40 research outputs found
The Relic Problem of String Gas Cosmology
We discuss the relic problem of string gas cosmology (SGC) using gravitinos
and magnetic monopoles as examples. Since SGC operates near or at the Hagedorn
temperature, relics are produced copiously; in the absence of dilution, their
abundances are too large. A subsequent phase of reheating can solve the
gravitino problem, but fails to dilute monopoles sufficiently. We propose a
subsequent phase of inflation as the most natural solution to the monopole
problem; unfortunately, inflation marginalizes almost all potential
achievements of SGC, with the exception of a possible explanation of the
dichotomy of space (why did only three dimensions inflate?).Comment: 5 pages, V2 and V3: added reference
On the Suppression of Parametric Resonance and the Viability of Tachyonic Preheating after Multi-Field Inflation
We investigate the feasibility of explosive particle production via
parametric resonance or tachyonic preheating in multi-field inflationary models
by means of lattice simulations. We observe a strong suppression of resonances
in the presence of four-leg interactions between the inflaton fields and a
scalar matter field, leading to insufficient preheating when more than two
inflatons couple to the same matter field. This suppression is caused by a
dephasing of the inflatons that increases the effective mass of the matter
field.
Including three-leg interactions leads to tachyonic preheating, which is not
suppressed by an increase in the number of fields. If four-leg interactions are
sub-dominant, we observe a slight enhancement of tachyonic preheating. Thus, in
order for preheating after multi-field inflation to be efficient, one needs to
ensure that three-leg interactions are present. If no tachyonic contributions
exist, we expect the old theory of reheating to be applicable.Comment: v2: reference added, identical with PRD version, 23 pages, 3 figure
Local random potentials of high differentiability to model the Landscape
We generate random functions locally via a novel generalization of Dyson
Brownian motion, such that the functions are in a desired differentiability
class, while ensuring that the Hessian is a member of the Gaussian orthogonal
ensemble (other ensembles might be chosen if desired). Potentials in such
higher differentiability classes are required/desirable to model string
theoretical landscapes, for instance to compute cosmological perturbations
(e.g., smooth first and second derivatives for the power-spectrum) or to search
for minima (e.g., suitable de Sitter vacua for our universe). Since potentials
are created locally, numerical studies become feasible even if the dimension of
field space is large (D ~ 100). In addition to the theoretical prescription, we
provide some numerical examples to highlight properties of such potentials;
concrete cosmological applications will be discussed in companion publications.Comment: V2: added discussion section to match published version (conclusions
unchanged); 25 pages, 5 figure
Multi-Field Inflation on the Landscape
We examine a wide class of multi-field inflationary models based on fields
that decay or stabilize during inflation in a staggered fashion. The fields
driving assisted inflation are on flat, short stretches, before they encounter
a sharp drop; whenever a field encounters such a drop due to its slow roll
evolution, its energy is transferred to other degrees of freedom, i.e.
radiation. The rate at which fields decay is determined dynamically and it is
not a free parameter in this class of models. To compute observables, we
generalize the analytic framework of staggered inflation, allowing for more
general initial conditions and varying potentials. By searching for generic
situations arising on the landscape, we arrive at a setup involving linear or
hilltop potentials and evenly spread out initial field values. This scenario is
not more fine tuned than large-field models, despite the fact that many more
degrees of freedom are involved. Further, the -problem can be alleviated.
The additional decrease of the potential energy caused by the decay of fields
provides leading order contribution to observables, such as the scalar and
tensor spectral index or the tensor to scalar ratio, for which we derive
general expressions. We compare the predictions with WMAP5 constraints and find
that hilltop potentials are borderline ruled out at the -level, while
linear potentials are in excellent agreement with observations. We further
comment on additional sources of gravitational waves and non-Gaussianities that
could serve as a smoking gun for staggered inflation.Comment: 29 pages, 2 figures; v2: slightly extended to match JCAP versio