arXiv subjects

Ismael Ayuso

Publications and source records attributed to Ismael Ayuso.

13 recordsLinked to original sources

arXiv2026

Degenerate and connection-dependent cosmological sectors in f(Q,C) gravity

We investigate cosmological solutions in $f(Q,C)$ gravity formulated within symmetric teleparallel geometry, where gravitation is described by the nonmetricity scalar $Q$ and the boundary term $C$, related to the Ricci scalar of General Relativity through $\mathring{R}=Q+C$ in the absence of curvature and torsion. The symmetry requirements of FLRW spacetime admit three distinct realizations of the affine connection, leading in principle to three different cosmological sectors within the same theory. We show that the connection field equations play a crucial role in determining the cosmological dynamics. In their simplest realization, these equations impose a constraint that renders the theory dynamically equivalent to $f(\mathring{R})$ gravity, causing the cosmological background equations associated with the three connection realizations to coincide. This defines a degenerate cosmological sector of $f(Q,C)$ gravity, in which three a priori distinct geometric constructions converge to the same cosmological dynamics. We then consider the complementary class of genuinely nonequivalent $f(Q,C)$ models, for which the choice of connection becomes physically relevant. In this regime, the connection sector introduces additional dynamical degrees of freedom and gives rise to novel cosmological phenomenology absent in $f(\mathring{R})$ gravity.

gr-qc↗

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

Insights in $f(Q)$ cosmology: the relevance of the connection

We explore the role of the affine connection in $f(Q)$ gravity, a modified theory where gravity is governed by non-metricity within the symmetric teleparallel framework. Although the connection is constrained to be flat and torsionless, it is not uniquely determined by the metric, allowing for multiple physically distinct formulations. We analyze three such connections compatible with a homogeneous and isotropic universe to show that they yield markedly different cosmological dynamics, even under the same functional form of $f(Q)$. Using both analytical and numerical methods, including a Born-Infeld type model of $f(Q)$, we demonstrate that specific connections can resolve cosmological singularities like the Big Bang and Big Rip, replacing them with smooth de Sitter phases. Others retain singularities but with notable modifications in their behavior. These findings highlight the physical relevance of connection choice in $f(Q)$ gravity and its potential to address fundamental cosmological questions.

gr-qc↗

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

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↗

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

Wormhole geometries induced by action-dependent Lagrangian theories

In this work, we explore wormhole geometries in a recently proposed modified gravity theory arising from a non-conservative gravitational theory, tentatively denoted action-dependent Lagrangian theories. The generalized gravitational field equation essentially depends on a background four-vector $λ^μ$, that plays the role of a coupling parameter associated with the dependence of the gravitational Lagrangian upon the action, and may generically depend on the spacetime coordinates. Considering wormhole configurations, by using "Buchdahl coordinates", we find that the four-vector is given by $λ_μ=\left(0,0,λ_θ,0\right)$, and that the spacetime geometry is severely restricted by the condition $g_{tt}g_{uu}=-1$, where $u$ is the radial coordinate. We find a plethora of specific asymptotically flat, symmetric and asymmetric, solutions with power law choices for the function $λ$, by generalizing the Ellis-Bronnikov solutions and the recently proposed black bounce geometries, amongst others. We show that these compact objects possess a far richer geometrical structure than their general relativistic counterparts.

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↗

arXiv2020

Extremal cosmological black holes in Horndeski gravity and the anti-evaporation regime

Extremal cosmological black holes are analysed in the framework of the most general second order scalar-tensor theory, the~so-called Horndeski gravity. Such~extremal black holes are a particular case of Schwarzschild-De Sitter black holes that arises when the black hole horizon and the cosmological one coincide. Such~metric is induced by a particular value of the effective cosmological constant and is known as Nariai spacetime. The~existence of this type of solutions is studied when considering the Horndeski Lagrangian and its stability is analysed, where the so-called anti-evaporation regime is studied. Contrary to other frameworks, the~radius of the horizon remains stable for some cases of the Horndeski Lagrangian when considering perturbations at linear order.

gr-qc↗

arXiv2019

What if Newton's Gravitational Constant was negative?

In this work, we seek a cosmological mechanism that may define the sign of the effective gravitational coupling constant, {\em G}. To this end, we consider general scalar-tensor gravity theories as they provide the field theory natural framework for the variation of the gravitational coupling. We find that models with a quadratic potential naturally stabilize the value of {\em G} into the positive branch of the evolution and further, that de Sitter inflation and a relaxation to General Relativity is easily attained.

gr-qc↗

arXiv2016

Commissioning and First Observations with Wide FastCam at the Telescopio Carlos Sánchez

The FastCam instrument platform, jointly developed by the IAC and the UPCT, allows, in real-time, acquisition, selection and storage of images with a resolution that reaches the diffraction limit of medium-sized telescopes. FastCam incorporates a specially designed software package to analyse series of tens of thousands of images in parallel with the data acquisition at the telescope. Wide FastCam is a new instrument that, using the same software for data acquisition, does not look for lucky imaging but fast observations in a much larger field of view. Here we describe the commissioning process and first observations with Wide FastCam at the Telescopio Carlos Sánchez (TCS) in the Observatorio del Teide.

astro-ph.IM↗

arXiv2015

On the consistency of universally non-minimally coupled $f(R,T,R_{μν}T^{μν})$ theories

We discuss the consistency of a recently proposed class of theories described by an arbitrary function of the Ricci scalar, the trace of the energy-momentum tensor and the contraction of the Ricci tensor with the energy-momentum tensor. We briefly discuss the limitations of including the energy-momentum tensor in the action, as it is a non fundamental quantity, but a quantity that should be derived from the action. The fact that theories containing non-linear contractions of the Ricci tensor usually leads to the presence of pathologies associated with higher-order equations of motion will be shown to constrain the stability of this class of theories. We provide a general framework and show that the conformal mode for these theories generally has higher-order equations of motion and that non-minimal couplings to the matter fields usually lead to higher-order equations of motion. In order to illustrate such limitations we explicitly study the cases of a canonical scalar field, a K-essence field and a massive vector field. Whereas for the scalar field cases it is possible to find healthy theories, for the vector field case the presence of instabilities is unavoidable.

hep-th↗

arXiv2014

Non-minimal scalar-tensor theories

We compute the spectrum of scalar models with a general coupling to the scalar curvature. We find that the perturbative states of these theories are given by two massive spin-0 modes in addition to one massless spin-2 state. This latter mode can be identified with the standard graviton field. Indeed, we are able to define an Einstein frame, where the dynamics of the massless spin-2 graviton is the one associated with the Einstein-Hilbert action. We also explore the interactions of all these degrees of freedom in the mentioned frame, since part of the coupling structure can be anticipated by geometrical arguments.

gr-qc↗