22 research outputs found
Non-gaussianity of inflationary field perturbations from the field equation
We calculate the tree-level bispectrum of the inflaton field perturbation
directly from the field equations, and construct the corresponding f_NL
parameter. Our results agree with previous ones derived from the Lagrangian. We
argue that quantum theory should only be used to calculate the correlators when
they first become classical a few Hubble times after horizon exit, the
classical evolution taking over thereafter.Comment: 16 pages, uses iopart.sty. v2: replaced with version accepted by
JCAP; minor changes of wording only. v3: supersedes version published by
journal; typo fixed in Eq. (20) and updated references. v4: sign errors in
Eqs. (32) and (38) correcte
Non-Gaussianity from Symmetry
We point out that a light scalar field fluctuating around a symmetry-enhaced
point can generate large non-Gaussianity in density fluctuations. We name such
a particle as an "ungaussiton", a scalar field dominantly produced by the
quantum fluctuations,generating sizable non-Gaussianity in the density
fluctuations. We derive a consistency relation between the bispectrum and the
trispectrum, tau_NL = 10^3 f_NL^(4/3), which can be extended to arbitrary high
order correlation functions. If such a relation is confirmed by future
observations, it will strongly support this mechanism.Comment: 26 pages, 1 figure;v2 discussion and references added. To appear in
JCA
Reconstructing Single Field Inflationary Actions From CMBR Data
This paper describes a general program for deriving the action of single
field inflation models with nonstandard kinetic energy terms using CMBR power
spectrum data. This method assumes that an action depends on a set of
undetermined functions, each of which is a function of either the inflaton wave
function or its time derivative. The scalar, tensor and non-gaussianity of the
curvature perturbation spectrum are used to derive a set of reconstruction
equations whose solution set can specify up to three of the undetermined
functions. The method is then used to find the undetermined functions in
various types of actions assuming power law type scalar and tensor spectra. In
actions that contain only two unknown functions, the third reconstruction
equation implies a consistency relation between the non-gaussianty, sound speed
and slow roll parameters. In particular we focus on reconstructing a
generalized DBI action with an unknown potential and warp factor. We find that
for realistic scalar and tensor spectra, the reconstructed warp factor and
potential are very similar to the theoretically derived result. Furthermore,
physical consistency of the reconstructed warp factor and potential imposes
strict constraints on the scalar and tensor spectral indices.Comment: 33 pages, 3 figures: v3 - References adde
Non-Gaussianity from massless preheating
Preheating can convert superhorizon fluctuations of light scalar fields
present at the end of inflation into observable density perturbations. We show
in detail how lattice field theory simulations and the separate universes
approximation can be used to calculate these perturbations and make predictions
for the nonlinearity parameter fNL . We also present a simple approximation
scheme that can reproduce these results analytically. Applying these methods to
the massless preheating model, we determine the parameter values that are ruled
out by too high levels of non-Gaussianity.Comment: 26 pages, 9 figures, 1 table; erratum adde
Non-Gaussianity from isocurvature perturbations
We develop a formalism to study non-Gaussianity in both curvature and
isocurvature perturbations. It is shown that non-Gaussianity in the
isocurvature perturbation between dark matter and photons leaves distinct
signatures in the CMB temperature fluctuations, which may be confirmed in
future experiments, or possibly, even in the currently available observational
data. As an explicit example, we consider the QCD axion and show that it can
actually induce sizable non-Gaussianity for the inflationary scale, H_{inf} =
O(10^9 - 10^{11})GeV.Comment: 24 pages, 6 figures; references added; version to appear in JCA
Generation and Characterization of Large Non-Gaussianities in Single Field Inflation
Inflation driven by a single, minimally coupled, slowly rolling field
generically yields a negligible primordial non-Gaussianity. We discuss two
distinct mechanisms by which a non-trivial potential can generate large
non-Gaussianities. Firstly, if the inflaton traverses a feature in the
potential, or if the inflationary phase is short enough so that initial
transient contributions to the background dynamics have not been erased, modes
near horizon-crossing can acquire significant non-Gaussianities. Secondly,
potentials with small-scale structure may induce significant non-Gaussianities
while the relevant modes are deep inside the horizon. The first case includes
the "step" potential we previously analyzed while the second "resonance" case
is novel. We derive analytic approximations for the 3-point terms generated by
both mechanisms written as products of functions of the three individual
momenta, permitting the use of efficient analysis algorithms. Finally, we
present a significantly improved approach to regularizing and numerically
evaluating the integrals that contribute to the 3-point function.Comment: 29 pp, 8 fig
Hunting for Primordial Non-Gaussianity in the Cosmic Microwave Background
Since the first limit on the (local) primordial non-Gaussianity parameter,
fNL, was obtained from COBE data in 2002, observations of the CMB have been
playing a central role in constraining the amplitudes of various forms of
non-Gaussianity in primordial fluctuations. The current 68% limit from the
7-year WMAP data is fNL=32+/-21, and the Planck satellite is expected to reduce
the uncertainty by a factor of four in a few years from now. If fNL>>1 is found
by Planck with high statistical significance, all single-field models of
inflation would be ruled out. Moreover, if the Planck satellite finds fNL=30,
then it would be able to test a broad class of multi-field models using the
four-point function (trispectrum) test of tauNL>=(6fNL/5)^2. In this article,
we review the methods (optimal estimator), results (WMAP 7-year), and
challenges (secondary anisotropy, second-order effect, and foreground) of
measuring primordial non-Gaussianity from the CMB data, present a science case
for the trispectrum, and conclude with future prospects.Comment: 33 pages, 4 figures. Invited review, accepted for publication in the
CQG special issue on nonlinear cosmological perturbations. (v2) References
added. More clarifications are added to the second-order effect and the
multi-field consistency relation, tauNL>=(6fNL/5)^2
Volume Modulus Inflation and the Gravitino Mass Problem
The Hubble constant during the last stages of inflation in a broad class of
models based on the KKLT mechanism should be smaller than the gravitino mass, H
<~ m_{3/2}. We point out that in the models with large volume of
compactification the corresponding constraint typically is even stronger, H <~
m_{3/2}^{3/2}, in Planck units. In order to address this problem, we propose a
class of models with large volume of compactification where inflation may occur
exponentially far away from the present vacuum state. In these models, the
Hubble constant during inflation can be many orders of magnitude greater than
the gravitino mass. We introduce a toy model describing this scenario, and
discuss its strengths and weaknesses.Comment: 24 pages, JHEP style; v2. refs adde
Non-Gaussianity as a Probe of the Physics of the Primordial Universe and the Astrophysics of the Low Redshift Universe
A new and powerful probe of the origin and evolution of structures in the
Universe has emerged and been actively developed over the last decade. In the
coming decade, non-Gaussianity, i.e., the study of non-Gaussian contributions
to the correlations of cosmological fluctuations, will become an important
probe of both the early and the late Universe. Specifically, it will play a
leading role in furthering our understanding of two fundamental aspects of
cosmology and astrophysics: (i) the physics of the very early universe that
created the primordial seeds for large-scale structures, and (ii) the
subsequent growth of structures via gravitational instability and gas physics
at later times. To date, observations of fluctuations in the Cosmic Microwave
Background (CMB) and the Large-Scale Structure of the Universe (LSS) have
focused largely on the Gaussian contribution as measured by the two-point
correlations (or the power spectrum) of density fluctuations. However, an even
greater amount of information is contained in non-Gaussianity and a large
discovery space therefore still remains to be explored. Many observational
probes can be used to measure non-Gaussianity, including CMB, LSS,
gravitational lensing, Lyman-alpha forest, 21-cm fluctuations, and the
abundance of rare objects such as clusters of galaxies and high-redshift
galaxies. Not only does the study of non-Gaussianity maximize the science
return from a plethora of present and future cosmological experiments and
observations, but it also carries great potential for important discoveries in
the coming decade.Comment: 8 pages, 1 figure. Science White Paper submitted to the Cosmology and
Fundamental Physics (CFP) Science Frontier Panel of the Astro 2010 Decadal
Survey (v2,v3,v4) More co-signers and references adde