183 research outputs found
Conditions for inflation in an initially inhomogeneous universe
Using a long wavelength iteration scheme to solve Einstein's equations near
the Big-Bang singularity of a universe driven by a massive scalar field, we
find how big initial quasi-isotropic inhomogeneities can be before they can
prevent inflation to set in.Comment: 9 pages, plain Tex, gr-qc/yymmnn
String-Dominated Cosmology
If string theory controls physics at the string scale, the dynamics of the
early universe before the GUT era will be governed by the low-energy string
equations of motion. Studying these equations for FRW spacetimes, we find that
depending on the initial conditions when the stringy era starts, and on the
time when it ends, there are a wide variety of qualitatively distinct types of
evolution. We classify these, and present the general solution to the equations
of motion
Can Gravitational Waves Prevent Inflation?
To investigate the cosmic no hair conjecture, we analyze numerically
1-dimensional plane symmetrical inhomogeneities due to gravitational waves in
vacuum spacetimes with a positive cosmological constant. Assuming periodic
gravitational pulse waves initially, we study the time evolution of those waves
and the nature of their collisions. As measures of inhomogeneity on each
hypersurface, we use the 3-dimensional Riemann invariant and the electric and magnetic parts of
the Weyl tensor. We find a temporal growth of the curvature in the waves'
collision region, but the overall expansion of the universe later overcomes
this effect. No singularity appears and the result is a ``no hair" de Sitter
spacetime. The waves we study have amplitudes between and widths between ,
where , the horizon scale of de Sitter spacetime. This
supports the cosmic no hair conjecture.Comment: LaTeX, 11 pages, 3 figures are available on request <To
[email protected] (Hisa-aki SHINKAI)>, WU-AP/29/9
Junction Conditions of Friedmann-Robertson-Walker Space-Times
We complete a classification of junctions of two Friedmann-Robertson-Walker
space-times bounded by a spherical thin wall. Our analysis covers super-horizon
bubbles and thus complements the previous work of Berezin, Kuzumin and Tkachev.
Contrary to sub-horizon bubbles, various topology types for super-horizon
bubbles are possible, regardless of the sign of the extrinsic curvature. We
also derive a formula for the peculiar velocity of a domain wall for all types
of junction.Comment: 7 pages, LaTeX, figures are not included (available on request by
regular mail), WU-AP/31/9
On the fate of vacuum bubbles on matter backgrounds
In this letter we discuss cosmological first order phase transitions with de
Sitter bubbles nucleating on (inhomogeneous) matter backgrounds. The de Sitter
bubble can be a toy model for an inflationary phase of universes like our own.
Using the thin wall approximation and the Israel junction method we trace the
classical evolution of the formed bubbles within a compound model. We first
address homogeneous ambient space (FRW model) and already find that bubbles
nucleated in a dust dominated background cannot expand. For an inhomogeneous
dust background (LTB model) we describe cases with at least initially expanding
bubbles. Yet, an ensuing passage of the bubble wall through ambient curvature
inhomogeneities remains unnoticed for observers inside the bubble. Notable
effects also for interior observers are found in the case of a rapid background
phase transition in a FRW model.Comment: 4 pages, 2 figures, proceedings (Annalen der Physik) of the
Grassmannian Conference in Fundamental Cosmology (Grasscosmofun'09) Szczecin,
Poland, 14-19 September 200
Choptuik scaling in null coordinates
A numerical simulation is performed of the gravitational collapse of a
spherically symmetric scalar field. The algorithm uses the null initial value
formulation of the Einstein-scalar equations, but does {\it not} use adaptive
mesh refinement. A study is made of the critical phenomena found by Choptuik in
this system. In particular it is verified that the critical solution exhibits
periodic self-similarity. This work thus provides a simple algorithm that gives
verification of the Choptuik results.Comment: latex (revtex), 6 figures included in the fil
Onset of inflation in inhomogeneous cosmology
We study how the initial inhomogeneities of the universe affect the onset of
inflation in the closed universe. We consider the model of a chaotic inflation
which is driven by a massive scalar field. In order to construct an
inhomogeneous universe model, we use the long wavelength approximation ( the
gradient expansion method ). We show the condition of the inhomogeneities for
the universe to enter the inflationary phase.Comment: 22 pages including 12 eps figures, RevTe
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