arXiv subjects

Juan Carlos Hidalgo

Publications and source records attributed to Juan Carlos Hidalgo.

At least 19 recordsLinked to original sources

arXiv2026

Spherically symmetric solutions with dust and radiation sources in Schouten-Codazzi Gravity

We examine in this paper exact solutions of Schouten-Codazzi gravity (SCG), a second order metric theory in which a Codazzi tensor is added to the Einstein field equations. We consider two spherically symmetric geometries that describe null radiation and inhomogeneous dust, corresponding to generalized Vaidya and Lemaître-Tolman-Bondi (LTB) spacetimes. The Codazzi tensor has to comply the Codazzi equations, this imposes restrictions on the geometry of spacetime that are determined up to two integration functions that naturally generate an effective ``cosmological constant'' and an additional degree of freedom. We show that for the Vaidya geometry, these Codazzi contributions modify the Misner-Sharp mass, while the LTB models have as an evolution equation a cubic polynomial whose coefficients depend on the Codazzi integration functions. This leads to a broader class of dynamics than those of General Relativity, and allows for an accelerated expansion and ever-expanding models without the introduction of ``dark energy''. Overall, our analysis suggests that Schouten-Codazzi gravity provides an appealing framework for applications in cosmology.

gr-qc↗

arXiv2026

On the Reliability of Quasars as Cosmological Distance Indicators

We assess the viability of quasars as cosmological distance indicators based on the non-linear $L_X$--$L_{\rm UV}$ relation. We calibrate this relation in a model-independent way by anchoring quasar luminosity distances to cosmic-chronometer $H(z)$ measurements over $z\leq 1.43$, and construct a quasar Hubble diagram extending up to $z\simeq 7.5$. We compare a traditional stepwise approach, in which the calibration is fixed before cosmological inference, with a joint QSO+SN calibration--cosmology framework where calibration and cosmological parameters are sampled simultaneously. The stepwise analysis, supplemented with DESI DR2 BAO measurements and Planck compressed CMB distance priors, is used as a diagnostic benchmark, while the joint framework, with and without the SH0ES $H_0$ information, provides our main cosmological results. We selected a low-$z$ quasar subsample ($z<1.43$), matching the redshift range of the cosmic-chronometer calibration, and found calibration parameters consistent with previous studies. However, the stepwise cosmological constraints can become unstable once quasars are included, reflecting the incomplete propagation of calibration uncertainties and calibration--cosmology degeneracies. In contrast, the joint analysis yields self-consistent constraints because the quasar calibration parameters are fitted simultaneously with the cosmological parameters, allowing these uncertainties and degeneracies to be propagated into the final posteriors. Our results indicate that the current limitations of quasar cosmology are driven mainly by intrinsic scatter and possible sample-dependent effects in the $L_X$--$L_{\rm UV}$ relation, rather than by a fundamental inconsistency with standard cosmology.

astro-ph.CO↗

arXiv2026

Bayesian analysis of $α$-Starobinsky model with Planck, ACT and DESI data

We present a joint Bayesian analysis to impose constraints on the generalized $α$-Starobinsky inflationary model using the high-precision cosmological datasets: Planck, CMB lensing from ACT DR6, and Baryon Acoustic Oscillations (BAO) from DESI DR2. For the parameter inference, we introduce an alternative sampling approach. Rather than imposing priors on the cosmological parameters of the inflationary potential $(V_0, \, α, \, N_*)$, we place priors directly on the primordial physical observables $(A_s,\, n_s,\, r)$ through analytical slow-roll consistency relations. Our pipeline internally maps these sampled observables to the corresponding $α$-Starobinsky parameters. These values are then passed to a modified version of $\tt{CLASS}$, which solves the exact inflationary dynamics numerically. This pipeline ensures that the final reported posteriors for the observables are computed exactly, completely free from the slow-roll approximation. Applying this methodology, we explore the viability of the $α$-Starobinsky model. We show that, when the full combined dataset is considered, the pure Starobinsky model (i.e., the canonical limit $α= 1$) shows signs of breaking down, since it requires a large number of $e$-folds of inflation after horizon crossing ($N_* > 60$) due to the shift in the scalar spectral index, $n_s$. In contrast, allowing the deformation parameter $α$ as a free parameter yields a clear $1σ$ preference for $\log_{10} α> 0$, present across all datasets. Notably, we also show that the addition of ACT DR6 lensing data introduces no significant impact on these primordial constraints, confirming that our robust posteriors are primarily driven by Planck and DESI measurements.

astro-ph.CO↗

arXiv2026

Peculiar velocity fields from analytic solutions of General Relativity

Peculiar velocities are analyzed through cosmological perturbations in the Newtonian longitudinal gauge characterized by irrotational shear-free congruences in an Eulerian frame. We show that non-trivial peculiar velocity fields can be generated through Lorentzian boosts in the non-relativistic limit, where the Eulerian frame is obtained from analytic solutions of Einstein's equations sourced by an irrotational shear-free fluid with nonzero energy flux. This approach provides a physically viable interpretation of these analytic solutions, which (in general) admit no isometries, thus allowing, in principle, for modeling time and space varying 3-dimensional fields of peculiar velocities that can be contrasted with observational data on our local cosmography. As a ``proof of concept'' we examine the peculiar velocities of varying dark matter and dark energy perfect fluids with respect to the CMB frame using a simple, spherically symmetric particular solution. The resulting peculiar velocities are qualitatively compatible with observational data on the CMB dipole.

gr-qc↗

arXiv2026

Non-Gaussian statistics in galaxy weak lensing: compressed three-point correlations and cosmological forecasts

Building on previous developments of a harmonic decomposition framework for computing the three-point correlation function (3PCF) of projected scalar fields over the sky, this work investigates how much cosmological information is contained in these higher-order statistics. We perform a forecast to determine the number of harmonic multipoles required to capture the full information content of the 3PCF in the context of galaxy weak lensing, finding that only the first few multipoles are sufficient to capture the additional cosmological information provided by the 3PCF. This study addresses a critical practical question: to what extent can the high-dimensional 3PCF signal be compressed without significant loss of cosmological information? Since the different multipoles contain highly redundant information, we apply a principal component analysis (PCA) which further reduces its dimensionality and preserving information. We also account for non-linear parameter degeneracies using the DALI method, an extension of Fisher forecasting that includes higher-order likelihood information. Under optimistic settings, we find that the 3PCF considerably improves the constraining power of the 2PCF for $Ω_m$, reaching a 20% improvement. Other parameters also benefit, mainly due to their degeneracy with the matter abundance. For example, with our chosen scale cuts for galaxy sources at $z = 0.5$, we find that $σ_8$ is more tightly constrained, whereas $S_8$ and $w_0$ are not. Finally, we construct analytical Gaussian covariance matrices that can serve as a first step toward developing semi-analytical, semi-empirical alternatives to sample covariances.

astro-ph.CO↗

arXiv2025

Revisiting the gravitational "arrow of time"

We address a long-standing misperception on the gravitational ``arrow of time'', a proposal by Penrose (also known as the ``Weyl-curvature hypothesis") that associates structure formation along timelike directions in which Weyl-curvature scalars become dominant over Ricci scalars. A counterexample of this hypothesis was found by Bonnor on a class of exact solutions describing heat conducting spheres collapsing in a Vaidya background. We show that this result does not hold in the same class of solutions considered as physically viable near FLRW cosmological models, with the heat conduction vector interpreted as a peculiar velocity field. We also discuss the similarities and differences between the gravitational ``arrow of time'' and the gravitational entropy formalism of Clifton, Ellis and Tavakol.

gr-qc↗

arXiv2025

Viability of generalized $α$-inflation from Planck, ACT, and DESI Data

We study inflationary constraints from reheating on two classes of single-field inflationary models: a generalized $α$-attractor with potential $V(ϕ) = V_0 \!\left(1 - \sech^p\!\left[ϕ/(\sqrt{6α}\,M_{Pl})\right]\right)$ and the $α$-Starobinsky model with potential $V(ϕ) = V_0 \!\left(1 - e^{- \sqrt{2/(3α)}\, ϕ/ M_{Pl}} \right)^2$. Using a semi-analytical relation that connects inflationary dynamics to reheating, we solve for the horizon-crossing field value and calculate the scalar spectral index $n_s$, the tensor-to-scalar ratio $r$, and the reheating temperature $T_{re}$. We consider three perturbative decay channels of the inflaton: gravitational, Yukawa (fermionic), and scalar. The parameter space spanned by $(α,p,ω_{re})$ for the first model and $(α,ω_{re})$ for the second is explored and compared with recent measurements from Planck and ACT DR6, as well as BAO from DESI DR2. For clarity, we emphasize that ACT DR6 alone is fully compatible with Planck regarding $n_s$. The upward shift toward $n_s \simeq 0.975$ appears only when DESI DR2 BAO are combined with CMB datasets (Planck+DESI, ACT+DESI, SPT-3G+DESI), and is likewise seen in the P--ACT--LB2+DESI combination. CMB-only combinations (e.g., Planck+ACT or Planck+ACT+SPT-3G) primarily refine constraints and do not by themselves raise $n_s$. Our comparisons and model assessments are therefore made with this distinction in mind: CMB-only constraints versus CMB+DESI combinations. We conclude that both models remain consistent with current CMB-only data in restricted regions of parameter space, and that residual tension with the higher $n_s$ favored by CMB+DESI persists.

astro-ph.CO↗

arXiv2025

Gravitational entropy of fluids with energy flux

We examine gravitational entropy growth within the formalism of Clifton, Ellis and Tavakol (CET) applied to a class of spherically symmetric exact solutions whose source is a shear-free fluid with energy flux in a comoving frame. By considering these solutions as potential cosmological models, we update previous literature that considered them only as restricted toy models of radiating spheres collapsing in a Vaidya background. In the present paper we examine the integrability of the CET entropy form in connection with Einstein's equations in the fluid flow approach, proving as well that all expanding configurations comply with the growth of CET gravitational entropy. Finally, we examine the connection between the CET gravitational entropy and the notion of a gravitational ``arrow of time'' based on the ratio of Weyl to Ricci curvature. Some of the solutions also provide potentially useful and viable inhomogeneous generalizations of FLRW models, thus suggesting an appealing potential for applications to current cosmological research.

gr-qc↗

arXiv2025

Primordial Black Hole Formation in a Scalar Field Dominated Universe: Investigation of the Critical nature of the Collapse

In this paper, we investigate the critical collapse leading to primordial black hole (PBH) formation in a universe dominated by a self-interacting scalar field with a quartic potential, comparing it to the well-known radiation-dominated case. Using fully relativistic nonlinear numerical simulations in spherical symmetry, based on the Misner--Sharp formalism, we analyze the dynamics near the collapse threshold and track the scaling of the black hole mass. Our results confirm that both the scalar field and radiation cases exhibit type II critical behavior with similar -- though not identical -- critical exponents, differing by about $2σ$. This suggests that, while a quartic scalar field effectively mimics a radiation fluid even in the nonlinear collapse regime, small differences in the critical exponent persist. Our findings provide direct numerical evidence for the near universality of the critical exponent in PBH formation, with only mild dependence on whether the collapse is driven by a scalar field or a perfect fluid.

astro-ph.CO↗

arXiv2025

Primordial Black Hole Formation in a Scalar Field Dominated Universe

We present a numerical code that solves the Misner-Sharp system for a spherically symmetric cosmological model containing both a scalar field and a perfect fluid. While the code is capable of exploring general scenarios involving an minimally coupled scalar field and perfect fluid, we focus on the regime where the scalar field dominates the dynamics, particularly in the post-inflationary scalar field-dominated scenario, where the universe is governed by a rapidly oscillating scalar field for a period lasting a few $e$-folds. We analyse the threshold for PBH formation under quadratic and quartic potentials, evolving configurations from superhorizon scales. Our results confirm that a quartic potential behavior is similar to the radiation-dominated universe, resulting in a PBH formation threshold close to the well-established value in radiation backgrounds. Conversely, in the quadratic case, we observe a significant deviation from the expected dust-like behaviour, due to wave-like effects opposing the gravitational collapse. While numerical limitations prevent us from evolving a wide range of initial conditions to determine a precise threshold for PBH formation, our findings suggest that PBH formation may be suppressed with respect to the pure dust scenario, allowing the formation of stable solitonic structures instead. This study highlights the importance of properly accounting for wave dynamics in oscillating scalar fields when characterising PBH formation.

astro-ph.CO↗

arXiv2025

Testing interacting dark energy with Stage IV cosmic shear surveys through differentiable neural emulators

We employ a novel framework for accelerated cosmological inference, based on neural emulators and gradient-based sampling methods, to forecast constraints on dark energy models from Stage IV cosmic shear surveys. We focus on dark scattering (DS), an interacting dark energy model with pure momentum exchange in the dark sector, and train COSMOPOWER emulators to accurately and efficiently model the DS non-linear matter power spectrum produced by the halo model reaction framework, including the effects of baryon feedback and massive neutrinos. We embed the emulators within a fully-differentiable pipeline for gradient-based cosmological inference for which the batch likelihood call is up to $O(10^5)$ times faster than with traditional approaches, producing parameter constraints from simulated Stage IV cosmic shear data running on a single graphics processing unit (GPU). We also perform model comparison on the output chains from the inference process, employing the learnt harmonic mean estimator implemented in the software HARMONIC. We investigate degeneracies between dark energy and systematics parameters and assess the impact of scale cuts on the final constraints. Assuming a DS model for the mock data vector, we find that a Stage IV survey cosmic shear analysis can constrain the DS amplitude parameter $A_{\mathrm{ds}}$ with an uncertainty roughly an order of magnitude smaller than current constraints from Stage III surveys, even after marginalising over baryonic feedback, intrinsic alignments and redshift distribution uncertainties. These results show great promise for constraining DS with Stage IV data; furthermore, our methodology can be straightforwardly extended to a wide range of dark energy and modified gravity models.

astro-ph.CO↗

arXiv2024

Modeling the 3-point correlation function of projected scalar fields on the sphere

One of the main obstacles for the signal extraction of the three point correlation function using photometric surveys, such as the Rubin Observatory Legacy Survey of Space and Time (LSST), will be the prohibitive computation time required for dealing with a vast quantity of sources. Brute force algorithms, which naively scales as $O(N^3)$ with the number of objects, can be further improved with tree methods but not enough to deal with large scale correlations of Rubin's data. However, a harmonic basis decomposition of these higher order statistics reduces the time dramatically, to scale as a two-point correlation function with the number of objects, so that the signal can be extracted in a reasonable amount of time. In this work, we aim to develop the framework to use these expansions within the Limber approximation for scalar (or spin-0) fields, such as galaxy counts, weak lensing convergence or aperture masses. We develop an estimator to extract the signal from catalogs and different phenomenological and theoretical models for its description. The latter includes halo model and standard perturbation theory, to which we add a simple effective field theory prescription based on the short range of non-locality of cosmic fields, significantly improving the agreement with simulated data. In parallel to the modeling of the signal, we develop a code that can efficiently calculate three points correlations of more than 200 million data points (a full sky simulation with Nside=4096) in $\sim$40 minutes, or even less than 10 minutes using an approximation in the searching algorithm, on a single high-performance computing node, enabling a feasible analysis for the upcoming LSST data.

astro-ph.CO↗

arXiv2024

Constraints on primordial black holes for nonstandard cosmologies

We study how the bounds on the abundance of Primordial Black Holes (PBHs) and the constraints on power spectrum are modified if a non-standard evolution phase takes place between the end of inflation and the Standard radiation-dominated (RD) universe after inflation. The constraints on PBH abundance and power spectrum are computed using the new, freely available, \href{https://github.com/TadeoDGAguilar/PBHBeta}{\faGithubSquare}~\texttt{PBHBeta} library, which accounts for the effects of non-standard expansion and specific criteria for PBH formation in such non-standard scenarios. As working examples, we consider three different cases: a pure matter-dominated (MD) phase, a scalar field-dominated ($φ$D) universe, and a stiff fluid-dominated (SD) scenario. While the background expansion is the same for the MD and $φ$D scenarios, the PBH formation criteria lead to different constraints to power spectrum. On the other hand, the duration of the non-standard expansion phase alters the bounds, with longer MD periods resulting in weaker constraints on power spectrum, and longer SD scenarios leading to an enhanced abundance due to the dust-like redshifting of PBHs. The modifications to the constraints are reported in all cases and we highlight those where the power spectrum may be significantly constrained.

astro-ph.CO↗

arXiv2024

Dark Scattering: accelerated constraints from KiDS-1000 with $\tt{ReACT}$ and $\tt{CosmoPower}$

We present constraints on the Dark Scattering model through cosmic shear measurements from the Kilo Degree Survey (KiDS-1000), using an accelerated pipeline with novel emulators produced with $\tt{CosmoPower}$. Our main emulator, for the Dark Scattering non-linear matter power spectrum, is trained on predictions from the halo model reaction framework, previously validated against simulations. Additionally, we include the effects of baryonic feedback from $\tt{HMcode2016}$, whose contribution is also emulated. We analyse the complete set of statistics of KiDS-1000, namely Band Powers, COSEBIs and Correlation Functions, for Dark Scattering in two distinct cases. In the first case, taking into account only KiDS cosmic shear data, we constrain the amplitude of the dark energy - dark matter interaction to be $\vert A_{\rm ds} \vert \lesssim 20$ $\rm b/GeV$ at 68% C.L. Furthermore, we add information from the cosmic microwave background (CMB) from Planck, along with baryon acoustic oscillations (BAO) from 6dFGS, SDSS and BOSS, approximating a combined weak lensing + CMB + BAO analysis. From this combination, we constrain $A_{\rm ds} = 10.6^{+4.5}_{-7.3}$ $\rm b/GeV$ at 68% C.L. We confirm that with this estimated value of $A_{\rm ds}$ the interacting model considered in this work offers a promising alternative to solve the $S_8$ tension.

astro-ph.CO↗

arXiv2024

Detecting the Stochastic Gravitational Wave Background from Primordial Black Holes in Slow-reheating Scenarios

After primordial inflation, the universe may have experienced a prolonged reheating epoch, potentially leading to a phase of matter domination supported by the oscillating inflaton field. During such an epoch, perturbations in the inflaton virialize upon reentering the cosmological horizon, forming inflaton structures. If the primordial overdensities are sufficiently large, these structures collapse to form primordial black holes (PBHs). To occur at a significant rate, this process requires an enhanced primordial power spectrum (PPS) at small scales. The enhancement of the PPS, as well as the formation and tidal interaction of the primordial structures, will in turn source a stochastic gravitational wave background(SGWB) that could be detected by current and/or future gravitational wave detectors. In this paper, we study the SGWB arising from these different sources during slow-reheating, focusing on a PPS that satisfies the requirements necessary for the formation of PBHs with a mass of $M_{\rm PBH}\simeq 10^{21}$ and that constitute the entirety of dark matter in the universe.

astro-ph.CO↗

arXiv2024

Inflationary models constrained by reheating

The study of reheating in inflationary models is crucial for understanding the early universe and gaining insights into inflationary dynamics and parameters. The reheating temperature $T_{re}$ and the duration of the reheating phase, quantified by the number of $e$-folds $N_{re}$, have significant implications for particle production, thermalization, and the primordial power spectrum. The duration of reheating affects particle abundance, including dark matter, and shapes the primordial power spectrum and cosmic microwave background anisotropies. By combining cosmological observations and theoretical considerations, we can constrain both $T_{re}$ and $N_{re}$, which in turn constrain the spectral index $n_s$, tensor-to-scalar ratio $r$, and inflation model parameters. Utilizing consistency relations among observables, such as $n_s$ and $r$, provides additional constraints on inflationary models and determines bounds for other observables like the running of the scalar spectral index. These bounds are valuable for assessing the viability of models and can serve to specify priors in Bayesian analyses of specific models. As an example of how to proceed, we study in detail a particular case of a generalized $α$-attractor model that accurately reproduces observed quantities. We present equations for the conditions of instantaneous reheating, establish consistency relations, and explore the generalized $α$-attractor model using cosmological data.

astro-ph.CO↗

arXiv2024

Solution for cosmological observables in the Starobinsky model of inflation

This paper focuses on the Starobinsky model of inflation. We derive solutions for various cosmological observables, such as the scalar spectral index $n_s$, the tensor-to-scalar ratio $r$ and their runnings, as well as the number of $e$-folds of inflation, reheating, and radiation with minimal assumptions. We establish an equation that connects inflation and reheating, which can be solved for the spectral index $n_s$. Using consistency relations of the model, we determine the other observables, the number of $e$-folds during inflation $N_k$, and the number of $e$-folds during reheating $N_{re}$. The impact of reheating on inflation is explored by constraining the equation of state parameter $ω_{re}$ at the end of reheating. We find remarkable agreement between the Starobinsky model and current measurements of the power spectrum of primordial curvature perturbations and the present bounds on the spectrum of primordial gravitational waves.

astro-ph.CO↗

arXiv2024

Primordial black hole formation during slow-reheating: A review

In this paper we review the possible mechanisms for the production of primordial black holes (PBHs) during a slow-reheating period {in which the energy transfer of the inflaton field to standard model particles becomes effective at slow temperatures}, offering a comprehensive examination of the theoretical foundations and conditions required for each of formation channel. In particular, we focus on post-inflationary scenarios where there are no self-resonances and the reheating epoch can be described {by the inflaton evolving in} a quadratic-like potential. In the hydrodynamical interpretation of this field during the slow-reheating epoch, the gravitational collapse of primordial fluctuations is subject to conditions on their sphericity, limits on their spin, as well as a maximum velocity dispersion. We show how to account for all conditions and show that PBHs form with different masses depending on the collapse mechanism. Finally we show, through an example, how PBH production serves to probe both the physics after primordial inflation, as well as the primordial powerspectrum at the smallest scales.

astro-ph.CO↗