arXiv subjects

César A. Valenzuela-Toledo

Publications and source records attributed to César A. Valenzuela-Toledo.

At least 19 recordsLinked to original sources

arXiv2026

MG-NECOLA: A Field-Level Emulator for $f(R)$ Gravity and Massive Neutrino Cosmologies

Accurate modeling of non-linear gravitational dynamics is essential for constraining extensions to the standard cosmological model using large-scale structure observations. While high-resolution $N$-body simulations provide the required fidelity, they are computationally prohibitive for the large ensembles needed to analyze Modified Gravity (MG) scenarios. We present MG-NECOLA, a field-level emulator based on a convolutional neural network that upgrades fast, approximate MG-PICOLA simulations to near--$N$-body accuracy at a fraction of the computational cost. Trained on a suite of QUIJOTE_MG simulations for $f(R)$ gravity, MG-NECOLA achieves nearly sub-percent accuracy ($\lesssim 1\%$) in both the matter power spectrum and bispectrum up to $k \simeq 1~h\,\mathrm{Mpc}^{-1}$. Crucially, although being trained on a fixed cosmology, the network generalizes robustly to cosmologies outside its training manifold keeping the error below $5\%$. It successfully recovers the General Relativity limit ($Λ$CDM) without introducing spurious MG signals and accurately captures the power suppression induced by massive neutrinos ($M_ν\leq 0.4$ eV), despite being trained on cosmologies with massless neutrinos. The pipeline delivers a speed-up factor of $\sim 1500\times$ relative to full $N$-body runs, generating a high-fidelity realization in O$(10^3)$ CPU seconds compared to O$(10^6)$ for the baseline. This accuracy-efficiency trade-off establishes MG-NECOLA as a powerful tool for generating the massive mock catalogs required for next-generation galaxy surveys.

astro-ph.CO↗

arXiv2026

Inconsistencies of Tsallis Cosmology within Horizon Thermodynamics and Holographic Scenarios

We investigate the cosmological implications of Tsallis entropy in two widely discussed frameworks: the Cai-Kim thermodynamic derivation of the Friedman equations and the Tsallis holographic dark energy (HDE) scenario, considering both the Hubble scale and the Granda-Oliveros (GO) cutoff as infrared regulators. In both cases, the dynamics introduce a nonextensivity parameter $δ$, with the standard Bekenstein-Hawking entropy-area relation recovered for $δ= 1$. While previous studies have suggested that only small deviations from extensivity are observationally allowed, typically requiring $|1 - δ| \lesssim 10^{-3}$, here we go further and perform a systematic consistency analysis across the entire expansion history. We show that even mild departures from $δ= 1$ lead to pathological behavior in the effective dark energy sector: its density can become negative or complex, its equation of state may diverge, or it can contribute an unacceptably large early-time fraction that spoils radiation domination and violates BBN and CMB constraints. Our results sharpen and unify earlier hints of tension, providing a clear physical interpretation in terms of corrections that grow uncontrollably with the expansion rate toward the past. We conclude that within both the Cai-Kim and HDE formulations, a viable cosmology emerges only in the extensive limit, effectively reducing the models to $Λ$CDM. More broadly, our findings emphasize the importance of dynamical consistency and cosmological viability tests, when assessing nonextensive entropy formalisms as potential frameworks for describing the Universe's dynamics.

astro-ph.CO↗

arXiv2025

MG-NECOLA: Fast Neural Emulators for Modified Gravity Cosmologies

Observations of the large-scale structure (LSS) provide a powerful test of gravity on cosmological scales, but high-resolution N-body simulations of modified gravity (MG) are prohibitively expensive. We present MG-NECOLA, a convolutional neural network that enhances fast MG-PICOLA simulations to near-N-body fidelity at a fraction of the cost. MG-NECOLA reproduces QUIJOTE-MG N-body results in the power spectrum and bispectrum with better than 1% accuracy down to non-linear scales ($k \simeq 1~h~\mathrm{Mpc}^{-1}$), while reducing computational time by several orders of magnitude. Importantly, although trained only on $f(R)$ models with massless neutrinos, the network generalizes robustly to scenarios with massive neutrinos, preserving accuracy to within 5% at non-linear scales. This combination of precision and robustness establishes MG-NECOLA as a practical emulator for producing large ensembles of high-fidelity simulations, enabling efficient exploration of modified gravity and beyond-$Λ$CDM cosmologies in upcoming surveys.

astro-ph.CO↗

arXiv2025

Anisotropic Universes in Light of Background Cosmological Observations

The cosmological principle is a cornerstone of the standard cosmological model. However, recent observations suggest potential deviations from this assumption, hinting at a small anisotropic expansion. Such an expansion can arise from sources that break rotational invariance. A minimal realization of this scenario is described by a Bianchi I geometry, where the degree of anisotropy is quantified by the shear parameter $Σ$. In this work, we constrain the present-day value of the shear, $Σ_0$, by confronting theoretical predictions with recent cosmological data. We implement various anisotropic models within the Boltzmann code \texttt{CLASS} and explore their parameter space using the sampler \texttt{MontePython}. Although our results show that $Σ_0$ is model-dependent, notably, in one specific scenario considering a homogeneous scalar field coupled to a 2-form field, $Σ_0 = 0$ is excluded at the $2σ$ confidence level, with mean value around $|Σ_0| \sim 10^{-4}$ while remaining consistent with observations. These findings challenge the conventional assumption that cosmic shear is negligible in the present universe. Moreover, the anisotropic expansion in this model is driven by a steep scalar field potential, a feature often found in supergravity-inspired scenarios. While anisotropic models offer interesting alternatives and could help explain some cosmological anomalies, they generally introduce additional parameters, making the standard $Λ$CDM model statistically favored in most cases. Still, they remain compatible with current observations and provide new perspectives on features not fully explained within the standard framework. These results highlight the importance of further exploring anisotropic cosmologies to better understand their implications.

astro-ph.CO↗

arXiv2024

Multifield curved solid: Early dark energy and perturbation instabilities

We introduce a multifield dark energy model with a nonflat field-space metric, in which one field is dynamical while the others have constant spatial gradients. The model is predictive at the background level, leading to an early dark energy component at high redshifts and a suppressed fraction of late-time anisotropy. Both features have simple expressions in terms of the curvature scale of the field-space, and correspond to stable points in the phase space of possible solutions. Because of the coupling between time and space-dependent scalar fields, vector field perturbations develop tachyonic instabilities at scales below the Hubble radius, thus being potentially observable in the number count of galaxies. Overall, the presence of a nontrivial field-space curvature also leads to the appearance of instabilities on scalar perturbations, which can impact the matter density distribution at large scales.

astro-ph.CO↗

arXiv2024

Late-Time Anisotropy Sourced by a 2-Form Field Non-Minimally Coupled to Cold Dark Matter

This paper investigates the cosmological dynamics arising from the interaction between a 2-form field and cold dark matter within a Bianchi I background. Employing a dynamical system analysis, we identify two attractors yielding to exponential expansion of the Universe, i.e., de-Sitter solutions. Notably, these solutions exhibit a pivotal distinction: one is indistinguishable from the cosmological constant scenario, while the other corresponds to an \emph{anisotropic de-Sitter} expansion sourced by the 2-form field. To validate the asymptotic behavior of our model, we conduct a numerical exploration of its expansion history. Our analysis reveals that the coupling between the dark sectors amplifies the shear during the matter-dominated epoch, offering a potential avenue to address certain observational discrepancies related to the structure formation process. Then, we constrain the parameter space of the model using recent observational datasets. Remarkably, we find that the current shear is precisely constrained to be approximately $Σ_0 \approx - 10^{-4}$. We also discuss some key differences in the expansionary dynamics sourced by the 2-form field compared to a 1-form field, i.e., a vector field, offering insights into their respective impacts on the support they provide to late-time anisotropy.

gr-qc↗

arXiv2024

Scrutinizing coupled vector dark energy in light of data

Since current challenges faced by $Λ$CDM might be hinting at new unravelled physics, here we investigate a plausible cosmological model where a vector field acts as source of dark energy. In particular, we examine whether an energy-momentum exchange between dark energy and dark matter could provide an explanation for current discrepancies in cosmological parameters. We carefully work out equations governing background and linear order perturbations and implement them in a Boltzmann code. We found that a negative coupling makes the dark energy equation of state less negative and closer to a cosmological constant during the matter dominated epoch than an uncoupled vector dark energy model. While the effect of the coupling is hardly noticeable through its effect on matter density perturbations, matter velocity perturbations and gravitational potentials are enhanced at late-times when dark energy dominates. Therefore, data of redshift space distortions help to narrow down these kinds of couplings in the dark sector. We computed cosmological constraints and found common parameters also present in $Λ$CDM are in good agreement with the Planck Collaboration baseline result. Our best fit for a negatively coupled vector field predicts a higher growth rate of matter perturbations at low redshift, thus exacerbating the disagreement with redshift space distortions data. While a positively coupled vector field can lead to power suppression of $P_{\rm{m}}(k,z=0)$ on small scales as well as a lower growth rate of matter perturbations than the standard model, it might compromise the goodness of fit to the CMB angular power spectrum on small scales. We conclude that our negatively coupled vector dark energy model does not solve current tensions (i.e., $H_0$ and $σ_8$). (abridged)

astro-ph.CO↗

arXiv2024

Anisotropic Dark Energy from String Compactifications

We explore the cosmological dynamics of a minimalistic yet generic string-inspired model for multifield dark energy. Adopting a supergravity four-dimensional viewpoint, we motivate the model's structure arising from superstring compactifications involving a chiral superfield and a pure $U(1)$ gauge sector. The chiral sector gives rise to a pair of scalar fields, such as the axio-dilaton, which are kinetically coupled. However, the scalar potential depends on only one of them, further entwined with the vector field through the gauge kinetic function. The model has two anisotropic attractor solutions that, despite a steep potential and thanks to multifield dynamics, could explain the current accelerated expansion of the Universe while satisfying observational constraints on the late-times cosmological anisotropy. Nevertheless, justifying the parameter space allowing for slow roll dynamics together with the correct cosmological parameters, would be challenging within the landscape of string theory. Intriguingly, we find that the vector field, particularly at one of the studied fixed points, plays a crucial role in enabling geodesic trajectories in the scalar field space while realizing slow-roll dynamics with a steep potential. This observation opens a new avenue for exploring multifield dark energy models within the superstring landscape.

hep-th↗

arXiv2023

Reconstructing the parameter space of non-analytical cosmological fixed points

Dynamical system theory is a widely used technique in the analysis of cosmological models. Within this framework, the equations describing the dynamics of a model are recast in terms of dimensionless variables, which evolve according to a set of autonomous first-order differential equations. The fixed points of this autonomous set encode the asymptotic evolution of the model. Usually, these points can be written as analytical expressions for the variables in terms of the parameters of the model, which allows a complete characterization of the corresponding parameter space. However, a thoroughly analytical treatment is impossible in some cases. In this work, we give an example of a dark energy model, a scalar field coupled to a vector field in an anisotropic background, where not all the fixed points can be analytically found. Then, we put forward a general scheme that provides a numerical description of the parameter space. This allows us to find interesting accelerated attractors of the system with no analytical representation. This work may serve as a template for the numerical analysis of highly complicated dynamical systems.

gr-qc↗

arXiv2022

An effective fluid description of scalar-vector-tensor theories under the sub-horizon and quasi-static approximations

We consider scalar-vector-tensor (SVT) theories with second-order equations of motion and tensor propagation speed equivalent to the speed of light. Under the sub-horizon and the quasi-static approximations we find analytical formulae for an effective dark energy fluid, i.e., sound speed, anisotropic stress as well as energy density and pressure. We took advantage of our general, analytical fluid description and showed that it is possible to design SVT cosmological models which are degenerate with $Λ$CDM at the background level while having gravity strength $G_{\rm eff}<G_{\rm N}$ at late-times as well as non-vanishing dark energy perturbations. We implemented SVT designer models in the widely used Boltzmann solver CLASS thus making it possible to test SVT models against astrophysical observations. Our effective fluid approach to SVT models reveals non trivial behaviour in the sound speed and the anisotropic stress well worth an investigation in light of current discrepancies in cosmological parameters such as $H_0$ and $σ_8$.

astro-ph.CO↗

arXiv2022

Structure formation in an anisotropic universe: Eulerian perturbation theory

We present an effective Eulerian description, in the non-relativistic regime, of the growth of cosmological perturbations around a homogeneous but anisotropic Bianchi I spacetime background. We assume a small deviation from isotropy, sourced at late times for example by dark energy anisotropic stress. We thus derive an analytic solution for the linear dark matter density contrast, and use it in a formal perturbative approach which allows us to derive a second order (non-linear) solution. As an application of the procedure followed here we derive analytic expressions for the power spectrum and the bispectrum of the dark matter density contrast. The power spectrum receives a quadrupolar correction as expected, and the bispectrum receives several angle-dependent corrections. Quite generally, we find that the contribution of a late-time phase of anisotropic expansion to the growth of structure peaks at a finite redshift between CMB decoupling and today, tough the exact redshift value is model-dependent.

astro-ph.CO↗

arXiv2021

Anisotropic $k$-essence

In this paper, we study the late time cosmology of a non-canonical scalar field ($k$-essence) coupled to a vector field in a Bianchi-I background. Specifically, we study three cases: canonical scalar field (quintessence) with exponential potential as a warm up, the dilatonic ghost condensate, and the Dirac Born Infeld field with exponentials throat and potential. By using a dynamical system approach, we show that anisotropic dark energy fixed points can be attractors for a suitable set of parameters of each model. We also numerically integrate the associated autonomous systems for particular initial conditions chosen in the deep radiation epoch. We find that the three models can give an account of an equation of state of dark energy close to $-1$ nowadays. However, a non-negligible spatial shear within the observational bounds nowadays is possible only for thequintessence and the Dirac Born Infeld field. We also find that the equation of state of dark energy and the shear oscillate around nowadays, whenever the coupling of the k-essence field to the vector field is strong enough. The reason of these oscillations is explained in the appendix.

gr-qc↗

arXiv2021

Anisotropic solid dark energy

In this paper, we study a triad of inhomogeneous scalar fields, known as "solid", as a source of homogeneous but anisotropic dark energy. By using a dynamical system approach, we find that anisotropic accelerated solutions can be realized as attractor points for suitable choices of the parameters of the model. We complement the dynamical analysis with a numerical solution whose initial conditions are set in the deep radiation epoch. The model can give an account of a non-negligible spatial shear within the observational bounds nowadays, even when the later is set to zero at early times. However, we find that there is a particular region in the parameter space of the model in which the Universe isotropizes. The anisotropic attractors, the particular isotropic region, and a nearly constant equation of state of dark energy very close to $- 1$ are key features of this scenario. Following a similar approach, we also analyzed the full isotropic version of the model. We find that the solid can be characterized by a constant equation of state and thus being able to simulate the behavior of a cosmological constant.

gr-qc↗

arXiv2021

On the coupling of vector fields to the Gauss-Bonnet invariant

Inflationary models including vector fields have attracted a great deal of attention over the past decade. Such an interest owes to the fact that they might contribute to, or even be fully responsible for, the curvature perturbation imprinted in the cosmic microwave background. However, the necessary breaking of the vector field's conformal invariance during inflation is not without problems. In recent years, it has been realized that a number of instabilities endangering the consistency of the theory arise when the conformal invariance is broken by means of a non-minimal coupling to gravity. In this paper, we consider a massive vector field non-minimally coupled to gravity through the Gauss-Bonnet invariant, and investigate whether the vector can play the role of a curvaton while evading the emergence of instabilities and preserves the large-scale isotropy.

gr-qc↗

arXiv2020

Dynamical analysis of cosmological models with non-Abelian gauge vector fields

In this paper we study some models where non-Abelian gauge vector fields endowed with a SU(2) group representation are the unique source of inflation and dark energy. These models were first introduced under the name of gaugeflation and gaugessence, respectively. Although several realizations of these models have been discussed, not all available parameters and initial conditions are known. In this work, we use a dynamical system approach to find the full parameter space of the massive version of each model. In particular, we found that the inclusion of the mass term increases the length of the inflationary period. Additionally, the mass term implies new behaviors for the equation of state of dark energy allowing to distinguish this from other prototypical models of accelerated expansion. We show that an axially symmetric gauge field can support an anisotropic accelerated expansion within the observational bounds.

gr-qc↗

arXiv2020

Topological mass generation and $2-$forms

In this work we revisit the topological mass generation of 2-forms and establish a connection to the unique derivative coupling arising in the quartic Lagrangian of the systematic construction of massive $2-$form interactions, relating in this way BF theories to Galileon-like theories of 2-forms. In terms of a massless $1-$form $A$ and a massless $2-$form $B$, the topological term manifests itself as the interaction $B\wedge F$, where $F = {\rm d} A$ is the field strength of the $1-$form. Such an interaction leads to a mechanism of generation of mass, usually referred to as "topological generation of mass" in which the single degree of freedom propagated by the $2-$form is absorbed by the $1-$form, generating a massive mode for the $1-$form. Using the systematical construction in terms of the Levi-Civita tensor, it was shown that, apart from the quadratic and quartic Lagrangians, Galileon-like derivative self-interactions for the massive 2-form do not exist. A unique quartic Lagrangian $ε^{μνρσ}ε^{αβγ}_{\;\;\;\;\;\;σ}\partial_μB_{αρ}\partial_νB_{βγ}$ arises in this construction in a way that it corresponds to a total derivative on its own but ceases to be so once an overall general function is introduced. We show that it exactly corresponds to the same interaction of topological mass generation. Based on the decoupling limit analysis of the interactions, we bring out supporting arguments for the uniqueness of such a topological mass term and absence of the Galileon-like interactions. Finally, we discuss some preliminary applications in cosmology.

hep-th↗

arXiv2019

de Sitter symmetries and inflationary scalar-vector models

In this paper, we study the correspondence between a field theory in de Sitter space in $D$-dimensions and a dual conformal field theory in a euclidean space in $(D-1)$-dimensions. In particular, we investigate the form in which this correspondence is established for a system of interacting scalar and a vector fields propagating in de Sitter space. We analyze some necessary (but not sufficient) conditions for which conformal symmetry is preserved in the dual theory in $(D-1)$-dimensions, making possible the establishment of the correspondence. The discussion that we address on this paper is framed on the context of inflationary cosmology, so, the results obtained here pose some relevant possibilities of application to the calculation of the fields's correlation functions and of the primordial curvature perturbation $ζ$, in inflationary models including coupled scalar and vector fields.

gr-qc↗

arXiv2019

Arbitrarily coupled $p-$forms in cosmological backgrounds

In this paper we consider a model based on interacting $p-$forms and explore some cosmological applications. Restricting to gauge invariant actions, we build a general Lagrangian allowing for arbitrary interactions between the $p-$forms (including interactions with a $0-$form, scalar field) in a given background in $D$ dimensions. For simplicity, we restrict the construction to up to first order derivatives of the fields in the Lagrangian. We discuss with detail the four dimensional case and devote some attention to the mechanism of topological mass generation originated by couplings of the form $B\wedge F$ between a $p-$form and a $(3-p)-$form. As a result, we show the system of the interacting $p-$forms $(p=1,2,3)$ is equivalent to a parity violating, massive, Proca vector field model. Finally, we discuss some cosmological applications. In a first case we study a very minimalistic system composed by a $3-$form coupled to a $0-$form. The $3-$form induces an effective potential which acts as a cosmological constant term suitable to drive the late time accelerated expansion of the universe dominated by dark energy. We study the dynamics of the system and determine its critical points and stability. Additionally, we study a system composed by a scalar field and a $1$-form. This case is interesting because the presence of a coupled $1-$form can generate non vanishing anisotropic signatures during the late time accelerated expansion. We discuss the evolution of cosmological parameters such as the equation of state in this model.

astro-ph.CO↗