arXiv subjects

Ruth Lazkoz

Publications and source records attributed to Ruth Lazkoz.

At least 19 recordsLinked to original sources

arXiv2026

Generalizing the CPL Parametrization through Dark Sector Interaction

We investigate a hierarchy of interacting dark energy (IDE) models featuring a non-gravitational coupling between dark matter and dark energy. Specifically, we examine scenarios where the background interaction kernel, $Q = 3H(δ+ ηa)ρ_\mathrm{de}$, allows for both constant and dynamical coupling parameters. Adopting the Chevallier-Polarski-Linder parametrization for the dark energy equation of state, $w_\mathrm{de} = w_0 + w_a(1-a)$, we derive closed analytical expressions for the energy densities of dark matter and dark energy. Afterwards, we obtain observational constraints using joint combinations of DESI DR2 baryon acoustic oscillations, Pantheon$+$ Type Ia supernovae, and Planck$+$ACT compressed cosmic microwave background likelihoods. For constant coupling models, we find parametric deviations from $Λ$ ranging from $2.7σ$ to $2.9σ$; however, for interactions with dynamical couplings, these significances are reduced to $1.3σ$--$1.5σ$. Ultimately, our Bayesian model comparison reveals that no investigated IDE scenario is statistically preferred over the concordance $Λ$CDM model. These results highlight the necessity of reporting Bayesian evidence alongside conventional frequentist maximum-likelihood analyses to ensure robust cosmological claims concerning dark energy evolution and interaction.

astro-ph.CO↗

arXiv2026

Is Chevallier-Polarski-Linder dark energy a mirage?

Recent observations from the Dark Energy Spectroscopic Instrument (DESI) raise doubts about the standard cosmological model, $Λ$CDM, suggesting a preference for an inherently dynamical dark energy component. The Chevallier-Polarski-Linder (CPL) parametrization -- a widely used two-parameter model for the dark energy equation of state -- displays marked early-time phantom behavior and a recent crossing of the phantom divide. These features suggest the convenience to check observationally the robustness of such evolution. To address this, we design two alternative families of two-parameter dark energy parametrizations which remain close to the original CPL but aim to soften its phantom character. Specifically, these models reproduce CPL-like behavior at low redshift but mitigate early phantom behavior through the use of smooth sigmoid transitions, yielding a more gradual evolution. By combining recent DESI data with constraints from the cosmic microwave background and Type Ia supernovae, we assess the viability of these models. Our analysis shows that CPL remains a strong and competitive parametrization, with the proposed alternatives only marginally favored or disfavored. We conclude that current observational data lack the statistical precision to decisively distinguish between CPL and similarly constructed parametrizations across the redshift range probed by late-time observables.

astro-ph.CO↗

arXiv2025

A Spectrum of Cosmological Rips and Their Observational Signatures

We present a unified dark energy framework capable of generating a continuous spectrum of cosmological ``rip'' scenarios -- including the Big Rip, Grand Rip, Mild Rip, Little Rip, Little Sibling of the Big Rip, and the newly found Dollhouse Rip -- while ensuring a physically consistent evolution across cosmic history. Building on earlier phenomenological proposals, we introduce a barotropic equation-of-state parameter with a sigmoid-like correction to guarantee a strictly positive dark energy density and to avoid early-time pathologies commonly present in previous models. Using this formulation, closed-form analytic expressions for the energy density can be obtained. This, in turn, enables a systematic classification of future singularities based on the signs and magnitudes of two key parameters of the model. We test these scenarios with state-of-the-art cosmological probes, including DESI DR2 BAO, cosmic chronometers, CMB compressed likelihoods, and the Pantheon+ supernovae sample. According to our Bayesian analysis, all rip scenarios yield best-fit parameters compatible with $Λ$CDM at the $1σ$ level, with Bayes factors weakly favoring $Λ$CDM. The mild, logarithmic evolution of the proposed dark energy density prevents current observations from distinguishing among the different future fates. We conclude that, for rip cosmologies to gain observational support over $Λ$CDM, they must display more accentuated late-time dynamical features -- such as perhaps rapid transitions or a phantom-divide crossing -- within the redshift range probed by present surveys.

astro-ph.CO↗

arXiv2025

Geometric acceleration in $f(Q,C)$ theories

The $f(Q,C)$ framework of gravity enables the depiction of an effective dark energy fluid that emerges from geometry itself, thus leading to modifications in the cosmological phenomenology of General Relativity. We pursue this approach to discover new and observationally supported (effective) evolving dark energy models. We propose a general $f(Q,C)$ formulation that cannot be simply split into separate functions of $Q$ and $C$, yet it still results in second-order field equations. By employing a particular type of connection, we derive guidelines for new cosmological models, including a variant of the DGP model that appears to be statistically favored over $Λ$CDM. Notably, we also demonstrate how to translate solutions within this $f(Q,C)$ framework to $f(Q)$ counterparts at the background level.

gr-qc↗

arXiv2024

Probing Interacting Dark Sector with the next generation of gravitational-wave detectors

We have probed the capability of third-generation Gravitational Waves (GW) interferometers, such as the Einstein Telescope and Cosmic Explorer, to constrain a cosmological model with an interacting dark sector. We focused on GW events with a detected electromagnetic counterpart being the $γ$ or X emission of Gamma-Ray Burst, and a Kilonova emission. We assume the first one to be detected by the THESEUS satellite, while the second one to be detected by the Vera Rubin Observatory. We probed three different interaction kernels and found that the posterior estimation of the cosmological parameters is biased due to the existing degeneracies between the dark and matter sectors. We also found that introducing an external prior on the matter density parameter breaks the degeneracy, removes the bias results, and improves the accuracy on the dark sector parameters.

gr-qc↗

arXiv2024

Characterization of wormhole space-times supported by a covariant action-dependent Lagrangian theory

In this work, we undertake an analysis of new wormhole solutions within an action-dependent Lagrangian framework. These geometries can be traversable and supported by a positive energy density. The modification of the gravitational field equations is produced by the inclusion in the gravitational Lagrangian linear of a background four-vector $λ_μ$. This new term expands significantly the conventional description of gravity making it highly non-linear, and therefore drawing general conclusions about legitimate forms of $λ_μ$ proves a formidable task in general. It is, then, customary to adopt an ansatz that strikes a balance between enabling new phenomenology while retaining a significant degree of generality on $λ_μ$. Ours is given by the choice $λ_μ=(0,λ_1(r), 0, 0)$, with an arbitrary $λ_1(r)$. By setting $λ_1(r)=-1/r$ we craft new families with physically desirable properties, but the wormholes thus generated turn out to be conical, as evidenced by an angle deficit, in a similar fashion to other known solution families. Under the general shape of $λ_1(r)$, we demonstrate that these solutions are not compatible with the Null Energy Condition (NEC) in general, as it happens to their General Relativity counterparts, except on specific occasions where the derivative of the redshift function of the metric diverges at the throat (however, in these latter cases, the traversability of the wormhole will be disrupted). On the other hand, it is possible to solve the conical character and satisfies the flatness condition for more general functions of $λ_1(r)$.

gr-qc↗

arXiv2023

Modified Gravity and Cosmology: An Update by the CANTATA Network

General Relativity and the $Λ$CDM framework are currently the standard lore and constitute the concordance paradigm. Nevertheless, long-standing open theoretical issues, as well as possible new observational ones arising from the explosive development of cosmology the last two decades, offer the motivation and lead a large amount of research to be devoted in constructing various extensions and modifications. All extended theories and scenarios are first examined under the light of theoretical consistency, and then are applied to various geometrical backgrounds, such as the cosmological and the spherical symmetric ones. Their predictions at both the background and perturbation levels, and concerning cosmology at early, intermediate and late times, are then confronted with the huge amount of observational data that astrophysics and cosmology are able to offer recently. Theories, scenarios and models that successfully and efficiently pass the above steps are classified as viable and are candidates for the description of Nature. This work is a Review of the recent developments in the fields of gravity and cosmology, presenting the state of the art, high-lighting the open problems, and outlining the directions of future research. Its realization was performed in the framework of the COST European Action ``Cosmology and Astrophysics Network for Theoretical Advances and Training Actions''.

gr-qc↗

arXiv2023

Dynamical systems analysis of $f(Q)$ gravity

Modified gravity theories can be used for the description of homogeneous and isotropic cosmological models through the corresponding field equations. These can be cast into systems of autonomous differential equations because of their sole dependence on a well chosen time variable, be it the cosmological time, or an alternative. For that reason a dynamical systems approach offers a reliable route to study those equations. Through a model independent set of variables we are able to study all $f(Q)$ modified gravity models. The drawback of the procedure is a more complicated constraint equation. However, it allows the dynamical system to be formulated in fewer dimensions than using other approaches. We focus on a recent model of interest, the power-exponential model, and generalise the fluid content of the model.

gr-qc↗

arXiv2022

Cosmological dynamical systems in modified gravity

The field equations of modified gravity theories, when considering a homogeneous and isotropic cosmological model, always become autonomous differential equations. This relies on the fact that in such models all variables only depend on cosmological time, or another suitably chosen time parameter. Consequently, the field equations can always be cast into the form of a dynamical system, a successful approach to study such models. We propose a perspective that is applicable to many different modified gravity models and relies on the standard cosmological density parameters only, making our choice of variables model independent. The drawback of our approach is a more complicated constraint equation. We demonstrate our procedure studying various modified gravity models and show how much generic information can be extracted before a specific model is considered.

gr-qc↗

arXiv2021

DGP and DGPish cosmologies from $f(Q)$ actions

In this work we explore and test new formulations of cosmological scenarios in $f(Q)$ theories. In these settings, the non-metricity scalar ($Q$) is the main source of gravity and Friedmann equations are modified to account for the associated degrees of freedom. This work focuses first on the derivation, and then theoretical and observational analysis of two such (new) exact cosmological models; they both display a non-standard behaviour in which an additional parameter encoding non-metricity effects acts in the fashion of a screened cosmological constant. One of the new settings has the same background evolution as the well know DGP cosmological model, while the other resembles the former considerably, although its origin is purely phenomenological. We use the Markov Chain Montecarlo method combined with standard statistical techniques to perform observational astrophysical tests relying upon background data, specifically these are Type Ia Supernovae luminosities and direct Hubble data (from cosmic clocks), along with Cosmic Microwave Background shift and Baryon Acoustic Oscillations data. In addition, we compute some of the cosmographic parameters and other discriminators with the purpose of refining our knowledge about these models in the light of their theoretical and observational signatures, and this allows for a better comparison with the (concordance) $Λ$CDM setup. We conclude that these scenarios do not show signatures indicating a departure from the $Λ$CDM behaviour.

astro-ph.CO↗

arXiv2021

ΛCDM suitably embedded in f(R) with a non-minimal coupling to matter

In this work, we further study a metric modified theory of gravity which contains a non-minimal coupling to matter, more precisely, we assume two functions of the scalar curvature, $f_1$ and $f_2$, where the first one generalises the Hilbert-Einstein action, while the second couples to the matter Lagrangian. On the one hand, assuming a $Λ$CDM background, we calculate analytical solutions for the functions $f_1$ and $f_2$. We consider two setups: on the first one, we fix $f_2$ and compute $f_1$ and on the second one, we fix $f_1$ and compute $f_2$. Moreover, we do the analysis for two different energy density contents, a matter dominated universe and a general perfect fluid with a constant equation of state fuelling the universe expansion. On the other hand, we complete our study by performing a cosmographic analysis for $f_1$ and $f_2$. We conclude that the gravitational coupling to matter can drive the accelerated expansion of the universe.

gr-qc↗

arXiv2020

Observational constraints on cosmological solutions of $f(Q)$ theories

Over the last years some interest has been gathered by $f(Q)$ theories, which are new candidates to replace Einstein's prescription for gravity. The non-metricity tensor $Q$ allows to put forward the assumption of a free torsionless connection and, consequently, new degrees of freedom in the action are taken into account. This work focuses on a class of $f(Q)$ theories, characterized by the presence of a general power-law term which adds up to the standard (linear in) $Q$ term in the action, and on new cosmological scenarios arising from them. Using the Markov chain Montecarlo method we carry out statistical tests relying upon background data such as Type Ia Supernovae luminosities and direct Hubble data (from cosmic clocks), along with Cosmic Microwave Background shift and Baryon Acoustic Oscillations data. This allows us to perform a multifaceted comparison between these new cosmologies and the (concordance) $Λ$CDM setup. We conclude that, at the current precision level, the best fits of our $f(Q)$ models correspond to values of their specific parameters which make them hardly distinguishable from our General Relativity "échantillon", that is $Λ$CDM.

astro-ph.CO↗

arXiv2019

Observational constraints of $f(Q)$ gravity

In this work, we consider an extension of symmetric teleparallel gravity, namely, $f(Q)$ gravity, where the fundamental block to describe spacetime is the nonmetricity, $Q$. Within this formulation of gravitation, we perform an observational analysis of several modified $f(Q)$ models using the redshift approach, where the $f(Q)$ Lagrangian is reformulated as an explicit function of the redshift, $f(z)$. Various different polynomial parameterizations of $f(z)$ are proposed, including new terms which would allow for deviations from the $Λ$CDM model. Given a variety of observational probes, such as the expansion rate data from early-type galaxies, Type Ia Supernovae, Quasars, Gamma Ray Bursts, Baryon Acoustic Oscillations data and Cosmic Microwave Background distance priors, we have checked the validity of these models at the background level in order to verify if this new formalism provides us with plausible alternative models to explain the late time acceleration of the universe. Indeed, this novel approach provides a different perspective on the formulation of observationally reliable alternative models of gravity.

gr-qc↗

arXiv2019

Evidence of Neutrino Enhanced Clustering in a Complete Sample of Sloan Survey Clusters, Implying $\sum m_ν= 0.119 \pm 0.034$ eV

The clustering amplitude of 7143 clusters from the Sloan Digital Sky Survey (SDSS) is found to increase with cluster mass, closely agreeing with the Gaussian random field hypothesis for structure formation. The amplitude of the observed cluster correlation exceeds the predictions from pure cold dark matter (CDM) simulation by $\simeq 6\%$ for the standard Planck-based values of the cosmological parameters. We show that this excess can be naturally accounted for by free streaming of light neutrinos, which opposes gravitational growth, so clusters formed at fixed mass are fewer and hence more biased than for a pure CDM density field. An enhancement of the cluster bias by 7\% matches the observations, corresponding to a total neutrino mass, $m_ν = 0.119 \pm 0.034$ eV at 67\% confidence level, for the standard relic neutrino density. If ongoing laboratory experiments favor a normal neutrino mass hierarchy then we may infer a somewhat larger total mass than the minimum oscillation based value, $\sum m_ν \simeq 0.056eV$, with 90\% confidence. Much higher precision can be achieved by applying our method to a larger sample of more distant clusters with weak lensing derived masses.

astro-ph.CO↗

arXiv2019

The Umami Chaplygin Model

In this work we study in detail a phenomenological generalization of the Chaplygin cosmological model, which we call as \textit{umami} Chaplygin model. We consider three different cosmological background scenarios in which our fluid can play three different roles: only as a dark energy component; as a dark matter and dark energy component; and as a dark plus baryonic matter and dark energy component. With such analysis we explore the possibility to unify the dark fluids under one single component within the context of General Relativity. We test this hypothesis against the main available data related to the cosmological background, namely: Type Ia supernovae; Baryon Acoustic Oscillations; Quasars; Gamma Ray Bursts; Hubble data from cosmic chronometers; and Cosmic Microwave Background. We eventually compare the statistically efficiency and reliability of our model to describe observational data with respect to the standard $Λ$CDM model by means of the Bayesian Evidence. Final results point to a positive (albeit not strong) evidence in favor of a possible unification of dark energy and dark matter with a the \textit{umami} fluid.

astro-ph.CO↗

arXiv2018

Modified gravity revealed along geodesic tracks

The study of the dynamics of a two-body system in modified gravity constitutes a more complex problem than in Newtonian gravity. Numerical methods are typically needed to solve the equations of geodesics. Despite the complexity of the problem, the study of a two-body system in $f(R)$ gravity leads to a new exciting perspective hinting the right strategy to adopt in order to probe modified gravity. Our results point out some differences between the {\em semiclassical} (Newtonian) approach, and the {\em relativistic} (geodesic) one thus suggesting that the latter represents the best strategy for future tests of modified theories of gravity. { Finally, we have also highlighted the capability of forthcoming observations to serve as smoking gun of modified gravity revealing a departure from GR or further reducing the parameter space of $f(R)$ gravity}. \keywords{$f(R)$ gravity \and binary system \and geodesics \and precession

gr-qc↗

arXiv2018

Analysis of the Yukawa gravitational potential in $f(R)$ gravity I: semiclassical periastron advance

The {\it concordance} cosmological model has been successfully tested throughout the last decades. Despite its successes, the fundamental nature of dark matter and dark energy is still unknown. Modifications of the gravitational action have been proposed as an alternative to these dark components. The straightforward modification of gravity is to generalize the action to a function, $f(R)$, of the scalar curvature. Thus one is able to describe the emergence and the evolution of the Large Scale Structure without any additional (unknown) dark component. In the weak field limit of the $f(R)$-gravity, a modified Newtonian gravitational potential arises. This gravitational potential accounts for an extra force, generally called fifth force, that produces a precession of the orbital motion even in the classic mechanical approach. We have shown that the orbits in the modified potential can be written as Keplerian orbits under some conditions on the strength and scale length of this extra force. Nevertheless, we have also shown that this extra term gives rise to the precession of the orbit. Thus, comparing our prediction with the measurements of the precession of some planetary motions, we have found that the strength of the fifth force must be in the range $[2.70-6.70]\times10^{-9}$ whit the characteristic scale length to fixed to the fiducial values of $\sim 5000$ AU.

gr-qc↗

arXiv2018

Analysis of the Yukawa gravitational potential in $f(R)$ gravity II: relativistic periastron advance

Alternative theories of gravity may serve to overcame several shortcomings of the standard cosmological model but, in their weak field limit, General Relativity must be recovered so as to match the tight constraints at the Solar System scale. Therefore, testing such alternative models at scales of stellar systems could give a unique opportunity to confirm or rule them out. One of the most straightforward modifications is represented by analytical $f(R)$-gravity models that introduce a Yukawa-like modification to the Newtonian potential thus modifying the dynamics of particles. Using the geodesics equations, we have illustrated the amplitude of these modifications. First, we have integrated numerically the equations of motion showing the orbital precession of a particle around a massive object. Second, we have computed an analytic expression for the periastron advance of systems having their semi-major axis much shorter than the Yukawa-scale length. Finally, we have extended our results to the case of a binary system composed of two massive objects. Our analysis provides a powerful tool to obtain constraints on the underlying theory of gravity using current and forthcoming datasets.

gr-qc↗