arXiv subjects

Diego Sáez-Chillón Gómez

Publications and source records attributed to Diego Sáez-Chillón Gómez.

32 records · Page 2Linked to original sources

arXiv2024

Exponential gravity with logarithmic corrections in the presence of axion dark matter

An exponential modified gravity with additional logarithmic corrections is considered with the presence of an axion-like scalar field in the role of dark matter. Axion fields are thought to become important at late-times when the axion-like scalar field oscillates around its vacuum expectation value, mimicking dark matter behaviour. The model is compared with the usual pressureless fluid description of dark matter. Both models are tested with observational data including some of the latest sources, providing similar fits in comparison with the $Λ$CDM model. Despite results are not statistically relevant to rule out any model, the number of free parameters still favours $Λ$CDM model, as shown by computing the goodness of the fits.

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arXiv2023

Exponential F(R) gravity with axion dark matter

The cosmological evolution within the framework of exponential $F(R)$ gravity is analysed by assuming two forms for dark matter: (a) a standard dust-like fluid and (b) an axion scalar field. As shown in previous literature, an axion-like field oscillates during the cosmological evolution but can play the role of dark matter when approaching the minimum of its potential. Both scenarios are confronted with recent observational data including the Pantheon Type Ia supernovae, Hubble parameter estimations (Cosmic Chronometers), Baryon Acoustic Oscillations and Cosmic Microwave Background distances. The models show great possibilities in describing these observations when compared with the $Λ$CDM model, supporting the viability of exponential $F(R)$ gravity. The differences between both descriptions of dark matter is analysed.

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arXiv2023

Shadows and photon rings of regular black holes and geonic horizonless compact objects

The optical appearance of a body compact enough to feature an unstable bound orbit, when surrounded by an accretion disk, is expected to be dominated by a luminous ring of radiation enclosing a central brightness depression typically known as the shadow. Despite observational limitations, the rough details of this picture have been now confirmed by the results of the EHT Collaboration on the imaging of the M87 and Milky Way supermassive central objects. However, the precise characterization of both features - ring and shadow - depends on the interaction between the background geometry and the accretion disk, thus being a fertile playground to test our theories on the nature of compact objects and the gravitational field itself in the strong-field regime. In this work we use both features in order to test a continuous family of solutions interpolating between regular black holes and horizonless compact objects, which arise within the Eddington-inspired Born-Infeld theory of gravity, a viable extension of Einstein's General Relativity (GR). To this end we consider seven distinctive classes of such configurations (five black holes and two traversable wormholes) and study their optical appearances under illumination by a geometrically and optically thin accretion disk, emitting monochromatically with three analytic intensity profiles previously suggested in the literature. We build such images and consider the sub-ring structure created by light rays crossing the disk more than once and existing on top of the main ring of radiation. We discuss in detail the modifications as compared to their GR counterparts, the Lyapunov exponents of unstable nearly-bound orbits, as well as the differences between black hole and traversable wormholes for the three intensity profiles.

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arXiv2023

Unifying inflation with early and late dark energy in Einstein-Gauss-Bonnet gravity

In the era of precision cosmology, different observational data has led to precise measurements of the Hubble constant that differ significantly, what has been called the Hubble tension problem. In order to solve such a discrepancy, many different solutions have been proposed, from systematic errors on the observational data to theoretical proposals that assume an early dark energy that might affect the universe expansion at the time of recombination. In this paper, a model of varying cosmological constant is proposed in the framework of Einstein-Gauss-Bonnet gravity. The corresponding gravitational action is reconstructed and such a model is shown to reproduce well the inflationary era together with dark energy epoch and at the same time to provide an explanation for the discrepancy on the Hubble constant predictions. The transition to a phantom epoch is also realised, avoiding the usual instability problems of ordinary scalar field models.

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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

Eternal vs singular observers in interacting dark energy-dark matter models

Interacting dark energy-dark matter models have been widely analyzed in the literature in an attempt to find traces of new physics beyond the usual cosmological ($Λ$CDM) models. Such a coupling between both dark components is usually introduced in a phenomenological way through a flux in the continuity equation. However, models with a Lagrangian formulation are also possible. A class of the latter assumes a conformal/disformal coupling that leads to a fifth force on the dark matter component, which consequently does not follow the same geodesics as the other (baryonic, radiation, and dark energy) matter sources. Here we analyze how the usual cosmological singularities of the standard matter frame are seen from the dark matter one, concluding that by choosing an appropriate coupling, dark matter observers will see no singularities but a non-beginning, non-ending universe. By considering two simple phenomenological models we show that such a type of coupling can fit observational data as well as the usual $Λ$CDM model.

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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

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

3+1 decomposition in modified gravities within the Palatini formalism and some applications

In the present paper, the 3+1 decomposition of the spacetime onto hypersurface(s) is analysed and established for theories within the Palatini formalism by considering a general function of the Ricci scalar in the gravitational action. The corresponding Gauss-Codazzi relations are obtained and the boundary term that has to be subtracted in the gravitational action is easily deduced. Then, these relations are applied to the so-called ADM decomposition to describe the foliation of the spacetime onto hypersurfaces of constant time within these theories. Finally, the junction conditions are also obtained by using a decomposition in Gaussian normal coordinates, which coincide with the conditions deduced previously through different approaches.

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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

Analyzing the $H_0$ tension in $F(R)$ gravity models

The Hubble constant tension problem is analysed in the framework of a class of modified gravity, the so-called $F(R)$ gravity. To do so, we explore two models: an exponential and a power-law $F(R)$ gravities, which includes an early dark energy (EDE) term in the latter. These models can describe both an early time inflationary epoch and the late time accelerating expansion of the universe. We confront both models with recent observational data including the Pantheon Type Ia supernovae sample, the latest measurements of the Hubble parameter $H(z)$ from differential ages of galaxies (cosmic chronometers) and separately from baryon acoustic oscillations. Standard rulers data set from the Cosmic Microwave Background radiation are also included in our analysis. The estimations of the Hubble constant appear to be essentially depending on the set of observational data and vary in the range from 68 to 70{.}3 km\,/(s$\cdot$Mpc). The fits of the other free parameters of the models are also obtained, revealing interesting consequences.

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arXiv2021

Variational principle and boundary terms in gravity à la Palatini

A general $f(\mathcal{R})$ gravitational theory is considered within the Palatini formalism. By applying the variational principle and the usual conditions on the boundary, we show explicitly that a surface term remains such that as in their metric-compatible counterparts, an additional surface term has to be added in the gravitational action, which plays a fundamental role when calculating the entropy of the black hole.

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arXiv2020

General slow-roll inflation in $f(R)$ gravity under the Palatini approach

Slow-roll inflation is analyzed in the context of modified gravity within the Palatini formalism. As shown in the literature, inflation in this framework requires the presence of non-traceless matter, otherwise it does not occur just as a consequence of the non-linear gravitational terms of the action. Nevertheless, by including a single scalar field that plays the role of the inflaton, slow-roll inflation can be performed in these theories, where the equations lead to an effective potential that modifies the dynamics. We obtain the general slow-roll parameters and analyze a simple model to illustrate the differences introduced by the gravitational terms under the Palatini approach, and the modifications on the spectral index and the tensor to scalar ratio predicted by the model.

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arXiv2020

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

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

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