1,066 research outputs found
Non-Gaussianity of scalar perturbations generated by conformal mechanisms
We consider theories which explain the flatness of the power spectrum of
scalar perturbations in the Universe by conformal invariance, such as conformal
rolling model and Galilean Genesis. We show that to the leading {\it
non-linear} order, perturbations in all models from this class behave in one
and the same way, at least if the energy density of the relevant fields is
small compared to the total energy density (spectator approximation). We then
turn to the intrinsic non-Gaussianities in these models (as opposed to
non-Gaussianities that may be generated during subsequent evolution). The
intrinsic bispectrum vanishes, so we perform the complete calculation of the
trispectrum and compare it with the trispecta of local forms in various limits.
The most peculiar feature of our trispectrum is a (fairly mild) singularity in
the limit where two momenta are equal in absolute value and opposite in
direction (folded limit). Generically, the intrinsic non-Gaussianity can be of
detectable size.Comment: 28 pages, 5 figures. Journal version. A comment on the size of the
non-Gaussianities inserted. Misprints corrected. A reference adde
Cosmological Inflation and the Quantum Measurement Problem
According to cosmological inflation, the inhomogeneities in our universe are
of quantum mechanical origin. This scenario is phenomenologically very
appealing as it solves the puzzles of the standard hot big bang model and
naturally explains why the spectrum of cosmological perturbations is almost
scale invariant. It is also an ideal playground to discuss deep questions among
which is the quantum measurement problem in a cosmological context. Although
the large squeezing of the quantum state of the perturbations and the
phenomenon of decoherence explain many aspects of the quantum to classical
transition, it remains to understand how a specific outcome can be produced in
the early universe, in the absence of any observer. The Continuous Spontaneous
Localization (CSL) approach to quantum mechanics attempts to solve the quantum
measurement question in a general context. In this framework, the wavefunction
collapse is caused by adding new non linear and stochastic terms to the
Schroedinger equation. In this paper, we apply this theory to inflation, which
amounts to solving the CSL parametric oscillator case. We choose the
wavefunction collapse to occur on an eigenstate of the Mukhanov-Sasaki variable
and discuss the corresponding modified Schroedinger equation. Then, we compute
the power spectrum of the perturbations and show that it acquires a universal
shape with two branches, one which remains scale invariant and one with nS=4, a
spectral index in obvious contradiction with the Cosmic Microwave Background
(CMB) anisotropy observations. The requirement that the non-scale invariant
part be outside the observational window puts stringent constraints on the
parameter controlling the deviations from ordinary quantum mechanics...
(Abridged).Comment: References added, minor corrections, conclusions unchange
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
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
Testing Multi-Field Inflation: A Geometric Approach
We develop an approach for linking the power spectra, bispectrum, and
trispectrum to the geometric and kinematical features of multifield
inflationary Lagrangians. Our geometric approach can also be useful in
determining when a complicated multifield model can be well approximated by a
model with one, two, or a handful of fields. To arrive at these results, we
focus on the mode interactions in the kinematical basis, starting with the case
of no sourcing and showing that there is a series of mode conservation laws
analogous to the conservation law for the adiabatic mode in single-field
inflation. We then treat the special case of a quadratic potential with
canonical kinetic terms, showing that it produces a series of mode sourcing
relations identical in form to that for the adiabatic mode. We build on this
result to show that the mode sourcing relations for general multifield
inflation are extension of this special case but contain higher-order covariant
derivatives of the potential and corrections from the field metric. In
parallel, we show how these interactions depend on the geometry of the
inflationary Lagrangian and on the kinematics of the associated field
trajectory. Finally, we consider how the mode interactions and effective number
of fields active during inflation are reflected in the spectra and introduce a
multifield consistency relation, as well as a multifield observable that can
potentially distinguish two-field scenarios from scenarios involving three or
more effective fields.Comment: 21 pages, 4 figures + tables. Revised to clarify several points and
reorganized Section III for pedagogical reasons. Error in one equation and
typos were corrected, as well as additional references adde
Testing Two-Field Inflation
We derive semi-analytic formulae for the power spectra of two-field inflation
assuming an arbitrary potential and non-canonical kinetic terms, and we use
them both to build phenomenological intuition and to constrain classes of
two-field models using WMAP data. Using covariant formalism, we first develop a
framework for understanding the background field kinematics and introduce a
"slow-turn" approximation. Next, we find covariant expressions for the
evolution of the adiabatic/curvature and entropy/isocurvature modes, and we
discuss how the mode evolution can be inferred directly from the background
kinematics and the geometry of the field manifold. From these expressions, we
derive semi-analytic formulae for the curvature, isocurvature, and cross
spectra, and the spectral observables, all to second-order in the slow-roll and
slow-turn approximations. In tandem, we show how our covariant formalism
provides useful intuition into how the characteristics of the inflationary
Lagrangian translate into distinct features in the power spectra. In
particular, we find that key features of the power spectra can be directly read
off of the nature of the roll path, the curve the field vector rolls along with
respect to the field manifold. For example, models whose roll path makes a
sharp turn 60 e-folds before inflation ends tend to be ruled out because they
produce strong departures from scale invariance. Finally, we apply our
formalism to confront four classes of two-field models with WMAP data,
including doubly quadratic and quartic potentials and non-standard kinetic
terms, showing how whether a model is ruled out depends not only on certain
features of the inflationary Lagrangian, but also on the initial conditions.
Ultimately, models must possess the right balance of kinematical and dynamical
behaviors, which we capture in a set of functions that can be reconstructed
from spectral observables.Comment: Revised to match accepted PRD version: Improved discussion of
background kinematics and multi-field effects, added tables summarizing key
quantities and their links to observables, more detailed figures, fixed typos
in former equations (103) and (117). 49 PRD pages, 11 figure
A Radiation Bounce from the Lee-Wick Construction?
It was recently realized that matter modeled by the scalar field sector of
the Lee-Wick Standard Model yields, in the context of a homogeneous and
isotropic cosmological background, a bouncing cosmology. However, bouncing
cosmologies induced by pressure-less matter are in general unstable to the
addition of relativistic matter (i.e. radiation). Here we study the possibility
of obtaining a bouncing cosmology if we add not only radiation, but also its
Lee-Wick partner, to the matter sector. We find that, in general, no bounce
occurs. The only way to obtain a bounce is to choose initial conditions with
very special phases of the radiation field and its Lee-Wick partner.Comment: 11 page
Pitfalls of a power-law parametrization of the primordial power spectrum for primordial black hole formation
Primordial Black Holes (PBHs) can form in the radiation dominated early Universe from the collapse of large density perturbations produced by inflation. A power-law parameterisation of the primordial power spectrum is often used to extrapolate from cosmological scales, where the amplitude of the perturbations is well-measured by Cosmic Microwave Background and Large Scale Structure observations, down to the small scales on which PBHs may form. We show that this typically leads to large errors in the amplitude of the fluctuations on small scales, and hence extremely inaccurate calculations of the abundance of PBHs formed
Translational Invariance and the Anisotropy of the Cosmic Microwave Background
Primordial quantum fluctuations produced by inflation are conventionally
assumed to be statistically homogeneous, a consequence of translational
invariance. In this paper we quantify the potentially observable effects of a
small violation of translational invariance during inflation, as characterized
by the presence of a preferred point, line, or plane. We explore the imprint
such a violation would leave on the cosmic microwave background anisotropy, and
provide explicit formulas for the expected amplitudes of
the spherical-harmonic coefficients.Comment: Notation improve
Path Integral for Inflationary Perturbations
The quantum theory of cosmological perturbations in single field inflation is
formulated in terms of a path integral. Starting from a canonical formulation,
we show how the free propagators can be obtained from the well known
gauge-invariant quadratic action for scalar and tensor perturbations, and
determine the interactions to arbitrary order. This approach does not require
the explicit solution of the energy and momentum constraints, a novel feature
which simplifies the determination of the interaction vertices. The constraints
and the necessary imposition of gauge conditions is reflected in the appearance
of various commuting and anti-commuting auxiliary fields in the action. These
auxiliary fields are not propagating physical degrees of freedom but need to be
included in internal lines and loops in a diagrammatic expansion. To illustrate
the formalism we discuss the tree-level 3-point and 4-point functions of the
inflaton perturbations, reproducing the results already obtained by the methods
used in the current literature. Loop calculations are left for future work.Comment: (v1) 28 pages, no figures; (v2) 29 pages, minor changes, matches
published versio
- …