Latest Papers in General Relativity

General Relativity And Quantum Cosmology

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Generalising the matter coupling in massive gravity: a search for new interactions (1902.01391v1)

A. Emir Gumrukcuoglu, Kazuya Koyama

2019-02-04

Massive gravity theory introduced by de Rham, Gabadadze, Tolley (dRGT) is restricted by several uniqueness theorems that protect the form of the potential and kinetic terms, as well as the matter coupling. These restrictions arise from the requirement that the degrees of freedom match the expectation from Poincar'e representations of a spin--2 field. Any modification beyond the dRGT form is known to invalidate a constraint that the theory enjoys and revive a dangerous sixth mode. One loophole is to exploit the effective nature of the theory by pushing the sixth mode beyond the strong coupling scale without completely removing it. In this paper, we search for modifications to dRGT action by coupling the matter sector to an arbitrary metric constructed out of the already existing degrees of freedom in the dRGT action. We formulate the conditions that such an extension should satisfy in order to prevent the sixth mode from contaminating the effective theory. Our approach provides a new perspective for the "composite coupling" which emerges as the unique extension up to four-point interactions.

Uniqueness of minimal loop quantum cosmology dynamics (1902.01386v1)

Jonathan Engle, Ilya Vilensky

2019-02-04

We show that the standard Hamiltonian of isotropic loop quantum cosmology is selected by physical criteria plus one choice: that it have a `minimal' number of terms. We also show the freedom, and boundedness of energy density, even when this choice is relaxed. A criterion used is covariance under dilations, the continuous diffeomorphisms remaining in this context, which are not canonical but conformally canonical transformations. We propose how to implement conformally canonical transformations in quantum theory. Removal of the infrared regulator yields independence of ordering ambiguities.

Red Giant evolution in Modified Gravity (1802.04001v4)

Sh. Najafi, M. T. Mirtorabi, Z. Ansari, D. F. Mota

2018-02-12

In this paper, we study the chameleon profile in inhomogeneous density distributions and find that the fifth force in thin shell near the surface is weaker from what expected in homogeneous density distributions. Also, we check the validity of quasi-static approximation for the chameleon scalar field in the astrophysical time scales. We have investigated the rolling down behaviour of the scalar field on its effective potential inside a one solar mass red giant star by using MESA code. We have found that the scalar field is fast enough to follow the minimum of the potential. This adiabatic behaviour reduces the fifth force and extends the screened regions to lower densities where the field has smaller mass and was expected to be unscreened. As a consequence, the star evolution is similar to what expected from standard general relativity. In addition, considering the stability of star, an approximate constraint on the coupling constant is found.

Reconstructing Gravity on Cosmological Scales (1902.01366v1)

Marco Raveri

2019-02-04

We present the data-driven reconstruction of gravitational theories and Dark Energy models on cosmological scales. We showcase the power of present cosmological probes at constraining these models and quantify the knowledge of their properties that can be acquired through state of the art data. This reconstruction exploits the power of the Effective Field Theory approach to Dark Energy and Modified Gravity phenomenology, which compresses the freedom in defining such models into a finite set of functions that can be reconstructed across cosmic times using cosmological data. We consider several model classes described within this framework and thoroughly discuss their phenomenology and data implications. We find that some models can alleviate the present discrepancy in the determination of the Hubble constant as inferred from the cosmic microwave background and as directly measured. This results in a statistically significant preference for the reconstructed theories over the standard cosmological model.

La cosmología y los matemáticos (Cosmology and mathematicians) (1902.01336v1)

José M. M. Senovilla

2019-02-04

Free translation of the original abstract in Spanish: Some of the most relevant milestones due to, or instigated by, mathematicians concerning the creation, development and advances of Cosmology as a scientific discipline are presented and discussed. In particular, the close relationship between Cosmology and Mathematics, derived from the geometrization of the former performed by Einstein after his General theory of Relativity, is described in detail with a thorough analysis of the contributions and collaborations by distinguished XX-th century mathematicians.

Unambiguous Phase Spaces for Subregions (1901.09857v2)

Josh Kirklin

2019-01-28

The covariant phase space technique is a powerful formalism for understanding the Hamiltonian description of covariant field theories. However, applications of this technique to problems involving subregions, such as the exterior of a black hole, have heretofore been plagued by boundary ambiguities. We provide a resolution of these ambiguities by directly computing the symplectic structure from the path integral, showing that it may be written as a contour integral around a partial Cauchy surface. This result has implications for gauge symmetry and entanglement.

Probing gravity at sub-femtometer scales through the pressure distribution inside the proton (1902.01318v1)

P. P. Avelino

2019-02-04

Recently, a measurement of the pressure distribution experienced by the quarks inside the proton has found a strong repulsive (positive) pressure at distances up to 0.6 femtometers from its center and a (negative) confining pressure at larger distances. In this paper we show that this measurement puts significant constraints on modified theories of gravity in which the strength of the gravitational interaction on microscopic scales is enhanced with respect to general relativity. We consider the particular case of Eddington-inspired Born-Infeld gravity, showing that strong limits on , the only additional parameter of the theory with respect to general relativity, may be derived from the quark pressure measurement (). Furthermore, we show how these limits may be significantly improved with precise measurements of the first and second moments of the pressure distribution inside the proton.

A Penrose-Type Inequality with Angular Momentum and Charge for Axisymmetric Initial Data (1902.00501v2)

Marcus Khuri, Benjamin Sokolowsky, Gilbert Weinstein

2019-02-01

A lower bound for the ADM mass is established in terms of angular momentum, charge, and horizon area in the context of maximal, axisymmetric initial data for the Einstein-Maxwell equations which satisfy the weak energy condition. If, on the horizon, the given data agree to a certain extent with the associated model Kerr-Newman data, then the inequality reduces to the conjectured Penrose inequality with angular momentum and charge. In addition, a rigidity statement is also proven whereby equality is achieved if and only if the data set arises from the canonical slice of a Kerr-Newman spacetime.

Structure and thermodynamics of charged nonrotating black holes in higher dimensions (1807.06298v2)

H. Benbellout, J. Diaz-Alonso, D. Rubiera-Garcia

2018-07-17

We analyze the structural and thermodynamic properties of -dimensional (), 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 behaviour (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 which are non topological 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 absence of a cosmological term). In these cases we obtain the behaviour 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.

Axial quasi-normal modes of scalarized neutron stars with massive self-interacting scalar field (1902.01277v1)

Zahra Altaha Motahar, Jose Luis Blázquez-Salcedo, Daniela D. Doneva, Jutta Kunz, Stoytcho S. Yazadjiev

2019-02-04

We study the axial quasi-normal modes of neutron stars in scalar-tensor theories with massive scalar field including a self-interacting term in the potential. Various realistic equations of state including nuclear, hyperonic and hybrid matter are employed. Although the effect of spontaneous scalarization of neutron stars can be very large, binary pulsar observations and gravitational wave detections significantly constrain the massless scalar-tensor theories. If we consider a properly chosen nonzero mass for the scalar field, though, the scalar-tensor parameters cannot be restricted by the observations, resulting in large deviation from pure general relativity. With this motivation in mind, we extend the universal relations for axial quasi-normal modes known in general relativity to neutron stars in massive scalar-tensor theories with self-interaction using a wide range of realistic EOS. We confirm the universality of the scaled frequency and damping time in terms of the compactness and scaled moment of inertia for neutron stars with and without massive scalarization.

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