5,372 research outputs found
Black Holes and Instabilities of Negative Tension Branes
We consider the collision in 2+1 dimensions of a black hole and a negative
tension brane on an orbifold. Because there is no gravitational radiation in
2+1 dimensions, the horizon area shrinks when part of the brane falls through.
This provides a potential violation of the generalized second law of
thermodynamics. However, tracing the details of the dynamical evolution one
finds that it does not proceed from equilibrium configuration to equilibrium
configuration. Instead, a catastrophic space-time singularity develops similar
to the `big crunch' of FRW space-times. In the context of classical
general relativity, our result demonstrates a new instability of constructions
with negative tension branes.Comment: 18 pages, 3 figures, uses RevTeX. Minor typos fixed. References and
one footnote adde
Cosmology Without Averaging
We construct cosmological models consisting of large numbers of identical,
regularly spaced masses. These models do not rely on any averaging procedures,
or on the existence of a global Friedmann-Robertson-Walker (FRW) background.
They are solutions of Einstein's equations up to higher order corrections in a
perturbative expansion, and have large-scale dynamics that are well modelled by
the Friedmann equation. We find that the existence of arbitrarily large density
contrasts does not change either the magnitude or scale of the background
expansion, at least when masses are regularly arranged, and up to the
prescribed level of accuracy. We also find that while the local space-time
geometry inside each cell can be described as linearly perturbed FRW, one could
argue that a more natural description is that of perturbed Minkowski space (in
which case the scalar perturbations are simply Newtonian potentials). We expect
these models to be of use for understanding and testing ideas about averaging
in cosmology, as well as clarifying the relationship between global
cosmological dynamics and the static space-times associated with isolated
masses.Comment: 24 pages, 3 figures. Corrected and expande
Friedmann-like universes with torsion
We consider spatially homogeneous and isotropic cosmologies with non-zero
torsion. Given the high symmetry of these universes, we adopt a specific form
for the torsion tensor that preserves the homogeneity and isotropy of the
spatial surfaces. Employing both covariant and metric-based techniques, we
derive the torsional versions of the continuity, the Friedmann and the
Raychaudhuri equations. These formulae demonstrate how, by playing the role of
the spatial curvature, or that of the cosmological constant, torsion can
drastically change the evolution of the classic homogeneous and isotropic
Friedmann universes. In particular, torsion alone can lead to exponential
expansion. For instance, in the presence of torsion, the Milne and the
Einstein-de Sitter universes evolve like the de Sitter model. We also show
that, by changing the expansion rate of the early universe, torsion can affect
the primordial nucleosynthesis of helium-4. We use this sensitivity to impose
strong cosmological bounds on the relative strength of the associated torsion
field, requiring that its ratio to the Hubble expansion rate lies in the narrow
interval () around zero. Interestingly, the introduction
of torsion can \textit{reduce} the production of primordial helium-4, unlike
other changes to the standard thermal history of an isotropic universe.
Finally, turning to static spacetimes, we find that there exist torsional
analogues of the classic Einstein static universe, with all three types of
spatial geometry. These models can be stable when the torsion field and the
universe's spatial curvature have the appropriate profiles.Comment: Revised article. Section on BBN limits on torsion added. References
added and update
Geodesics, the Equivalence Principle and Singularities in Higher-dimensional General Relativity and Braneworlds
The geodesics of a spacetime seldom coincide with those of an embedded
submanifold of codimension one. We investigate this issue for
higher-dimensional general relativity-like models, firstly in the simpler case
without branes to isolate which features are already present, and then in the
more complicated case with branes. The framework in which we consider branes is
general enough to include asymmetric braneworlds but not thick branes. We apply
our results on geodesics to study both the equivalence principle and
cosmological singularities. Among the models we study these considerations
favour symmetric braneworlds with a negative bulk cosmological constant.Comment: 20 pages, 2 figures. Accepted by JCAP. Minor proofreading
corrections; several references adde
Geometry of crossing null shells
New geometric objects on null thin layers are introduced and their importance
for crossing null-like shells are discussed. The Barrab\`es--Israel equations
are represented in a new geometric form and they split into decoupled system of
equations for two different geometric objects: tensor density
and vector field . Continuity properties of these objects through a crossing
sphere are proved. In the case of spherical symmetry Dray--t'Hooft--Redmount
formula results from continuity property of the corresponding object.Comment: 24 pages, 1 figur
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