367 research outputs found
Remarks on the Entropy of Non-Stationary Black Holes
The definition of entropy obtained for stationary black holes is extended in
this paper to the case of non-stationary black holes. Entropy is defined as a
macroscopical thermodynamical quantity which satisfies the first principle of
thermodynamics. In the non-stationary case a volume term appears since the
solution does not admit a Killing vector
Covariant Charges in Chern-Simons AdS_3 Gravity
We try to give hereafter an answer to some open questions about the
definition of conserved quantities in Chern-Simons theory, with particular
reference to Chern-Simons AdS_3 Gravity. Our attention is focused on the
problem of global covariance and gauge invariance of the variation of Noether
charges. A theory which satisfies the principle of covariance on each step of
its construction is developed, starting from a gauge invariant Chern-Simons
Lagrangian and using a recipe developed in gr-qc/0110104 and gr-qc/0107074 to
calculate the variation of conserved quantities. The problem to give a
mathematical well-defined expression for the infinitesimal generators of
symmetries is pointed out and it is shown that the generalized Kosmann lift of
spacetime vector fields leads to the expected numerical values for the
conserved quantities when the solution corresponds to the BTZ black hole. The
fist law of black holes mechanics for the BTZ solution is then proved and the
transition between the variation of conserved quantities in Chern-Simons AdS_3
Gravity theory and the variation of conserved quantities in General Relativity
is analysed in detail.Comment: 30 pages, no figures. References adde
Modified Gauss-Bonnet theory as gravitational alternative for dark energy
We suggest the modified gravity where some arbitrary function of Gauss-Bonnet
(GB) term is added to Einstein action as gravitational dark energy. It is shown
that such theory may pass solar system tests. It is demonstrated that modified
GB gravity may describe the most interesting features of late-time cosmology:
the transition from deceleration to acceleration, crossing the phantom divide,
current acceleration with effective (cosmological constant, quintessence or
phantom) equation of state of the universe.Comment: LaTeX file, 5 pages, version to appear in PL
A non-geodesic motion in the R^-1 theory of gravity tuned with observations
In the general picture of high order theories of gravity, recently, the R^-1
theory has been analyzed in two different frameworks. In this letter a third
context is added, considering an explicit coupling between the R^-1 function of
the Ricci scalar and the matter Lagrangian. The result is a non-geodesic motion
of test particles which, in principle, could be connected with Dark Matter and
Pioneer anomaly problems.Comment: Accepted for Modern Physics Letters
Curvature scalar instability in f(R) gravity
An instability in the presence of matter in theories of gravity which include
a 1/R correction in the gravitational action has been found by Dolgov and
Kawasaki. In the present paper this instability is discussed for f(R) gravity
in general. We focus on the Palatini formalism of the theory and it is shown
that no such instability occurs in this version of f(R) gravity. The reasons
for the appearance of the instability in the metric but not in the Palatini
formalism are fully investigated.Comment: typos corrected, replaced to match published versio
Higher-order gravity and the cosmological background of gravitational waves
The cosmological background of gravitational waves can be tuned by the
higher-order corrections to the gravitational Lagrangian. In particular, it can
be shown that assuming , where indicates a generic
(eventually small) correction to the Hilbert-Einstein action in the Ricci
scalar , gives a parametric approach to control the evolution and the
production mechanism of gravitational waves in the early Universe.Comment: 6 pages, 8 figure
Dark Energy Dominance and Cosmic Acceleration in First Order Formalism
The current accelerated universe could be produced by modified gravitational
dynamics as it can be seen in particular in its Palatini formulation. We
analyze here a specific non-linear gravity-scalar system in the first order
Palatini formalism which leads to a FRW cosmology different from the purely
metric one. It is shown that the emerging FRW cosmology may lead either to an
effective quintessence phase (cosmic speed-up) or to an effective phantom
phase. Moreover, the already known gravity assisted dark energy dominance
occurs also in the first order formalism. Finally, it is shown that a dynamical
theory able to resolve the cosmological constant problem exists also in this
formalism, in close parallel with the standard metric formulation.Comment: 21 pages, LaTeX file, no figures. Replaced version to be published on
Phys. Rev.
- …