arXiv subjects

Diego Rubiera-Garcia

Publications and source records attributed to Diego Rubiera-Garcia.

At least 55 records · Page 3Linked to original sources

arXiv2023

Modified Gravity and Cosmology: An Update by the CANTATA Network

General Relativity and the $Λ$CDM framework are currently the standard lore and constitute the concordance paradigm. Nevertheless, long-standing open theoretical issues, as well as possible new observational ones arising from the explosive development of cosmology the last two decades, offer the motivation and lead a large amount of research to be devoted in constructing various extensions and modifications. All extended theories and scenarios are first examined under the light of theoretical consistency, and then are applied to various geometrical backgrounds, such as the cosmological and the spherical symmetric ones. Their predictions at both the background and perturbation levels, and concerning cosmology at early, intermediate and late times, are then confronted with the huge amount of observational data that astrophysics and cosmology are able to offer recently. Theories, scenarios and models that successfully and efficiently pass the above steps are classified as viable and are candidates for the description of Nature. This work is a Review of the recent developments in the fields of gravity and cosmology, presenting the state of the art, high-lighting the open problems, and outlining the directions of future research. Its realization was performed in the framework of the COST European Action ``Cosmology and Astrophysics Network for Theoretical Advances and Training Actions''.

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arXiv2023

Geodesic completeness of effective null geodesics in regular space-times with non-linear electrodynamics

We study the completeness of light trajectories in certain spherically symmetric regular geometries found in Palatini theories of gravity threaded by non-linear (electromagnetic) fields, which makes their propagation to happen along geodesics of an effective metric. Two types of geodesic restoration mechanisms are employed: by pushing the focal point to infinite affine distance, thus unreachable in finite time by any sets of geodesics, or by the presence of a defocusing surface associated to the development of a wormhole throat. We discuss several examples of such geometries to conclude the completeness of all such effective paths. Our results are of interest both for the finding of singularity-free solutions and for the analysis of their optical appearances e.g. in shadow observations.

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arXiv2023

Imaging compact boson stars with hot-spots and thin accretion disks

In this work we consider the observational properties of compact boson stars with self-interactions orbited by isotropically emitting (hot-spot) sources and optically thin accretion disks. We consider two families of boson stars supported by quartic and sixth-order self-interaction potentials, and choose three samples of each of them in growing compactness; only those with large enough compactness are capable to hold light-rings, namely, null bound orbits. For the hot-spots, using inclination angles $θ=\{20^\circ, 50^\circ, 80^\circ \}$ we find a secondary track plunge-through image of photons crossing the interior of the boson star, which can be further decomposed into additional images if the star is compact enough. For accretion disks we find that the latter class of stars actually shows a sequence of additional secondary images in agreement with the hot-spot analysis, a feature absent in typical black hole space-times. Furthermore, we also find a shadow-like central brightness depression for some of these stars in both axial observations and at the inclination angles above. We discuss our findings in relation to the capability of boson stars to effectively act as black hole mimickers in their optical appearances as well as potential observational discriminators.

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arXiv2022

Shadows of boson and Proca stars with thin accretion disks

In this work, we obtain the shadow images of spherically symmetric scalar boson and Proca stars using analytical fittings of numerical solutions, when illuminated by a geometrically thin accretion disk. We chose a sample of four boson and four Proca stars with radii ranging from more compact configurations with $R\sim 9M$ to more dilute configurations with $R\sim 20M$, where $M$ is the total mass of the bosonic star. In these configurations, the absence of the photon sphere (the locus of unstable bound geodesics) makes the optical appearance of these stars to be dominated by a single luminous ring enclosing a central brightness depression, and no further light rings are available. We show that if one considers face-on observations and a disk model whose emission is truncated at some finite radius at which the luminosity attains its maximum value, both the size of the shadow, as well as the luminosity and depth of the bright region, are heavily influenced by the emission profile, with the choice of the type and parameters of the bosonic stars in our samples having a sub-dominant influence. These differences are nonetheless significantly magnified when one allows the accretion disk to extend close enough to the center of the star. Our results point out that even though bosonic stars are horizonless and do not have a photon sphere, some of them may be able to produce conventional black hole shadow-like images provided that their compactness is large enough, thus being potentially consistent with current and future observations.

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arXiv2022

Regular black holes in Palatini gravity

Palatini (or metric-affine) theories of gravity are characterized by having {\it a priori} independent metric and affine structures. The theories built in this framework have their field equations obtained as independent variations of the action with respect to the metric, the affine connection, and the other fields. In this work we consider the issue of singularity-removal in several members of a family of theoretically consistent and observationally viable subclass of them, built as contractions of the Ricci tensor with the metric (Ricci-based gravities or RBGs, for short). Several types of (spherically symmetric) solutions are considered from combinations of the gravity and matter sectors satisfying basic energy conditions, discussing the modifications to the horizons and to the innermost structure of the solutions, the main player in town being the presence in some cases of a bouncing behavior in the radial function. We use a full weaponry of criteria to test whether singularity-removal has been achieved: completeness of (null and time-like) geodesic paths, impact of unbound curvature divergences, analysis of causal contact upon congruences of geodesics, paths of observers with (bound) acceleration, and propagation of (scalar) waves. We further elaborate on the (three) main avenues by which such a regularity of the corresponding space-times is achieved, and comment on the lack of correlation between the (in)completeness of geodesics and the blow up of curvature scalars, something too frequently and too carelessly assumed in the literature on the subject. We conjecture that a larger flexibility in the underlying geometrical ingredients to formulate our gravitational theories may hold (part of) the clue to resolve black hole singularities.

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arXiv2022

Junction conditions of Palatini $f\left(\mathcal R,T\right)$ gravity

We work out the junction conditions for the Palatini $f(\mathcal{R},T)$ extension of General Relativity, where $f$ is an arbitrary function of the curvature scalar $\mathcal{R}$ of an independent connection, and of the trace $T$ of the stress-energy tensor of the matter fields. We find such conditions on the allowed discontinuities of several geometrical and matter quantities, some of which depart from their metric counterparts, and in turn extend their Palatini $f(\mathcal{R})$ versions via some new $T$-dependent terms. Moreover, we also identify some "exceptional cases" of $f(\mathcal{R},T)$ Lagrangians such that some of these conditions can be discarded, thus allowing for further discontinuities in $\mathcal{R}$ and $T$ and, in contrast with other theories of gravity, they are shown to not give rise to extra components in the matter sector e.g. momentum fluxes and double gravitational layers. We discuss how these junction conditions, together with the non-conservation of the stress-energy tensor ascribed to these theories, may induce non-trivial changes in the shape of specific applications such as traversable thin-shell wormholes.

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arXiv2022

Multi-ring images of thin accretion disk of a regular naked compact object

We discuss the importance of multi-ring images in the optical appearance of a horizonless spherically symmetric compact object, when illuminated by an optically thin accretion disk. Such an object corresponds to a sub-case of an analytically tractable extension of the Kerr solution dubbed as the {\it eye of the storm} by Simpson and Visser in [JCAP \textbf{03} (2022) 011], which merits in removing curvature singularities via an asymptotically Minkowski core, while harbouring both a critical curve and an infinite potential barrier at the center for null geodesics. This multi-ring structure is induced by light rays winding several times around the object, and whose luminosity is significantly boosted as compared to the Schwarzschild solution by the modified shape of the potential. Using three toy-profiles for the emission of an infinitely thin disk, truncated at its inner edge (taking its maximum value there) and having different decays with the distance, we discuss the image created by up to eight rings superimposed on top of the direct emission of the disk as its edge is moved closer to the center of the object. Our results point out to the existence of multi-ring images with a non-negligible luminosity in shadow observations when one allows for the existence of other compact objects in the cosmic zoo beyond the Schwarzschild solution. Such multi-ring images could be detectable within the future projects on very-long baseline interferometry.

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arXiv2022

An infinite class of exact rotating black hole metrics of modified gravity

We build an infinite class of exact axisymmetric solutions of a metric-affine gravity theory, namely, Eddington-inspired Born-Infeld gravity, coupled to an anisotropic fluid as a matter source. The solution-generating method employed is not unique of this theory but can be extended to other Ricci-Based Gravity theories (RBGs), a class of theories built out of contractions of the Ricci tensor with the metric. This method exploits a correspondence between the space of solutions of General Relativity and that of RBGs, and is independent of the symmetries of the problem. For the particular case in which the fluid is identified with non-linear electromagnetic fields we explicitly derive the corresponding axisymmetric solutions. Finally, we use this result to work out the counterpart of the Kerr-Newman black hole when Maxwell electrodynamics is set on the metric-affine side. Our results open up an exciting new avenue for testing new gravitational phenomenology in the fields of gravitational waves and shadows out of rotating black holes.

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arXiv2022

Light ring images of double photon spheres in black hole and wormhole space-times

The silhouette of a black hole having a critical curve (an unstable bound photon orbit) when illuminated by an optically thin accretion disk whose emission is confined to the equatorial plane shows a distinctive central brightness depression (the shadow) whose outer edge consists of a series of strongly lensed, self-similar rings superimposed with the disk's direct emission. While the size and shape of the critical curve depend only on the background geometry, the pattern of bright and dark regions (including the size and depth of the shadow itself) in the image is strongly influenced by the (astro)physics of the accretion disk. This aspect makes it difficult to extract clean and clear observational discriminators between the Kerr black hole and other compact objects. In the presence of a second critical curve, however, observational differences become apparent. In this work we shall consider some spherically symmetric black hole and wormhole geometries characterized by the presence of a second critical curve, via a uniparametric family of extensions of the Schwarzschild metric. By assuming three toy models of geometrically thin accretion disks, we show the presence of additional light rings in the intermediate region between the two critical curves. The observation of such rings could represent a compelling evidence for the existence of black hole mimickers having multiple critical curves.

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arXiv2021

Pre-main sequence evolution of low-mass stars in Eddington-inspired Born-Infeld gravity

We study three aspects of the early-evolutionary phases in low-mass stars within Eddington-inspired Born-Infeld (EiBI) gravity, a viable extension of General Relativity. These aspects are concerned with the Hayashi tracks (i.e. the effective temperature-luminosity relation); the minimum mass required to belong to the main sequence; and the maximum mass allowed for a fully convective star within the main sequence. In all cases we find a dependence of these quantities not only on the theory's parameter, but also on the star's central density, a feature previously found in Palatini $f(R)$ gravity. Using this, we investigate the evolution of these quantities with the (sign of the) EiBI parameter, finding a shift in the Hayashi tracks in opposite directions in the positive/negative branches of it, and an increase (decrease) for positive (negative) parameter in the two masses above. We use these results to ellaborate on the chances to seek for traces of new physics in low-mass stars within this theory.

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arXiv2021

Singularity-free and cosmologically viable Born-Infeld gravity with scalar matter

The early Cosmology driven by a single scalar field, both massless and massive, in the context of Eddington-inspired Born-Infeld gravity, is explored. We show the existence of nonsingular solutions of bouncing and loitering type (depending on the sign of the gravitational theory's parameter, $ε$) replacing the Big Bang singularity, and discuss their properties. In addition, in the massive case we find some new features of the cosmological evolution depending on the value of the mass parameter, including asymmetries in the expansion/contraction phases, or a continuous transition between a contracting phase to an expanding one via an intermediate loitering phase. We also provide a combined analysis of cosmic chronometers, standard candles, BAO, and CMB data to constrain the model, finding that for roughly $\vert ε\vert \lesssim 5\cdot 10^{-8} \text{ m}^2$ the model is compatible with the latest observations while successfully removing the Big Bang singularity. This bound is several orders of magnitude stronger than the most stringent constraints currently available in the literature

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arXiv2021

New light rings from multiple critical curves as observational signatures of black hole mimickers

We argue that the appearance of additional light rings in a shadow observation - beyond the infinite sequence of exponentially demagnified self-similar rings foreseen in the Kerr solution - would make a compelling case for the existence of black hole mimickers having multiple critical curves. We support this claim by discussing three different scenarios of spherically symmetric wormhole geometries having two such critical curves, and explicitly work out the optical appearance of one such object when surrounded by an optically and geometrically thin accretion disk.

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arXiv2021

Charged BTZ-type solutions in Eddington-inspired Born-Infeld gravity

We construct an axially symmetric solution of Eddington-inspired Born-Infeld gravity coupled to an electromagnetic field in 2+1 dimensions including a (negative) cosmological constant term. This is achieved by using a recently developed mapping procedure that allows to generate solutions in certain families of metric-affine gravity theories starting from a known seed solution of General Relativity, which in the present case corresponds to the electrically charged Banados-Teitelboim-Zanelli (BTZ) solution. We discuss the main features of the new configurations, including the modifications to the ergospheres and horizons, the emergence of wormhole structures, and the consequences for the regularity (or not) of these space-times via geodesic completeness.

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arXiv2021

Sudden singularities in generalized hybrid metric-Palatini cosmologies

In this work, we explore cosmological sudden singularities arising in the dynamically equivalent scalar-tensor representation of generalized hybrid metric-Palatini gravity. Using a FLRW background, we show that the structure of the field equations prevents sudden singularities from arising at time derivatives of the scale factor of orders lower than four, but that they are allowed to appear for time derivatives of higher orders. Imposing an ansatz for the scale factor, we provide an explicit solution where these sudden singularities appear in the fourth-order time derivative of the scale factor. A comparison of the Hubble and deceleration parameters arising from this model with the experimental measurements from the Planck Satellite allow us to impose constraints on the time span for which the occurrence of sudden singularities becomes likely in our universe, as measured from the Big Bang.

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arXiv2021

Shadows and optical appearance of black bounces illuminated by a thin accretion disk

We study the light rings and shadows of an uniparametric family of spherically symmetric geometries interpolating between the Schwarzschild solution, a regular black hole, and a traversable wormhole, and dubbed as black bounces, all of them sharing the same critical impact parameter. We consider the ray-tracing method in order to study the impact parameter regions corresponding to the direct, lensed, and photon ring emission, finding a broadening of all these regions for black bounce solutions as compared to the Schwarzschild one. Using this, we determine the optical appearance of black bounces when illuminated by three standard toy models of optically and geometrically thin accretion disks viewed in face-on orientation.

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arXiv2021

Parameterized non-relativistic limit of stellar structure equations in Ricci-based gravity theories

We present the non-relativistic limit of the stellar structure equations of Ricci-based gravities, a family of metric-affine theories whose Lagrangian is built via contractions of the metric with the Ricci tensor of an a priori independent connection. We find that this limit is characterized by four parameters that arise in the expansion of several geometric quantities in powers of the stress-energy tensor of the matter fields. We discuss the relevance of this result for the phenomenology of non-relativistic stars, such as main-sequence stars as well as several substellar objects.

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arXiv2021

Imprints from a Riemann-Cartan space-time on the energy levels of Dirac spinors

In this work, we investigate the effects of the torsion-fermionic interaction on the energy levels of fermions within a Riemann-Cartan geometry using a model-independent approach. We consider the case of fermions minimally coupled to the background torsion as well as non-minimal extensions via additional couplings with the vector and axial fermionic currents which include parity-breaking interactions. In the limit of zero-curvature, and for the cases of constant and spherically symmetric torsion, we find a Zeeman-like effect on the energy levels of fermions and anti-fermions depending on whether they are aligned/anti-aligned with respect to the axial vector part of the torsion (or to specific combination of torsion quantities), and determine the corresponding fine-structure energy transitions. We also discuss non-minimal couplings between fermionic fields and torsion within the Einstein-Cartan theory and its extension to include the (parity-breaking) Holst term. Finally we elaborate on the detection of torsion effects related to the splitting of energy levels in astrophysics, cosmology and solid state physics using current capabilities.

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arXiv2021

Double shadows of reflection-asymmetric wormholes supported by positive energy thin-shells

We consider reflection-asymmetric thin-shell wormholes within Palatini $f(\mathcal{R})$ gravity using a matching procedure of two patches of electrovacuum space-times at a hypersurface (the shell) via suitable junction conditions. The conditions for having (linearly) stable wormholes supported by positive-energy matter sources are determined. We also identify some subsets of parameters able to locate the shell radius above the event horizon (when present) but below the photon sphere (on both sides). We illustrate with an specific example that such two photon spheres allow an observer on one of the sides of the wormhole to see another (circular) shadow in addition to the one generated by its own photon sphere, which is due to the photons passing above the maximum of the effective potential on its side and bouncing back across the throat due to a higher effective potential on the other side. We finally comment on the capability of these double shadows to seek for traces of new gravitational physics beyond that described by General Relativity.

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