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 $-1/2 \xi R \phi^2$. 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 $\mu(z)$ and the lookback time $t_L(z)$ 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 $1\sigma-3\sigma$ confidence levels, between our bounds and supernovae data from the gold and combined samples is made for the recent past. Our analyses indicate, with $3\sigma$ confidence levels, the fulfillment of both the weak energy condition (WEC) and dominant energy condition (DEC) for $z \leq 1$ and $z \lesssim 0.8$, respectively. In addition, they suggest a possible recent violation of the null energy condition (NEC) with $3\sigma$, 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 $3\sigma$ in the recent past ($z \leq 1$), but interestingly the $q(z)$-best-fit curve crosses the SEC-fulfillment divider at $z \simeq 0.67$, which is a value very close to the beginning of the epoch of cosmic acceleration predicted by the standard concordance flat $\Lambda$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