13 research outputs found
Non-Gaussianities in Multifield Inflation: Superhorizon Evolution, Adiabaticity, and the Fate of fnl
We explore the superhorizon generation of large fnl of the local form in two
field inflation. We calculate the two- and three-point observables in a general
class of potentials which allow for an analytic treatment using the delta N
formalism. Motivated by the conservation of the curvature perturbation outside
the horizon in the adiabatic mode and also by the observed adiabaticity of the
power spectrum, we follow the evolution of fnl^{local} until it is driven into
the adibatic solution by passing through a phase of effectively single field
inflation. We find that although large fnl^{local} may be generated during
inflation, such non-gaussianities are transitory and will be exponentially
damped as the cosmological fluctuations approach adiabaticity.Comment: v3: Typos corrected, minor changes to match published version,
references added, 18 pages, 1 figure. v2: Changed sign of fnl to match WMAP
convention, minor changes throughout, references added, 18 pages, 1 figure.
v1: 17 pages, 1 figur
Axion Inflation and Gravity Waves in String Theory
The majority of models of inflation in string theory predict an absence of
measurable gravitational waves, r << 10^{-3}. The most promising proposals for
making string theoretic models that yield measurable tensor fluctuations
involve axion fields with slightly broken shift symmetry. We consider such
models in detail, with a particular focus on the N-flation scenario and on
axion valley/natural inflation models. We find that in Calabi-Yau threefold
compactifications with logarithmic Kahler potentials K it appears to be
difficult to meet the conditions required for axion inflation in the
supergravity regime. However, in supergravities with an (approximately)
quadratic shift-symmetric K, axion inflation may be viable. Such Kahler
potentials do arise in some string models, in specific limits of the moduli
space. We describe the most promising classes of models; more detailed study
will be required before one can conclude that working models exist.Comment: 30 + 16 pages, 5 figures. Added references, corrected typo
Constraining the Inflationary Equation of State
We explore possible constraints on the inflationary equation state: p=w\rho.
While w must be close to -1 for those modes that contribute to the observed
power spectrum, for those modes currently out of experimental reach, the
constraints on w are much weaker, with only w<-1/3 as an a priori requirement.
We find, however, that limits on the reheat temperature and the inflationary
energy scale constrain w further, though there is still ample parameter space
for a vastly different (accelerating) equation of state between the end of
quasi-de Sitter inflation and the beginning of the radiation-dominated era. In
the event that such an epoch of acceleration could be observed, we review the
consequences for the primordial power spectrum.Comment: 12 pages, 2 figur
Volume Weighted Measures of Eternal Inflation in the Bousso-Polchinski Landscape
We consider the cosmological dynamics associated with volume weighted
measures of eternal inflation, in the Bousso-Polchinski model of the string
theory landscape. We find that this measure predicts that observers are most
likely to find themselves in low energy vacua with one flux considerably larger
than the rest. Furthermore, it allows for a satisfactory anthropic explanation
of the cosmological constant problem by producing a smooth, and approximately
constant, distribution of potentially observable values of Lambda. The low
energy vacua selected by this measure are often short lived. If we require
anthropically acceptable vacua to have a minimum life-time of 10 billion years,
then for reasonable parameters a typical observer should expect their vacuum to
have a life-time of approximately 12 billion years. This prediction is model
dependent, but may point toward a solution to the coincidence problem of
cosmology.Comment: 35 pages, 8 figure
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Features of de Sitter Vacua in M-Theory
We compute the masses of all moduli in the unstable deSitter vacua arising in the toy model of cosmological M-theory flux compactifications on the G{sub 2} holonomy manifolds of [1]. The slow-roll parameters in the tachyonic directions are shown to be too large to be useful for conventional models of inflation. However, it appears that we can find fast roll regimes which could, under certain conditions, account for the current dark energy driven accelerated expansion of the universe
The Inflationary Wavefunction and its Initial Conditions
We explore the effect of initial conditions on the inflationary wavefunction
and their consequences for the observed spectrum of primordial fluctuations. In
a class of models with a sudden transition into inflation we find that, for a
reasonable set of assumptions about the reheat temperature and the number of
e-foldings, it is possible for initial conditions set by a pre-inflationary
epoch to have an observable effect.Comment: 17 pages, 1 figure, numerical example and analysis thereof added in
this versio
Islands in the landscape
The string theory landscape consists of many metastable de Sitter vacua,
populated by eternal inflation. Tunneling between these vacua gives rise to a
dynamical system, which asymptotically settles down to an equilibrium state. We
investigate the effects of sinks to anti-de Sitter space, and show how their
existence can change probabilities in the landscape. Sinks can disturb the
thermal occupation numbers that would otherwise exist in the landscape and may
cause regions that were previously in thermal contact to be divided into
separate, thermally isolated islands.Comment: 31 pages, 8 figure