6,034 research outputs found
Inflationary cosmology of the extreme cosmic string
Starting with a study of the cosmological solution to the Einstein equations
for the internal spacetime of an extreme supermassive cosmic string kink, and
by evaluating the probability measure for the formation of such a kink in
semiclassical approximation using a minisuperspace with the appropriate
symmetry, we have found a set of arguments in favor of the claim that the
kinked extreme string can actually be regarded as a unbounded chain of pairs of
Planck- sized universes. Once one such universe pairs is created along a
primordial phase transition at the Planck scale, it undergoes an endless
process of continuous self-regeneration driven by chaotic inflation in each of
the universes forming the pair.Comment: 15 pages, RevTex, to appear in Int. J. Mod. Phys.
Inflation and Large Internal Dimensions
We consider some aspects of inflation in models with large internal
dimensions. If inflation occurs on a 3D wall after the stabilization of
internal dimensions in the models with low unification scale (M ~ 1 TeV), the
inflaton field must be extremely light. This problem may disappear In models
with intermediate (M ~10^{11} GeV) to high (M ~ 10^{16} GeV) unification scale.
However, in all of these cases the wall inflation does not provide a complete
solution to the horizon and flatness problems. To solve them, there must be a
stage of inflation in the bulk before the compactification of internal
dimensions.Comment: 4 pages, revtex, minor modification
Inflation with
We discuss various models of inflationary universe with . A
homogeneous universe with may appear due to creation of the
universe "from nothing" in the theories where the effective potential becomes
very steep at large , or in the theories where the inflaton field
nonminimally couples to gravity. Inflation with generally requires
intermediate first order phase transition with the bubble formation, and with a
second stage of inflation inside the bubble. It is possible to realize this
scenario in the context of a theory of one scalar field, but typically it
requires artificially bent effective potentials and/or nonminimal kinetic
terms. It is much easier to obtain an open universe in the models involving two
scalar fields. However, these models have their own specific problems. We
propose three different models of this type which can describe an open
homogeneous inflationary universe.Comment: 29 pages, LaTeX, parameters of one of the models are slightly
modifie
Second Order Gauge-Invariant Perturbations during Inflation
The evolution of gauge invariant second-order scalar perturbations in a
general single field inflationary scenario are presented. Different second
order gauge invariant expressions for the curvature are considered. We evaluate
perturbatively one of these second order curvature fluctuations and a second
order gauge invariant scalar field fluctuation during the slow-roll stage of a
massive chaotic inflationary scenario, taking into account the deviation from a
pure de Sitter evolution and considering only the contribution of super-Hubble
perturbations in mode-mode coupling. The spectra resulting from their
contribution to the second order quantum correlation function are nearly
scale-invariant, with additional logarithmic corrections to the first order
spectrum. For all scales of interest the amplitude of these spectra depend on
the total number of e-folds. We find, on comparing first and second order
perturbation results, an upper limit to the total number of e-folds beyond
which the two orders are comparable.Comment: 17 pages, 6 figures. Final version to appear in Phys. Rev.
Pre-Big-Bang Requires the Universe to be Exponentially Large From the Very Beginning
We show that in a generic case of the pre-big-bang scenario, inflation will
solve cosmological problems only if the universe at the onset of inflation is
extremely large and homogeneous from the very beginning. The size of a
homogeneous part of the universe at the beginning of the stage of pre-big-bang
(PBB) inflation must be greater than , where is the
stringy length. The total mass of an inflationary domain must be greater than
, where . If the universe is initially
radiation dominated, then its total entropy at that time must be greater than
. If the universe is closed, then at the moment of its formation it
must be uniform over causally disconnected domains. The natural
duration of the PBB stage in this scenario is . We argue that the
initial state of the open PBB universe could not be homogeneous because of
quantum fluctuations. Independently of the issue of homogeneity, one must
introduce two large dimensionless parameters, , and , in order to solve the flatness problem in the PBB cosmology. A regime
of eternal inflation does not occur in the PBB scenario. This should be
compared with the simplest versions of the chaotic inflation scenario, where
the regime of eternal inflation may begin in a universe of size
with vanishing initial radiation entropy, mass , and geometric entropy
O(1). We conclude that the current version of the PBB scenario cannot replace
usual inflation even if one solves the graceful exit problem in this scenario.Comment: 14 pages, a discussion of the flatness problem in the PBB cosmology
is adde
Towards the Theory of Cosmological Phase Transitions
We discuss recent progress (and controversies) in the theory of finite
temperature phase transitions. This includes the structure of the effective
potential at a finite temperature, the infrared problem in quantum statistics
of gauge fields, the theory of formation of critical and subcritical bubbles
and the theory of bubble wall propagation.Comment: 50 p
Quintessential inflation from 5D warped product spaces on a dynamical foliation
Assuming the existence of a 5D purely kinetic scalar field on the class of
warped product spaces we investigate the possibility of mimic both an
inflationary and a quintessential scenarios on 4D hypersurfaces, by
implementing a dynamical foliation on the fifth coordinate instead of a
constant one. We obtain that an induced chaotic inflationary scenario with a
geometrically induced scalar potential and an induced quasi-vacuum equation of
state on 4D dynamical hypersurfaces is possible. While on a constant foliation
the universe can be considered as matter dominated today, in a family of 4D
dynamical hypersurfaces the universe can be passing for a period of accelerated
expansion with a deceleration parameter nearly -1. This effect of the dynamical
foliation results negligible at the inflationary epoch allowing for a chaotic
scenario and becomes considerable at the present epoch allowing a
quintessential scenario.Comment: 7 pages, 1 figure Accepted for publication in Modern Physics Letters
Quantum Cosmology with Yang-Mills Fields
We examine an extension of the ideas of quantum cosmology and, in particular,
the proposal of Hartle and Hawking for the boundary conditions of the Universe,
to models which incorporate Yang-Mills fields. Inhomogeneous perturbations
about a homogeneous, isotropic minisuperspace background model are considered,
by expanding the Yang-Mills fields in harmonics of the spatial directions which
are taken to be three-spheres. The expansions are made explicit for
gauge fields thereby obtaining formulae compatible with the formalism
conventionally used in quantum cosmology. We apply these results to the gauge
group and derive the Lagrangian and the semi-classical wave function
for this special case.Comment: 33 pages, ENSLAPP-A-434/93, TUM-TH-160/93, NTUA 43/9
Causality and Cosmic Inflation
In the context of inflationary models with a pre-inflationary stage, in which
the Einstein equations are obeyed, the weak energy condition is satisfied, and
spacetime topology is trivial, we argue that homogeneity on super-Hubble scales
must be assumed as an initial condition. Models in which inflation arises from
field dynamics in a Friedman-Robertson-Walker background fall into this class
but models in which inflation originates at the Planck epoch, {\it eg.} chaotic
inflation, may evade this conclusion. Our arguments rest on causality and
general relativistic constraints on the structure of spacetime. We discuss
modifications to existing scenarios that may avoid the need for initial
large-scale homogeneity.Comment: 4 pages, 3 figures, RevTeX, expanded and sharpened discussion of
result, figures improved, references adde
The Development of Equilibrium After Preheating
We present a fully nonlinear study of the development of equilibrium after
preheating. Preheating is the exponentially rapid transfer of energy from the
nearly homogeneous inflaton field to fluctuations of other fields and/or the
inflaton itself. This rapid transfer leaves these fields in a highly nonthermal
state with energy concentrated in infrared modes. We have performed lattice
simulations of the evolution of interacting scalar fields during and after
preheating for a variety of inflationary models. We have formulated a set of
generic rules that govern the thermalization process in all of these models.
Notably, we see that once one of the fields is amplified through parametric
resonance or other mechanisms it rapidly excites other coupled fields to
exponentially large occupation numbers. These fields quickly acquire nearly
thermal spectra in the infrared, which gradually propagates into higher
momenta. Prior to the formation of total equilibrium, the excited fields group
into subsets with almost identical characteristics (e.g. group effective
temperature). The way fields form into these groups and the properties of the
groups depend on the couplings between them. We also studied the onset of chaos
after preheating by calculating the Lyapunov exponent of the scalar fields.Comment: 15 pages, 23 figure
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