7 research outputs found
The Cosmology of Massless String Modes
We consider the spacetime dynamics of a gas of closed strings in the context
of General Relativity in a background of arbitrary spatial dimensions. Our
motivation is primarily late time String Gas Cosmology, where such a spacetime
picture has to emerge after the dilaton has stabilized. We find that after
accounting for the thermodynamics of a gas of strings, only string modes which
are massless at the self-dual radius are relevant, and that they lead to a
dynamics which is qualitatively different from that induced by the modes
usually considered in the literature. In the context of an ansatz with three
large spatial dimensions and an arbitrary number of small extra dimensions, we
obtain isotropic stabilization of these extra dimensions at the self-dual
radius. This stabilization occurs for fixed dilaton, and is induced by the
special string states we focus on. The three large dimensions undergo a regular
Friedmann-Robertson-Walker expansion. We also show that this framework for
late-time cosmology is consistent with observational bounds.Comment: 15 pages, no figures, references added (again
Moduli Stabilization in Brane Gas Cosmology with Superpotentials
In the context of brane gas cosmology in superstring theory, we show why it
is impossible to simultaneously stabilize the dilaton and the radion with a
general gas of strings (including massless modes) and D-branes. Although this
requires invoking a different mechanism to stabilize these moduli fields, we
find that the brane gas can still play a crucial role in the early universe in
assisting moduli stabilization. We show that a modest energy density of
specific types of brane gas can solve the overshoot problem that typically
afflicts potentials arising from gaugino condensation.Comment: minor changes to match the journal versio
Inflation and Brane Gases
We investigate a new way of realizing a period of cosmological inflation in
the context of brane gas cosmology. It is argued that a gas of co-dimension one
branes, out of thermal equilibrium with the rest of the matter, has an equation
of state which can - after stabilization of the dilaton - lead to power-law
inflation of the bulk. The most promising implementation of this mechanism
might be in Type IIB superstring theory, with inflation of the three large
spatial dimensions triggered by ``stabilized embedded 2-branes''. Possible
applications and problems with this proposal are discussed.Comment: 7 pages, uses REVTeX, version to appear in Phys. Rev.
The accelerating universe and a limiting curvature proposal
We consider the hypothesis of a limiting minimal curvature in gravity as a
way to construct a class of theories exhibiting late-time cosmic acceleration.
Guided by the minimal curvature conjecture (MCC) we are naturally lead to a set
of scalar tensor theories in which the scalar is non-minimally coupled both to
gravity and to the matter Lagrangian. The model is compared to the Lambda Cold
Dark Matter concordance model and to the observational data using the gold
SNeIa sample of Riess et. al. (2004). An excellent fit to the data is achieved.
We present a toy model designed to demonstrate that such a new, possibly
fundamental, principle may be responsible for the recent period of cosmological
acceleration. Observational constraints remain to be imposed on these models.Comment: 22 pages, 7 figures; revised version to appear in JCAP; references
adde
String windings in the early universe
We study string dynamics in the early universe. Our motivation is the
proposal of Brandenberger and Vafa, that string winding modes may play a key
role in decompactifying three spatial dimensions. We model the universe as a
homogeneous but anisotropic 9-torus filled with a gas of excited strings. We
adopt initial conditions which fix the dilaton and the volume of the torus, but
otherwise assume all states are equally likely. We study the evolution of the
system both analytically and numerically to determine the late-time behavior.
We find that, although dynamical evolution can indeed lead to three large
spatial dimensions, such an outcome is not statistically favored.Comment: 26 pages, LaTeX, 4 eps figure
Towards an Observational Appraisal of String Cosmology
We review the current observational status of string cosmology when
confronted with experimental datasets. We begin by defining common
observational parameters and discuss how they are determined for a given model.
Then we review the observable footprints of several string theoretic models,
discussing the significance of various potential signals. Throughout we comment
on present and future prospects of finding evidence for string theory in
cosmology, and on significant issues for the future.Comment: Review accepted for publication in the CQG focus issue on string
cosmology. Minor clarifications and references adde
Predictions for Nongaussianity from Nonlocal Inflation
In our previous work the nonlinearity parameter f_NL, which characterizes
nongaussianity in the cosmic microwave background, was estimated for a class of
inflationary models based on nonlocal field theory. These models include p-adic
inflation and generically have the remarkable property that slow roll inflation
can proceed even with an extremely steep potential. Previous calculations found
that large nongaussianity is possible; however, the technical complications
associated with studying perturbations in theories with infinitely many
derivatives forced us to provide only an order of magnitude estimate for f_NL.
We reconsider the problem of computing f_NL in nonlocal inflation models,
showing that a particular choice of field basis and recent progress in
cosmological perturbation theory makes an exact computation possible. We
provide the first quantitatively accurate computation of the bispectrum in
nonlocal inflation, confirming our previous claim that it can be observably
large. We show that the shape of the bispectrum in this class of models makes
it observationally distinguishable from Dirac-Born-Infeld inflation models.Comment: 26 pages, 5 figures; references added, sign convention for f_NL
clarified, minor correction