arXiv subjects

Víctor H. Cárdenas

Publications and source records attributed to Víctor H. Cárdenas.

At least 19 recordsLinked to original sources

arXiv2026

Beyond Comoving Volume: Horizon Flux and Matter Creation in Modified Cosmology from the Unified First Law of Thermodynamics

We explore the derivation of the Friedmann equations from a thermodynamic perspective, applying the unified first law of thermodynamics to the apparent horizon of a flat Friedmann-Lemaître-Robertson-Walker (FLRW) universe. We extend this framework to incorporate gravitationally induced particle creation, treating the region enclosed by the apparent horizon as an open thermodynamic system. A crucial aspect of our analysis is the recognition that the apparent horizon volume is not comoving; this requires consistent accounting of particle exchange across the moving boundary. We demonstrate that the evolution of the particle number, and explicitly the matter entropy, can be decomposed into two distinct physical contributions: genuine bulk particle production and a net flux induced by the dynamics of the horizon itself. Finally, we derive the Generalized Second Law (GSL) in this setting, showing transparently how the total entropy budget is balanced by horizon thermodynamics, bulk creation, and boundary fluxes.

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arXiv2026

Cosmological Stealth fields and Non-Equilibrium thermodynamics

We investigate the connection between cosmological stealth scalar fields and non-equilibrium thermodynamics in a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) background. We consider a non-minimally coupled scalar field whose energy-momentum tensor vanishes identically, allowing the field to evolve on a dissipative cosmological background without producing gravitational backreaction. We show that the stealth condition leads to a generalized Riccati equation for the scalar-field kinematics, where the dissipative pressure acts as a thermodynamic driving term. In terms of the variable $y=\dotϕ/(Hϕ)$, the system admits two thermodynamic branches: a stable attractor selected by entropy production and an unstable repeller. We also construct the corresponding phase-space structure in the $(y,ω_{\rm eff})$ plane and identify a near-critical regime associated with $ζ=1/4$. Finally, we reconstruct the stealth potential and present a bulk viscous realization in which irreversible entropy production drives the universe toward an asymptotic de Sitter state while the stealth field tracks the dissipative background. We further develop the linear scalar perturbations of the model and show that, although the stealth field is gravitationally invisible at the background level, its perturbation couples directly to the dissipative-pressure perturbation. This coupling provides a mechanism for distinguishing dissipative and non-dissipative sources that share the same background expansion history. Our results suggest that stealth fields can be interpreted as dynamically non-trivial thermodynamic trackers of non-equilibrium cosmological evolution.

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arXiv2026

Matter Creation in Unimodular Gravity Cosmology

We investigate matter creation in the cosmological framework of unimodular gravity. We keep the energy density $ρ$, pressure $p$, particle number density $n$, and particle number $N=nV$ as the physical thermodynamic variables of the matter sector, and introduce particle creation directly through the balance equation $N^μ{}_{;μ}=nΓ$. Combining this open-system formulation with the unimodular diffusion term $Q$, we derive the exact identity $nT\dotσ=-3Hp_c-(ρ+p)Γ-\dot Q$, which shows that the creation pressure $p_c$, the creation rate $Γ$, and the diffusion term $Q$ are, in general, three independent sources of entropy production per particle. Adopting the standard postulate that particle creation is itself adiabatic, the first two terms cancel identically and the relation reduces to the particular case $nT\dotσ=-\dot Q$, so that the entropy per particle is constant only when $Q$ is constant. Thus, even when the creation process associated with $Γ$ is adiabatic in the usual Prigogine sense, the full matter evolution in unimodular gravity is non-adiabatic whenever $\dot Q\neq0$. The general identity, however, remains available as a starting point for relaxing this postulate in future work.

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arXiv2026

Cosmo-Learn: code for learning cosmology using different methods and mock data

We present cosmo_learn, an open-source python-based software package designed to simulate cosmological data and perform data-driven inference using a range of modern statistical and machine learning techniques. Motivated by the growing complexity of cosmological models and the emergence of observational tensions, cosmo_learn provides a standardized and flexible framework for benchmarking cosmological inference methods. The package supports realistic noise modeling for key observables in the late Universe, including cosmic chronometers, supernovae Ia, baryon acoustic oscillations, redshift space distortions, and gravitational wave bright sirens. We demonstrate the internal consistency of the simulated data with the input cosmology via residuals and parameter recovery using a fiducial $w$CDM model. Built-in learning and inference modules include traditional Markov Chain Monte Carlo, as well as more recent approaches such as genetic algorithms, Gaussian processes, Bayesian ridge regression, and artificial neural networks. These methods are implemented in a modular and extensible architecture designed to facilitate comparisons across inference strategies in a common pipeline. By providing a flexible and transparent simulation and learning environment, cosmo_learn supports both educational and research efforts at the intersection of cosmology, statistics, and machine learning.

astro-ph.CO↗

arXiv2025

Revealing evolution of Dark Energy density from observations

We present a model--independent reconstruction of the normalized dark energy density function, $X(z) \equiv ρ_{\mathrm{de}}(z)/ρ_{\mathrm{de}}(0)$, derived directly from the DES-SN5YR Type~Ia supernova sample. The analysis employs an inversion formalism that relates the derivative of the distance modulus, $μ^{\prime}(z)$, to the expansion history, allowing the data to determine the shape of $X(z)$ without assuming a specific equation--of--state or dark energy density parameterization. A statistically optimized binning of the supernova sample (using 17 intervals following the Freedman--Diaconis criterion and 34 following Scott's rule) ensures a stable estimation of $μ^{\prime}(z)$ and a controlled propagation of uncertainties throughout the inversion process. The resulting $X(z)$ remains statistically consistent with a constant value within one standard deviation across the entire redshift range, showing no significant evidence for an evolving dark energy component at present. In a direct comparison among $Λ$CDM, CPL, and the quadratic $X^2(z)$ parameterization -- where CPL and $X^2(z)$ each introduce two additional free parameters relative to $Λ$CDM -- the CPL model attains the best statistical agreement with the data, albeit only marginally and strictly within this restricted model set. These outcomes indicate that current observations are compatible with an almost constant dark energy density ($w \simeq -1$), while the inversion framework remains sensitive to subtle departures that forthcoming high--precision surveys could resolve.

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arXiv2025

Non-geometrical perturbation on homogeneous stealth dust

In this work, we study a non-geometrical perturbation to the stealth field, which means the background remains invariant. The stealh is homogeneous in a universe whose source is dust and demand that perturbation unchanged density. As a regular procedure, we introduce a parameter $λ$ to perturb the scalar field equation and get an intriguing expression of the equation, similar to a series expansion in $λ$. From this procedure, we distinguish and approach to discriminate solutions, and the numerical solutions show that the most significant contribution to the solution comes from the linear term of $λ$.

gr-qc↗

arXiv2025

Matter creation, adiabaticity and phantom behavior

We present a novel cosmological framework that unifies matter creation dynamics with thermodynamic principles. Starting with a single-component fluid characterized by a constant equation of state parameter, $ω$, we introduce a generalized second law of thermodynamics by considering the entropy associated with the cosmic horizon. Imposing an adiabatic expansion condition uniquely determines the particle creation rate, $Γ$, a feature unprecedented in previous matter creation models. This mechanism yields a cosmology featuring phantom-like expansion while relying solely on a single constituent, which can be either a quintessence-like fluid or a non-exotic, non-relativistic dark matter component. Remarkably, this framework avoids the need for exotic physics while providing a consistent explanation for the accelerated expansion of the universe. Our results open new pathways for understanding the interplay between horizon thermodynamics, particle creation, and cosmic evolution, offering fresh insights into the nature of dark energy and its potential thermodynamic origins.

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arXiv2024

Probing Dark Energy Evolution Post-DESI 2024

We study the evidence for dark energy (DE) evolution at low redshift, using baryonic acoustic oscillations (BAOs) from the DESI Early Data Release, Pantheon+ Type Ia supernovae (SNe-Ia), and redshift space distortions (RSDs) to constrain cosmological parameters. Furthermore, we make use of the angular acoustic scale to analyse the effect of introducing condensed CMB information on the cosmological parameters informing DE evolution. The analysis is divided into cases based on the variability of priors inferred from early-time physics. Using a quadratic parametrisation, $X(z)$, for DE density, we find evidence for DE evolution in all cases, both with and without the angular acoustic scale. We reconstruct $X(z)$ using best fit parameters and find that DE density starts to exhibit dynamical behaviour at $z \sim 0.5 $, assuming negative values beyond $z\sim 1.5$. The data show no significant preference for $X(z)$CDM over a $Λ$CDM, with both models performing equally well according to our chosen metrics of reduced $χ^2$ and the Durbin-Watson statistic.

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arXiv2024

Generalized second law of thermodynamics for the matter creation scenario and emergence of phantom regime

This work is focused on the exploration of the thermodynamics foundations of the matter creation scenario when a generalized form of the second law of thermodynamics for this scheme is implemented. In this scenario we consider an expanding cosmology in which the created matter is trapped by the apparent horizon. The scheme leads to phantom evolution but at first glance it lacks of physical consistency. However, the inclusion of chemical potential into the description solves the thermodynamics issues of the model and determines the behavior of the cosmic fluid, in other words, the cosmic fluid now can behave as phantom dark energy or as quintessence one.

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arXiv2024

Emulating dark energy models with known equation of state via the created cold dark matter scenario

In this work we establish only at background dynamics level, the equivalence between the Created Cold Dark Matter (CCDM) model and diverse dark energy (DE) models. We find that for a barotropic or linear equation of state (EoS) for the DE, $p=w ρ$, and standard matter sector the corresponding CCDM model is described by the same functional structure of the ratio of particle production, $Γ$. For a different EoS the functional form of the $Γ$ term is not longer maintained, however, in the case of a polytropic EoS given by the Chaplygin gas the resulting $Γ$ term can be written as the one obtained in the barotropic case under certain considerations.

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arXiv2023

Dark energy by natural evolution: Constraining dark energy using Approximate Bayesian Computation

We look at dark energy from a biology inspired viewpoint by means of the Approximate Bayesian Computation (ABC) and late time cosmological observations. We find that dynamical dark energy comes out on top, or in the ABC language naturally selected, over the standard $Λ$CDM cosmological scenario. We confirm this conclusion is robust to whether baryon acoustic oscillations and Hubble constant priors are considered. Our results show that the algorithm prefers low values of the Hubble constant, consistent or at least a few standard deviation away from the cosmic microwave background estimate, regardless of the priors taken initially in each model. This supports the result of the traditional MCMC analysis and could be viewed as strengthening evidence for dynamical dark energy being a more favorable model of late time cosmology.

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arXiv2023

A fractal LTB model cannot explain Dark Energy

We revisited the problem of describing, on average, a fractal distribution of matter using a Lemaitre-Tolman-Bondi (LTB) solution. Here we study the fractal structure of our local universe having a fractal dimension and a scale transition, ensuring an homogeneous bang-time function. We test our model with the latest type Ia supernova data, the Pantheon compilation, and discuss problems and possible improvements for it, concluding that a fractal transition in LTB cosmology cannot be used to explain the effects of dark energy without requiring an inhomogeneous big-bang, but it can be useful to study structures at low scales.

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arXiv2022

Parametric and nonparametric methods hint dark energy evolution

We study dark energy through the viewpoints of parametric and nonparametric analyses of late-time cosmological data. We consider four Hubble parameter priors reflecting the Hubble tension and make use of two phenomenological functions, namely, a normalized dark energy density and a compactified dark energy equation of state. We predict the shape of both functions and present new constraints on the dark energy equation of state. The results hint at dark energy evolution regardless of the choice of the method and of the priors. The fact that similar evolutions for the dark energy densities are found through drastically different approaches suggests that the features found in this paper are driven by the data, and are not artifact of the reconstruction methods applied.

astro-ph.CO↗

arXiv2021

Matter production effects and interacting scenario within a reconstructed mimetic cosmology for late times

In this work we explore two possible scenarios that can be considered to extend a recent proposed model by the authors known as reconstructed mimetic cosmology. This study is complemented with an statistical analysis for each case. The first scenario considers the inclusion of matter production as a possible source of cosmic expansion in the reconstructed mimetic model, at effective level was found that this construction can cross the phantom divide, the model evolves from quintessence to phantom dark energy. The second scenario corresponds to a construction of an interacting scheme for the dark sector which is described by the unified mimetic model. The resulting interaction term (not imposed by an Ansatz), $Q$, exhibits changes of sign leading to the violation of the second law along the cosmic evolution and non adiabaticity; the temperatures for the components of the dark sector are computed and such components are shown to be out of thermal equilibrium.

gr-qc↗

arXiv2021

Reconstructing Mimetic Cosmology

We explore the mimetic gravity formulation with the inclusion of a scalar field potential namely, $V(ϕ)$. However, we are not considering any {\it a priori} specific form this term. By means of the Chevallier-Polarski-Linder parametrization for the parameter state of the fluid we can construct an explicit function for such potential in terms of the cosmological redshift and obtain analytical solutions in the mimetic gravity approach. We revise some cosmological implications of these results and additionally we perform a numerical reconstruction for the potential $V$ as a function of the mimetic scalar field, $ϕ$.

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arXiv2020

Cosmic expansion with matter creation and bulk viscosity

We explore the cosmological implications at effective level of matter creation effects in a dissipative fluid for a FLRW geometry; we also perform a statistical analysis for this kind of model. By considering an inhomogeneous Ansatz for the particle production rate we obtain that for created matter of dark matter type we can have a quintessence scenario or a future singularity known as little rip; in dependence of the value of a constant parameter, $η$, which characterizes the matter production effects. The dimensionless age of this kind of Universe is computed, showing that this number is greater than the standard cosmology value, this is typical of universes with presence of dark energy. The inclusion of baryonic matter is studied. By implementing the construction of the particle production rate for a dissipative fluid by considering two approaches for the expression of the bulk viscous pressure; we find that in Eckart model we have a big rip singularity leading to a catastrophic matter production and in the truncated version of the Israel-Stewart model such rate remains bounded leading to a quintessence scenario. For a non adiabatic dissipative fluid, we obtain a positive temperature and the cosmic expansion obeys the second law of thermodynamics.

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arXiv2020

Challenging matter creation models in the phantom divide

We perform a study both statistical and theoretical for cosmological models of matter creation and their ability to describe effective phantom models of dark energy. Such models are beyond the $Λ$CDM model since the resulting cosmic expansion is not adiabatic. In fact, we show that this approach exhibits transient phantom/quintessence scenarios at present time and tends to the standard cosmological model at some stage of the cosmic evolution. We discuss some generalities of the thermodynamics properties for this type of cosmological model; we emphasize on the behavior of the temperature associated to dark matter, which keeps positive along cosmic evolution together with the entropy. The enrichment of this type of model by means of the incorporation of cosmological constant and dissipative effects in the fluid description to explore their cosmological consequences in the expansion of the universe is considered. Finally, a generalization for the matter production rate as an inhomogeneous expression of the Hubble parameter and its derivatives is discussed; as in all the cases examined, such election leads to an effective phantom/quintessence behavior.

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arXiv2018

Dark energy and Dark matter interaction in light of the second law of thermodynamics

In the context of thermodynamics we discuss the way inevitable emerge an interaction between dark components, and in this way, provide a mechanism to understand the limits of the LCDM model and the class of interaction models between dark components. Using observational data we have tested two models of explicit interaction between dark components and reconstructed the evolution of temperatures for both components. We found that observations suggest the interaction exist with energy flowing from dark energy to dark matter. The best fit also suggest a phantom equation of state parameter for dark energy. We discuss the results having in mind the constraints imposed by thermodynamics.

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