422 research outputs found
Decaying Cosmological Constant of the Inflating Branes in the Randall-Sundrum -Oda Model
We examine the issue of the cosmological constant in the
branes scenario, extending on two recent models by I.Oda and Randall-Sundrum.
The exact solution in a closed form is found in the slow roll approximation of
the radion. Defining an effective expansion rate, which depends on the location
of each brane in the fifth dimension and demanding stability for this case we
show that each positive tension brane has a localized, decaying cosmological
constant (the opposite process applies to the negative energy branes [4]) . The
reason is that the square of the effective expansion rate enters as a source
term in the Einstein equations for the branes.Thus the brane has two scale
factors depending on time and the fifth dimnesion respectively .The brane will
roll along the fifth dimension in order to readjust its effective expansion
rate in such a way that it compensates for its internal energy changes due to
inflation and possible phase transitions.Comment: 9 pages, comments and ref.added, solution replaced with the exact
one, submitted to PR
Type 0 Brane Inflation from Mirage Cosmology
We consider a three-dimensional brane-universe moving in a Type 0 String
background. The motion induces on the brane a cosmological evolution which, for
some range of the parameters, exhibits an inflationary phase.Comment: 11 pages, latex, one figur
Brane Induced Gravity: Codimension-2
We review the results of arXiv:hep-th/0703190, on brane induced gravity (BIG)
in 6D. Among a large diversity of regulated codimension-2 branes, we find that
for near-critical tensions branes live inside very deep throats which
efficiently compactify the angular dimension. In there, 4D gravity first
changes to 5D, and only later to 6D. The crossover from 4D to 5D is independent
of the tension, but the crossover from 5D to 6D is not. This shows how the
vacuum energy problem manifests in BIG: instead of tuning vacuum energy to
adjust the 4D curvature, generically one must tune it to get the desired
crossover scales and the hierarchy between the scales governing the 4D \to 5D
\to 6D transitions. In the near-critical limit, linearized perturbation theory
remains under control below the crossover scale, and we find that linearized
gravity around the vacuum looks like a scalar-tensor theory.Comment: 16 pages latex, 2 .eps figs, based on the talks given at the "Sowers
Workshop", Virginia Tech, May 14-18, 2007, "Cosmology and Strings" workshop
at ICTP, Trieste, Italy, July 9-13, 2007, "Dark Energy In the Universe",
Hakone, Japan, Sep 1-4, 2007 and "Zagreb Workshop 2007", Zagreb, Croatia, Nov
9-11, 2007; v2: added reference
Probing Gravity with Spacetime Sirens
A gravitational observatory such as LISA will detect coalescing pairs of
massive black holes, accurately measure their luminosity distance and help
identify a host galaxy or an electromagnetic counterpart. If dark energy is a
manifestation of modified gravity on large scales, gravitational waves from
cosmologically-distant spacetime sirens are direct probes of this new physics.
For example, a gravitational Hubble diagram based on black hole pair luminosity
distances and host galaxy redshifts could reveal a large distance
extra-dimensional leakage of gravity. Various additional signatures may be
expected in a gravitational signal propagated over cosmological scales.Comment: 11 pages, 1 figure, accepted for publication in ApJ Letter
Braneworlds and Dark Energy
In the Randall-Sundrum scenario, we analyse the dynamics of an AdS5
braneworld when conformal matter fields propagate in five dimensions. We show
that conformal fields of weight -4 are associated with stable geometries which
describe the dynamics of inhomogeneous dust, generalized dark radiation and
homogeneous polytropic dark energy on a spherically symmetric 3-brane embedded
in the compact AdS5 orbifold. We discuss aspects of the radion stability
conditions and of the localization of gravity in the vicinity of the brane.Comment: 12 pages, latex, 3 eps figures. Talk given at the Seventh Workshop on
Quantum Field Theory under the Influence of External Conditions, IEEC, CSIC
and University of Barcelona, Barcelona, Catalonia, Spain, 5-9 September 2005.
Revised version published in the workshop proceedings, J. Phys. A:
Mathematical and General. Typos corrected and some changes introduced for
clarit
Cosmological Solution in M-theory on S^1/Z_2
We provide the first example of a cosmological solution of the Horava-Witten
supergravity. This solution is obtained by exchanging the role of time with the
radial coordinate of the transverse space to the five-brane soliton. On the
boundary this corresponds to rotating an instanton solution into a tunneling
process in a space with Lorentzian signature, leading to an expanding universe.
Due to the freedom to choose different non-trivial Yang-Mills backgrounds on
the boundaries, the two walls of the universe ( visible and hidden worlds)
expand differently. However at late times the anisotropy is washed away by
gravitational interactions.Comment: 10 pages, latex, no figur
String cosmology versus standard and inflationary cosmology
This paper presents a review of the basic, model-independent differences
between the pre-big bang scenario, arising naturally in a string cosmology
context, and the standard inflationary scenario. We use an unconventional
approach in which the introduction of technical details is avoided as much as
possible, trying to focus the reader's attention on the main conceptual aspects
of both scenarios. The aim of the paper is not to conclude in favour either of
one or of the other scenario, but to raise questions that are left to the
reader's meditation. Warnings: the paper does not contain equations, and is not
intended as a complete review of all aspects of string cosmology.Comment: 22 pages, Latex (IOP Style), three figures included using epsfig. To
appear in Class. Quantum Grav. (Topical Review Section). Two misprints
correcte
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