43 research outputs found
Rapid roll Inflation with Conformal Coupling
Usual inflation is realized with a slow rolling scalar field minimally
coupled to gravity. In contrast, we consider dynamics of a scalar with a flat
effective potential, conformally coupled to gravity. Surprisingly, it contains
an attractor inflationary solution with the rapidly rolling inflaton field. We
discuss models with the conformal inflaton with a flat potential (including
hybrid inflation). There is no generation of cosmological fluctuations from the
conformally coupled inflaton. We consider realizations of modulated
(inhomogeneous reheating) or curvaton cosmological fluctuations in these
models. We also implement these unusual features for the popular
string-theoretic warped inflationary scenario, based on the interacting D3-anti
D3 branes. The original warped brane inflation suffers a large inflaton mass
due to conformal coupling to 4-dimensional gravity. Instead of considering this
as a problem and trying to cure it with extra engineering, we show that warped
inflation with the conformally coupled, rapidly rolling inflaton is yet
possible with N=37 efoldings, which requires low energy scales 1-100 TeV of
inflation. Coincidentally, the same warping numerology can be responsible for
the hierarchy. It is shown that the scalars associated with angular isometries
of the warped geometry of compact manifold (e.g. S^3 of KS geometry) have
solutions identical to conformally coupled modes and also cannot be responsible
for cosmological fluctuations. We discuss other possibilities.Comment: 15 pages, version accepted for publication in PR
Equation of state and Beginning of Thermalization After Preheating
We study the out-of-equilibrium nonlinear dynamics of fields after
post-inflationary preheating. During preheating, the energy in the homogeneous
inflaton is exponentially rapidly transfered into highly occupied
out-of-equilibrium inhomogeneous modes, which subsequently evolve towards
equilibrium. The infrared modes excited during preheating evolve towards a
saturated distribution long before thermalization completes. We compute the
equation of state during and immediately after preheating. It rapidly evolves
towards radiation domination long before the actual thermal equilibrium is
established. The exact time of this transition is a non-monotonic function of
the coupling between the inflaton and the decay products, and it varies only
very weakly (around 10^(-35) s) as this coupling changes over several orders of
magnitude. This result is applied to refine the relation between the number of
efoldings N and the physical wavelength of perturbations generated during
inflation. We also discuss the implications for the theory of modulated
perturbations from preheating. We finally argue that many questions of the
thermal history of the universe should be addressed in terms of
pre-thermalization, illustrating this point with a calculation of perturbative
production of gravitinos immediately after chaotic inflation. We also highlight
the effects of three-legs inflaton interactions on the dynamics of preheating
and thermalization in an expanding universe.Comment: 15 pages, 13 figure
Exact identification of the radion and its coupling to the observable sector
Braneworld models in extra dimensions can be tested in laboratory by the
coupling of the radion to the Standard Model fields. The identification of the
radion as a canonically normalized field involves a careful General Relativity
treatment: if a bulk scalar is responsible for the stabilization of the system,
its fluctuations are entangled with the perturbations of the metric and they
also have to be taken into account (similarly to the well-developed theory of
scalar metric perturbations in 4D cosmology with a scalar field). Extracting a
proper dynamical variable in a warped geometry/scalar setting is a nontrivial
task, performed so far only in the limit of negligible backreaction of the
scalar field on the background geometry. We perform the general calculation,
diagonalizing the action up to second order in the perturbations and
identifying the physical eigenmodes of the system for any amplitude of the bulk
scalar. This computation allows us to derive a very simple expression for the
exact coupling of the eigenmodes to the Standard Model fields on the brane,
valid for an arbitrary background configuration. As an application, we discuss
the Goldberger-Wise mechanism for the stabilization of the radion in the
Randall-Sundrum type models. The existing studies, limited to small amplitude
of the bulk scalar field, are characterized by a radion mass which is
significantly below the physical scale at the observable brane. We extend them
beyond the small backreaction regime. For intermediate amplitudes, the radion
mass approaches the electroweak scale, while its coupling to the observable
brane remains nearly constant. At very high amplitudes, the radion mass instead
decreases, while the coupling sharply increases. Severe experimental
constraints are expected in this regime.Comment: 20 pages, 6 figure
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
Reheating after Inflation
The theory of reheating of the Universe after inflation is developed. The
transition from inflation to the hot Universe turns out to be strongly
model-dependent and typically consists of several stages. Immediately after
inflation the field begins rapidly rolling towards the minimum of its
effective potential. Contrary to some earlier expectations, particle production
during this stage does not lead to the appearance of an extra friction term
in the equation of motion of the field . Reheating
becomes efficient only at the next stage, when the field rapidly
oscillates near the minimum of its effective potential. We have found that
typically in the beginning of this stage the classical inflaton field
very rapidly (explosively) decays into -particles or into other bosons
due to broad parametric resonance. This stage cannot be described by the
standard elementary approach to reheating based on perturbation theory. The
bosons produced at this stage, as well as some part of the classical field
which survives the stage of explosive reheating, should further decay
into other particles, which eventually become thermalized. The last stages of
decay can be described in terms of perturbation theory. Complete reheating is
possible only in those theories where a single massive -particle can
decay into other particles. This imposes strong constraints on the structure of
inflationary models. On the other hand, this means that a scalar field can be a
cold dark matter candidate even if it is strongly coupled to other fields.Comment: 7 pages, 1 figure, LaTeX, UH-IfA-94/35; SU-ITP-94-13; YITP/U-94-15
(paper replaced by its version to be published in Phys. Rev. Lett.
Inhomogeneous Fragmentation of the Rolling Tachyon
Dirac-Born-Infeld type effective actions reproduce many aspects of string
theory classical tachyon dynamics of unstable Dp-branes. The inhomogeneous
tachyon field rolling from the top of its potential forms topological defects
of lower codimensions. In between them, as we show, the tachyon energy density
fragments into a p-dimensional web-like high density network evolving with
time. We present an analytic asymptotic series solution of the non-linear
equations for the inhomogeneous tachyon and its stress energy. The generic
solution for a tachyon field with a runaway potential in arbitrary dimensions
is described by the free streaming of noninteracting massive particles whose
initial velocities are defined by the gradients of the initial tachyon profile.
Thus, relativistic particle mechanics is a dual picture of the tachyon field
effective action. Implications of this picture for inflationary models with a
decaying tachyon field are discussed.Comment: 10 pages, 1 figur
On the Theory of Fermionic Preheating
In inflationary cosmology, the particles constituting the Universe are
created after inflation due to their interaction with moving inflaton field(s)
in the process of preheating. In the fermionic sector, the leading channel is
out-of equilibrium particle production in the non-perturbative regime of
parametric excitation, which respects Pauli blocking but differs significantly
from the perturbative expectation. We develop theory of fermionic preheating
coupling to the inflaton, without and with expansion of the universe, for light
and massive fermions, to calculate analytically the occupation number of
created fermions, focusing on their spectra and time evolution. In the case of
large resonant parameter we extend for rermions the method of successive
parabolic scattering, earlier developed for bosonic preheating. In an expanding
universe parametric excitation of fermions is stochastic. Created fermions very
quickly, within tens of inflaton oscillations, fill up a sphere of radius
in monetum space. We extend our formalism to the production of
superheavy fermions and to `instant' fermion creation.Comment: 14 pages, latex, 12 figures, submitted for publicatio
Dynamics of Symmetry Breaking and Tachyonic Preheating
We reconsider the old problem of the dynamics of spontaneous symmetry
breaking using 3d lattice simulations, and develop a theory of tachyonic
preheating, which occurs due to the spinodal instability of the scalar field.
Tachyonic preheating is so efficient that symmetry breaking typically completes
within a single oscillation of the field distribution as it rolls towards the
minimum of its effective potential. As an application of this theory we
consider preheating in the hybrid inflation scenario, including SUSY-motivated
F-term and D-term inflationary models. We show that preheating in hybrid
inflation is typically tachyonic and the stage of oscillations of a homogeneous
component of the scalar fields driving inflation ends after a single
oscillation. Our results may also be relevant for the theory of the formation
of disoriented chiral condensates in heavy ion collisions.Comment: 7 pages, 6 figures. Higher quality figures and computer generated
movies in gif format illustrating our results can be found at
http://physics.stanford.edu/gfelder/hybri
Theory and Numerics of Gravitational Waves from Preheating after Inflation
Preheating after inflation involves large, time-dependent field
inhomogeneities, which act as a classical source of gravitational radiation.
The resulting spectrum might be probed by direct detection experiments if
inflation occurs at a low enough energy scale. In this paper, we develop a
theory and algorithm to calculate, analytically and numerically, the spectrum
of energy density in gravitational waves produced from an inhomogeneous
background of stochastic scalar fields in an expanding universe. We derive some
generic analytical results for the emission of gravity waves by stochastic
media of random fields, which can test the validity/accuracy of numerical
calculations. We contrast our method with other numerical methods in the
literature, and then we apply it to preheating after chaotic inflation. In this
case, we are able to check analytically our numerical results, which differ
significantly from previous works. We discuss how the gravity wave spectrum
builds up with time and find that the amplitude and the frequency of its peak
depend in a relatively simple way on the characteristic spatial scale amplified
during preheating. We then estimate the peak frequency and amplitude of the
spectrum produced in two models of preheating after hybrid inflation, which for
some parameters may be relevant for gravity wave interferometric experiments.Comment: 28 pages, 10 figures, refs added, published versio