Junction conditions of Palatini f (R,T) gravity

We work out the junction conditions for the Palatini f(R, T) extension of general relativity, where f is an arbitrary function of the curvature scalar 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(R) versions via some new T-dependent terms. Moreover, we also identify some "exceptional cases " of f(R,T) Lagrangians such that some of these conditions can be discarded, thus allowing for further discontinuities in 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 nonconservation of the stress-energy tensor ascribed to these theories, may induce nontrivial changes in the shape of specific applications such as traversable thin-shell wormholes

Nonsingular charged black holes \`{a} la Palatini

We 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 formulated \`{a} la Palatini and show that nonrotating, electrically charged black holes develop a compact core at the Planck density which is nonsingular if the mass spectrum satisfies a certain discreteness condition. We also find that the area of the core is proportional to the number of charges times the Planck area.Comment: 10 single column page

New scalar compact objects in Ricci-based gravity theories

Taking 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 mapping the free-field static, spherically symmetric solution of General Relativity (GR) into quadratic f(R) gravity and the Eddington-inspired Born-Infeld gravity. The obtained solutions have some distinctive feature below the would-be Schwarzschild radius of a configuration with the same mass, though in this case no horizon is present. The compact objects found include wormholes, compact balls, shells of energy with no interior, and a new kind of object which acts as a kind of wormhole membrane. The latter object has Euclidean topology but connects antipodal points of its surface by transferring particles and null rays across its interior in virtually zero affine time. We point out the relevance of these results regarding the existence of compact scalar field objects beyond General Relativity that may effectively act as black hole mimickers

Geometric inequivalence of metric and Palatini formulations of General Relativity

Projective invariance is a symmetry of the Palatini version of General Relativity which is not present in the metric formulation. The fact that the Riemann tensor changes nontrivially under projective transformations implies that, unlike in the usual metric approach, in the Palatini formulation this tensor is subject to a gauge freedom, which allows some ambiguities even in its scalar contractions. In this sense, we show that for the Schwarzschild solution there exists a projective gauge in which the (affine) Kretschmann scalar, K≡RαβμνRαβμν, can be set to vanish everywhere. This puts forward that the divergence of curvature scalars may, in some cases, be avoided by a gauge transformation of the connection.C. B. is funded by the National Scientific and Technical Re-search Council (CONICET). AD and AJC are supported by a PhD contract of the program FPU 2015 (Spanish Ministry of Econ-omy and Competitiveness) with references FPU15/05406 and FPU15/02864, respectively. GJO is funded by the Ramon y Cajal contract RYC-2013-13019 (Spain). DRG is funded by the Atracción de Talento Investigador programme of the Comunidad de Madrid No. 2018-T1/TIC-10431, and acknowledges support from the Fundação para a Ciência e a Tecnologia (FCT, Portugal) research grants Nos. PTDC/FIS-OUT/29048/2017 and PTDC/FIS-PAR/31938/2017. Thiswork is supported by the Spanish projects FIS2017-84440-C2-1-P, FIS2014-57387-C3-1-P (MINECO/FEDER, EU) and i-LINK1215 (CSIC), the project H2020-MSCA-RISE-2017 Grant FunFiCO-777740, the project SEJI/2017/042 (Generalitat Valenciana), the Consolider Program CPANPHY-1205388, and the Severo Ochoa grant SEV-2014-0398 (Spain)

Structure and thermodynamics of charged nonrotating black holes in higher dimensions.

We analyze the structural and thermodynamic properties of D-dimensional (D >= 4), asymptotically flat or anti-de Sitter, electrically charged black hole solutions, resulting from the minimal coupling of general nonlinear electrodynamics to general relativity. This analysis deals with static spherically symmetric (elementary) configurations with spherical horizons. Our methods are based on the study of the behavior (in vacuum and on the boundary of their domain of definition) of the Lagrangian density functions characterizing the nonlinear electrodynamic models in flat spacetime. These functions are constrained by some admissibility conditions endorsing the physical consistency of the corresponding theories, which are classified in several families, some of them supporting elementary solutions in flat space that are nontopological solitons. This classification induces a similar one for the elementary black hole solutions of the associated gravitating nonlinear electrodynamics, whose geometrical structures are thoroughly explored. A consistent thermodynamic analysis can be developed for the subclass of families whose associated black hole solutions behave asymptotically as the Schwarzschild metric (in the absence of a cosmological term). In these cases we obtain the behavior of the main thermodynamic functions, as well as important finite relations among them. In particular, we find the general equation determining the set of extreme black holes for every model, and a general Smarr formula, valid for the set of elementary black hole solutions of such models. We also consider the one-parameter group of scale transformations, which are symmetries of the field equations of any nonlinear electrodynamics in flat spacetime. These symmetries are respected by the minimal coupling to gravitation and induce representations of the group in the spaces of solutions of the different models, characterized by their thermodynamic functions. Exploiting this fact we find the expression of the equation of state of the set of black hole solutions associated with any model. These results are generalized to asymptotically anti-de Sitter solutions

Stellar structure models in modified theories of gravity: lessons and challenges.

The understanding of stellar structure represents the crossroads of our theories of the nuclear force and the gravitational interaction under the most extreme conditions observably accessible. It provides a powerful probe of the strong field regime of General Relativity, and opens fruitful avenues for the exploration of new gravitational physics. The latter can be captured via modified theories of gravity, which modify the Einstein-Hilbert action of General Relativity and/or some of its principles. These theories typically change the Tolman-Oppenheimer-Volkoff equations of stellar's hydrostatic equilibrium, thus having a large impact on the astrophysical properties of the corresponding stars and opening a new window to constrain these theories with present and future observations of different types of stars. For relativistic stars, such as neutron stars, the uncertainty on the equation of state of matter at supranuclear densities intertwines with the new parameters coming from the modified gravity side, providing a whole new phenomenology for the typical predictions of stellar structure models, such as mass-radius relations, maximum masses, or moment of inertia. For non-relativistic stars, such as white, brown and red dwarfs, the weakening/strengthening of the gravitational force inside astrophysical bodies via the modified Newtonian (Poisson) equation may induce changes on the star's mass, radius, central density or luminosity, having an impact, for instance, in the Chandrasekhar's limit for white dwarfs, or in the minimum mass for stable hydrogen burning in high-mass brown dwarfs. This work aims to provide a broad overview of the main such results achieved in the recent literature for many such modified theories of gravity, by combining the results and constraints obtained from the analysis of relativistic and non-relativistic stars in different scenarios. Moreover, we will build a bridge between the efforts of the community working on different theories, formulations, types of stars, theoretical modelings, and observational aspects, highlighting some of the most promising opportunities in the field. (C) 2020 Elsevier B.V. All rights reserved

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

We present the nonrelativistic 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 nonrelativistic stars, such as main-sequence stars as well as several substellar objects

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. Using analytical models accounting for the most relevant physics of these processes, we find in all cases 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 elaborate on the chances to seek for traces of new physics in low-mass stars within this theory, and the limitations and difficulties faced by this approach