arXiv subjects

Ramón Herrera

Publications and source records attributed to Ramón Herrera.

At least 19 recordsLinked to original sources

arXiv2026

Gravitational Wave Propagation in K-essence Cosmology: Theory and Observational Constraints

Gravitational waves (GWs) provide a powerful, theory-independent probe of the dynamical structure of spacetime and the cosmological background. We study linearized GW propagation in k-essence cosmology, where a non-canonical scalar field describes the dark sector. In the high-frequency (short-wavelength) approximation on a Friedmann--Lema\^ıtre--Robertson--Walker (FLRW) background, and restricting to the transverse-traceless tensor sector, we derive a modified evolution equation for tensor perturbations. The GW speed remains strictly luminal, consistent with multimessenger bounds such as GW170817, but the interaction with the background field $\barϕ$ induces a time-dependent effective mass-like term $m_{\rm eff}$. This background-induced mass modifies the dispersion relation without introducing additional propagating degrees of freedom, leading to a cumulative, frequency-dependent phase shift in the waveform over cosmological distances. We show that $m_{\rm eff}$ is uniquely determined by background cosmological parameters and can be written as a redshift-dependent function, $m_{\rm eff}(z)$, directly linking GW observables to scalar-field dynamics, while the GW luminosity distance remains identical to its electromagnetic counterpart, preserving standard-siren consistency. We test the scenario through a joint Bayesian analysis that combines cosmic chronometers (CC), BAO, Pantheon+SH0ES, and standard-siren data from GWTC-2.1/3/4. The reconstruction is consistent with current constraints and reproduces the late-time expansion history, while the evolution of $m_{\rm eff}(z)$ offers a new mechanism that may help alleviate the $H_0$ tension.

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arXiv2026

Reconstructing inflation in Einstein-Gauss-Bonnet gravity in light of ACT data

During the inflationary epoch, we investigate the reconstruction of the background variables within the framework of Einstein-Gauss-Bonnet gravity, considering the scalar spectral index $n_s(N)$ and the tensor-to-scalar ratio $r(N)$, where $N$ denotes the number of $e-$folds. Under a general formalism, we determine the effective potential and the coupling function associated with the Gauss-Bonnet term as functions of the cosmological parameters $n_s(N)$ and $r(N)$, respectively. To implement the reconstruction methodology for the background variables, we study an example in which the attractors for the index $n_s$ and the ratio $r$ are in agreement with Atacama Cosmology Telescope (ACT) data. In this context, explicit expressions for the effective potential $V(ϕ)$ and the coupling parameter $ξ(ϕ)$ are reconstructed. Moreover, the reconstruction based on observational parameters shows that $V(ϕ)\not\propto 1/ξ(ϕ)$, in contrast to the assumption adopted in the literature for the study of the evolution of the universe in Einstein-Gauss-Bonnet gravity.

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arXiv2025

Cosmological dynamics of interacting dark matter-dark energy in generalized Rastall gravity

In this work, we investigate late-time interacting cosmologies within the framework of generalized Rastall gravity, where the interaction arises naturally from the non-conservation of the energy-momentum tensor. We formulate the background evolution of the dark sector as an autonomous dynamical system, defining interaction terms $Q_1=α\,\dot{f}$ and $Q_2=-\dot{f}\,(1+α)$, with $α$ a constant parameter and $f$ a time-dependent function. Three interaction cases are studied: $f \propto ρ_m$, $f \propto ρ_{de}$, and $f \propto ρ_m + ρ_{de}$, assuming a constant dark-energy equation of state $w_{de}$. For each scenario, we derive the closed dynamical system in terms of the density parameters $(Ω_{de}, Ω_m)$, identify its fixed points, and analyze their stability across the parameter space. In this context, the phase-space exhibits a standard cosmological dynamics: an unstable radiation point, a transient matter saddle, and a stable late-time attractor with accelerated expansion. In addition, we utilize a joint likelihood analysis with Cosmic Chronometers, PantheonPlus, and DESI data to obtain marginalized parameter estimates at the $68\%$ and $95\%$ confidence levels, constraining the parameter space in each interaction model.

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arXiv2025

Interacting dilatonic ghost condensate as dark energy model

In this paper, we examine the cosmological evolution of a dilatonic ghost condensate field responsible for dark energy, which interacts with dark matter through a source term. We explore three different interaction models to describe the present universe. For each interaction model, we perform a detailed phase-space analysis, obtaining the stability conditions, and identifying the critical points. Furthermore, we compare our interaction models with the most recent Hubble parameter and supernova Ia data as functions of redshift. Additionally, we investigate the conditions for scaling regimes in these models and analyze the successful transition toward an attractor point to characterize the behavior of dark matter.

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arXiv2024

Constant-roll inflation with a complex scalar field

We consider inflation with a constant rate of rolling in which a complex scalar field plays the role of inflaton during the inflationary epoch. We implement the inflationary analysis for an accredited angular speed $\dotθ$ which satisfies our dynamical equations. Scalar and tensorial perturbations generated in the framework of constant roll inflation with a complex field are studied. In this respect, we find analytically solutions to the gauge invariant fluctuations, with which an expression for the scalar power spectrum together with its scalar index spectral in this scenario were found. By comparing the obtained results with the observations coming from the cosmic microwave background anisotropies, the constraints on the parameters space of the model and also its predictions are analyzed and discussed.

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arXiv2022

Warm inflation with bulk viscous pressure for different solutions of an anisotropic universe

We study a warm inflationary model for different expansions assuming an anisotropic universe described by Bianchi I metric. The universe is filled with a scalar field or inflaton, radiation, and bulk viscous pressure. We carry out the inflationary analysis for different solutions of such universe in two different cases of the bulk viscosity coefficient $ξ$ and the dissipation coefficient $Γ$ as constant and variable parameters, respectively. We compare the obtained results with the recent observations, in order to find the observational constraints on the parameters space of the models. Moreover, we attempt to present a better judgment among the considered models by calculation of the non-linear parameter $f_{NL}$ describing the non-Gaussianity property of the models. Additionally, we investigate the warm inflationary models with viscous pressure from the Weak Gravity Conjecture approach, considering the swampland criteria.

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arXiv2021

Reconstructing inflation in scalar-torsion $f(T,ϕ)$ gravity

It is investigated the reconstruction during the slow-roll inflation in the most general class of scalar-torsion theories whose Lagrangian density is an arbitrary function $f(T,ϕ)$ of the torsion scalar $T$ of teleparallel gravity and the inflaton $ϕ$. For the class of theories with Lagrangian density $f(T,ϕ)=-M_{pl}^{2} T/2 - G(T) F(ϕ) - V(ϕ)$, with $G(T)\sim T^{s+1}$ and the power $s$ as constant, we consider a reconstruction scheme for determining both the non-minimal coupling function $F(ϕ)$ and the scalar potential $V(ϕ)$ through the parametrization (or attractor) of the scalar spectral index $n_{s}(N)$ and the tensor-to-scalar ratio $r(N)$ as functions of the number of $e-$folds $N$. As specific examples, we analyze the attractors $n_{s}-1 \propto 1/N$ and $r\propto 1/N$, as well as the case $r\propto 1/N (N+γ)$ with $γ$ a dimensionless constant. In this sense and depending on the attractors considered, we obtain different expressions for the function $F(ϕ)$ and the potential $V(ϕ)$, as also the constraints on the parameters present in our model and its reconstruction.

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arXiv2020

On (non-)dynamical dark energy

The current Universe is composed by a mixture of relativistic species, baryonic matter, dark matter and dark energy which evolve in a non-trivial way at perturbative level. An advanced description of the cosmological dynamics should include non-standard features beyond the simplistic approach idealized by the standard cosmology in which cosmic components do not interact, are adiabatic and dissipationless. We promote a full perturbative analysis of linear scalar perturbations of a non-interacting cosmological model containing baryons, dark matter (both pressureless) and a scalar field allowing for the presence of relative entropic perturbations between the three fluids. Assuming an effective scalar-field sound speed equal to one and neglecting anisotropic stresses we establish a new set of equations for the scalar cosmological perturbations. As a consequence of this new approach, we show that tiny departures from a constant scalar field equation of state $w_S=-1$ damage structure formation in a non-acceptable manner. Hence, by strongly constraining $w_S$ our results provide compelling evidence in favor of the standard cosmological model and rule out a large class of dynamical dark energy models

astro-ph.CO↗

arXiv2019

Reconstruction and exact solutions for cosmological perturbations from a generalized gravity theory

Scalar and tensor cosmological perturbations during an inflationary universe scenario in the context of the a generalized gravity theory are studied. This analyze is carried out considering an ansatz on the variables associated to scalar and tensor perturbation ($z_s$ and $z_t$) in the Jordan frame. In this context, we analyze two different Ansatze for the ratio $z_s/z_t$, and we study in great detail the analytical and exact solutions for the cosmological perturbations together with the corresponding reconstruction of the background variables. Recent observational data from the Planck 2018 results are employed to constrain the parameters of each of the models.

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arXiv2019

Constraining chameleon field driven warm inflation with Planck 2018 data

We investigate warm inflationary scenario in which the accelerated expansion of the early Universe is driven by chameleon-like scalar fields. Due to the non-minimal coupling between the scalar field and the matter sector, the energy-momentum tensor of each fluid component is not conserved anymore, and the generalized balance equation is obtained. The new source term in the energy equation can be used to model warm inflation. On the other hand, if the coupling function varies slowly, the model reduces to the standard model used for the description of cold inflation. To test the validity of the warm chameleon inflation model, the results for warm inflationary scenarios are compared with the observational Planck2018 Cosmic Microwave Background data. In this regard, the perturbation parameters such as the amplitude of scalar perturbations, the scalar spectral index and the tensor-to-scalar ratio are derived at the horizon crossing in two approximations, corresponding to the weak and strong dissipative regimes. As a general result it turns out that the theoretical predictions of the chameleon warm inflationary scenario are consistent with the Planck 2018 observations.

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arXiv2018

Large-scale (in-) stability Analysis of an Exactly Solved Coupled Dark-Energy Model

Assuming a non-gravitational interaction amongst the dark fluids of our universe namely, the dark matter and dark energy, we study a specific interaction model in the background of a spatially flat Friedmann-Lemaître-Robertson-Walker geometry. The interaction model, as we found, solves the background evolution in an analytic way when the dark energy takes a constant barotropic equation of state, $w_x$. In particular, we analyze two separate interaction scenarios, namely, when the dark energy is a fluid other than the vacuum energy (i.e., $w_x \neq -1$) and when it is vacuum energy itself (i.e., $w_x = -1$). We found that the interacting model with $w_x \neq -1$ produces stable perturbation at large scales for $w_x < -1$ with the coupling strength $ξ<0$. Both the scenarios have been constrained with the latest astronomical data having distinct origin. The analyses show that a very small interaction with coupling strength is allowed and within 68.3\% confidence-region, $ξ=0$ is recovered. The analyses further show that a large coupling strength significantly affects the large scale dynamics of the universe while according to the observational data the interaction models are very well consistent with the $Λ$-cosmology. Furthermore, we observe that for the vacuum interaction scenario, the tension on $H_0$ is not released while for the interacting dark energy scenario with $w_x < -1$, the tension on $H_0$ seems to be released partially because of the high error bars in $H_0$. Finally, we close the work with the Bayesian evidence which shows that the $Λ$CDM cosmology is favored over the two interacting scenarios.

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arXiv2016

Warm intermediate inflationary Universe model in the presence of a Generalized Chaplygin Gas

A warm intermediate inflationary model in the context of Generalized Chaplygin Gas is investigated. We study this model in the weak and strong dissipative regimes, considering a generalized form of the dissipative coefficient $Γ=Γ(T,ϕ)$, and we describe the inflationary dynamics in the slow-roll approximation. We find constraints on the parameters in our model considering the Planck 2015 data, together with the condition for warm inflation $T>H$, and the conditions for the weak and strong dissipative regimes.

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arXiv2015

Warm intermediate inflation in the Randall-Sundrum II model in the light of Planck 2015 and BICEP2 results: A general dissipative coefficient

A warm inflationary Universe in the Randall-Sundrum II model during intermediate inflation is studied. For this purpose, we consider a general form for the dissipative coefficient $Γ(T,ϕ)=C_ϕ\,\frac{T^{m}}{ϕ^{m-1}}$, and also analyze this inflationary model in the weak and strong dissipative regimes. We study the evolution of the Universe under the slow-roll approximation and find solutions to the full effective Friedmann equation in the brane-world framework. In order to constrain the parameters in our model, we consider the recent data from the BICEP2-Planck 2015 data together with the necessary condition for warm inflation $T>H$, and also the condition from the weak (or strong) dissipative regime.

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arXiv2015

Emergent Cosmology, Inflation and Dark Energy

A new class of gravity-matter models defined in terms of two independent non-Riemannian volume forms (alternative generally covariant integration measure densities) on the space-time manifold are studied in some detail. These models involve an additional $R^2$ (square of the scalar curvature) term as well as scalar matter field potentials of appropriate form so that the pertinent action is invariant under global Weyl-scale symmetry. Scale invariance is spontaneously broken upon integration of the equations of motion for the auxiliary volume-form degrees of freedom. After performing transition to the physical Einstein frame we obtain: (i) An effective potential for the scalar field with two flat regions which allows for a unified description of both early universe inflation as well as of present dark energy epoch; (ii) For a definite parameter range the model possesses a non-singular "emergent universe" solution which describes an initial phase of evolution that precedes the inflationary phase; (iii) For a reasonable choice of the parameters the present model conforms to the Planck Collaboration data.

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arXiv2014

General dissipative coefficient in warm intermediate inflation in loop quantum cosmology in light of Planck and BICEP2

In this paper, we study a warm intermediate inflationary model with a general form for the dissipative coefficient $Γ(T,ϕ)=C_ϕ\,T^{m}/ϕ^{m-1}$ in the context of loop quantum cosmology. We examine this model in the weak and strong dissipative regimes. In general, we discuss in great detail the characteristics of this model in the slow-roll approximation. Also, we assume that the modifications to perturbation equations result exclusively from Hubble rate. In this approach, we use recent astronomical observations from Planck and BICEP2 experiments to restrict the parameters in our model.

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arXiv2013

Cosmology with Ricci dark energy

We assume the cosmological dark sector to consist of pressureless matter and holographic dark energy with a cutoff length proportional to the Ricci scale. The requirement of separate energy-momentum conservation of the components is shown to establish a relation between the matter fraction and the (necessarily time-dependent) equation-of-state parameter of the dark energy. Focusing on intrinsically adiabatic pressure perturbations of the dark-energy component, the matter perturbations are found as linear combinations of the total energy-density perturbations of the cosmic medium and the relative (nonadiabatic) perturbations of the components. The resulting background dynamics is consistent with observations from supernovae of type Ia, baryonic acoustic oscillations and the differential age of old objects. The perturbation dynamics, on the other hand, is plagued by instabilities which excludes any phantom-type equation of state. The only stable configuration is singled out by a fixed relation between the present matter fraction $Ω_{m0}$ and the present value $ω_{0}$ of the equation-of-state parameter of the dark energy. However, this instability-avoiding configuration is only marginally consistent with the observationally preferred background values of the mentioned parameters.

astro-ph.CO↗

arXiv2012

The generalized second law and the emergent universe

We explore whether the generalized second law of thermodynamics is fulfilled in the transition from a generic initial Einstein static phase to the inflationary phase, with constant Hubble rate, and from the end of the latter to the conventional thermal radiation dominated era of expansion. As it turns out, the said law is satisfied provided the radiation component does not contribute largely to the total energy of the static phase.

gr-qc↗

arXiv2012

Parameterizing the Deceleration Parameter

We propose and constrain with the latest observational data three parameterizations of the deceleration parameter, valid from the matter era to the far future. They are well behaved and do not diverge at any redshift. On the other hand, they are model independent in the sense that in constructing them the only assumption made was that the Universe is homogeneous and isotropic at large scales.

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