discordances can still be in part the result of systematic errors, their persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the necessity for new physics or generalisations beyond the standard model. In this paper, we focus on the $5.0\,σ$ tension between the {\it Planck} CMB estimate of the Hubble constant $H_0$ and the SH0ES collaboration measurements. After showing the $H_0$ evaluations made from different teams using different methods and geometric calibrations, we list a few interesting new physics models that could alleviate this tension and discuss how the next decade's experiments will be crucial. Moreover, we focus on the tension of the {\it Planck} CMB data with weak lensing measurements and redshift surveys, about the value of the matter energy density $Ω_m$, and the amplitude or rate of the growth of structure ($σ_8,fσ_8$). We list a few interesting models proposed for alleviating this tension, and we discuss the importance of trying to fit a full array of data with a single model and not just one parameter at a time. Additionally, we present a wide range of other less discussed anomalies at a statistical significance level lower than the $H_0$--$S_8$ tensions which may also constitute hints towards new physics, and we discuss possible generic theoretical approaches that can collectively explain the non-standard nature of these signals.[Abridged]

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arXiv2017

Clustering dark energy and halo abundances

Within the standard paradigm, dark energy is taken as a homogeneous fluid that drives the accelerated expansion of the universe and does not contribute to the mass of collapsed objects such as galaxies and galaxy clusters. The abundance of galaxy clusters -- measured through a variety of channels -- has been extensively used to constrain the normalization of the power spectrum: it is an important probe as it allows us to test if the standard $Λ$CDM model can indeed accurately describe the evolution of structures across billions of years. It is then quite significant that the Planck satellite has detected, via the Sunyaev-Zel'dovich effect, less clusters than expected according to the primary CMB anisotropies. One of the simplest generalizations that could reconcile these observations is to consider models in which dark energy is allowed to cluster, i.e., allowing its sound speed to vary. In this case, however, the standard methods to compute the abundance of galaxy clusters need to be adapted to account for the contributions of dark energy. In particular, we examine the case of clustering dark energy -- a dark energy fluid with negligible sound speed -- with a redshift-dependent equation of state. We carefully study how the halo mass function is modified in this scenario, highlighting corrections that have not been considered before in the literature. We address modifications in the growth function, collapse threshold, virialization densities and also changes in the comoving scale of collapse and mass function normalization. Our results show that clustering dark energy can impact halo abundances at the level of 10\%--30\%, depending on the halo mass, and that cluster counts are modified by about 30\% at a redshift of unity.

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arXiv2014

Structure formation in a $Λ$ $viscous$ CDM universe

The possibility of dark matter being a dissipative component represents an option for the standard view where cold dark matter (CDM) particles behave on large scales as an ideal fluid. By including a physical mechanism to the dark matter description like viscosity we construct a more realistic model for the universe. Also, the known small scale pathologies of the standard CDM model either disappear or become less severe. We study clustering properties of a $Λ$CDM-like model in which dark matter is described as a bulk viscous fluid. The linear power spectrum, the nonlinear spherical "top hat" collapse and the mass functions are presented. We use the analysis with such structure formation tools in order to place an upper bound on the magnitude of the dark matter's viscosity.

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arXiv2014

The impact of dark energy perturbations on the growth index

We show that in clustering dark energy models the growth index of linear matter perturbations, $γ$, can be much lower than in $Λ$CDM or smooth quintessence models and present a strong variation with redshift. We find that the impact of dark energy perturbations on $γ$ is enhanced if the dark energy equation of state has a large and rapid decay at low redshift. We study four different models with these features and show that we may have $0.33<γ\left(z\right)<0.48$ at $0<z<3$. We also show that the constant $γ$ parametrization for the growth rate, $f=d\lnδ_{m}/d\ln a=Ω_{m}^γ$, is a few percent inaccurate for such models and that a redshift dependent parametrization for $γ$ can provide about four times more accurate fits for $f$. We discuss the robustness of the growth index to distinguish between General Relativity with clustering dark energy and modified gravity models, finding that some $f\left(R\right)$ and clustering dark energy models can present similar values for $γ$.

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arXiv2009

The signature of dark energy perturbations in galaxy cluster surveys

All models of dynamical dark energy possess fluctuations, which affect the number of galaxy clusters in the Universe. We have studied the impact of dark energy clustering on the number of clusters using a generalization of the spherical collapse model and the Press-Schechter formalism. Our statistical analysis is performed in a 7-parameter space using the Fisher matrix method, for several hypothetical Sunyaev-Zel'dovich and weak lensing (shear maps) surveys. In some scenarios, the impact of these fluctuations is large enough that their effect could already be detected by existing instruments such as the South Pole Telescope, when its data is combined with WMAP and SDSS. Future observations could go much further and probe the nature of dark energy by distinguishing between different models on the basis of their perturbations, not only their expansion histories.

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