13 research outputs found
Non-Gaussianities in N-flation
We compute non-Gaussianities in N-flation, a string motivated model of
assisted inflation with quadratic, separable potentials and masses given by the
Marcenko-Pastur distribution. After estimating parameters characterizing the
bi- and trispectrum in the horizon crossing approximation, we focus on the
non-linearity parameter , a measure of the bispectrum; we compute its
magnitude for narrow and broad spreads of masses, including the evolution of
modes after horizon crossing. We identify additional contributions due to said
evolution and show that they are suppressed as long as the fields are evolving
slowly. This renders -flation indistinguishable from simple
single-field models in this regime. Larger non-Gaussianities are expected to
arise for fields that start to evolve faster, and we suggest an analytic
technique to estimate their contribution. However, such fast roll during
inflation is not expected in N-flation, leaving (p)re-heating as the main
additional candidate for generating non-Gaussianities.Comment: 27 pages, 4 figures, extended references to match version accepted in
JCA
Duality Cascade in Brane Inflation
We show that brane inflation is very sensitive to tiny sharp features in
extra dimensions, including those in the potential and in the warp factor. This
can show up as observational signatures in the power spectrum and/or
non-Gaussianities of the cosmic microwave background radiation (CMBR). One
general example of such sharp features is a succession of small steps in a
warped throat, caused by Seiberg duality cascade using gauge/gravity duality.
We study the cosmological observational consequences of these steps in brane
inflation. Since the steps come in a series, the prediction of other steps and
their properties can be tested by future data and analysis. It is also possible
that the steps are too close to be resolved in the power spectrum, in which
case they may show up only in the non-Gaussianity of the CMB temperature
fluctuations and/or EE polarization. We study two cases. In the slow-roll
scenario where steps appear in the inflaton potential, the sensitivity of brane
inflation to the height and width of the steps is increased by several orders
of magnitude comparing to that in previously studied large field models. In the
IR DBI scenario where steps appear in the warp factor, we find that the
glitches in the power spectrum caused by these sharp features are generally
small or even unobservable, but associated distinctive non-Gaussianity can be
large. Together with its large negative running of the power spectrum index,
this scenario clearly illustrates how rich and different a brane inflationary
scenario can be when compared to generic slow-roll inflation. Such distinctive
stringy features may provide a powerful probe of superstring theory.Comment: Corrections in Eq.(5.47), Eq (5.48), Eq(5.49) and Fig
Large Non-Gaussianities in Single Field Inflation
We compute the 3-point correlation function for a general model of inflation
driven by a single, minimally coupled scalar field. Our approach is based on
the numerical evaluation of both the perturbation equations and the integrals
which contribute to the 3-point function. Consequently, we can analyze models
where the potential has a "feature", in the vicinity of which the slow roll
parameters may take on large, transient values. This introduces both scale and
shape dependent non-Gaussianities into the primordial perturbations. As an
example of our methodology, we examine the ``step'' potentials which have been
invoked to improve the fit to the glitch in the for ,
present in both the one and three year WMAP data sets. We show that for the
typical parameter values, the non-Gaussianities associated with the step are
far larger than those in standard slow roll inflation, and may even be within
reach of a next generation CMB experiment such as Planck. More generally, we
use this example to explain that while adding features to potential can improve
the fit to the 2-point function, these are generically associated with a
greatly enhanced signal at the 3-point level. Moreover, this 3-point signal
will have a very nontrivial shape and scale dependence, which is correlated
with the form of the 2-point function, and may thus lead to a consistency check
on the models of inflation with non-smooth potentials.Comment: 23 pages JHEP-style, 7 Figures. Updated with improved results.
Accepted for publication by JCA
Comparing Brane Inflation to WMAP
We compare the simplest realistic brane inflationary model to recent
cosmological data, including WMAP 3-year cosmic microwave background (CMB)
results, Sloan Digital Sky Survey luminous red galaxies (SDSS LRG) power
spectrum data and Supernovae Legacy Survey (SNLS) Type 1a supernovae distance
measures. Here, the inflaton is simply the position of a -brane which is
moving towards a -brane sitting at the bottom of a throat (a warped,
deformed conifold) in the flux compactified bulk in Type IIB string theory. The
analysis includes both the usual slow-roll scenario and the Dirac-Born-Infeld
scenario of slow but relativistic rolling. Requiring that the throat is inside
the bulk greatly restricts the allowed parameter space. We discuss possible
scenarios in which large tensor mode and/or non-Gaussianity may emerge. Here,
the properties of a large tensor mode deviate from that in the usual slow-roll
scenario, providing a possible stringy signature. Overall, within the brane
inflationary scenario, the cosmological data is providing information about the
properties of the compactification of the extra dimensions.Comment: 45 pages 11 figure
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
Single-field inflation, anomalous enhancement of superhorizon fluctuations, and non-Gaussianity in primordial black hole formation
We show a text-book potential for single-field inflation, namely, the
Coleman-Weinberg model can induce double inflation and formation of primordial
black holes (PBHs), because fluctuations that leave the horizon near the end of
first inflation are anomalously enhanced at the onset of second inflation when
the time-dependent mode turns to a growing mode rather than a decaying mode.
The mass of PBHs produced in this mechanism lies in several discrete ranges
depending on the model parameters. We also calculate the effects of
non-Gaussian statistics due to higher-order interactions on the abundance of
PBHs, which turns out to be small.Comment: 22pages, 8figure
Large Nongaussianity from Nonlocal Inflation
We study the possibility of obtaining large nongaussian signatures in the
Cosmic Microwave Background in a general class of single-field nonlocal
hill-top inflation models. We estimate the nonlinearity parameter f_{NL} which
characterizes nongaussianity in such models and show that large nongaussianity
is possible. For the recently proposed p-adic inflation model we find that
f_{NL} ~ 120 when the string coupling is order unity. We show that large
nongaussianity is also possible in a toy model with an action similar to those
which arise in string field theory.Comment: 27 pages, no figures. Added references and some clarifying remark
Predictions for Nongaussianity from Nonlocal Inflation
In our previous work the nonlinearity parameter f_NL, which characterizes
nongaussianity in the cosmic microwave background, was estimated for a class of
inflationary models based on nonlocal field theory. These models include p-adic
inflation and generically have the remarkable property that slow roll inflation
can proceed even with an extremely steep potential. Previous calculations found
that large nongaussianity is possible; however, the technical complications
associated with studying perturbations in theories with infinitely many
derivatives forced us to provide only an order of magnitude estimate for f_NL.
We reconsider the problem of computing f_NL in nonlocal inflation models,
showing that a particular choice of field basis and recent progress in
cosmological perturbation theory makes an exact computation possible. We
provide the first quantitatively accurate computation of the bispectrum in
nonlocal inflation, confirming our previous claim that it can be observably
large. We show that the shape of the bispectrum in this class of models makes
it observationally distinguishable from Dirac-Born-Infeld inflation models.Comment: 26 pages, 5 figures; references added, sign convention for f_NL
clarified, minor correction
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
Probing Inflation with CMB Polarization
International audienceWe summarize the utility of precise cosmic microwave background (CMB)polarization measurements as probes of the physics of inflation. We focus onthe prospects for using CMB measurements to differentiate various inflationarymechanisms. In particular, a detection of primordial B-mode polarization woulddemonstrate that inflation occurred at a very high energy scale, and that theinflaton traversed a super-Planckian distance in field space. We explain howsuch a detection or constraint would illuminate aspects of physics at thePlanck scale. Moreover, CMB measurements can constrain the scale-dependence andnon-Gaussianity of the primordial fluctuations and limit the possibility of asignificant isocurvature contribution. Each such limit provides crucialinformation on the underlying inflationary dynamics. Finally, we quantify theseconsiderations by presenting forecasts for the sensitivities of a futuresatellite experiment to the inflationary parameters