arXiv subjects

David Tamayo

Publications and source records attributed to David Tamayo.

8 recordsLinked to original sources

arXiv2025

Thermodynamics of sign-switching dark energy models

We perform a comprehensive thermodynamic analysis of three sign-switching dark energy models in a flat FLRW cosmology: graduated dark energy (gDE), sign-switching cosmological constant ($\Lambda_s$), and smoothed sign-switching cosmological constant ($\Lambda_t$). We systematically derive key cosmological thermodynamic quantities -- horizon temperature, horizon entropy, internal entropy, total entropy, and their first and second derivatives -- using the Generalised Second Law (GSL) as the fundamental evaluation criterion. We first confirm the compliance of the $\Lambda$CDM model with the GSL, establishing a baseline for comparison. We find that despite their unconventional negative-to-positive energy density transitions, both $\Lambda_s$ and $\Lambda_t$ remain thermodynamically consistent. In contrast, gDE exhibits significant issues: divergences in its equation-of-state lead to infinite horizon temperature and entropy derivatives; and asymptotically, the horizon temperature diverges while entropy approaches zero, causing entropy reduction and violating the GSL. We highlight a key insight: models with divergences in the product of the dark energy equation-of-state parameter and its energy density ($w_x \Omega_x$) inevitably produce thermodynamic inconsistencies in standard cosmology. This thermodynamic approach provides a complementary criterion alongside observational constraints for evaluating the physical viability of cosmological models.

astro-ph.CO↗

arXiv2025

Equivalence of Dark Energy Models: A Theoretical and Bayesian Perspective

We explore the background equivalence among three dark energy models by constructing explicit mappings between dynamical dark energy (DDE), interacting dark energy (IDE), and running vacuum (RV). In our approach, the dark sector functions that characterize each model-such as the equation of state parameter $\bar{w}(a)$ for DDE, the interaction term $Q$ for IDE, and the functional form $\Lambda(H)$ for RV-are transformed into one another under specific assumptions. Extending previous work by von Marttens et al. (2020), we demonstrate that running vacuum models, characterized by $\Lambda(H) = a_0 + a_1 \dot{H} + a_2 H^2$, can be reinterpreted as an interacting dark energy model with $Q = 3H\gamma \hat{\rho}_c$, which in turn is equivalent to a dynamic dark energy model with an appropriately defined $\bar{w}(a)$. Using Bayesian analysis with Type Ia supernovae, Baryon Acoustic Oscillations, and Cosmic Chronometers, our observational constraints confirm that these theoretical equivalences hold at the background level. This study underscores the importance of seeking convergence in dark energy models, facilitating a better understanding of the dark sector.

astro-ph.CO↗

arXiv2024

Coupled Multi Scalar Field Dark Energy

The main aim of this paper is to present the multi scalar field components as candidates to be the dark energy of the universe and their observational constraints. We start with the canonical Quintessence and Phantom fields with quadratic potentials and show that a more complex model should bear in mind to satisfy current cosmological observations. Then we present some implications for a combination of two fields, named as Quintom models. We consider two types of models, one as the sum of the quintessence and phantom potentials and other including an interacting term between fields. We find that adding one extra degree of freedom, by the interacting term, the dynamics enriches considerably and could lead to an improvement in the fit of $-2\lnΔ\Like_{\rm max}= 5.19$, compared to $Λ$CDM. The resultant effective equation of state is now able to cross the phantom divide line, and in several cases present an oscillatory or discontinuous behavior, depending on the interaction value. The parameter constraints of the scalar field models (quintessence, phantom, quintom and interacting quintom) were performed using Cosmic Chronometers, Supernovae Ia and Baryon Acoustic Oscillations data; and the Log-Bayes factors were computed to compare the performance of the models. We show that single scalar fields may face serious troubles and hence the necessity of a more complex models, i.e. multiple fields.

astro-ph.CO↗

arXiv2022

Thermodynamics of viscous dark energy for the late future time universe

In this work we explore the thermodynamic aspects of dark energy for late future time universe in two different scenarios: as a perfect fluid with constant and variable equation of state parameter; and as dissipative fluid described by a barotropic equation of state with bulk viscosity in the framework of the Eckart theory and the full Israel-Stewart theory. We explore cosmological solutions for a flat, homogeneous and isotropic universe; and we assume the late future time behavior when the dark energy dominates the cosmic evolution. When modeled as a perfect fluid with a dynamical equation of state, $p=w(a)ρ$, the dark energy has an energy density, temperature and entropy well defined and an interesting result is that there is no entropy production even though been dynamical. For dissipative dark energy, in the Eckart theory two cases are studied: $ξ=const.$ and $ξ=(β/\sqrt{3}) ρ^{1/2}$; it is found that the entropy grows exponentially for the first case and as a power-law for the second. In the Israel-Stewart theory we consider a $ξ=ξ_0 ρ^{1/2}$ and a relaxation time $τ= ξ/ρ$; an analytical Big Rip solution is obtained with a power-law entropy. In all cases a power-law relation between temperature and energy density is obtained. In order to maintain the second law of thermodynamics theoretical constraints for the equation of state are found in the different dark energy models studied. A barotropic dark fluid with $w<-1$ is thermodynamically difficult to support, but the overall effect of bulk viscosity in certain cases allows a phantom regime without thermodynamic anomalies.

gr-qc↗

arXiv2021

Bayesian model selection on Scalar $ε$-Field Dark Energy

The main aim of this paper is to analyse minimally-coupled scalar-fields -- quintessence and phantom -- as the main candidates to explain the accelerated expansion of the universe and compare its observables to current cosmological observations; as a byproduct we present its python module. This work includes a parameter $ε$ which allows to incorporate both quintessence and phantom fields within the same analysis. Examples of the potentials, so far included, are $V(ϕ)=V_0ϕ^μe^{βϕ^α}$ and $V(ϕ)=V_0(\cosh(αϕ)+β)$ with $α$, $μ$ and $β$ being free parameters, but the analysis can be easily extended to any other scalar field potential. Additional to the field component and the standard content of matter, the study also incorporates the contribution from spatial curvature ($Ω_k$), as it has been the focus in recent studies. The analysis contains the most up-to-date datasets along with a nested sampler to produce posterior distributions along with the Bayesian evidence, that allows to perform a model selection. In this work we constrain the parameter-space describing the two generic potentials, and among several combinations, we found that the best-fit to current datasets is given by a model slightly favouring the quintessence field with potential $V(ϕ)=V_0ϕ^μe^{βϕ}$ with $β=0.22\pm 1.56$, $μ= -0.41\pm 1.90$, and slightly negative curvature $Ω_{k,0}=-0.0016\pm0.0018$, which presents deviations of $1.6σ$ from the standard $Λ$CDM model. Even though this potential contains three extra parameters, the Bayesian evidence $\mathcal{B}_{Λ, ϕ} =2.0$ is unable to distinguish this model compared to the $Λ$CDM with curvature ($Ω_{k,0}=0.0013\pm0.0018$). The potential that provides the minimal Bayesian evidence corresponds to $V(ϕ)=V_0 \cosh(αϕ)$ with $α=-0.61\pm 1.36$.

gr-qc↗

arXiv2019

Fourier-series expansion of the dark-energy equation of state

The dark energy component of the universe still remains as a mystery, however, several papers based on observational data have shown that its equation of state may have an oscillatory behaviour. In this paper, we provide a general description for the dark-energy equation-of-state $w(z)$ in the form of Fourier series. This description generalises some previous dynamical dark energy models and is in agreement with the $w(z)$ reconstructions. We make use of a modified version of a simple and fast Markov Chain Monte Carlo code to constraint the model parameters. For the analysis we use data from supernovae type-Ia , baryon acoustic oscillations, $H(z)$ measurements and cosmic microwave background. We provide a comparison of the proposed model with $Λ$CDM, $w$CDM and the standard Taylor approximation. The Fourier series expansion of $w(z)$ is preferred from $Λ$CDM at more than $3σ$ significance level based on the improvement in the fit alone. We use the Akaike criteria to perform the model comparison and found that, even though there are extra parameters, there is a slight preference of the Fourier series compared with the $Λ$CDM model. The preferred shape of $w(z)$ found here puts in jeopardy the single scalar field models, as they as they cannot reproduce the crossing the phantom divide line $w=-1$.

astro-ph.CO↗

arXiv2019

Dynamical Systems Analysis in Post-Friedmann Parametrizations of Modified Theories of Gravity

We carry out a dynamical analysis of first order perturbations for Cold Dark Matter, $Λ$ Cold Dark Matter, and a couple of Modified Gravity models using the Parametrized Post-Friedmann formalism. We use normalized variables to set the proper dynamical system of equations through which we make the analysis in order to shed some light on the dynamics of such perturbations inside these models. For Modified Gravity models, we use the scale-independent and -dependent parametrizations, in particular, two $f(R)$ and two Chameleon-like models are considered within the quasi-static approximation. Given the employed formalism, we found that the critical points and stability features of the dynamical systems for Modified Gravity models are the same as those found in the standard $Λ$ Cold Dark Matter model. However, the behavior around the critical points suffers important modifications in some specific cases. We explicitly find that signatures of these Modified Gravity models mainly arise on the velocity perturbations, while the density contrast and the curvature potentials turn out to be less sensitive to the parametrization taken into consideration. We also provide a percentage estimation of the extent of modification in the perturbations in the Modified Gravity models considered in comparison to the standard $Λ$ Cold Dark Matter model along the expansion history and for a couple of wavenumbers.

gr-qc↗

arXiv2018

Non-linear coupling in the dark sector as a running vacuum model

In this work we study a phenomenological non-gravitational interaction between dark matter and dark energy. The scenario studied in this work extends the usual interaction model proportional to the derivative of the dark component density adding to the coupling a non-linear term of the form $Q = ρ'/3(α+ βρ)$. This dark sector interaction model could be interpreted as a particular case of a running vacuum model of the type $Λ(H) = n_0 + n_1 H^2 + n_2 H^4$ in which the vacuum decays into dark matter. For a flat FRW Universe filled with dark energy, dark matter and decoupled baryonic matter and radiation we calculate the energy density evolution equations of the dark sector and solve them. The different sign combinations of the two parameters of the model show clear qualitative different cosmological scenarios, from basic cosmological insights we discard some of them. The linear scalar perturbation equations of the dark matter were calculated. Using the CAMB code we calculate the CMB and matter power spectra for some values of the parameters $α$ and $β$ and compare it with $Λ$CDM. The model modify mainly the lower multipoles of the CMB power spectrum remaining almost the same the high ones. The matter power spectrum for low wave numbers is not modified by the interaction but after the maximum it is clearly different. Using observational data from Planck, and various galaxy surveys we obtain the constraints of the parameters, the best fit values obtained are the combinations $α= (3.7 \pm 7 )\times 10^{-4} $, $-(1.5\times10^{-5} {\rm eV}^{-1})^{4} \ll β< (0.07 {\rm eV}^{-1})^4$.

astro-ph.CO↗