6,524 research outputs found
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 from Extra Dimensions
The radial mode of n extra compact dimensions (the radion, b) can cause
inflation in theories where the fundamental gravity scale, M, is smaller than
the Planck scale M_P. For radion potentials V(b) with a simple polynomial form,
to get the observed density perturbations, the energy scale of V(b) must
greatly exceed M ~ 1 TeV: V(b)^{1/4} = M_v ~ 10^{-4} M_P. This gives a large
radion mass and reheat temperature ~ 10^9 GeV, thus avoiding the moduli
problem. Such a value of M_v can be consistent with the classical treatment if
the new dimensions started sufficiently small. A new possibility is that b
approaches its stable value from above during inflation. The same conclusions
about M_v may hold even if inflation is driven by matter fields rather than by
the radion.Comment: 4 pages, 4 figures, uses epsf.te
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
Realizing Scale-invariant Density Perturbations in Low-energy Effective String Theory
We discuss the realization of inflation and resulting cosmological
perturbations in the low-energy effective string theory. In order to obtain
nearly scale-invariant spectra of density perturbations and a suppressed
tensor-to-scalar ratio, it is generally necessary that the dilaton field
is effectively decoupled from gravity together with the existence of a slowly
varying dilaton potential. We also study the effect of second-order corrections
to the tree-level action which are the sum of a Gauss-Bonnet term coupled to
and a kinetic term . We find that it is possible to
realize observationally supported spectra of scalar and tensor perturbations
provided that the correction is dominated by the term even in
the absence of the dilaton potential. When the Gauss-Bonnet term is dominant,
tensor perturbations exhibit violent negative instabilities on small-scales
about a de Sitter background in spite of the fact that scale-invariant scalar
perturbations can be achieved.Comment: 13 pages; v2: minor corrections, refs. added, version to appear in
PR
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
Coleman-Weinberg Potential In Good Agreement With WMAP
We briefly summarize and update a class of inflationary models from the early
eighties based on a quartic (Coleman-Weinberg) potential for a gauge singlet
scalar (inflaton) field. For vacuum energy scales comparable to the grand
unification scale, the scalar spectral index n_s=0.94-0.97, in very good
agreement with the WMAP three year results. The tensor to scalar ratio r<~0.14,
while alpha=dn/dlnk is =~-10^-3. An SO(10) version naturally explains the
observed baryon asymmetry via non-thermal leptogenesis.Comment: v1: 6 pages, 1 table. v2: minor corrections. v3: 8 pages, added some
details, comments, references and 3 figures. v4: minor corrections, published
versio
Comment on âStreaming potential dependence on water-content in Fontainebleau sand' by V. AllĂšgre, L. Jouniaux, F. Lehmann and P. Sailhac
AllĂšgre et al. recently presented new experimental data regarding the dependence of the streaming potential coupling coefficient with the saturation of the water phase. Such experiments are important to model the self-potential response associated with the flow of water in the vadose zone and the electroseismic/seismoelectric conversions in unsaturated porous media. However, the approach used to interpret the data is questionable and the conclusions reached by AllĂšgre et al. likely incorrec
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
Supersymmetric And Smooth Hybrid Inflation In The Light Of WMAP3
In their minimal form both supersymmetric and smooth hybrid inflation yield a
scalar spectral index n_s close to 0.98, to be contrasted with the result
n_s=0.951+0.015-0.019 from WMAP3. To realize better agreement, following
hep-ph/0604198, we extend the parameter space of these models by employing a
non-minimal Kaehler potential. We also discuss non-thermal leptogenesis by
inflaton decay and obtain new bounds in these models on the reheat temperature
to explain the observed baryon asymmetry.Comment: 21 pages, 12 figures, revtex4. v2: minor revisions, additional
references, to appear in PR
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
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