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Accelerated observers and the notion of singular spacetimeOct 24 2017Jan 18 2018Geodesic completeness is typically regarded as a basic criterion to determine whether a given spacetime is regular or singular. However, the principle of general covariance does not privilege any family of observers over the others and, therefore, observers ... More

A generalized Weyl structure with arbitrary non-metricityJun 12 2019A Weyl structure is usually defined by an equivalence class of pairs $({\bf g}, \boldsymbol{\omega})$ related by Weyl transformations, which preserve the relation $\nabla {\bf g}=\boldsymbol{\omega}\otimes{\bf g}$, where ${\bf g}$ and $\boldsymbol{\omega}$ ... More

Global Monopole in Palatini f(R) gravityDec 02 2018Mar 10 2019We consider the space-time metric generated by a global monopole in an extension of General Relativity (GR) of the form $f(\mathcal{R})=\mathcal{R}-\lambda \mathcal{R}^2$. The theory is formulated in the metric-affine (or Palatini) formalism and exact ... More

Novel couplings between nonmetricity and matterJan 03 2019We present a novel theory of gravity, namely, an extension of symmetric teleparallel gravity. This is done by introducing a new class of theories where the nonmetricity $Q$ is coupled nonminimally to the matter Lagrangian. This nonminimal coupling entails ... More

On gravitational waves in Born-Infeld inspired non-singular cosmologiesJul 27 2017Jul 03 2018We study the evolution of gravitational waves for non-singular cosmological solutions within the framework of Born-Infeld inspired gravity theories, with special emphasis on the Eddington-inspired Born-Infeld theory. We review the existence of two types ... More

Born-Infeld inspired modifications of gravityApr 11 2017May 17 2017General Relativity has shown an outstanding observational success in the scales where it has been directly tested. However, modifications have been intensively explored in the regimes where it seems either incomplete or signals its own limit of validity. ... More

A correspondence between modified gravity and General Relativity with scalar fieldsOct 09 2018Jan 22 2019We describe a novel procedure to map the field equations of nonlinear Ricci-based metric-affine theories of gravity, coupled to scalar matter described by a given Lagrangian, into the field equations of General Relativity coupled to a different scalar ... More

Coupling matter in modified $Q$-gravityJun 27 2018We present a novel theory of gravity by considering an extension of symmetric teleparallel gravity. This is done by introducing, in the framework of the metric-affine formalism, a new class of theories where the nonmetricity $Q$ is nonminimally coupled ... More

New scalar compact objects in Ricci-based gravity theoriesJun 11 2019Taking advantage of a previously developed method, which allows to map solutions of General Relativity into a broad family of theories of gravity based on the Ricci tensor (Ricci-based gravities), we find new exact analytical scalar field solutions by ... More

Limit to General Relativity in f(R) theories of gravityDec 07 2006Jan 09 2007We discuss two aspects of f(R) theories of gravity in metric formalism. We first study the reasons to introduce a scalar-tensor representation for these theories and the behavior of this representation in the limit to General Relativity, f(R)--> R. We ... More

Introduction to Modified Gravity: From the Cosmic Speedup Problem to Quantum Gravity PhenomenologyDec 09 2011These notes represent a summary of the introductory part of a course on modified gravity delivered at several Spanish Universities (Granada, Valencia, and Valladolid), at the University of Wisconsin-Milwaukee (WI, USA), and at the Karl-Franzens Universitaet ... More

Non-singular Universes a la PalatiniJan 25 2011It has recently been shown that f(R) theories formulated in the Palatini variational formalism are able to avoid the big bang singularity yielding instead a bouncing solution. The mechanism responsible for this behavior is similar to that observed in ... More

Cosmology in Palatini theories of gravityJul 19 2012We discuss recent results on the cosmology of extended theories of gravity formulated in the Palatini approach, i.e., assuming that metric and connection are independent fields. In particular, we focus on the attempts to explain the cosmic speedup with ... More

Palatini approach to bouncing cosmologies and DSR-like effectsNov 23 2011It is shown that a quadratic gravitational Lagrangian in the Palatini formulation is able to capture different aspects of quantum gravity phenomenology in a single framework. In particular, in this theory field excitations propagating with different energy-densities ... More

Re-examination of Polytropic Spheres in Palatini f(R) GravityOct 20 2008We investigate spherically symmetric, static matter configurations with polytropic equation of state for a class of f(R) models in Palatini formalism and show that the surface singularities recently reported in the literature are not physical in the case ... More

Nonsingular black holes in Palatini extensions of General RelativityJul 22 2016An introduction to extended theories of gravity formulated in metric-affine (or Palatini) spaces is presented. Focusing on spherically symmetric configurations with electric fields, we will see that in these theories the central singularity present in ... More

Introduction to Palatini theories of gravity and nonsingular cosmologiesDec 27 2012These notes are a summary of lectures given at the Instituto de Astronomia of the Universidad Nacional Autonoma de Mexico (UNAM), the Dipartimento di Fisica of the Universita degli studi di Salerno (Italy), and the Instituto de Fisica Corpuscular of the ... More

Palatini Approach Beyond Einstein's GravityDec 07 2011I review recent results obtained for extensions of general relativity formulated within the Palatini formalism, an approach in which metric and connection are treated as independent geometrical entities. The peculiar dynamics of these theories, governed ... More

Post-Newtonian constraints on f(R) cosmologies in metric formalismMay 26 2005We compute the complete post-Newtonian limit of the metric form of f(R) gravities using a scalar-tensor representation. By comparing the predictions of these theories with laboratory and solar system experiments, we find a set of inequalities that any ... More

Palatini Approach to Modified Gravity: f(R) Theories and BeyondJan 20 2011We review the recent literature on modified theories of gravity in the Palatini approach. After discussing the motivations that lead to consider alternatives to Einstein's theory and to treat the metric and the connection as independent objects, we review ... More

Hydrogen atom in Palatini theories of gravityFeb 27 2008Jun 03 2008We study the effects that the gravitational interaction of $f(R)$ theories of gravity in Palatini formalism has on the stationary states of the Hydrogen atom. We show that the role of gravity in this system is very important for lagrangians $f(R)$ with ... More

Violation of the Equivalence Principle in Modified Theories of GravityNov 30 2006Jan 09 2007We study modified theories of gravity of the f(R) type in Palatini formalism. For a generic f(R) lagrangian, we show that the metric can be solved as the product of a scalar function times a rank-two tensor (or auxiliary metric). The scalar function is ... More

Post-Newtonian constraints on f(R) cosmologies in Palatini formalismMay 26 2005We compute the complete post-Newtonian limit of the Palatini form of f(R) gravities using a scalar-tensor representation. By comparing the predictions of these theories with laboratory and solar system experiments, we find a set of inequalities that any ... More

Birkhoff's theorem and perturbations in $f(R)$ theoriesJul 19 2012Invited contribution to Annalen der Physik (Expert Opinion).

Palatini Actions and Quantum Gravity PhenomenologyJan 14 2011Oct 05 2011We show that an invariant an universal length scale can be consistently introduced in a generally covariant theory through the gravitational sector using the Palatini approach. The resulting theory is able to capture different aspects of quantum gravity ... More

New Phenomenology for Palatini f(R) Theories: Non-singular UniversesOct 20 2009Oct 24 2009We study modified theories of gravity of the f(R) type in Palatini formalism. We first consider the stability of atoms when the Palatini gravitational interaction is taken into account in the derivation of the non-relativistic Schrodinger equation. We ... More

The gravity lagrangian according to solar system experimentsMay 20 2005May 22 2005In this work we show that the gravity lagrangian f(R) at relatively low curvatures in both metric and Palatini formalisms is a bounded function that can only depart from the linearity within the limits defined by well known functions. We obtain those ... More

Nonlinear Gravity Theories in the Metric and Palatini FormalismsMar 22 2004May 03 2004We study nonlinear gravity theories in both the metric and the Palatini (metric-affine) formalisms. The nonlinear character of the gravity lagrangian in the metric formalism causes the appearance of a scalar source of matter in Einstein's equations that ... More

Covariant Effective Action for Loop Quantum Cosmology a la PalatiniJun 17 2008Dec 09 2008In loop quantum cosmology, non-perturbative quantum gravity effects lead to the resolution of the big bang singularity by a quantum bounce without introducing any new degrees of freedom. Though fundamentally discrete, the theory admits a continuum description ... More

Isotropic and Anisotropic Bouncing Cosmologies in Palatini GravityMay 22 2010We study isotropic and anisotropic (Bianchi I) cosmologies in Palatini $f(R)$ and $f(R,R_{\mu\nu}R^{\mu\nu})$ theories of gravity and consider the existence of non-singular bouncing solutions in the early universe. We find that all $f(R)$ models with ... More

Reissner-Nordström black holes in extended Palatini theoriesJul 25 2012We study static, spherically symmetric solutions with an electric field in an extension of general relativity (GR) containing a Ricci-squared term and formulated in the Palatini formalism. We find that all the solutions present a central core whose area ... More

Hamiltonian Formulation of Palatini f(R) theories a la Brans-DickeJan 18 2011We study the Hamiltonian formulation of f(R) theories of gravity both in metric and in Palatini formalism using their classical equivalence with Brans-Dicke theories with a non-trivial potential. The Palatini case, which corresponds to the w=-3/2 Brans-Dicke ... More

Nonsingular black holes in quadratic Palatini gravityDec 02 2011Jul 25 2012We find that if general relativity is modified at the Planck scale by a Ricci-squared term, electrically charged black holes may be nonsingular. These objects concentrate their mass in a microscopic sphere of radius $r_{core}\approx N_q^{1/2}l_P/3$, where ... More

The quantum, the geon, and the crystalJul 28 2015Effective geometries arising from a hypothetical discrete structure of space-time can play an important role in the understanding of the gravitational physics beyond General Relativity. To discuss this question, we make use of lessons from crystalline ... More

Non-Riemannian geometry: towards new avenues for the physics of modified gravityJun 06 2015Less explored than their metric (Riemannian) counterparts, metric-affine (or Palatini) theories bring an unexpected phenomenology for gravitational physics beyond General Relativity. Lessons of crystalline structures, where the presence of defects in ... More

Nonsingular charged black holes à la PalatiniJul 18 2012We argue that the quantum nature of matter and gravity should lead to a discretization of the allowed states of the matter confined in the interior of black holes. To support and illustrate this idea, we consider a quadratic extension of General Relativity ... More

Nonsingular Black Holes in Palatini Extensions of General RelativityJan 11 2013We discuss static, spherically symmetric solutions with an electric field in a quadratic extension of general relativity formulated in the Palatini approach (assuming that metric and connection are independent fields). Unlike the usual metric formulation ... More

Importance of torsion and invariant volumes in Palatini theories of gravityJun 18 2013Oct 26 2013We study the field equations of extensions of General Relativity formulated within a metric-affine formalism setting torsion to zero (Palatini approach). We find that different (second-order) dynamical equations arise depending on whether torsion is set ... More

Geons as wormholes of modified gravityJan 02 2016Wormholes may arise as solutions of extensions of General Relativity without violation of the energy conditions. Working in a Palatini approach we consider classical geometries supporting such wormholes. It is shown that the resulting space-times represent ... More

Palatini $f(R)$ Black Holes in Nonlinear ElectrodynamicsOct 04 2011The electrically charged Born-Infeld black holes in the Palatini formalism for $f(R)$ theories are analyzed. Specifically we study those supported by a theory $f(R)=R\pm R^2/R_P$, where $R_P$ is Planck's curvature. These black holes only differ from their ... More

Nonsingular Black Holes in $f(R)$ TheoriesSep 08 2015We study the structure of a family of static, spherically symmetric space-times generated by an anisotropic fluid and governed by a particular type of $f(R)$ theory. We find that for a range of parameters with physical interest, such solutions represent ... More

Brane-world and loop cosmology from a gravity-matter coupling perspectiveMay 28 2014We show that the effective brane-world and the loop quantum cosmology background expansion histories can be reproduced from a modified gravity perspective in terms of an $f(R)$ gravity action plus a $g(R)$ term non-minimally coupled with the matter Lagrangian. ... More

Semiclassical geons at particle acceleratorsJun 27 2013Jan 31 2014We point out that in certain four-dimensional extensions of general relativity constructed within the Palatini formalism stable self-gravitating objects with a discrete mass and charge spectrum may exist. The incorporation of nonlinearities in the electromagnetic ... More

Vanishing efficiency of speeded-up quantum Otto enginesJun 18 2019We assess the energy cost of shortcuts to adiabatic expansions or compressions of a harmonic oscillator, the power strokes of a quantum Otto engine. Difficulties to identify the cost stem from the interplay between different parts of the total system ... More

Black hole formation from a null fluid in extended Palatini gravitySep 15 2012We study the formation and perturbation of black holes by null fluxes of neutral matter in a quadratic extension of General Relativity formulated a la Palatini. Working in a spherically symmetric space-time, we obtain an exact analytical solution for ... More

Short distances, black holes, and TeV gravityJan 17 2007The Hawking effect can be rederived in terms of two-point functions and in such a way that it makes it possible to estimate, within the conventional semiclassical theory, the contribution of ultrashort distances at $I^+$ to the Planckian spectrum. Thermality ... More

Short-distance contribution to the spectrum of Hawking radiationNov 30 2006Nov 06 2007The Hawking effect can be rederived in terms of two-point functions and in such a way that it makes it possible to estimate, within the conventional semiclassical theory, the contribution of ultrashort distances to the Planckian spectrum. For Schwarzschild ... More

Inflation, Renormalization, and CMB AnisotropiesFeb 20 2010In single-field, slow-roll inflationary models, scalar and tensorial (Gaussian) perturbations are both characterized by a zero mean and a non-zero variance. In position space, the corresponding variance of those fields diverges in the ultraviolet. The ... More

Microscopic wormholes and the geometry of entanglementFeb 20 2014Jun 02 2014It has recently been suggested that Einstein-Rosen (ER) bridges can be interpreted as maximally entangled states of two black holes that form a complex Einstein-Podolsky-Rosen (EPR) pair. This relationship has been dubbed as the ER = EPR correlation. ... More

Impact of curvature divergences on physical observers in a wormhole space-time with horizonsFeb 04 2016The impact of curvature divergences on physical observers in a black hole space-time which, nonetheless, is geodesically complete is investigated. This space-time is an exact solution of certain extensions of General Relativity coupled to Maxwell's electrodynamics ... More

Geometric aspects of charged black holes in Palatini theoriesJun 06 2015Charged black holes in gravity theories in the Palatini formalism present a number of unique properties. Their innermost structure is topologically nontrivial, representing a wormhole supported by a sourceless electric flux. For certain values of their ... More

Bouncing Cosmologies in Palatini $f(R)$ GravityJul 02 2009Oct 24 2009We consider the early time cosmology of f(R) theories in Palatini formalism and study the conditions that guarantee the existence of homogeneous and isotropic models that avoid the Big Bang singularity. We show that for such models the Big Bang singularity ... More

Nonsingular BTZ-type solutions of $2+1$ Born-Infeld gravitySep 19 2016We study Born-Infeld gravity coupled to a static, nonrotating electric field in $2+1$ dimensions and find exact analytical solutions. Two families of such solutions represent geodesically complete, and hence nonsingular, spacetimes. Another family represents ... More

Wormholes as a cure for black hole singularitiesJan 02 2016Using exactly solvable models, it is shown that black hole singularities in different electrically charged configurations can be cured. Our solutions describe black hole space-times with a wormhole giving structure to the otherwise point-like singularity. ... More

Mapping Ricci-based theories of gravity into general relativityJan 31 2018We show that the space of solutions of a wide family of Ricci-based metric-affine theories of gravity can be put into correspondence with the space of solutions of general relativity (GR). This allows us to use well-established methods and results from ... More

Two-point functions with an invariant Planck scale and thermal effectsApr 03 2008Nonlinear deformations of relativistic symmetries at the Planck scale are usually addressed in terms of modified dispersion relations. We explore here an alternative route by directly deforming the two-point functions of an underlying field theory. The ... More

Dynamical Aspects of Generalized Palatini Theories of GravityJul 16 2009We study the field equations of modified theories of gravity in which the lagrangian is a general function of the Ricci scalar and Ricci-squared terms in Palatini formalism. We show that the independent connection can be expressed as the Levi-Civita connection ... More

Minimum main sequence mass in quadratic Palatini $f(\mathcal{R})$ gravityJun 11 2019General Relativity yields an analytical prediction of a minimum required mass of roughly $\sim 0.08-0.09 M_{\odot}$ for a star to stably burn sufficient hydrogen to fully compensate photospheric losses and, therefore, to belong to the main sequence. Those ... More

Crystal clear lessons on the microstructure of space-time and modified gravityDec 15 2014Jun 12 2015We argue that a microscopic structure for space-time such as that expected in a quantum foam scenario, in which microscopic wormholes and other topological structures should play a relevant role, might lead to an effective metric-affine geometry at larger ... More

Born-Infeld gravity and its functional extensionsJun 04 2014Aug 04 2014We investigate the dynamics of a family of functional extensions of the (Eddington-inspired) Born-Infeld gravity theory, constructed with the inverse of the metric and the Ricci tensor. We provide a generic formal solution for the connection and an Einstein-like ... More

Inflation, Quantum Field Renormalization, and CMB AnisotropiesMay 16 2010We point out that if quantum field renormalization is taken into account the predictions of slow-roll inflation for both the scalar and tensorial power spectra change significantly for wavelengths that today are at observable scales

Inflation, quantum fields, and CMB anisotropiesSep 01 2009Inflationary cosmology has proved to be the most successful at predicting the properties of the anisotropies observed in the cosmic microwave background (CMB). In this essay we show that quantum field renormalization significantly influences the generation ... More

Insensitivity of Hawking radiation to an invariant Planck-scale cutoffJun 29 2009A disturbing aspect of Hawking's derivation of black hole radiance is the need to invoke extreme conditions for the quantum field that originates the emitted quanta. It is widely argued that the derivation requires the validity of the conventional relativistic ... More

The Power Spectrum in de Sitter Inflation, RevisitedMay 30 2008We find that the amplitude of quantum fluctuations of the invariant de Sitter vacuum coincides exactly with that of the vacuum of a comoving observer for a massless scalar (inflaton) field. We propose redefining the actual physical power spectrum as the ... More

Avoiding the Big Bang Singularity with Palatini f(R) TheoriesFeb 20 2010We show that there exist modified theories of gravity in which the metric satisfies second-order equations and in which the Big Bang singularity is replaced by a cosmic bounce without violating any energy condition. In fact, the bounce is possible even ... More

Born-Infeld-$f(R)$ gravityMar 28 2014Jul 04 2014Motivated by the properties of matter quantum fields in curved space-times, we work out a gravity theory that combines the Born-Infeld gravity Lagrangian with an $f(R)$ piece. To avoid ghost-like instabilities, the theory is formulated within the Palatini ... More

Classical resolution of black hole singularities via wormholesApr 27 2015Mar 05 2016In certain extensions of General Relativity, wormholes generated by spherically symmetric electric fields can resolve black hole singularities without necessarily removing curvature divergences. This is shown by studying geodesic completeness, the behavior ... More

Quadratic Palatini gravity and stable black hole remnantsNov 25 2013We present a four-dimensional Planck-scale corrected quadratic extension of General Relativity (GR) where no a priori relation between metric and connection is imposed (Palatini formalism). Static spherically symmetric electrovacuum solutions are obtained ... More

Black holes in extended gravity theories in Palatini formalismJan 14 2013We consider several physical scenarios where black holes within classical gravity theories including $R^2$ and Ricci-squared corrections and formulated \`a la Palatini can be analytically studied.

Cascading dust inflation in Born-Infeld gravitySep 02 2015Sep 04 2015In the framework of Born-Infeld inspired gravity theories, which deviates from General Relativity (GR) in the high curvature regime, we discuss the viability of Cosmic Inflation without scalar fields. For energy densities higher than the new mass scale ... More

Tensor perturbations in a general class of Palatini theoriesApr 01 2015Sep 29 2015We study a general class of gravitational theories formulated in the Palatini approach and derive the equations governing the evolution of tensor perturbations. In the absence of torsion, the connection can be solved as the Christoffel symbols of an auxiliary ... More

Small and hollow magnetic monopolesJul 03 2018We deal with the presence of magnetic monopoles in a non Abelian model that generalizes the standard 't~Hooft-Polyakov model in three spatial dimensions. We investigate the energy density of the static and spherically symmetric solutions to find first ... More

Remarks on the renormalization of primordial cosmological perturbationsAug 03 2011We briefly review the need to perform renormalization of inflationary perturbations to properly work out the physical power spectra. We also summarize the basis of (momentum-space) renormalization in curved spacetime and address several misconceptions ... More

Revising the observable consequences of slow-roll inflationNov 05 2009We study the generation of primordial perturbations in a (single-field) slow-roll inflationary universe. In momentum space, these (Gaussian) perturbations are characterized by a zero mean and a non-zero variance $\Delta^2(k, t)$. However, in position ... More

Stellar Structure Equations in Extended Palatini GravityNov 04 2012We consider static spherically symmetric stellar configurations in Palatini theories of gravity in which the Lagrangian is an unspecified function of the form $f(R,R_{\mu\nu}R^{\mu\nu})$. We obtain the Tolman-Oppenheimer-Volkov equations corresponding ... More

Black hole radiance, short distances, and TeV gravityApr 06 2006Sep 11 2006Using a derivation of black hole radiance in terms of two-point functions one can provide a quantitative estimate of the contribution of short distances to the spectrum. Thermality is preserved for black holes with $\kappa l_P <<1$. However, deviations ... More

Semiclassical geons as solitonic black hole remnantsJun 11 2013We find that the end state of black hole evaporation could be represented by non-singular and without event horizon stable solitonic remnants with masses of the order the Planck scale and up to 16 units of charge. Though these objects are locally indistinguishable ... More

Geodesic completeness in a wormhole spacetime with horizonsAug 13 2015The geometry of a spacetime containing a wormhole generated by a spherically symmetric electric field is investigated in detail. These solutions arise in high-energy extensions of General Relativity formulated within the Palatini approach and coupled ... More

Black holes in five-dimensional Palatini $f(R)$ gravity and implications for the AdS/CFT correspondenceMay 01 2014We show that theories having second-order field equations in the context of higher-dimensional modified gravity are not restricted to the family of Lovelock Lagrangians, but can also be obtained if no a priori assumption on the relation between the metric ... More

Infrared lessons for ultraviolet gravity: the case of massive gravity and Born-InfeldAug 31 2014Nov 12 2014We generalize the ultraviolet sector of gravitation via a Born-Infeld action using lessons from massive gravity. The theory contains all of the elementary symmetric polynomials and is treated in the Palatini formalism. We show how the connection can be ... More

Reply to "Comment on 'Insensitivity of Hawking radiation to an invariant Planck-scale cutoff' "May 13 2010We clarify the relationship between the conclusions of the previous Comment of A. Helfer [1] and that of our Brief Report [arXiv:0906.5315]

Revising the predictions of inflation for the cosmic microwave background anisotropiesJan 05 2009Jul 21 2009We point out that if quantum field renormalization is taken into account, and the counterterms are evaluated at the Hubble-radius crossing time or few e-foldings after it, the predictions of slow-roll inflation for both the scalar and tensorial power ... More

Static Spherically Symmetric Solutions in Extended Palatini GravityNov 04 2012We consider static spherically symmetric stellar configurations in Palatini theories of gravity in which the Lagrangian is an unspecified function of the form f(R,R_{\mu\nu}R^{\mu\nu}). We obtain the Tolman-Oppenheimer-Volkov equations corresponding to ... More

What is a singular black hole beyond General Relativity?Feb 04 2017Exploring the characterization of singular black hole spacetimes, we study the relation between energy density, curvature invariants, and geodesic completeness using a quadratic $f(R)$ gravity theory coupled to an anisotropic fluid. Working in a metric-affine ... More

Melvin Universe in Born-Infeld gravityApr 08 2015We consider a magnetic flux pointing in the $z$ direction of an axially symmetric space-time (Melvin Universe) in a Born-Infeld-type extension of General Relativity (GR) formulated in the Palatini approach. Large magnetic fields could have been produced ... More

Geonic black holes and remnants in Eddington-inspired Born-Infeld gravityNov 04 2013Apr 28 2014We show that electrically charged solutions within the Eddington-inspired Born-Infeld theory of gravity replace the central singularity by a wormhole supported by the electric field. As a result, the total energy associated with the electric field is ... More

Little Rip, $Λ$CDM and singular dark energy cosmology from Born-Infeld-$f(R)$ gravityApr 10 2014May 13 2014We study late-time cosmic accelerating dynamics from Born-Infeld-$f(R)$ gravity in a simplified conformal approach. We find that a variety of cosmic efects such as Little Rip, $\Lambda$CDM universe and dark energy cosmology with finite-time future singularities ... More

Hawking radiation by Kerr black holes and conformal symmetryJun 23 2010Nov 22 2010The exponential blueshift associated with the event horizon of a black hole makes conformal symmetry play a fundamental role in accounting for its thermal properties. Using a derivation based on two-point functions, we show that the spectrum of thermal ... More

Enriched Phenomenology in Extended Palatini TheoriesFeb 20 2010We show that extended theories of gravity with Lagrangian f(R,R_{\mu\nu}R^{\mu\nu}) in the Palatini formulation possess a phenomenology much richer than the simpler f(R) or f(R_{\mu\nu}R^{\mu\nu}) theories. In fact, we find that the scalars R and Q=R_{\mu\nu}R^{\mu\nu} ... More

Topological properties of a dense atomic lattice gasMar 09 2017We investigate the existence of topological phases in a dense two-dimensional atomic lattice gas. The coupling of the atoms to the radiation field gives rise to dissipation and a non-trivial coherent long-range exchange interaction whose form goes beyond ... More

Global Monopole in Palatini f(R) gravityDec 02 2018We consider the space-time metric generated by a global monopole in an extension of General Relativity (GR) of the form $f(\mathcal{R})=\mathcal{R}-\lambda \mathcal{R}^2$. The theory is formulated in the metric-affine (or Palatini) formalism and exact ... More

Rydberg ringsAug 25 2010Atoms in highly excited Rydberg states exhibit remarkable properties and constitute a powerful tool for studying quantum phenomena in strongly interacting many-particle systems. We investigate alkali atoms that are held in a ring lattice and excited to ... More

A note on the uniqueness of the canonical connection of a naturally reductive spaceOct 31 2012We extend the result in J. Reine Angew. Math. 664, 29-53, to the non-compact case. Namely, we prove that the canonical connection on a simply connected and irreducible naturally reductive space is unique, provided the space is not a sphere, a compact ... More

Nonsingular electrovacuum solutions with dynamically generated cosmological constantJun 28 2013Oct 26 2013We consider static spherically symmetric configurations in a Palatini extension of General Relativity including $R^2$ and Ricci-squared terms, which is known to replace the central singularity by a wormhole in the electrovacuum case. We modify the matter ... More

Braneworld solutions for modified theories of gravity with non-constant curvatureMay 23 2015We study braneworld models in the presence of scalar field in a five-dimensional geometry with a single extra dimension of infinite extent, with gravity modified to include a function of the Ricci scalar. We develop a procedure that allows to obtain analytical ... More

Thick brane in $f(R)$ gravity with Palatini dynamicsNov 04 2014Nov 23 2015This work deals with modified gravity in five dimensional spacetime. We study a thick Palatini $f(R)$ brane, that is, a braneworld scenario described by an anti-de Sitter warped geometry with a single extra dimension of infinite extent, sourced by real ... More

Classical resolution of black hole singularities in arbitrary dimensionJul 28 2015A metric-affine approach is employed to study higher-dimensional modified gravity theories involving different powers and contractions of the Ricci tensor. It is shown that the field equations are \emph{always} second-order, as opposed to the standard ... More

Early-time cosmic dynamics in $f(R)$ and $f(|\hatΩ|)$ extensions of Born-Infeld gravityNov 23 2014We consider two types of modifications of Born-Infeld gravity in the Palatini formulation and explore their dynamics in the early universe. One of these families considers $f(R)$ corrections to the Born-Infeld Lagrangian, which can be seen as modifications ... More

Hybrid modified gravity unifying local tests, galactic dynamics and late-time cosmic accelerationMay 16 2013Aug 14 2013The non-equivalence between the metric and Palatini formalisms of $f(R)$ gravity is an intriguing feature of these theories. However, in the recently proposed hybrid metric-Palatini gravity, consisting of the superposition of the metric Einstein-Hilbert ... More

Mapping nonlinear gravity into General Relativity with nonlinear electrodynamicsJul 17 2018Oct 26 2018We show that families of nonlinear gravity theories formulated in a metric-affine approach and coupled to a nonlinear theory of electrodynamics can be mapped into General Relativity (GR) coupled to another nonlinear theory of electrodynamics. This allows ... More