8 research outputs found
Higher order corrections to Heterotic M-theory inflation
We investigate inflation driven by dynamical five-branes in Heterotic
M-theory using the scalar potential derived from the open membrane instanton
sector. At leading order the resulting theory can be mapped to power law
inflation, however more generally one may expect higher order corrections to be
important. We consider a simple class of such corrections, which imposes tight
bounds on the number of branes required for inflation.Comment: 10 pages, 2 figure
Energy Transfer in Multi Field Inflation and Cosmological Perturbations
In cascade inflation and some other string inflation models, collisions of
mobile branes with other branes or orbifold planes occur and lead to
interesting cosmological signatures. The fundamental M/string-theory
description of these collisions is still lacking but it is clear that the
inflaton looses part of its energy to some form of brane matter, e.g. a
component of tensionless strings. In the absence of a fundamental description,
we assume a general barotropic fluid on the brane, which absorbs part of the
inflaton's energy. The fluid is modeled by a scalar with a suitable exponential
potential to arrive at a full-fledged field theory model. We study numerically
the impact of the energy transfer from the inflaton to the scalar on curvature
and isocurvature perturbations and demonstrate explicitly that the curvature
power spectrum gets modulated by oscillations which damp away toward smaller
scales. Even though, the contribution of isocurvature perturbations decays
toward the end of inflation, they induce curvature perturbations on scales that
exit the horizon before the collision. We consider cases where the scalar
behaves like radiation, matter or a web of cosmic strings and discuss the
differences in the resulting power spectra.Comment: v1: 25 pages, 7 figures; v2: references added;v3: typo corrected,
accepted for publication to JCA
Inflation Assisted by Heterotic Axions
We explore the possibility of obtaining inflation in weakly coupled heterotic
string theory, where the model dependent axions are responsible for driving
inflation. This model can be considered as a certain extrapolation of
-inflation, and is an attempt to explicitly realize the so
called N-flation proposal in string theory. The instanton generated potential
for the axions essentially has two parameters; a natural mass scale and the
string coupling . For isotropic compactifications leading to of order
axions in the four dimensional spectrum we find that with
the observed temperature fluctuations in the
CMB are correctly reproduced. We assume an initially random distribution for
the vevs of the axions. The spectral index, , is generically more red
than for -inflation. The greater the vevs, the more red the
spectral index becomes. Allowing for a wide range of vevs 55 -foldings from
the end of inflation, we find . The
tensor-to-scalar ratio, , is more sensitive to the vevs, but typically
smaller than in -inflation. Furthermore, in the regime where the
leading order theory is valid, is bounded by . The spectral index
and the tensor-to-scalar ratio are correlated. For example,
corresponds to .Comment: 1+21 pages, 2 figures, v2: Typos corrected, v3: Typos, very minor
corrections, reference added, to appear in JCA
Non-Gaussianities in N-flation
We compute non-Gaussianities in N-flation, a string motivated model of
assisted inflation with quadratic, separable potentials and masses given by the
Marcenko-Pastur distribution. After estimating parameters characterizing the
bi- and trispectrum in the horizon crossing approximation, we focus on the
non-linearity parameter , a measure of the bispectrum; we compute its
magnitude for narrow and broad spreads of masses, including the evolution of
modes after horizon crossing. We identify additional contributions due to said
evolution and show that they are suppressed as long as the fields are evolving
slowly. This renders -flation indistinguishable from simple
single-field models in this regime. Larger non-Gaussianities are expected to
arise for fields that start to evolve faster, and we suggest an analytic
technique to estimate their contribution. However, such fast roll during
inflation is not expected in N-flation, leaving (p)re-heating as the main
additional candidate for generating non-Gaussianities.Comment: 27 pages, 4 figures, extended references to match version accepted in
JCA
Spinflation from Geometric Tachyon
We study the assisted inflation scenario from the rolling of BPS D3-brane
into the NS5-branes, on a transverse geometry of , coupled to
four dimensional gravity. We assume that the branes are distributed along
and the probe D3-branes spin along plane. Qualitatively this process is
similar to that of N-tachyon assisted inflation on unstable D-branes. We
further study the spinflation scenario numerically and analyze its effect.Comment: 18pages, 9 figures, added clarifications, to appear in JHE
Chasing Brane Inflation in String-Theory
We investigate the embedding of brane anti-brane inflation into a concrete
type IIB string theory compactification with all moduli fixed. Specifically, we
are considering a D3-brane, whose position represents the inflaton , in a
warped conifold throat in the presence of supersymmetrically embedded D7-branes
and an anti D3-brane localized at the tip of the warped conifold cone. After
presenting the moduli stabilization analysis for a general D7-brane embedding,
we concentrate on two explicit models, the Ouyang and the Kuperstein
embeddings. We analyze whether the forces, induced by moduli stabilization and
acting on the D3-brane, might cancel by fine-tuning such as to leave us with
the original Coulomb attraction of the anti D3-brane as the driving force for
inflation. For a large class of D7-brane embeddings we obtain a negative
result. Cancelations are possible only for very small intervals of
around an inflection point but not globally. For the most part of its motion
the inflaton then feels a steep, non slow-roll potential. We study the
inflationary dynamics induced by this potential.Comment: 34 pages, 4 figures. Final version published in JCA
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