438 research outputs found
A Note on Tubular Brane Dynamics
We present new time dependent solutions for the dynamics of tubular
D2-branes. We comment on the connection to cosmic string dynamics and
explicitly give a few simple examples of oscillating and rotating brane
configurations.Comment: 1+6 pages, 2 Figure
The Initial State of a Primordial Anisotropic Stage of Inflation
We investigate the possibility that the inflationary period in the early
universe was preceded by a primordial stage of strong anisotropy. In particular
we focus on the simplest model of this kind, where the spacetime is described
by a non-singular Kasner solution that quickly evolves into an isotropic de
Sitter space, the so-called Kasner-de Sitter solution. The initial Big Bang
singularity is replaced, in this case, by a horizon. We show that the extension
of this metric to the region behind the horizon contains a timelike singularity
which will be visible by cosmological observers. This makes it impossible to
have a reliable prediction of the quantum state of the cosmological
perturbations in the region of interest. In this paper we consider the
possibility that this Kasner-de Sitter universe is obtained as a result of a
quantum tunneling process effectively substituting the region behind the
horizon by an anisotropic parent vacuum state, namely a dimensional
spacetime compactified over an internal flat torus, , which we take it to
be of the form or . As a
first approximation to understand the effects of this anisotropic initial
state, we compute the power spectrum of a massless scalar field in these
backgrounds. In both cases, the spectrum converges at small scales to the
isotropic scale invariant form and only present important deviations from it at
the largest possible scales. We find that the decompactification scenario from
leads to a suppressed and slightly anisotropic power spectrum
at large scales which could be related to some of the anomalies present in the
current CMB data. On the other hand, the spectrum of the universe with a parent vacuum presents an enhancement in power at large scales not
consistent with observations.Comment: 36 pages, 7 figures, references adde
Bubbles from Nothing
Within the framework of flux compactifications, we construct an instanton
describing the quantum creation of an open universe from nothing. The solution
has many features in common with the smooth 6d bubble of nothing solutions
discussed recently, where the spacetime is described by a 4d compactification
of a 6d Einstein-Maxwell theory on S^2 stabilized by flux. The four-dimensional
description of this instanton reduces to that of Hawking and Turok. The choice
of parameters uniquely determines all future evolution, which we additionally
find to be stable against bubble of nothing instabilities.Comment: 19 pages, 6 figure
Cosmic Superstrings Stabilized by Fermions
We show that there exist massive perturbative states of the ten dimensional
Green-Schwarz closed superstring that are stabilized against collapse due to
presence of fermionic zero modes on its worldsheet. The excited fermionic
degrees of freedom backreact on the spacetime motion of the string in the same
way as a neutral persistent current would, rendering these string loops stable.
We point out that the existence of these states could have important
consequences as stable loops of cosmological size as well as long lived states
within perturbative string theory.Comment: 10 page
Decay of flux vacua to nothing
We construct instanton solutions describing the decay of flux
compactifications of a gauge theory by generalizing the Kaluza-Klein
bubble of nothing. The surface of the bubble is described by a smooth
magnetically charged solitonic brane whose asymptotic flux is precisely that
responsible for stabilizing the 4d compactification. We describe several
instances of bubble geometries for the various vacua occurring in a
Einstein-Maxwell theory namely, AdS_4 x S^2, R^{1,3} x S^2, and dS_4 x S^2.
Unlike conventional solutions, the bubbles of nothing introduced here occur
where a {\em two}-sphere compactification manifold homogeneously degenerates.Comment: 31 pages, 15 figure
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