4,018 research outputs found
Large-scale anomalies in the cosmic microwave background as signatures of non-Gaussianity
We derive a general expression for the probability of observing deviations
from statistical isotropy in the cosmic microwave background (CMB) if the
primordial fluctuations are non-Gaussian and extend to superhorizon scales. The
primary motivation is to properly characterize the monopole and dipole
modulations of the primordial power spectrum that are generated by the coupling
between superhorizon and subhorizon perturbations. Unlike previous proposals
for generating the hemispherical power asymmetry, we do not assume that the
power asymmetry results from a single large superhorizon mode. Instead, we
extrapolate the observed power spectrum to superhorizon scales and compute the
power asymmetry that would result from a specific realization of non-Gaussian
perturbations on scales larger than the observable universe. Our study
encompasses many of the scenarios that have been put forward as possible
explanations for the CMB hemispherical power asymmetry. We confirm our analytic
predictions for the probability of a given power asymmetry by comparing them to
numerical realizations of CMB maps. We find that non-local models of
non-Gaussianity and scale-dependent local non-Gaussianity produce
scale-dependent modulations of the power spectrum, thereby potentially
producing both a monopolar and a dipolar power modulation on large scales. We
then provide simple examples of finding the posterior distributions for the
parameters of the bispectrum from the observed monopole and dipole modulations.Comment: 21 pages, 11 figures; v2: minor changes to match the PRD accepted
versio
Local non-Gaussianity from inflation
The non-Gaussian distribution of primordial perturbations has the potential
to reveal the physical processes at work in the very early Universe. Local
models provide a well-defined class of non-Gaussian distributions that arise
naturally from the non-linear evolution of density perturbations on
super-Hubble scales starting from Gaussian field fluctuations during inflation.
I describe the delta-N formalism used to calculate the primordial density
perturbation on large scales and then review several models for the origin of
local primordial non-Gaussianity, including the cuvaton, modulated reheating
and ekpyrotic scenarios. I include an appendix with a table of sign conventions
used in specific papers.Comment: 21 pages, 1 figure, invited review to appear in Classical and Quantum
Gravity special issue on non-linear and non-Gaussian cosmological
perturbation
Characterizing Magnetohydrodynamic Turbulence in the Small Magellanic Cloud
We investigate the nature and spatial variations of turbulence in the Small
Magellanic Cloud (SMC) by applying several statistical methods on the neutral
hydrogen (HI) column density image of the SMC and a database of isothermal
numerical simulations. By using the 3rd and 4th statistical moments we derive
the spatial distribution of the sonic Mach number (M_s) across the SMC. We find
that about 90% of the HI in the SMC is subsonic or transonic. However, edges of
the SMC `bar' have M_s=4 and may be tracing shearing or turbulent flows. Using
numerical simulations we also investigate how the slope of the spatial power
spectrum depends on both sonic and Alfven Mach numbers. This allows us to gauge
the Alfven Mach number of the SMC and conclude that its gas pressure dominates
over the magnetic pressure. The super-Alfvenic nature of the HI gas in the SMC
is also highlighted by the bispectrum, a three-point correlation function which
characterizes the level of non-Gaussianity in wave modes. We find that the
bispectrum of the SMC HI column density displays similar large-scale
correlations as numerical simulations, however it has localized enhancements of
correlations. In addition, we find a break in correlations at a scale of 160
pc. This may be caused by numerous expanding shells of a similar size
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
Classical non-Gaussianity from non-linear evolution of curvature perturbations
We study the non-linear evolution of the curvature perturbations during
matter dominated era. We show that regardless of the origin of the primordial
perturbation, the Bardeen potential and curvature receive sizable contributions
from the classical non-linear evolution effects, and quantify them exactly. On
the super-horizon scales we have squeezed peak of the bispectrum with
magnitude, in terms of the local non-linear parameters of Bardeen curvature,
1/6 < f_{NL} 19/15, and of Bardeen potential, -1/4 < f_{NL} < 7/5, depending on
the configuration of momenta. On the sub-horizon scales the bispectrum show
equilateral shape, and can serve as a potential probe of general relativity.Comment: (v1) 8 pages, 2 figures; (v2) 10 pages, more discussions and
clarifications, including explicit comparison with other studie
- …