4 research outputs found
Looking Beyond Inflationary Cosmology
In spite of the phenomenological successes of the inflationary universe
scenario, the current realizations of inflation making use of scalar fields
lead to serious conceptual problems which are reviewed in this lecture. String
theory may provide an avenue towards addressing these problems. One particular
approach to combining string theory and cosmology is String Gas Cosmology. The
basic principles of this approach are summarized.Comment: invited talk at "Theory Canada 1" (Univ. of British Columbia,
Vancouver, Canada, June 2 - 4, 2005) (references updated
Cosmological String Gas on Orbifolds
It has long been known that strings wound around incontractible cycles can
play a vital role in cosmology. In particular, in a spacetime with toroidal
spatial hypersurfaces, the dynamics of the winding modes may help yield three
large spatial dimensions. However, toroidal compactifications are
phenomenologically unrealistic. In this paper we therefore take a first step
toward extending these cosmological considerations to -dimensional toroidal
orbifolds. We use numerical simulation to study the timescales over which
"pseudo-wound" strings unwind on these orbifolds with trivial fundamental
group. We show that pseudo-wound strings can persist for many ``Hubble times''
in some of these spaces, suggesting that they may affect the dynamics in the
same way as genuinely wound strings. We also outline some possible extensions
that include higher-dimensional wrapped branes.Comment: 14 pages, 8 eps fig
From Big Crunch to Big Bang
We consider conditions under which a universe contracting towards a big
crunch can make a transition to an expanding big bang universe. A promising
example is 11-dimensional M-theory in which the eleventh dimension collapses,
bounces, and re-expands. At the bounce, the model can reduce to a weakly
coupled heterotic string theory and, we conjecture, it may be possible to
follow the transition from contraction to expansion. The possibility opens the
door to new classes of cosmological models. For example, we discuss how it
suggests a major simplification and modification of the recently proposed
ekpyrotic scenario.Comment: 16 pages, compressed and RevTex file, including three postscript
figure files. Minor changes, version to appear in Physical Review