arXiv subjects

J. Alberto Vazquez

Publications and source records attributed to J. Alberto Vazquez.

At least 19 recordsLinked to original sources

arXiv2026

Cosmological Parameter Estimation using Particle Swarm Optimization

The quest for a theoretical framework and ingredients that capture our current understanding of the cosmos has motivated the design of a large number of highly informative experiments, generating an abundant flow of data. Faced with this amount of data and the need for thorough analysis, the main aim of this work is to present and assess the Particle Swarm Optimization (PSO) algorithm as a complementary tool to conventional cosmological data analysis techniques. PSO is one of the most representative bio inspired algorithms, offering strong robustness for high-dimensional or complex problems, while remaining relatively simple to implement and requiring only a few hyperparameters. In this study, we employ two standard variants of the canonical PSO algorithm, global best and local best, to investigate dark energy models using Type Ia Supernovae and Baryon Acoustic Oscillation measurements, focusing in particular on the DESI and DESI+Union3 datasets. Our findings demonstrate that PSO effectively recovers the best fit parameters from observational data and show that, under suitable conditions, PSO can achieve results comparable to those of traditional MCMC techniques but in a significantly reduced computation time. Moreover, the solutions obtained with PSO can be used as high quality initial conditions for MCMC analyzes, thereby accelerating their convergence.

astro-ph.CO↗

arXiv2026

Background-level reconstruction of scalar-field potentials from dark-energy histories and comparison with analytic potential families

We present a unified \emph{background-level} framework that maps a prescribed late-time dark-energy density history $ρ_{\rm de}(z)$ onto an effective scalar-field description in a spatially flat FLRW universe. Working directly with $ρ_{\rm de}(z)$, we reconstruct the associated field trajectory $ϕ(z)$, and field-space potential $V(ϕ)$, together with a null energy condition (NEC) consistency check. We apply the method to three benchmark histories: (i) the Chevallier--Polarski--Linder (CPL) form; (ii) a smooth mirror AdS$\rightarrow$dS sign-switching profile in which $ρ_{\rm de}$ crosses zero at $z_\dagger$, interpolating between a positive late-time plateau and a negative high-$z$ plateau ($Λ_{\rm s}$CDM-like at the background level); and (iii) a shifted-$\tanh$ emergent profile that remains positive definite and approaches $ρ_{\rm de}\to 0^{+}$ at high redshift. Finally, treating the reconstructed potential, $V_{\rm tar}(ϕ)$, as a target, we perform Bayesian model comparison directly in \emph{potential space} and rank representative analytic potential families by their Bayesian evidence. For CPL (restricting to the single-valued phantom branch for the potential-space comparison), the exponential potential has the highest evidence in the baseline analysis, while the shifted-$\tanh$ and hilltop quartic forms remain close competitors; for the sign-switching $\tanh$ target, the shifted-$\tanh$ potential is strongly preferred, and the emergent profile yields the same qualitative ranking. These results provide a practical dictionary between phenomenological expansion histories and the scalar-field potential shapes required to reproduce them at the background level.

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arXiv2026

How Holographic is the Dark Energy? A Spline Nodal reconstruction approach

In this work, we explore the generalized holographic dark energy (HDE) scenario. We relate the HDE density to the future-horizon scale via a non-parametric function, which is reconstructed via spline-based nodal interpolation. We perform a Bayesian analysis to assess the model consistency with current observations, including baryon acoustic oscillations (BAO) from the Dark Energy Spectroscopic Instrument (DESI) DR1, Type Ia supernovae (SNe Ia) from the Union3 and Pantheon+ compilations, and local measurements of the Hubble constant, $H_0$, from SH0ES. We show that under specific conditions, the model reduces to $Λ$CDM with one node. We find strong statistical evidence against the standard HDE model, and in contrast, the reconstructed HDE model, with three nodes, provides a better fit to the data than the $Λ$CDM model, indicating a strong statistical preference for the reconstructed model.

astro-ph.CO↗

arXiv2026

Dissipative Cosmology and the Nature of Dark Energy: Insights from Bulk Viscosity with DESI DR2 observations

We explore a cosmological model in which dark energy is described by a bulk viscous fluid, providing a dissipative mechanism for late-time cosmic acceleration. Considering both minimally and non-minimally coupled scenarios, we constrain the model using SNe Ia, DESI DR2 BAO, and Planck 2018 CMB data. We find that viscous effects can successfully mimic dynamical dark energy and yield improved fits over $Λ$CDM, particularly in the interacting non-minimal case. Our results demonstrate that dissipative processes offer a viable and physically motivated alternative to the cosmological constant in explaining the current accelerated expansion of the universe.

astro-ph.CO↗

arXiv2026

$Λ_{\rm s}$CDM cosmology from a type-II minimally modified gravity

We integrate $Λ_{\rm s}$CDM, a promising scenario for alleviating cosmological tensions, into VCDM, a type-II minimally modified gravity. This promotes the scenario to a fully predictive model (dubbed $Λ_{\rm s}$VCDM) that specifies the cosmological evolution self-consistently, including through the late-time AdS-to-dS transition epoch. In this theory, an auxiliary scalar field generates an effective cosmological constant with either a constant or a linear potential. This allows an abrupt mirror AdS-to-dS transition via a piecewise-linear potential with a sudden slope change. To remove the associated sudden singularity and ensure stable evolution, we smooth the junction using a blended sigmoid interpolant, obtaining rapid but continuous transitions. We identify two qualitatively distinct smooth mirror AdS-to-dS realisations of $Λ_{\rm s}$: (i) an agitated transition, in which the potential interpolates between equal-magnitude AdS and dS plateaus and $Λ_{\rm s}$ develops a central bump; and (ii) a quiescent transition, in which the potential remains continuous but changes slope across the transition layer, so that $Λ_{\rm s}(a)$ can remain monotone, with possible shallow shoulders, and a central bump is not automatic. Depending on type and sharpness, a finite-width transition can induce a transient accelerated-expansion interval ($\ddot a>0$) around $z\sim 1.5-2$, in addition to present-day acceleration, and, if the background enters a region where $V_{,ϕϕ}>2/3$, a nested super-acceleration episode. These distinct transient histories can imprint signatures on background and perturbation evolution. Our construction enables a self-consistent observational assessment of smooth $Λ_{\rm s}$CDM realisations and motivates multi-probe analyses to test transition dynamics and reassess cosmological tensions.

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arXiv2025

Exploring the Growth-Index ($γ$) Tension with $Λ_{\rm s}$CDM

Recent observational analyses have revealed a significant tension in the growth index $γ$, which characterizes the growth rate of cosmic structures. Specifically, when treating $γ$ as a free parameter within $Λ$CDM framework, a combination of Planck and $ fσ_8 $ data yields $γ\approx 0.64$, in $\sim4σ$ tension with the theoretically expected value $γ\approx 0.55$ (assuming general relativity). This discrepancy, closely related to the $ S_8 $ tension, poses a new challenge to the standard cosmological model by suggesting that it predicts an excessive growth of structure. In this work, we demonstrate that the $Λ_{\rm s}$CDM framework (featuring a rapid sign-switching cosmological constant (mirror AdS-to-dS transition) in the late universe at redshift $ z_\dagger \sim 2 $) can simultaneously alleviate the $ γ$, $ H_0 $, and $ S_8 $ tensions. We also examined a scenario with fixed $ z_\dagger = 1.7 $, previously identified as a sweet spot for alleviating multiple major cosmological tensions (including those in $ H_0 $, $ M_B $, and $ S_8 $) finding that it completely eliminates both the $ γ$ and $ H_0 $ tensions, although it is statistically disfavored by our dataset combinations. Our findings suggest that $Λ_{\rm s}$CDM is a promising model, providing a potential unified resolution to multiple major cosmological tensions.

astro-ph.CO↗

arXiv2024

Cosmological constraints on $Λ_{\rm s}$CDM scenario in a type II minimally modified gravity

The idea of a rapid sign-switching cosmological constant (mirror AdS-dS transition) in the late universe at $z\sim1.7$, known as the $Λ_{\rm s}$CDM model, has significantly improved the fit to observational data and provides a promising scenario for alleviating major cosmological tensions, such as the $H_0$ and $S_8$ tensions. However, in the absence of a fully predictive model, implementing this fit required conjecturing that the dynamics of the linear perturbations are governed by general relativity. Recent work embedding the $Λ_{\rm s}$CDM model with the Lagrangian of a type II minimally modified gravity known as VCDM has propelled $Λ_{\rm s}$CDM to a fully predictive model, removing the uncertainty related to the aforementioned assumption; we call this new model $Λ_{\rm s}$VCDM. In this work, we demonstrate that not only does $Λ_{\rm s}$CDM fit the data better than the standard $Λ$CDM model, but the new model, $Λ_{\rm s}$VCDM, performs even better in alleviating cosmological tensions while also providing a better fit to the data, including CMB, BAO, SNe Ia, and cosmic shear measurements. Our findings highlight the $Λ_{\rm s}$CDM framework, particularly the $Λ_{\rm s}$VCDM model, as a compelling alternative to the standard $Λ$CDM model, especially by successfully alleviating the $H_0$ tension. Additionally, these models predict higher values for $σ_8$, indicating enhanced structuring, albeit with lower present-day matter density parameter values and consequently reduced $S_8$ values, alleviating the $S_8$ tension as well. This demonstrates that the data are well fit by a combination of background and linear perturbations, both having dynamics differing from those of $Λ$CDM. This paves the way for further exploration of new ways for embedding the sign-switching cosmological constant into other models.

astro-ph.CO↗

arXiv2023

Model-independent reconstruction of the Interacting Dark Energy Kernel: Binned and Gaussian process

The cosmological dark sector remains an enigma, offering numerous possibilities for exploration. One particularly intriguing option is the (non-minimal) interaction scenario between dark matter and dark energy. In this paper, to investigate this scenario, we have implemented Binned and Gaussian model-independent reconstructions for the interaction kernel alongside the equation of state; while using data from BAOs, Pantheon+ and Cosmic Chronometers. In addition to the reconstruction process, we conducted a model selection to analyze how our methodology performed against the standard $Λ$CDM model. The results revealed a slight indication, of at least 1$σ$ confidence level, for some oscillatory dynamics in the interaction kernel and, as a by-product, also in the DE and DM. A consequence of this outcome is the possibility of a sign change in the direction of the energy transfer between DE and DM and a possible transition from a negative DE energy density in early-times to a positive one at late-times. While our reconstructions provided a better fit to the data compared to the standard model, the Bayesian Evidence showed an intrinsic penalization due to the extra degrees of freedom. Nevertheless these reconstructions could be used as a basis for other physical models with lower complexity but similar behavior.

astro-ph.CO↗

arXiv2023

Bayesian analysis for rotational curves with $\ell$-boson stars as a dark matter component

Using Low Brightness Surface Galaxies (LBSG) rotational curves we inferred the free parameters of $\ell$-boson stars as a dark matter component. The $\ell$-boson stars are numerical solutions to the non-relativistic limit of the Einstein-Klein-Gordon system, the Schrödinger-Poisson (SP) system. These solutions are parametrized by an angular momentum number $\ell = (N-1)/2$ and an excitation number $n$. We perform a bayesian analysis by modifying the SimpleMC code to perform the parameter inference, for the cases with $\ell = 0$, $\ell = 1$ and multi-states of $\ell$-boson stars. We used the Akaike information criterion (AIC), Bayesian information criterion and the Bayes factor to compare the excited state ($\ell$=1) and the multi-state case with the ground state ($\ell$=0) as the base model due to its simplicity. We found that the data in most galaxies in the sample favours the multi-states case and that the scalar field mass tends to be slightly bigger than the ground state case.

astro-ph.CO↗

arXiv2023

$Λ_{\rm s}$CDM model: A promising scenario for alleviation of cosmological tensions

We present a comprehensive analysis of the $Λ_{\rm s}$CDM model, which explores the recent conjecture suggesting a rapid transition of the Universe from anti-de Sitter vacua to de Sitter vacua (viz., the cosmological constant switches sign from negative to positive) at redshift ${z_\dagger\sim 2}$, inspired by the graduated dark energy (gDE) model. Our analysis shows that, predicting $z_\dagger\approx1.7$, $Λ_{\rm s}$CDM simultaneously addresses the major cosmological tensions of the standard $Λ$CDM model, viz., the Hubble constant $H_0$, the Type Ia Supernovae absolute magnitude $M_{\rm B}$, and the growth parameter $S_8$ tensions, along with other less significant tensions such as the BAO Lyman-$α$ discrepancy.

astro-ph.CO↗

arXiv2023

Relaxing cosmological tensions with a sign switching cosmological constant: Improved results with Planck, BAO, and Pantheon data

We present a further observational analysis of the $Λ_{\rm s}$CDM model proposed in Akarsu et al. [Phys. Rev. D 104, 123512 (2021)]. This model is based on the recent conjecture suggesting the Universe has transitioned from anti-de Sitter vacua to de Sitter vacua (viz., the cosmological constant switches sign from negative to positive), at redshift ${z_\dagger\sim2}$, inspired by the graduated dark energy model proposed in Akarsu et al. [Phys. Rev. D 101, 063528 (2020)]. $Λ_{\rm s}$CDM was previously claimed to simultaneously relax five cosmological discrepancies, namely, the $H_0$, $S_8$, and $M_B$ tensions along with the Ly-$α$ and $ω_{\rm b}$ anomalies, which prevail within the standard $Λ$CDM model as well as its canonical/simple extensions. In the present work, we extend the previous analysis by constraining the model using the Pantheon data (with and without the SH0ES $M_B$ prior) and/or the completed BAO data along with the full Planck CMB data. We find that $Λ_{\rm s}$CDM exhibits a better fit to the data compared to $Λ$CDM, and simultaneously relaxes the six discrepancies of $Λ$CDM, viz., the $H_0$, $M_B$, $S_8$, Ly-$α$, $t_0$, and $ω_{\rm b}$ discrepancies, all of which are discussed in detail. When the $M_B$ prior is included in the analyses, $Λ_{\rm s}$CDM performs significantly better in relaxing the $H_0$, $M_B$, and $S_8$ tensions with the constraint ${z_\dagger\sim1.8}$ even when the Ly-$α$ data (which imposed the $z_\dagger\sim2$ constraint in the previous studies) are excluded. In contrast, the presence of the $M_B$ prior causes only negligible improvements for $Λ$CDM. Thus, the $Λ_{\rm s}$CDM model provides remedy to various cosmological tensions simultaneously, only that the galaxy BAO data hinder its success to some extent.

astro-ph.CO↗

arXiv2023

Model selection applied to reconstructions of the Dark Energy

The main aim of this paper is to perform a model comparison for some reconstructions of the key properties that describe the dark energy of the Universe i.e. energy density and the equation of state (EoS). We carry out this process by using a binning and a linear interpolation methodologies, and on top of that, we incorporate a correlation function mechanism. An extension of the two of them was also introduced, where internal amplitudes are allowed to vary in height as well as in position. The reconstructions were made with data from the Hubble parameter, Supernovae Type Ia and Baryon Acoustic Oscillations (H+SN+BAO), all of which span a range from $z=0.01$ to $z=2.34$. First we perform the parameter estimation for each of the reconstructions to then provide a model selection through the Bayesian Evidence. Throughout our process we found a better fit to the data, up to $4σ$ compared to $Λ$CDM, and the presence of some interesting features, i.e. an oscillatory behaviour at late times, a decrease in the dark energy density component at early times and a transition to the phantom divide-line in the EoS. To discern these features from noisy contributions, we include a principal component analysis and found that some of these characteristics should be taken into account to satisfy current observations.

astro-ph.CO↗

arXiv2021

Inflationary Cosmology: From Theory to Observations

The main aim of this paper is to provide a qualitative introduction to the cosmic inflation and its relationship with current cosmological observations. The inflationary model solves many of the fundamental problems that challenge the Standard Big Bang cosmology i.e. Flatness, Horizon and Monopole problem, and additionally provides an explanation for the initial conditions observed throughout the Large-Scale Structure of the Universe, such as galaxies. In this review we describe the general solutions carry out by a single scalar field. Then with the use of current surveys, we show the constraints imposed on the inflationary parameters $(n_{\rm s},r)$ which allow us to make the connection between theoretical and observational cosmology. In this way, with the latest results, it is possible to choose or at least to constrain the right inflationary model, parameterised by a single scalar field potential $V(ϕ)$.

astro-ph.CO↗

arXiv2021

Relaxing cosmological tensions with a sign switching cosmological constant

Inspired by the recent conjecture originated from graduated dark energy that the Universe has recently transitioned from anti-de Sitter vacua to de Sitter vacua, we extend the $Λ$CDM model by a cosmological constant ($Λ_{\rm s}$) that switches sign at a certain redshift $z_\dagger$, and we call this model $Λ_{\rm s}$CDM. We discuss the construction and theoretical features of this model and find out that, when the consistency of $Λ_{\rm s}$CDM with the CMB data is ensured, (i) $z_\dagger\gtrsim1.1$ is implied by the condition that the Universe monotonically expands, (ii) $H_0$ and $M_B$ (type Ia supernovae absolute magnitude) values are inversely correlated with $z_\dagger$ and reach $H_0\approx74.5~{\rm km\, s^{-1}\, Mpc^{-1}}$ and $M_B\approx-19.2\,{\rm mag}$ for $z_\dagger=1.5$, in agreement with the SH0ES measurements, and (iii) $H(z)$ presents an excellent fit to the Ly-$α$ measurements provided that $z_\dagger\lesssim 2.34$. We further investigate the model constraints by using the full Planck CMB data set, with and without BAO data. We find that the CMB data alone does not constrain $z_\dagger$, but the CMB+BAO data set favors the sign switch of $Λ_{\rm s}$ providing the constraint: $z_\dagger=2.44\pm0.29$ (68% C.L.). Our analysis reveals that the lower and upper limits of $z_\dagger$ are controlled by the Galaxy and Ly-$α$ BAO measurements, respectively, and the larger $z_{\dagger}$ values imposed by the Galaxy BAO data prevent the model from achieving the highest local $H_0$ measurements. In general, $Λ_{\rm s}$CDM (i) relaxes the $H_0$ tension while being fully consistent with the TRGB measurements, (ii) relaxes the $M_B$ tension, (iii) removes the discrepancy with the Ly-$α$ measurements, (iv) relaxes the $S_8$ tension, and (v) finds a better agreement with the BBN constraints on the physical baryon density. [Abridged]

astro-ph.CO↗

arXiv2021

Simple-graduated dark energy and spatial curvature

In this work, we first discuss the possibility that dark energy models with negative energy density values in the past can alleviate the $H_0$ tension, as well as the discrepancy with the baryon acoustic oscillation (BAO) Lyman-$α$ data, both which prevail within the $Λ$CDM model. We then investigate whether two minimal extensions of the $Λ$CDM model, together or separately, can successfully realize such a scenario: (i) the spatial curvature, which, in the case of spatially closed universe, mimics a negative density source and (ii) simple-graduated dark energy (gDE), which promotes the null inertial mass density of the usual vacuum energy to an arbitrary constant--if negative, the corresponding energy density decreases with redshift similar to the phantom models, but unlike them crosses below zero at a certain redshift. We find that, when the Planck data are not included in the observational analysis, the models with simple-gDE predict interesting and some significant deviations from the $Λ$CDM model. In particular, a spatially closed universe along with a simple-gDE of positive inertial mass density, which work in contrast to each other, results in minor improvement to the $H_0$ tension. The joint dataset, including the Planck data, presents no evidence for a deviation from spatial flatness but almost the same evidence for a cosmological constant and the simple-gDE with an inertial mass density of order $\mathcal{O}(10^{-12})\,\rm eV^4$. The latter case predicts almost no deviation from the $Λ$CDM model up until today--so that it results in no improvement regarding the BAO Ly-$α$ data--except that it slightly aggravates the $H_0$ tension. We also study via dynamical analysis the history of the Universe in the models, as the simple-gDE results in futures different than the de Sitter future of the $Λ$CDM model.

astro-ph.CO↗

arXiv2021

Cosmological parameter inference with Bayesian statistics

Bayesian statistics and Markov Chain Monte Carlo (MCMC) algorithms have found their place in the field of Cosmology. They have become important mathematical and numerical tools, especially in parameter estimation and model comparison. In this paper, we review some fundamental concepts to understand Bayesian statistics and then introduce MCMC algorithms and samplers that allow us to perform the parameter inference procedure. We also introduce a general description of the standard cosmological model, known as the $Λ$CDM model, along with several alternatives, and current datasets coming from astrophysical and cosmological observations. Finally, with the tools acquired, we use an MCMC algorithm implemented in python to test several cosmological models and find out the combination of parameters that best describes the Universe.

astro-ph.CO↗

arXiv2021

Dark Matter with N-Body Numerical Simulations

The development of numerical N -body simulations have allowed to study formation process and evolution of galaxies at different scales. This paper presents the fundamental concepts of N-body systems applied to the cosmological evolution of the $Λ$-Cold Dark Matter ($Λ$CDM) model. In order to perform structure formation in the Universe, we provide an introduction to the basic equations and their implementation on the GADGET-2 software. We also present a simple guide to modify this code. First, we briefly describe the dark matter in the Universe as well as the theoretical and experimental basis of the $Λ$CDM model. Then, we focus on the simulation codes and provide the equations that govern most of the N-body simulations to model the dark matter. We describe the Smoothed Particle Hydrodynamics method used for simulating the gas, star dynamics and structure formation in these simulations. Then, cautiously, we guide the reader to the installation of GADGET-2 on a Linux-based computer, as well as to carry out a couple of examples to operate the code. Finally, by using a computational cluster, we show several results of a large structure simulation, analyse the outputs to display the matter power spectrum, and compare the outcome with theoretical predictions.

astro-ph.CO↗

arXiv2020

Scalar field emulator via anisotropically deformed vacuum energy: Application to dark energy

We introduce a generalization of the usual vacuum energy, called `deformed vacuum energy', which yields anisotropic pressure whilst preserving zero inertial mass density. It couples to the shear scalar in a unique way, such that they together emulate the canonical scalar field with an arbitrary potential. This opens up a new avenue by reconsidering cosmologies based on canonical scalar fields, along with a bonus that the kinetic term of the scalar field is replaced by an observable, the shear scalar. We further elaborate the aspects of this approach in the context of dark energy.

gr-qc↗