82 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 of Nothing in Flux Compactifications
We construct a simple AdS_4 x S^1 flux compactification stabilized by a
complex scalar field winding the extra dimension and demonstrate an instability
via nucleation of a bubble of nothing. This occurs when the Kaluza -- Klein
dimension degenerates to a point, defining the bubble surface. Because the
extra dimension is stabilized by a flux, the bubble surface must be charged, in
this case under the axionic part of the complex scalar. This smooth geometry
can be seen as a de Sitter topological defect with asymptotic behavior
identical to the pure compactification. We discuss how a similar construction
can be implemented in more general Freund -- Rubin compactifications.Comment: 16 pages, 5 figures References adde
Inflation in Random Landscapes with two energy scales
We investigate inflation in a multi-dimensional landscape with a hierarchy of
energy scales, motivated by the string theory, where the energy scale of Kahler
moduli is usually assumed to be much lower than that of complex structure
moduli and dilaton field. We argue that in such a landscape, the dynamics of
slow-roll inflation is governed by the low-energy potential, while the initial
condition for inflation are determined by tunneling through high-energy
barriers. We then use the scale factor cutoff measure to calculate the
probability distribution for the number of inflationary e-folds and the
amplitude of density fluctuations , assuming that the low-energy landscape
is described by a random Gaussian potential with a correlation length much
smaller than . We find that the distribution for has a unique
shape and a preferred domain, which depends on the parameters of the low-energy
landscape. We discuss some observational implications of this distribution and
the constraints it imposes on the landscape parameters.Comment: 39 pages, 3 figures; (v2) minor change
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