Gravitation

About: Gravitation is a research topic. Over the lifetime, 29306 publications have been published within this topic receiving 821510 citations.

Thermodynamic route to field equations in Lanczos-Lovelock gravity

Abstract: Spacetimes with horizons show a resemblance to thermodynamic systems and one can associate the notions of temperature and entropy with them. In the case of Einstein-Hilbert gravity, it is possible to interpret Einstein's equations as the thermodynamic identity TdS=dE+PdV for a spherically symmetric spacetime and thus provide a thermodynamic route to understand the dynamics of gravity. We study this approach further and show that the field equations for the Lanczos-Lovelock action in a spherically symmetric spacetime can also be expressed as TdS=dE+PdV with S and E given by expressions previously derived in the literature by other approaches. The Lanczos-Lovelock Lagrangians are of the form L=Q{sub a}{sup bcd}R{sup a}{sub bcd} with {nabla}{sub b}Q{sub a}{sup bcd}=0. In such models, the expansion of Q{sub a}{sup bcd} in terms of the derivatives of the metric tensor determines the structure of the theory and higher order terms can be interpreted as quantum corrections to Einstein gravity. Our result indicates a deep connection between the thermodynamics of horizons and the allowed quantum corrections to standard Einstein gravity, and shows that the relation TdS=dE+PdV has a greater domain of validity than Einstein's field equations.

The Page curve of Hawking radiation from semiclassical geometry

Abstract: We consider a gravity theory coupled to matter, where the matter has a higher-dimensional holographic dual. In such a theory, finding quantum extremal surfaces becomes equivalent to finding the RT/HRT surfaces in the higher-dimensional theory. Using this we compute the entropy of Hawking radiation and argue that it follows the Page curve, as suggested by recent computations of the entropy and entanglement wedges for old black holes. The higher-dimensional geometry connects the radiation to the black hole interior in the spirit of ER=EPR. The black hole interior then becomes part of the entanglement wedge of the radiation. Inspired by this, we propose a new rule for computing the entropy of quantum systems entangled with gravitational systems which involves searching for "islands" in determining the entanglement wedge.

Cosmology of f ( R ) gravity in the metric variational approach

Abstract: We consider the cosmologies that arise in a subclass of $f(R)$ gravity with $f(R)=R+{\ensuremath{\mu}}^{2n+2}/(\ensuremath{-}R{)}^{n}$ and $n\ensuremath{\in}(\ensuremath{-}1,0)$ in the metric (as opposed to the Palatini) variational approach to deriving the gravitational field equations. The calculations of the isotropic and homogeneous cosmological models are undertaken in the Jordan frame and at both the background and the perturbation levels. For the former, we also discuss the connection to the Einstein frame in which the extra degree of freedom in the theory is associated with a scalar field sharing some of the properties of a ``chameleon'' field. For the latter, we derive the cosmological perturbation equations in general theories of $f(R)$ gravity in covariant form and implement them numerically to calculate the cosmic microwave background (CMB) temperature and matter power spectra of the cosmological model. The CMB power is shown to reduce at low $l$'s, and the matter power spectrum is almost scale independent at small scales, thus having a similar shape to that in standard general relativity. These are in stark contrast with what was found in the Palatini $f(R)$ gravity, where the CMB power is largely amplified at low $l$'s and the matter spectrum is strongly scale dependent at small scales. These features make the present model more adaptable than that arising from the Palatini $f(R)$ field equations, and none of the data on background evolution, CMB power spectrum, or matter power spectrum currently rule it out.

Evading equivalence principle violations, cosmological, and other experimental constraints in scalar field theories with a strong coupling to matter

Abstract: We show that, as a result of nonlinear self-interactions, it is feasible, at least in light of the bounds coming from terrestrial tests of gravity, measurements of the Casimir force and those constraints imposed by the physics of compact objects, big-bang nucleosynthesis and measurements of the cosmic microwave background, for there to exist, in our Universe, one or more scalar fields that couple to matter much more strongly than gravity does. These scalar fields behave like chameleons: changing their properties to fit their surroundings. As a result these scalar fields can be not only very strongly coupled to matter, but also remain relatively light over solar-system scales. These fields could also be detected by a number of future experiments provided they are properly designed to do so. These results open up an altogether new window, which might lead to a completely different view of the r\^ole played by light scalar fields in particle physics and cosmology.

The Conformal frame freedom in theories of gravitation

Abstract: It has frequently been claimed in the literature that the classical physical predictions of scalar–tensor theories of gravity depend on the conformal frame in which the theory is formulated. We argue that this claim is false, and that all classical physical predictions are conformal-frame invariants. We also respond to criticisms by Vollick (2003 Preprint gr-qc/0312041), in which this issue arises, of our recent analysis of the Palatini form of 1/R gravity.