90 research outputs found
Nongaussianity from Particle Production During Inflation
In a variety of models the motion of the inflaton may trigger the production
of some non-inflaton particles during inflation, for example via parametric
resonance or a phase transition. Such models have attracted interest recently
for a variety of reasons, including the possibility of slowing the motion of
the inflaton on a steep potential. In this review we show that interactions
between the produced particles and the inflaton condensate can lead to a
qualitatively new mechanism for generating cosmological fluctuations from
inflation. We illustrate this effect using a simple prototype model g^2
(\phi-\phi_0)^2\chi^2 for the interaction between the inflaton, \phi, and
iso-inflaton, \chi. Such interactions are quite natural in a variety of
inflation models from supersymmetry and string theory. Using both lattice field
theory simulations and analytical calculations, we study the quantum production
of \chi particles and their subsequent rescatterings off the condensate
\phi(t), which generates bremsstrahlung radiation of light inflaton
fluctuations \delta\phi. This mechanism leads to observable features in the
primordial power spectrum. We derive observational constraints on such features
and discuss their implications for popular models of inflation, including
brane/axion monodromy. Inflationary particle production also leads to a very
novel kind of nongaussian signature which may be observable in future missions.
We argue that this mechanism provides a simple and well-motivated option to
generate large nongaussianity, without fine-tuning the inflationary trajectory
or appealing to re-summation of an infinite series of high dimension operators.Comment: 53 pages, 15 figures. Invited review article, published in Advances
in Astronom
Dynamics with Infinitely Many Derivatives: Variable Coefficient Equations
Infinite order differential equations have come to play an increasingly
significant role in theoretical physics. Field theories with infinitely many
derivatives are ubiquitous in string field theory and have attracted interest
recently also from cosmologists. Crucial to any application is a firm
understanding of the mathematical structure of infinite order partial
differential equations. In our previous work we developed a formalism to study
the initial value problem for linear infinite order equations with constant
coefficients. Our approach relied on the use of a contour integral
representation for the functions under consideration. In many applications,
including the study of cosmological perturbations in nonlocal inflation, one
must solve linearized partial differential equations about some time-dependent
background. This typically leads to variable coefficient equations, in which
case the contour integral methods employed previously become inappropriate. In
this paper we develop the theory of a particular class of linear infinite order
partial differential equations with variable coefficients. Our formalism is
particularly well suited to the types of equations that arise in nonlocal
cosmological perturbation theory. As an example to illustrate our formalism we
compute the leading corrections to the scalar field perturbations in p-adic
inflation and show explicitly that these are small on large scales.Comment: 26 pages, 2 figure
Nongaussianity from Tachyonic Preheating in Hybrid Inflation
In a previous work we showed that large nongaussianities and
nonscale-invariant distortions in the CMB power spectrum can be generated in
hybrid inflation models, due to the contributions of the tachyon (waterfall)
field to the second order curvature perturbation. Here we clarify, correct, and
extend those results. We show that large nongaussianity occurs only when the
tachyon remains light throughout inflation, whereas n=4 contamination to the
spectrum is the dominant effect when the tachyon is heavy. We find constraints
on the parameters of warped-throat brane-antibrane inflation from
nongaussianity. For F-term and D-term inflation models from supergravity, we
obtain nontrivial constraints from the spectral distortion effect. We also
establish that our analysis applies to complex tachyon fields.Comment: 16 pages, 2 figures. Comments added to introductio
Signatures of anisotropic sources in the squeezed-limit bispectrum of the cosmic microwave background
The bispectrum of primordial curvature perturbations in the squeezed
configuration, in which one wavenumber, , is much smaller than the other
two, , plays a special role in constraining the physics
of inflation. In this paper we study a new phenomenological signature in the
squeezed-limit bispectrum: namely, the amplitude of the squeezed-limit
bispectrum depends on an angle between and such that
, where are the Legendre polynomials. While
is related to the usual local-form parameter as , the higher-multipole coefficients, , , etc., have not been
constrained by the data. Primordial curvature perturbations sourced by
large-scale magnetic fields generate non-vanishing , , and .
Inflation models whose action contains a term like generate
. A recently proposed "solid inflation" model generates . A cosmic-variance-limited experiment measuring temperature anisotropy of
the cosmic microwave background up to is able to measure
these coefficients down to , , and (68% CL). We also find that and , and and , are
nearly uncorrelated. Measurements of these coefficients will open up a new
window into the physics of inflation such as the existence of vector fields
during inflation or non-trivial symmetry structure of inflaton fields. Finally,
we show that the original form of the Suyama-Yamaguchi inequality does not
apply to the case involving higher-spin fields, but a generalized form does.Comment: 31 pages, 6 figures. Accepted for publication in JCA
Tachyon Defect Formation and Reheating in Brane-Antibrane Inflation
We study analytically the dynamical formation of lower dimensional branes at
the endpoint of brane-antibrane inflation through the condensation of
topological defects of the tachyon field which describes the instability of the
initial state. We then use this information to quantify the efficiency of the
reheating which is due to the coupling of time dependent tachyon background to
massless gauge fields which will be localized on the final state branes. We
improve upon previous estimates indicating that this can be an efficient
reheating mechanism for observers on the brane.Comment: 9 pages. Talk given at the 26th annual
Montreal-Rochester-Syracuse-Toronto Conference on High-Energy Physics: MRST
200
Gravity waves and non-Gaussian features from particle production in a sector gravitationally coupled to the inflaton
We study the possibility that particle production during inflation could
source observable gravity waves on scales relevant for Cosmic Microwave
Background experiments. A crucial constraint on such scenarios arises because
particle production can also source inflaton perturbations, and might ruin the
usual predictions for a nearly scale invariant spectrum of nearly Gaussian
curvature fluctuations. To minimize this effect, we consider two models of
particle production in a sector that is only gravitationally coupled to the
inflaton. For a single instantaneous burst of massive particle production, we
find that localized features in the scalar spectrum and bispectrum might be
observable, but gravitational wave signatures are unlikely to be detectable
(due to the suppressed quadrupole moment of non-relativistic quanta) without
invoking some additional effects. We also consider a model with a rolling
pseudoscalar that leads to a continuous production of relativistic gauge field
fluctuations during inflation. Here we find that gravitational waves from
particle production can actually exceed the usual inflationary vacuum
fluctuations in a regime where non-Gaussianity is consistent with observational
limits. In this model observable B-mode polarization can be obtained for any
choice of inflaton potential, and the amplitude of the signal is not
necessarily correlated with the scale of inflation
Observable non-gaussianity from gauge field production in slow roll inflation, and a challenging connection with magnetogenesis
In any realistic particle physics model of inflation, the inflaton can be
expected to couple to other fields. We consider a model with a dilaton-like
coupling between a U(1) gauge field and a scalar inflaton. We show that this
coupling can result in observable non-gaussianity, even in the conventional
regime where inflation is supported by a single scalar slowly rolling on a
smooth potential: the time dependent inflaton condensate leads to amplification
of the large-scale gauge field fluctuations, which can feed-back into the
scalar/tensor cosmological perturbations. In the squeezed limit, the resulting
bispectrum is close to the local one, but it shows a sizable and characteristic
quadrupolar dependence on the angle between the shorter and the larger modes in
the correlation. Observable non-gaussianity is obtained in a regime where
perturbation theory is under control. If the gauge field is identified with the
electromagnetic field, the model that we study is a realization of the
magnetogenesis idea originally proposed by Ratra, and widely studied. This
identification (which is not necessary for the non-gaussianity production) is
however problematic in light of a strong coupling problem already noted in the
literature.Comment: 28 pages, no figures. Final versio
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