arXiv subjects

Alvaro de la Cruz-Dombriz

Publications and source records attributed to Alvaro de la Cruz-Dombriz.

At least 19 recordsLinked to original sources

arXiv2026

Constraining $f(R)$ gravity and evolving dark energy via large-scale structure and phase-space trajectories

We present a joint observational analysis confronting viable $f(R)$ modified gravity theories, specifically the Hu \& Sawicki and Starobinsky models, with background and large-scale structure (LSS) data. Utilizing Monte-Carlo Markov chain (MCMC) sampling across datasets including baryon acoustic oscillations (BAO), type Ia supernovae (SNeIa), cosmic microwave background (CMB) distance priors, and linear growth measurements ($fσ_8$, $f$, $σ_8$), we place tight constraints on the model parameters governing deviations from General Relativity. For the full dataset combination, we obtain $\log_{10} b_\mathrm{HS} = -6.325_{-1.138}^{+1.216}$ for the Hu \& Sawicki model and $b_\mathrm{S} = (0.8\pm61.0)\times10^{-4}$ for the Starobinsky model. Model comparison based on the Akaike Information Criterion indicates that these $f(R)$ extensions are statistically favored over flat $Λ\text{CDM}$ ($|Δ\text{AIC}| \ge 3.99$) for the combined data. However, when considering the Bayesian Information Criterion, the evidence for support is significantly reduced. Furthermore, we construct two-dimensional phase-space diagrams in the $(μ, γ)$ and $(μ, Σ)$ planes across several redshifts, establishing a novel diagnostic null-test allowing us to probe for deviations from $Λ\text{CDM}$, corresponding to the fixed point $(1,1)$ in both planes, using LSS observables. Should future weak-lensing and galaxy surveys provide data points with $μ-1<0$ and $γ-1>1$ or $Σ-1 < 0 $, then the aforementioned models could be directly ruled out.

physics.gen-ph↗

arXiv2026

Cosmological structure growth in energy-momentum squared gravity

We investigate the cosmological evolution of matter perturbations in the modified gravity model $f(R,T^2)$, where $T^2=T_{μν}T^{μν}$ denotes the quadratic contraction of the energy--momentum tensor. Using the gauge-invariant 1+3 covariant formalism, we study the evolution of the matter density contrast and analyze several growth observables, including the growth factor, the growth index, and the weighted growth rate $fσ_8$. We consider representative values $n=1/2$ and $n=1/4$, which probe different regimes of the matter--geometry coupling. We show that the growth index decreases with increasing redshift and approaches the standard matter-dominated behavior at early times, while mild scale-dependent deviations from the $Λ$CDM model emerge at late times. The model predicts small departures from General Relativity for $n=1/4$, whereas stronger deviations appear for $n=1/2$ and larger values of the coupling parameter $α$. We further compare the theoretical predictions for $fσ_8$ with current observational data and find that viable parameter choices remain within the observational $\pm2σ$ bounds. These results indicate that $f(R,T^2)$ gravity can provide a viable description of late-time cosmic acceleration and large-scale structure formation while remaining consistent with current growth observations.

astro-ph.CO↗

arXiv2025

Close Hyperbolic Encounters In f(R) Gravity

We explore the dynamics and gravitational-wave emission from black hole pairs on unbound orbits undergoing close hyperbolic encounters (CHEs) in dense astrophysical environments. While General Relativity predicts gravitational Bremsstrahlung radiation occurring at periastron, the contribution of the scalar gravitational-wave mode in fully general f(R) gravity remains largely unexplored. We characterize the f(R) scalar mode gravitational radiation for both non-precessing and precessing hyperbolic orbits, identifying potential detection signatures for advanced gravitational wave observatories. By systematically varying orbital precession and eccentricity, we examine their influence on the emitted gravitational waves. We derive potentially detectable time delay and scalar-to-tensor amplitude ratio estimates for representative astrophysical environments and determine optimal orbital configurations for the detection of f(R) scalar gravitational waves from hyperbolic encounters. Our results provide a theoretical framework for scalar-mode signals and observables, establishing CHEs as a promising probe of f(R) gravity with future detectors.

gr-qc↗

arXiv2025

Toward a realistic Buchdahl limit in f(R) theories of gravity

The so-called Buchdahl limit is not yet fully understood in the context of theories of gravity beyond the Einsteinian framework. In this investigation, we generalize this limit for the case of static, spherically-symmetric, relativistic compact stars in f(R) theories of gravity within the metric formalism. We present a comprehensive analysis that ensures regularity, thermodynamic stability, fulfillment of all required junction conditions, recovery of the Newtonian potential at long distances and a correct extraction of the asymptotic mass. Our results are exemplified for the f(R)=R+ alpha R^2 Starobinsky model and several realistic equations of state describing neutron-star matter. We also compare these results with the case of compact stars immersed ad hoc in an Schwarzschild exterior vacuum, although this scenario does not fulfill all necessary f(R) junction conditions. To test the validity of viable f(R) models, we show that such stars can indeed host additional energetic content, so their gravitational redshift can be greater than 2, which is prohibited in General Relativity.

gr-qc↗

arXiv2023

Constraining disformal couplings with Redshift Space Distortion

We study a quintessence model for which the scalar field is disformally coupled to dark matter. The background mimics the LCDM cosmological evolution and the quintessence potential is not specified. A disformal effect due to the quintessential mass is seen in the growth rate of the cosmological structure on large scales. The disformal parameter renders no appreciable effect on the evolution of the total matter perturbation. An analysis of the conformal parameter and quintessential mass is investigated using the Redshift Space Distortion data to find the best-fit values that might explain the well-known sigma-8 tension.

gr-qc↗

arXiv2022

Imprint of f(R) gravity in the cosmic magnification

f(R) gravity is one of the simplest viable modifications to General Relativity: it passes local astrophysical tests, predicts both the early-time cosmic inflation and the late-time cosmic acceleration, and also describes dark matter. In this paper, we probe cosmic magnification on large scales in f(R) gravity, using the well-known Hu-Sawicki model as an example. Our results indicate that at redshifts z < 3, values of the model exponent n > 1 lead to inconsistent behaviour in the evolution of scalar perturbations. Moreover, when relativistic effects are taken into account in the large scale analysis, our results show that as z increases, large-scale changes in the cosmic magnification angular power spectrum owing to integral values of n tend to share a similar pattern, while those of decimal values tend to share another. This feature could be searched for in the experimental data, as a potential "smoking gun" for the given class of gravity models. Furthermore, we found that at z = 1 and lower, relativistic effects lead to a suppression of the cosmic magnification on large scales in f(R) gravity, relative to the concordance model; whereas, at z > 1, relativistic effects lead to a relative boost of the cosmic magnification. In general, relativistic effects enhance the potential of the cosmic magnification as a cosmological probe.

astro-ph.CO↗

arXiv2021

Disformal couplings in a $Λ$CDM background cosmology

The coupled quintessence model with disformal couplings is treated here to mimic the $Λ$CDM background. Using this approach, the quintessence potential does not have to be specified. The model considers a generic fluid coupled to the quintessence, which is specified to be either dark matter or a relativistic fluid. The background consists of a cosmological constant and another uncoupled generic fluid, to cater for three studied scenarios. The cosmological dynamics is investigated for the coupled quintessence model, whose disformal couplings depend on the equation of state of both generic fluids. The scenario, whereby both generic fluids are dark matter, was further analyzed for an expansion history of the mimicking model. The analysis confirms that the mass scale of the quintessence influences the disformal characteristics of the dynamical system, which is portrayed by the evolution of an effective conformal coupling.

gr-qc↗

arXiv2020

Improvements in cosmological constraints from breaking growth degeneracy

The key probes of the growth of large-scale structure are its rate $f$ and amplitude $σ_8$. Redshift space distortions in the galaxy power spectrum allow us to measure only the combination $fσ_8$, which can be used to constrain the standard cosmological model or alternatives. By using measurements of the galaxy-galaxy lensing cross-correlation spectrum or of the galaxy bispectrum, it is possible to break the $fσ_8$ degeneracy and obtain separate estimates of $f$ and $σ_8$ from the same galaxy sample. Currently there are only a handful of such separate measurements, but even this allows for improved constraints on cosmological models. We explore how having a larger and more precise sample of such measurements in the future could constrain further cosmological models. We consider what can be achieved by a future nominal sample that delivers a $\sim 1\%$ constraint on $f$ and $σ_8$ separately, compared to the case with a similar precision on the combination $fσ_8$. For the six cosmological parameters of $Λ$CDM, we find improvements of $\sim\! 5$--$50\%$ on their constraints. For modified gravity models in the Horndeski class, the improvements on these standard parameters are $\sim\! 0$--$15\%$. However, the precision on the sum of neutrino masses improves by 65\% and there is a significant increase in the precision on the background and perturbation Horndeski parameters.

astro-ph.CO↗

arXiv2020

Scalar perturbations in $f(T)$ gravity using the $1+3$ covariant approach

The cosmological scalar perturbations of standard matter are investigated in the context of extended teleparallel $f(T)$ gravity theories using the $1+3$ covariant formalism. After a review of the background, gravitational field equations of $f(T)$ gravity and the introduction of the covariant perturbation variables, the usual scalar and harmonic decomposition have been performed, and the analysis of the growth of the density contrasts in the quasi-static approximation for two non-interacting fluids scenarios, namely torsion-dust and torsion-radiation mixtures is presented for the generic $f(T)$ gravity theory. Special applications to two classes of $f(T)$ gravity toy models, namely $f(T) = μT_0 (T/T_0)^n$ and $f(T) = T + μT_0 (-T/T_0)^n$, have then been made within the observationally viable regions of their respective parameter spaces, and the growth of the matter density contrast for both torsion-dust and torsion-radiation epochs of the Universe has been examined. The exact solutions of the dust perturbations, with growing amplitudes in cosmic time, are obtained for some limiting cases of n. Similarly, the long- and short-wavelength modes in the torsion-radiation case are treated, with the amplitudes either oscillating or monotonically growing with time. Overall, it is noted that $f(T)$ models contain a richer set of observationally viable structure growth scenarios that can be tested against up- and-coming observational data and can accommodate currently known features of the large-scale structure power spectrum in the general relativistic and $Λ$CDM limits.

gr-qc↗

arXiv2019

Optimising growth of structure constraints on modified gravity

We use growth of structure data to constrain the effective field theory of dark energy. Considering as case study Horndeski theories with the speed of gravitational waves equal to that of light, we show how constraints on the free parameters and the large-scale structure phenomenological functions can be improved by two ingredients: firstly by complementing the set of redshift-space distortions data with the three recent measurements of the growth rate $f$ and the amplitude of matter fluctuations $σ_8$ from the VIPERS and SDSS collaborations; secondly by applying a local Solar System bound on the variation of the Newton constant. This analysis allows us to conclude that: $i)$ despite firmly restricting the predictions of weaker gravity, the inclusion of the Solar System bound does not prevent suppressed growth relative to the standard model $Λ$CDM at low redshifts; $ii)$ the same bound in conjunction with the growth of structure data strongly restricts the redshift evolution of the gravitational slip parameter to be close to unity and the present value is constrained to one at the $10^{-3}$ level; $iii)$ the growth of structure data favours a fifth force contribution to the effective gravitational coupling at low redshifts and at more than one sigma at present time.

astro-ph.CO↗

arXiv2019

Analysis of branon dark matter and extra-dimensional models with AMS-02

In the following work, we compute the positron production from branon dark matter annihilations in order to constrain extra-dimensional theories. By having assumed that the positron fraction measured by AMS-02 is well explained just with astrophysical sources, exclusion diagrams for the branon mass and the tension of the brane, the two parameters characterising the branon phenomenology become possible. Our analysis has been performed for a minimal and a medium diffusion model in one extra dimension for both pseudo-isothermal and Navarro Frenck White dark matter haloes. Our constraints in the dark matter mass candidate range between 200 GeV and 100 TeV. Combined with previous cosmological analyses and experimental data in colliders, it allows us to set bounds on the parameter space of branons. In particular, we have discarded regions in the mass-tension diagram up to a branon mass of 28 TeV for the pseudo-Isothermal prole and minimal diffusion, and 63 TeV for the Navarro-Frenck-White profile and medium diffusion.

hep-ph↗

arXiv2019

Black holes, gravitational waves and fundamental physics: a roadmap

The grand challenges of contemporary fundamental physics---dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem---all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress.

gr-qc↗

arXiv2018

Cosmological bouncing solutions in extended teleparallel gravity theories

In the context of extended Teleparallel gravity theories with a 3+1 dimensions Gauss-Bonnet analog term, we address the possibility of these theories reproducing several well-known cosmological bouncing scenarios in a four-dimensional Friedmann-Lemaître-Robertson-Walker geometry. We shall study which types of gravitational Lagrangians are capable of reconstructing bouncing solutions provided by analytical expressions for symmetric, oscillatory, superbounce, the matter bounce and singular bounce. Some of the Lagrangians discovered are both analytical at the origin having Minkowski and Schwarzschild as vacuum solutions. All these results open the possibility up for such theories to be competitive candidates of extended theories of gravity in cosmological scales.

gr-qc↗

arXiv2017

Cosmological reconstructed solutions in extended teleparallel gravity theories with a teleparallel Gauss-Bonnet term

In the context of extended Teleparallel gravity theories with a 3+1 dimensions Gauss-Bonnet analog term, we address the possibility of these theories reproducing several well-known cosmological solutions. In particular when applied to a Friedmann-Lemaître-Robertson-Walker geometry in four-dimensional spacetime with standard fluids exclusively. We study different types of gravitational Lagrangians and reconstruct solutions provided by analytical expressions for either the cosmological scale factor or the Hubble parameter. We also show that it is possible to find Lagrangians of this type without a cosmological constant such that the behaviour of the LCDM model is precisely mimicked. The new Lagrangians may also lead to other phenomenological consequences opening up the possibility for new theories to compete directly with other extensions of General Relativity.

gr-qc↗

arXiv2017

The realistic models of relativistic stars in f(R) = R + alpha R^2 gravity

In the context of f(R)=R + alpha R^2 gravity, we study the existence of neutron and quark stars with no intermediate approximations in the generalised system of Tolman-Oppenheimer-Volkov equations. Analysis shows that for positive alpha's the scalar curvature does not drop to zero at the star surface (as in General Relativity) but exponentially decreases with distance. Also the stellar mass bounded by star surface decreases when the value alpha increases. Nonetheless distant observers would observe a gravitational mass due to appearance of a so-called gravitational sphere around the star. The non-zero curvature contribution to the gravitational mass eventually is shown to compensate the stellar mass decrease for growing alpha's. We perform our analysis for several equations of state including purely hadronic configurations as well as hyperons and quark stars. In all cases, we assess that the relation between the parameter $α$ and the gravitational mass weakly depend upon the chosen equation of state. Another interesting feature is the increase of the star radius in comparison to General Relativity for stars with masses close to maximal, whereas for intermediate masses around 1.4-1.6 solar masses, the radius of star depends upon alpha very weakly. Also the decrease in the mass bounded by star surface may cause the surface redshift to decrease in R^2-gravity when compared to Einsteinian predictions. This effect is shown to hardly depend upon the observed gravitational mass. Finally, for negative values of alpha our analysis shows that outside the star the scalar curvature has damped oscillations but the contribution of the gravitational sphere into the gravitational mass increases indefinitely with radial distance putting into question the very existence of such relativistic stars.

gr-qc↗

arXiv2016

On neutron stars in f(R) theories: small radii, large masses and large energy emitted in a merger

In the context of f(R) gravity theories, we show that the apparent mass of a neutron star as seen from an observer at infinity is numerically calculable but requires careful matching, first at the star's edge, between interior and exterior solutions, none of them being totally Schwarzschild-like but presenting instead small oscillations of the curvature scalar R; and second at large radii, where the Newtonian potential is used to identify the mass of the neutron star. We find that for the same equation of state, this mass definition is always larger than its general relativistic counterpart. We exemplify this with quadratic $R^2$ and Hu-Sawicki-like modifications of the standard General Relativity action. Therefore, the finding of two-solar mass neutron stars basically imposes no constraint on stable f(R) theories. However, star radii are in general smaller than in General Relativity, which can give an observational handle on such classes of models at the astrophysical level. Both larger masses and smaller matter radii are due to much of the apparent effective energy residing in the outer metric for scalar-tensor theories. Finally, because the f(R) neutron star masses can be much larger than General Relativity counterparts, the total energy available for radiating gravitational waves could be of order several solar masses, and thus a merger of these stars constitutes an interesting wave source.

gr-qc↗

arXiv2016

Spotting deviations from R^2 inflation

We discuss the soundness of inflationary scenarios in theories beyond the Starobinsky model, namely a class of theories described by arbitrary functions of the Ricci scalar and the K-essence field. We discuss the pathologies associated with higher-order equations of motion which will be shown to constrain the stability of this class of theories. We provide a general framework to calculate the slow-roll parameters and the corresponding mappings to the theory parameters. For paradigmatic gravitational models within the class of theories under consideration we illustrate the power of the Planck/Bicep2 latest results to constrain such gravitational Lagrangians. Finally, bounds for potential deviations from Starobinsky-like inflation are derived.

gr-qc↗

arXiv2016

Limitations of cosmography in extended theories of gravity

The cosmographic approach, which only relies upon the homogeneity and isotropy of the Universe on large scales, has become an essential tool in dealing with an increasing number of theoretical possibilities for explaining the late-time acceleration of the Universe, ranging from Modified Gravity theories to Dark Energy alternatives passing from testing the cosmological concordance Lambda-CDM model. Despite its generality, we show that this method has a number of shortcomings when trying to adequately reconstruct theories with higher-order derivatives in either the gravitational or the matter sector. Herein some paradigmatic examples of such an inability, explanations of the limitations and prospective cures will be presented.

gr-qc↗