24,158 research outputs found
Gauss-Bonnet gravity, brane world models, and non-minimal coupling
We study the case of brane world models with an additional Gauss-Bonnet term
in the presence of a bulk scalar field which interacts non-minimally with
gravity, via a possible interaction term of the form . The
Einstein equations and the junction conditions on the brane are formulated, in
the case of the bulk scalar field. Static solutions of this model are obtained
by solving numerically the Einstein equations with the appropriate boundary
conditions on the brane. Finally, we present graphically and comment these
solutions for several values of the free parameters of the model.Comment: 13 pages,4 figures, published versio
Nonlocal Cosmology
We explore nonlocally modified models of gravity, inspired by quantum loop
corrections, as a mechanism for explaining current cosmic acceleration. These
theories enjoy two major advantages: they allow a delayed response to cosmic
events, here the transition from radiation to matter dominance, and they avoid
the usual level of fine tuning; instead, emulating Dirac's dictum, the required
large numbers come from the large time scales involved. Their solar system
effects are safely negligible, and they may even prove useful to the black hole
information problem.Comment: Expanded(!) version, to appear in Phys. Rev. Letter
Interacting dark energy, holographic principle and coincidence problem
The interacting and holographic dark energy models involve two important
quantities. One is the characteristic size of the holographic bound and the
other is the coupling term of the interaction between dark energy and dark
matter. Rather than fixing either of them, we present a detailed study of
theoretical relationships among these quantities and cosmological parameters as
well as observational constraints in a very general formalism. In particular,
we argue that the ratio of dark matter to dark energy density depends on the
choice of these two quantities, thus providing a mechanism to change the
evolution history of the ratio from that in standard cosmology such that the
coincidence problem may be solved. We investigate this problem in detail and
construct explicit models to demonstrate that it may be alleviated provided
that the interacting term and the characteristic size of holographic bound are
appropriately specified. Furthermore, these models are well fitted with the
current observation at least in the low red-shift region.Comment: 20 pages, 3 figure
Integrability of the N-body problem in (2+1)-AdS gravity
We derive a first order formalism for solving the scattering of point sources
in (2+1) gravity with negative cosmological constant. We show that their
physical motion can be mapped, with a polydromic coordinate transformation, to
a trivial motion, in such a way that the point sources move as time-like
geodesics (in the case of particles) or as space-like geodesics (in the case of
BTZ black holes) of a three-dimensional hypersurface immersed in a
four-dimensional Minkowskian space-time, and that the two-body dynamics is
solved by two invariant masses, whose difference is simply related to the total
angular momentum of the system.Comment: 15 pages, LaTeX, no figure
Rippled Cosmological Dark Matter from Damped Oscillating Newton Constant
Let the reciprocal Newton 'constant' be an apparently non-dynamical
Brans-Dicke scalar field damped oscillating towards its General Relativistic
VEV. We show, without introducing additional matter fields or dust, that the
corresponding cosmological evolution averagely resembles, in the Jordan frame,
the familiar dark radiation -> dark matter -> dark energy domination sequence.
The fingerprints of our theory are fine ripples, hopefully testable, in the FRW
scale factor; they die away at the General Relativity limit. The possibility
that the Brans-Dicke scalar also serves as the inflaton is favorably examined.Comment: RevTex4, 12 pages, 5 figures; Minor revision, References adde
Energy conditions bounds and their confrontation with supernovae data
The energy conditions play an important role in the understanding of several
properties of the Universe, including the current accelerating expansion phase
and the possible existence of the so-called phantom fields. We show that the
integrated bounds provided by the energy conditions on cosmological observables
such as the distance modulus and the lookback time are not
sufficient (nor necessary) to ensure the local fulfillment of the energy
conditions, making explicit the limitation of these bounds in the confrontation
with observational data. We recast the energy conditions as bounds on the
deceleration and normalized Hubble parameters, obtaining new bounds which are
necessary and sufficient for the local fulfillment of the energy conditions. A
statistical confrontation, with confidence levels, between
our bounds and supernovae data from the gold and combined samples is made for
the recent past. Our analyses indicate, with confidence levels, the
fulfillment of both the weak energy condition (WEC) and dominant energy
condition (DEC) for and , respectively. In addition,
they suggest a possible recent violation of the null energy condition (NEC)
with , i.e. a very recent phase of super-acceleration. Our analyses
also show the possibility of violation of the strong energy condition
(\textbf{SEC}) with in the recent past (), but
interestingly the -best-fit curve crosses the SEC-fulfillment divider at
, which is a value very close to the beginning of the epoch of
cosmic acceleration predicted by the standard concordance flat CDM
scenario.Comment: 7 pages, 3 figures. V2: Version to appear in Phys.Rev.D, analyses
extended to 1sigma, 2sigma and 3sigma confidence levels, references added,
minors change
Hawking radiation, Unruh radiation and the equivalence principle
We compare the response function of an Unruh-DeWitt detector for different
space-times and different vacua and show that there is a {\it detailed}
violation of the equivalence principle. In particular comparing the response of
an accelerating detector to a detector at rest in a Schwarzschild space-time we
find that both detectors register thermal radiation, but for a given,
equivalent acceleration the fixed detector in the Schwarzschild space-time
measures a higher temperature. This allows one to locally distinguish the two
cases. As one approaches the horizon the two temperatures have the same limit
so that the equivalence principle is restored at the horizon.Comment: 9 pages. Added references and added discussion. To be published in
PR
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