Results 1  10
of
39
Dark Energy from Structure  A Status Report
 GEN. REL. GRAV., DARK ENERGY SPECIAL ISSUE
, 2007
"... The effective evolution of an inhomogeneous universe model in any theory of gravitation may be described in terms of spatially averaged variables. In Einstein’s theory, restricting attention to scalar variables, this evolution can be modeled by solutions of a set of Friedmann equations for an effe ..."
Abstract

Cited by 58 (9 self)
 Add to MetaCart
The effective evolution of an inhomogeneous universe model in any theory of gravitation may be described in terms of spatially averaged variables. In Einstein’s theory, restricting attention to scalar variables, this evolution can be modeled by solutions of a set of Friedmann equations for an effective volume scale factor, with matter and backreaction source terms. The latter can be represented by an effective scalar field (‘morphon field’) modeling Dark Energy. The present work provides an overview over the Dark Energy debate in connection with the impact of inhomogeneities, and formulates strategies for a comprehensive quantitative evaluation of backreaction effects both in theoretical and observational cosmology. We recall the basic steps of a description of backreaction effects in relativistic cosmology that lead to refurnishing the standard cosmological equations, but also lay down a number of challenges and unresolved issues in connection with their observational interpretation. The present status of this subject is intermediate: we have a good qualitative understanding of backreaction effects pointing to a global instability of the standard
Orbifold Physics and de Sitter Spacetime
, 2004
"... It is generally believed the way to resolve the black hole information paradox in string theory is to embed the black hole in antideSitter spacetime — without of course claiming that SchwarzschildAdS is a realistic spacetime. Here we propose that, similarly, the best way to study topologically non ..."
Abstract

Cited by 11 (6 self)
 Add to MetaCart
(Show Context)
It is generally believed the way to resolve the black hole information paradox in string theory is to embed the black hole in antideSitter spacetime — without of course claiming that SchwarzschildAdS is a realistic spacetime. Here we propose that, similarly, the best way to study topologically nontrivial versions of de Sitter spacetime from a stringy point of view is to embed them in an antide Sitter orbifold bulk, again without claiming that this is literally how de Sitter arises in string theory. Our results indicate that string theory may rule out the more complex spacetime topologies which are compatible with local de Sitter geometry, while still allowing the simplest versions. 1. AdS Embedding as a Toolbox It is well known that it is difficult to obtain realistic curved spacetimes from string theory. This is not necessarily a drawback in itself, but it does impede the analysis of important questions which string theory might be expected to answer. Ingenious ways of answering these questions have been found, without actually exhibiting the desired spacetimes explicitly. An outstanding example of this is the black hole information paradox. We do not yet
Optical phenomena in the field of braneworld Kerr black holes
"... We study the influence of the tidal charge parameter of the braneworld models onto some optical phenomena in rotating black hole spacetimes. The escape photon cones are determined for special families of locally nonrotating, circular geodetical and radially free falling observers. The silhuette of ..."
Abstract

Cited by 7 (2 self)
 Add to MetaCart
(Show Context)
We study the influence of the tidal charge parameter of the braneworld models onto some optical phenomena in rotating black hole spacetimes. The escape photon cones are determined for special families of locally nonrotating, circular geodetical and radially free falling observers. The silhuette of a rotating black hole, the shape of an equatorial thin accretion disk and time delay effect for direct and indirect images of a radiation hot spot orbiting the black hole are given and classified in terms of the black hole rotational and tidal parameters. It is shown that rising of negativelyvalued tidal parameter, with rotational parameter fixed, generally strenghtens the relativistic effects and suppresses the rotation induced asymmetries in the optical phenomena.
Braneworld Stars: Anisotropy Minimally Projected Onto the Brane
, 909
"... In the context of the RandallSundrum braneworld, an exhaustive and detailed description of the approach based in the minimal anisotropic consequence onto the brane, which has been successfully used to generate exact interior solutions to Einstein’s field equations for static and nonuniform branewo ..."
Abstract

Cited by 6 (2 self)
 Add to MetaCart
In the context of the RandallSundrum braneworld, an exhaustive and detailed description of the approach based in the minimal anisotropic consequence onto the brane, which has been successfully used to generate exact interior solutions to Einstein’s field equations for static and nonuniform braneworld stars with local and nonlocal bulk terms, is carefully presented. It is shown that this approach allows the generation of a braneworld version for any known general relativistic solution. As a demonstration, the braneworld version of the Schwarzschild’s interior solution is generated using this method. 1
Cosmic acceleration from asymmetric branes
, 2004
"... We consider a single 3brane sitting in between two different five dimensional spacetimes. On each side of the brane, the bulk is a solution to GaussBonnet gravity, although the bare cosmological constant, fundamental Planck scale, and GaussBonnet coupling can differ. This asymmetry leads to weigh ..."
Abstract

Cited by 6 (1 self)
 Add to MetaCart
We consider a single 3brane sitting in between two different five dimensional spacetimes. On each side of the brane, the bulk is a solution to GaussBonnet gravity, although the bare cosmological constant, fundamental Planck scale, and GaussBonnet coupling can differ. This asymmetry leads to weighted junction conditions across the brane and interesting brane cosmology. We focus on two special cases: a generalized RandallSundrum model without any GaussBonnet terms, and a stringy model, without any bare cosmological constants, and positive GaussBonnet coupling. Even though we assume there is no vacuum energy on the brane, we find late time de Sitter cosmologies can occur. Remarkably, in certain parameter regions, this acceleration is preceded by a period of matter/radiation domination, with H 2 ∝ ρ, all the way back to nucleosynthesis.
An exact selfsimilar solution for an expanding ball of radiation”, grqc/0402046
, 2004
"... We give an exact solution of the 5D Einstein equations which in 4D can be interpreted as a spherically symmetric dissipative distribution of matter, with heat flux, whose effective density and pressure are nonstatic, nonuniform, and satisfy the equation of state of radiation. The matter satisfies th ..."
Abstract

Cited by 4 (2 self)
 Add to MetaCart
(Show Context)
We give an exact solution of the 5D Einstein equations which in 4D can be interpreted as a spherically symmetric dissipative distribution of matter, with heat flux, whose effective density and pressure are nonstatic, nonuniform, and satisfy the equation of state of radiation. The matter satisfies the usual energy and thermodynamic conditions. The energy density and temperature are related by the StefanBoltzmann law. The solution admits a homothetic Killing vector in 5D, which induces the existence of selfsimilar symmetry in 4D, where the line element as well as the dimensionless matter quantities are invariant under a simple “scaling” group. PACS: 04.50.+h; 04.20.Cv
DECELERATION PARAMETER Q(Z) IN FOUR AND FIVE DIMENSIONAL GEOMETRIES, AND IMPLICATIONS OF GRAVITON MASS IN MIMICKING DE IN BOTH GEOMETRIES 1
"... The case for a four dimensional graviton mass (non zero) influencing reacceleration of the universe in both four and five dimensions is stated, with particular emphasis upon if four and five dimensional geometries as given below give us new physical insight as to cosmological evolution. The author f ..."
Abstract

Cited by 2 (2 self)
 Add to MetaCart
(Show Context)
The case for a four dimensional graviton mass (non zero) influencing reacceleration of the universe in both four and five dimensions is stated, with particular emphasis upon if four and five dimensional geometries as given below give us new physical insight as to cosmological evolution. The author finds that both cases give equivalent reacceleration one billion years ago which leads to an inquiry if other criteria as to cosmology can determine the benefits of adding additional dimensions to cosmology models 1
Inquiry as to if higher dimensions can be used to unify DM and DE if Massive
"... gravitons are stable. ..."
(Show Context)
Brane waves
"... In braneworld cosmology gravitational waves can propagate in the higher dimensions (i.e., in the ‘bulk’). In some appropriate regimes the bulk gravitational waves may be approximated by plane waves. We systematically study fivedimensional gravitational waves that are algebraically special and of t ..."
Abstract

Cited by 1 (1 self)
 Add to MetaCart
(Show Context)
In braneworld cosmology gravitational waves can propagate in the higher dimensions (i.e., in the ‘bulk’). In some appropriate regimes the bulk gravitational waves may be approximated by plane waves. We systematically study fivedimensional gravitational waves that are algebraically special and of type N. In the most physically relevant case the projected nonlocal stress tensor on the brane is formally equivalent to the energymomentum tensor of a null fluid. Some exact solutions are studied to illustrate the features of these branes. More importantly, it is possible that these bulk gravitational plane waves may act as initial conditions for the gravitational field in the bulk, thereby enabling the field equations to be integrated on the brane. In braneworld cosmology, matter fields and gauge interactions are confined to a fourdimensional brane moving in a higherdimensional “bulk ” spacetime. This paradigm is motivated by string and Mtheory; in particular, generalized RandallSundrumtype models [1] are relatively simple phenomenological fivedimensional (5D) models which capture some of the essential features of the dimensional reduction of HoˇravaWitten theory [2]. In a recent analysis of the asymptotic dynamical evolution of perfect fluid braneworld cosmological models close to the initial singularity, it was found that for an appropriate range of the equation of state parameter an isotropic singularity is a pastattractor [3]. It was subsequently argued that the initial cosmological singularity is isotropic in braneworld cosmological models. The 5D field equations are (5) GAB = −Λ5 (5) gAB + κ 2 5 (5) TAB, Λ5 = − 6
October 10, 2008 7:23 WSPC/INSTRUCTION FILE schee˙stuchlik Optical phenomena in the field of braneworld Kerr black holes
"... We study the influence of the tidal charge parameter of the braneworld models onto some optical phenomena in rotating black hole spacetimes. The escape photon cones are determined for special families of locally nonrotating, circular geodetical and radially free falling observers. The silhuette of ..."
Abstract
 Add to MetaCart
(Show Context)
We study the influence of the tidal charge parameter of the braneworld models onto some optical phenomena in rotating black hole spacetimes. The escape photon cones are determined for special families of locally nonrotating, circular geodetical and radially free falling observers. The silhuette of a rotating black hole, the shape of an equatorial thin accretion disk and time delay effect for direct and indirect images of a radiation hot spot orbiting the black hole are given and classified in terms of the black hole rotational and tidal parameters. It is shown that rising of negativelyvalued tidal parameter, with rotational parameter fixed, generally strenghtens the relativistic effects and suppresses the rotation induced asymmetries in the optical phenomena.