1,401 research outputs found

### Detecting extra dimensions with gravity wave spectroscopy: the black string brane-world

Using the black string between two branes as a model of a brane-world black
hole, we compute the gravity wave perturbations and identify the features
arising from the additional polarizations of the graviton. The standard
four-dimensional gravitational wave signal acquires late-time oscillations due
to massive modes of the graviton. The Fourier transform of these oscillations
shows a series of spikes associated with the masses of the Kaluza-Klein modes,
providing in principle a spectroscopic signature of extra dimensions.Comment: 4 pages, 5 figures. Comments on the frequency and detectability of
the massive mode signal added, and title modified. Version accepted for
publication in Phys. Rev. Let

### Stress effects in structure formation

Residual velocity dispersion in cold dark matter induces stresses which lead
to effects that are absent in the idealized dust model. A previous Newtonian
analysis showed how this approach can provide a theoretical foundation for the
phenomenological adhesion model. We develop a relativistic kinetic theory
generalization which also incorporates the anisotropic velocity dispersion that
will typically be present. In addition to density perturbations, we consider
the rotational and shape distortion properties of clustering. These quantities
together characterize the linear development of density inhomogeneity, and we
find exact solutions for their evolution. As expected, the corrections are
small and arise only in the decaying modes, but their effect is interesting.
One of the modes for density perturbations decays less rapidly than the
standard decaying mode. The new rotational mode generates precession of the
axis of rotation. The new shape modes produce additional distortion that
remains frozen in during the subsequent (linear) evolution, despite the rapid
decay of the terms that caused it.Comment: significantly improved discussion of kinetic theory of CDM velocity
dispersion; to appear Phys. Rev.

### Initial Kaluza-Klein fluctuations and inflationary gravitational waves in braneworld cosmology

We study the spectrum of gravitational waves generated from inflation in the
Randall-Sundrum braneworld. Since the inflationary gravitational waves are of
quantum-mechanical origin, the initial configuration of perturbations in the
bulk includes Kaluza-Klein quantum fluctuations as well as fluctuations in the
zero mode. We show, however, that the initial fluctuations in Kaluza-Klein
modes have no significant effect on the late time spectrum, irrespective of the
energy scale of inflation and the equation of state parameter in the
post-inflationary stage. This is done numerically, using the Wronskian
formulation.Comment: 7 pages, 6 figure

### Inflation driven by causal heat flux

We find a simple inflationary solution in an inhomogeneous spacetime with
heat flux. The heat flux obeys a causal transport equation, and counteracts the
inflationary decrease of energy density. At late times, the heat flux tends to
zero and the fluid approaches the equation of state $p=-\rho$.Comment: Latex 5 pages; to appear Gen. Rel. Gra

### Structure formation on the brane: A mimicry

We show how braneworld cosmology with bulk matter can explain structure
formation. In this scenario, the nonlocal corrections to the Friedmann
equations supply a Weyl fluid that can dominate over matter at late times due
to the energy exchange between the brane and the bulk. We demonstrate that the
presence of the Weyl fluid radically changes the perturbation equations, which
can take care of the fluctuations required to account for the large amount of
inhomogeneities observed in the local universe. Further, we show how this Weyl
fluid can mimic dark matter. We also investigate the bulk geometry responsible
for the scenario.Comment: 7 pages. Matches published versio

### Exact isotropic cosmologies with local fractal number counts

We construct an exact relativistic cosmology in which an inhomogeneous but
isotropic local region has fractal number counts and matches to a homogeneous
background at a scale of the order of $10^2$ Mpc. We show that Einstein's
equations and the matching conditions imply either a nonlinear Hubble law or a
very low large-scale density.Comment: revised version, to appear Class. Q. Grav.; minor corrections
following eqn 16, additional comments on relation to other work, some new
reference

### Anisotropic stresses in inhomogeneous universes

Anisotropic stress contributions to the gravitational field can arise from
magnetic fields, collisionless relativistic particles, hydrodynamic shear
viscosity, gravitational waves, skew axion fields in low-energy string
cosmologies, or topological defects. We investigate the effects of such
stresses on cosmological evolution, and in particular on the dissipation of
shear anisotropy. We generalize some previous results that were given for
homogeneous anisotropic universes, by including small inhomogeneity in the
universe. This generalization is facilitated by a covariant approach. We find
that anisotropic stress dominates the evolution of shear, slowing its decay.
The effect is strongest in radiation-dominated universes, where there is slow
logarithmic decay of shear.Comment: 7 pages Revte

### Lie symmetries for equations in conformal geometries

We seek exact solutions to the Einstein field equations which arise when two
spacetime geometries are conformally related. Whilst this is a simple method to
generate new solutions to the field equations, very few such examples have been
found in practice. We use the method of Lie analysis of differential equations
to obtain new group invariant solutions to conformally related Petrov type D
spacetimes. Four cases arise depending on the nature of the Lie symmetry
generator. In three cases we are in a position to solve the master field
equation in terms of elementary functions. In the fourth case special solutions
in terms of Bessel functions are obtained. These solutions contain known models
as special cases.Comment: 19 pages, To appear in J. Phys.

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