15 research outputs found
(Anti-)de Sitter Black Hole Thermodynamics and the Generalized Uncertainty Principle
We extend the derivation of the Hawking temperature of a Schwarzschild black
hole via the Heisenberg uncertainty principle to the de Sitter and anti-de
Sitter spacetimes. The thermodynamics of the Schwarzschild-(anti-)de Sitter
black holes is obtained from the generalized uncertainty principle of string
theory and non-commutative geometry. This may explain why the thermodynamics of
(anti-)de Sitter-like black holes admits a holographic description in terms of
a dual quantum conformal field theory, whereas the thermodynamics of
Schwarzschild-like black holes does not.Comment: 10 pages, revtex
Solutions of Higher Dimensional Gauss-Bonnet FRW Cosmology
We examine the effect on cosmological evolution of adding a Gauss-Bonnet term
to the standard Einstein-Hilbert action for a (1 + 3)+ d dimensional
Friedman-Robertson-Walker (FRW) metric. By assuming that the additional
dimensions compactify as a power law as the usual 3 spatial dimensions expand,
we solve the resulting dynamical equations and find that the solution may be of
either de Sitter or Kasner form depending upon whether the Gauss-Bonnet term or
the Einstein term dominates.Comment: 10 pages, references added/corrected, accepted for publication in
General Relativity and Gravitatio
Expansion-induced contribution to the precession of binary orbits
We point out the existence of new effects of global spacetime expansion on
local binary systems. In addition to a possible change of orbital size, there
is a contribution to the precession of elliptic orbits, to be added to the
well-known general relativistic effect in static spacetimes, and the
eccentricity can change. Our model calculations are done using geodesics in a
McVittie metric, representing a localized system in an asymptotically
Robertson-Walker spacetime; we give a few numerical estimates for that case,
and indicate ways in which the model should be improved.Comment: revtex, 7 pages, no figures; revised for publication in Classical and
Quantum Gravity, with minor changes in response to referees' comment
Semiclassical States in Quantum Cosmology: Bianchi I Coherent States
We study coherent states for Bianchi type I cosmological models, as examples
of semiclassical states for time-reparametrization invariant systems. This
simple model allows us to study explicitly the relationship between exact
semiclassical states in the kinematical Hilbert space and corresponding ones in
the physical Hilbert space, which we construct here using the group averaging
technique. We find that it is possible to construct good semiclassical physical
states by such a procedure in this model; we also discuss the sense in which
the original kinematical states may be a good approximation to the physical
ones, and the situations in which this is the case. In addition, these models
can be deparametrized in a natural way, and we study the effect of time
evolution on an "intrinsic" coherent state in the reduced phase space, in order
to estimate the time for this state to spread significantly.Comment: 21 pages, 1 figure; Version to be published in CQG; The discussion
has been slightly reorganized, two references added, and some typos correcte
Testing General Relativity with Present and Future Astrophysical Observations
One century after its formulation, Einstein's general relativity has maderemarkable predictions and turned out to be compatible with all experimentaltests. Most of these tests probe the theory in the weak-field regime, and thereare theoretical and experimental reasons to believe that general relativityshould be modified when gravitational fields are strong and spacetime curvatureis large. The best astrophysical laboratories to probe strong-field gravity areblack holes and neutron stars, whether isolated or in binary systems. We reviewthe motivations to consider extensions of general relativity. We present a(necessarily incomplete) catalog of modified theories of gravity for whichstrong-field predictions have been computed and contrasted to Einstein'stheory, and we summarize our current understanding of the structure anddynamics of compact objects in these theories. We discuss current bounds onmodified gravity from binary pulsar and cosmological observations, and wehighlight the potential of future gravitational wave measurements to inform uson the behavior of gravity in the strong-field regime
Effect of Global Expansion on Gravitational Lensing
Traditionally it had been thought that the cosmological constant played no role in gravitational lensing, but recently it has been shown that the cosmological constant effectively changes the impact parameter and thus does have an effect on lensing. In this paper, we will examine the effects of a FRW cosmology on null geodesics via numerical integration of the geodesics of the McVittie metric