19 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
A Critical Review of Classical Bouncing Cosmologies
Given the proliferation of bouncing models in recent years, we gather and
critically assess these proposals in a comprehensive review. The Planck data
shows an unmistakably red, quasi scale-invariant, purely adiabatic primordial
power spectrum and no primary non-Gaussianities. While these observations are
consistent with inflationary predictions, bouncing cosmologies aspire to
provide an alternative framework to explain them. Such models face many
problems, both of the purely theoretical kind, such as the necessity of
violating the NEC and instabilities, and at the cosmological application level,
as exemplified by the possible presence of shear. We provide a pedagogical
introduction to these problems and also assess the fitness of different
proposals with respect to the data. For example, many models predict a slightly
blue spectrum and must be fine-tuned to generate a red spectral index; as a
side effect, large non-Gaussianities often result.
We highlight several promising attempts to violate the NEC without
introducing dangerous instabilities at the classical and/or quantum level. If
primordial gravitational waves are observed, certain bouncing cosmologies, such
as the cyclic scenario, are in trouble, while others remain valid. We conclude
that, while most bouncing cosmologies are far from providing an alternative to
the inflationary paradigm, a handful of interesting proposals have surfaced,
which warrant further research. The constraints and lessons learned as laid out
in this review might guide future research.Comment: 60 pages, 20 figures, review articl
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
Preheating after Multi-field Inflation
In this note I study preheating after multi-field inflation to assess the
feasibility of parametric resonance. An intuitive argument for the suppression
of resonances due to dephasing of fields in generic multi-field models is
presented. This effect is absent in effective single field models, rendering
them inappropriate for the study of preheating.Comment: 4 pages, proceedings for work presented at Cargese 2008, COSMO08 and
UniverseNet second annual meetin
Non-Gaussianities and Preheating from N-flation and Magnetogenesis via rotating cosmic string loops
In this thesis we examine multi-field inflationary models of the early Universe. Since non-Gaussianities may allow for the possibility to discriminate between models of inflation, we compute deviations from a Gaussian spectrum of primordial perturbations by extending the delta-N formalism. We use N-flation as a concrete model; our findings show that these models are generically indistinguishable as long as the slow roll approximation is still valid. Besides computing non-Guassinities, we also investigate Preheating after multi-field inflation. Within the framework of N-flation, we find that preheating via parametric resonance is suppressed, an indication that it is the old theory of preheating that is applicable.
In addition to studying non-Gaussianities and preheatng in multi-field inflationary models, we study magnetogenesis in the early universe. To this aim, we propose a mechanism to generate primordial magnetic fields via rotating cosmic string loops. Magnetic fields in the micro-Gauss range have been observed in galaxies and clusters, but their origin has remained elusive. We consider a network of strings and find that rotating cosmic string loops, which are continuously produced in such networks, are viable candidates for magnetogenesis with relevant strength and length scales, provided we use a high string tension and an efficient dynamo