1,721,035 research outputs found
Horizon thermodynamics and spacetime mappings
No full textWhen black holes are dynamical, event horizons are replaced by apparent and trapping horizons. Conformal and Kerr-Schild transformations are widely used in relation to dynamical black holes, and we study the behavior under such transformations of quantities related to the thermodynamics of these horizons, such as the Misner-Sharp-Hernandez mass (internal energy), the Kodama vector, surface gravity, and temperature. The transformation properties are not those expected on the basis of naive arguments. © 2014 American Physical Society
Thermodynamics of apparent horizons under conformal and Kerr-Schild maps1
No full textWe derive the transformation properties of the internal energy and Kodama temperature of dynamical (spherically symmetric) black hole solutions generated through spacetime mappings. We use the Sultana-Dyer black hole and the Reissner-Nordström solution to provide prototypical examples testing our transformation formulae
Coupling to the curvature for a scalar field from the equivalence principle
No full textBy applying the equivalence principle to the propagation of scalar waves in a curved spacetime, the value 1/6 for the coupling constant between the field and the Ricci scalar is derived. No coupling to higher-order scalars constructed from the Riemann tensor is allowed
Huygens' principle and characteristic propagation property for waves in curved space-times
No full textFields satisfying a wave equation on a curved space‐time can be accompanied by tails propagating inside the light cone. A formal characterization of tails is given, and the relationship of the tail‐free property with the characteristic propagation property (CPP) discussed in the literature is clarified. The scalar wave equation satisfies the CPP independently of the tail‐free property only in the case of minimal coupling in two space‐time dimensions. The peculiar feature of electromagnetic radiation, for which pure gauge tails in the potential can occur, is pointed out
On the tail problem in cosmology
No full textThe tail problem for the propagation of a scalar field is considered in a cosmological background, taking a Robertson-Walker spacetime as a specific example. The explicit radial dependence of the general solution of the Klein-Gordon equation with non-minimal coupling is derived, and the inapplicability of the standard calculation of the reflection and transmission coefficients to the study of scattering of waves by the cosmological curvature is discussed
The equivalence principle determines the coupling constant to the curvature for a scalar field
No full textAveraging inhomogeneities in scalar-tensor cosmology
No full textThe backreaction of inhomogeneities on the cosmic dynamics is studied in the context of scalar-tensor gravity. Due to terms of indefinite sign in the non-canonical effective energy tensor of the Brans-Dicke-like scalar field, extra contributions to the cosmic acceleration can arise. Brans-Dicke and metric f(R) gravity are presented as specific examples. Certain representation problems of the formalism peculiar to these theories are pointed out
New approach to the thermodynamics of scalar-tensor gravity
Full text linkWe discuss and expand a new approach to the thermodynamics of scalar-tensor gravity and its diffusion toward general relativity (seen as an equilibrium state) proposed in a previous paper [Phys. Rev. D 103, L121501 (2021)PRVDAQ2470-001010.1103/PhysRevD.103.L121501, upon which we build. We describe scalar-tensor gravity as an effective dissipative fluid and apply Eckart's first order thermodynamics to it, obtaining explicitly effective quantities such as heat flux, "temperature of gravity,"viscosities, entropy density, plus an equation describing the "diffusion"to Einstein gravity. These quantities, still missing in the usual thermodynamics of spacetime, are obtained with minimal assumptions. Furthermore, we examine certain exact solutions of scalar-tensor gravity to test the proposed formalism and gain some physical insight on the "approach to equilibrium"for this class of theories
Theory of gravitation theories: A No-progress report
No full textAlready in the 1970s there where attempts to present a set of ground rules, sometimes referred to as a theory of gravitation theories, which theories of gravity should satisfy in order to be considered viable in principle and, therefore, interesting enough to deserve further investigation. From this perspective, an alternative title of this paper could be "Why Are We Still Unable to Write a Guide on How to Propose Viable Alternatives to General Relativity?". Attempting to answer this question, it is argued here that earlier efforts to turn qualitative statements, such as the Einstein equivalence principle, into quantitative ones, such as the metric postulates, stand on rather shaky ground probably contrary to popular belief - as they appear to depend strongly on particular representations of the theory. This includes ambiguities in the identification of matter and gravitational fields, dependence of frequently used definitions ( such as those of the stress - energy tensor or classical vacuum) on the choice of variables, etc. Various examples are discussed and possible approaches to this problem are pointed out. In the course of this study, several common misconceptions related to the various forms of the equivalence principle, the use of conformal frames and equivalence between theories are clarified
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