64 research outputs found
Enhancing the tensor-to-scalar ratio in simple inflation
We show that in theories with a nontrivial kinetic term the contribution of
the gravitational waves to the CMB fluctuations can be substantially larger
than that is naively expected in simple inflationary models. This increase of
the tensor-to-scalar perturbation ratio leads to a larger B-component of the
CMB polarization, thus making the prospects for future detection much more
promising. The other important consequence of the considered model is a higher
energy scale of inflation and hence higher reheating temperature compared to a
simple inflation.Comment: 9 pages, 1 figure and references are added, discussion is slightly
extended, published versio
Nongaussian Isocurvature Perturbations from Inflation
We present a class of very simple inflationary models of two scalar fields
which leads to nongaussian isothermal perturbations with "blue" spectrum, n >
1. One of the models is inspired by supersymmetric theories where light scalar
fields naturally acquire masses of the order of the Hubble constant H during
inflation. Another model presumes that one of the fields has a nonminimal
interaction with gravity. By a slight modification of parameters of these
models one can obtain either gaussian isothermal perturbations, or nongaussian
adiabatic perturbations with n > 1.Comment: Few misprints are corrected; results are not changed. It is noted
that our results for the most natural range of parameters correspond to the
spectral index of isothermal perturbations n ~1.
Evolution of Cosmological Perturbations in the Universe dominated by Multiple Scalar Fields
By efforts of several authors, it is recently established that the dynamical
behavior of the cosmological perturbation on superhorizon scales is well
approximated in terms of that in the long wavelength limit, and the latter can
be constructed from the evolution of corresponding exactly homogeneous
universe. Using these facts, we investigate the evolution of the cosmological
perturbation on superhorizon scales in the universe dominated by oscillating
multiple scalar fields which are generally interacting with each other, and the
ratio of whose masses is incommensurable. Since the scalar fields oscillate
rapidly around the local minimum of the potential, we use the action angle
variables. We found that this problem can be formulated as the canonical
perturbation theory in which the perturbed part appearing as the result of the
expansion of the universe and the interaction of the scalar fields is bounded
by the negative power ot time. We show that by constructing the canonical
transformations properly, the transformed hamiltonian becomes simple enough to
be solved. As the result of the invetigation using the long wavelength limit
and the canonical perturbation theory, under the sufficiently general
conditions, we prove that for the adiabatic growing mode the Bardeen parameter
stays constant and that for all the other modes the Bardeen parameter decays.
From the viewpoint of the ergodic theory, it is discussed that as for the
Bardeen parameter, the sigularities appear probabilistically. This analysis
serves the understanding of the evolution of the cosmological perturbations on
superhorizon scales during reheating.Comment: 31 Pages; Latex, No figure
Post-Inflation Reheating in an Expanding Universe
An analytic means of studying the resonant decay of the inflaton field is
developed for the case of background expansion, . It is shown that
the parametric resonance in the inflaton's decay need not disappear when the
expansion of the universe is taken into account, although the total number of
particles produced is fewer than in the case.Comment: 18pp. Plain LaTeX; no figures. Final revised version. To appear in
Physical Review D, 15 February 199
Stationarity of Inflation and Predictions of Quantum Cosmology
We describe several different regimes which are possible in inflationary
cosmology. The simplest one is inflation without self-reproduction of the
universe. In this scenario the universe is not stationary. The second regime,
which exists in a broad class of inflationary models, is eternal inflation with
the self-reproduction of inflationary domains. In this regime local properties
of domains with a given density and given values of fields do not depend on the
time when these domains were produced. The probability distribution to find a
domain with given properties in a self-reproducing universe may or may not be
stationary, depending on the choice of an inflationary model. We give examples
of models where each of these possibilities can be realized, and discuss some
implications of our results for quantum cosmology. In particular, we propose a
new mechanism which may help solving the cosmological constant problem.Comment: 30 pages, Stanford preprint SU-ITP-94-24, LaTe
On classical anisotropies in models of Open Inflation
In the simplest model of open inflation there are two inflaton fields
decoupled from each other. One of them, the tunneling field, produces a first
stage of inflation which prepares the ground for the nucleation of a highly
symmetric bubble. The other, a free field, drives a second period of slow roll
inflation inside the bubble. However, the second field also evolves during the
first stage of inflation, which to some extent breaks the needed symmetry. We
show that this generates large supercurvature anisotropies which, together with
the results of Tanaka and Sasaki, rule out this class of simple models (unless,
of course, is sufficiently close to one.) The problem does not arise
in modified models where the second field does not evolve in the first stage of
inflation.Comment: 6 pages, LaTeX, 2 figures. Factor 3/5 in figure labels. References
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Initial condition of scalar perturbation in inflation
A formula for the power spectrum of curvature perturbations having any
initial conditions in inflation is obtained. Based on the physical conditions
before inflation, the possibility exists that the initial state of scalar
perturbations is not only the Bunch-Davies state, but also a more general state
(a squeezed state). For example, the derived formula for the power spectrum is
calculated using simple toy cosmological models. When there exists a
radiation-dominated period before inflation, the behavior of the scalar
perturbation is revealed not to vary greatly; however, from large scales to
small scales the power spectrum of the curvature perturbations oscillates
around the normal value. In addition, when inflation has a large break and the
breaking time is a radiation- dominated period, a large enhancement is revealed
to occur which depends on the length of the breaking time.Comment: 24 pages,3 figue
The Trans-Planckian Problem of Inflationary Cosmology
In most current models of inflation based on a weakly self-coupled scalar
matter field minimally coupled to gravity, the period of inflation lasts so
long that, at the beginning of the inflationary period, the physical
wavelengths of comoving scales which correspond to the present large-scale
structure of the Universe were smaller than the Planck length. Thus, the usual
computations of the spectrum of fluctuations in these models involve
extrapolating low energy physics (both in the matter and gravitational sector)
into regions where this physics is not applicable. In this paper we demonstrate
that the usual predictions of inflation for the spectrum of cosmological
fluctuations do indeed depend on the hidden assumptions about super-Planck
scale physics. We introduce a class of modified dispersion relations to mimic
possible effects of super-Planck scale physics, and show that in some cases
important deviations from the usual predictions of inflation are obtained. Some
implications of this result for the unification of fundamental physics and
early Universe cosmology are discussed.Comment: 16 pages, 2 figures. One important correction in the Corley/Jacobson
case with b_m>0 and some misprints corrected. Version published in PR
Bubble fluctuations in inflation
In the context of the open inflationary universe, we calculate the amplitude
of quantum fluctuations which deform the bubble shape. These give rise to
scalar field fluctuations in the open Friedman-Robertson-Walker universe which
is contained inside the bubble. One can transform to a new gauge in which
matter looks perfectly smooth, and then the perturbations behave as tensor
modes (gravitational waves of very long wavelength). For , where
is the density parameter, the microwave temperature anisotropies
produced by these modes are of order . Here, is the expansion rate during inflation, is
the intrinsic radius of the bubble at the time of nucleation, is the
bubble wall tension and labels the different multipoles (). The
gravitational backreaction of the bubble has been ignored. In this
approximation, , and the new effect can be much larger than the
one due to ordinary gravitational waves generated during inflation (unless, of
course, gets too close to one, in which case the new effect
disappears).Comment: 17 pages, 3 figs, LaTeX, epsfig.sty, available at
ftp://ftp.ifae.es/preprint/ft/uabft387.p
Metric Perturbations in Dilaton-Driven Inflation
We compute the spectrum of scalar and tensor metric perturbations generated,
as amplified vacuum fluctuations, during an epoch of dilaton-driven inflation
of the type occurring naturally in string cosmology. In the tensor case the
computation is straightforward while, in the scalar case, it is made delicate
by the appearance of a growing mode in the familiar longitudinal gauge. In
spite of this, a reliable perturbative calculation of perturbations far outside
the horizon can be performed by resorting either to appropriate gauge invariant
variables, or to a new coordinate system in which the growing mode can be
"gauged down". The simple outcome of this complicated analysis is that both
scalar and tensor perturbations exhibit nearly Planckian spectra, whose common
"temperature" is related to some very basic parameters of the string-cosmology
background.Comment: 34 pages, latex, no figure
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