1,203 research outputs found
Covariant perturbations of domain walls in curved spacetime
A manifestly covariant equation is derived to describe the perturbations in a
domain wall on a given background spacetime. This generalizes recent work on
domain walls in Minkowski space and introduces a framework for examining the
stability of relativistic bubbles in curved spacetimes.Comment: 15 pages,ICN-UNAM-93-0
Gravity Waves from Instantons
We perform a first principles computation of the spectrum of gravity waves
produced in open inflationary universes. The background spacetime is taken to
be the continuation of an instanton saddle point of the Euclidean no boundary
path integral. The two-point tensor correlator is computed directly from the
path integral and is shown to be unique and well behaved in the infrared. We
discuss the tensor contribution to the cosmic microwave background anisotropy
and show how it may provide an observational discriminant between different
types of primordial instantons.Comment: 19 pages, RevTex file, including two postscript figure file
Solutions to the cosmological constant problems
We critically review several recent approaches to solving the two
cosmological constant problems. The "old" problem is the discrepancy between
the observed value of and the large values suggested by particle
physics models. The second problem is the "time coincidence" between the epoch
of galaxy formation and the epoch of -domination t_\L. It is
conceivable that the "old" problem can be resolved by fundamental physics
alone, but we argue that in order to explain the "time coincidence" we must
account for anthropic selection effects. Our main focus here is on the
discrete- models in which can change through nucleation of
branes. We consider the cosmology of this type of models in the context of
inflation and discuss the observational constraints on the model parameters.
The issue of multiple brane nucleation raised by Feng {\it et. al.} is
discussed in some detail. We also review continuous-\L models in which the
role of the cosmological constant is played by a slowly varying potential of a
scalar field. We find that both continuous and discrete models can in principle
solve both cosmological constant problems, although the required values of the
parameters do not appear very natural. M-theory-motivated brane models, in
which the brane tension is determined by the brane coupling to the four-form
field, do not seem to be viable, except perhaps in a very tight corner of the
parameter space. Finally, we point out that the time coincidence can also be
explained in models where is fixed, but the primordial density
contrast is treated as a random variable.Comment: 30 pages, 3 figures, two notes adde
Cosmological Perturbations from the No Boundary Euclidean Path Integral
We compute, from first principles, the quantum fluctuations about instanton
saddle points of the Euclidean path integral for Einstein gravity coupled to a
scalar field. The Euclidean two-point correlator is analytically continued into
the Lorentzian region where it describes the quantum mechanical vacuum
fluctuations in the state described by no boundary proposal initial conditions.
We concentrate on the density perturbations in open inflationary universes
produced from cosmological instantons, describing the differences between
non-singular Coleman-De Luccia and singular Hawking-Turok instantons. We show
how the Euclidean path integral uniquely specifies the fluctuations in both
cases.Comment: 21 pages, RevTex file, including five postscript figure file
Perturbations of a topological defect as a theory of scalar fields interacting with an external vector potential
This is a revised version of gr-qc/930403
Stable and Unstable Circular Strings in Inflationary Universes
It was shown by Garriga and Vilenkin that the circular shape of nucleated
cosmic strings, of zero loop-energy in de Sitter space, is stable in the sense
that the ratio of the mean fluctuation amplitude to the loop radius is
constant. This result can be generalized to all expanding strings (of non-zero
loop-energy) in de Sitter space. In other curved spacetimes the situation,
however, may be different.
In this paper we develop a general formalism treating fluctuations around
circular strings embedded in arbitrary spatially flat FRW spacetimes. As
examples we consider Minkowski space, de Sitter space and power law expanding
universes. In the special case of power law inflation we find that in certain
cases the fluctuations grow much slower that the radius of the underlying
unperturbed circular string. The inflation of the universe thus tends to wash
out the fluctuations and to stabilize these strings.Comment: 15 pages Latex, NORDITA 94/14-
Instantons for Vacuum Decay at Finite Temperature in the Thin Wall Limit
In dimensions, false vacuum decay at zero temperature is dominated by
the symmetric instanton, a sphere of radius , whereas at
temperatures , the decay is dominated by a `cylindrical' (static)
symmetric instanton. We study the transition between these two regimes
in the thin wall approximation. Taking an symmetric ansatz for the
instantons, we show that for and new periodic solutions exist in a
finite temperature range in the neighborhood of . However,
these solutions have higher action than the spherical or the cylindrical one.
This suggests that there is a sudden change (a first order transition) in the
derivative of the nucleation rate at a certain temperature , when the
static instanton starts dominating. For , on the other hand, the new
solutions are dominant and they smoothly interpolate between the zero
temperature instanton and the high temperature one, so the transition is of
second order. The determinantal prefactors corresponding to the `cylindrical'
instantons are discussed, and it is pointed out that the entropic contributions
from massless excitations corresponding to deformations of the domain wall give
rise to an exponential enhancement of the nucleation rate for .Comment: 24 pages, 7 figures available upon request, DAMTP-R-94/
Bubble collisions and measures of the multiverse
To compute the spectrum of bubble collisions seen by an observer in an
eternally-inflating multiverse, one must choose a measure over the diverging
spacetime volume, including choosing an "initial" hypersurface below which
there are no bubble nucleations. Previous calculations focused on the case
where the initial hypersurface is pushed arbitrarily deep into the past.
Interestingly, the observed spectrum depends on the orientation of the initial
hypersurface, however one's ability observe the effect rapidly decreases with
the ratio of inflationary Hubble rates inside and outside one's bubble. We
investigate whether this conclusion might be avoided under more general
circumstances, in particular placing the observer's bubble near the initial
hypersurface. We find that it is not. As a point of reference, a substantial
appendix reviews relevant aspects of the measure problem of eternal inflation.Comment: 24 pages, two figures, plus 16-page appendix with one figure; v2:
minor improvements and clarifications, conclusions unchanged (version to
appear in JCAP
Bubble wall perturbations coupled with gravitational waves
We study a coupled system of gravitational waves and a domain wall which is
the boundary of a vacuum bubble in de Sitter spacetime. To treat the system, we
use the metric junction formalism of Israel. We show that the dynamical degree
of the bubble wall is lost and the bubble wall can oscillate only while the
gravitational waves go across it. It means that the gravitational backreaction
on the motion of the bubble wall can not be ignored.Comment: 23 pages with 3 eps figure
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