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.

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

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.

Extraposition: A new perspective

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A classification of spherically symmetric spacetimes

A complete classification of locally spherically symmetric four-dimensional Lorentzian spacetimes is given in terms of their local conformal symmetries. The general solution is given in terms of canonical metric types and the associated conformal Lie algebras. The analysis is based upon the local conformal decomposition into 2+2 reducible spacetimes and the Petrov type. A variety of physically meaningful example spacetimes are discussed

Black holes and wormholes in AdS branes

In this work we have derived a class of geometries which describe black holes and wormholes in Randall-Sundrum-type brane models, focusing mainly on asymptotically anti-de Sitter backgrounds. We show that by continuously deforming the usual four dimensional vacuum background, a specific family of solutions is obtained. Maximal extensions of the solutions are presented, and their causal structures are discussed.Comment: 7 pages, 4 figures. Published version in Physical Review

New Vistas in Braneworld Cosmology

Traditionally, higher-dimensional cosmological models have sought to provide a description of the fundamental forces in terms of a unifying geometrical construction. In this essay we discuss how, in their present incarnation, higher-dimensional `braneworld' models might provide answers to a number of cosmological puzzles including the issue of dark energy and the nature of the big-bang singularity.Comment: Honorable mention in the 2002 Essay Competition of the Gravity Research Foundation. 10 pages, 2 figure

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