14 research outputs found
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
On the Physical Significance of Infra-red Corrections to Inflationary Observables
Inflationary observables, like the power spectrum, computed at one- and
higher-order loop level seem to be plagued by large infra-red corrections. In
this short note, we point out that these large infra-red corrections appear
only in quantities which are not directly observable. This is in agreement with
general expectations concerning infra-red effects.Comment: 11 pages; LateX file; 5 figures. Some coefficients in Eq.(A6)
corrected; References adde
Classical approximation to quantum cosmological correlations
We investigate up to which order quantum effects can be neglected in
calculating cosmological correlation functions after horizon exit. As a toy
model, we study theory on a de Sitter background for a massless
minimally coupled scalar field . We find that for tree level and one loop
contributions in the quantum theory, a good classical approximation can be
constructed, but for higher loop corrections this is in general not expected to
be possible. The reason is that loop corrections get non-negligible
contributions from loop momenta with magnitude up to the Hubble scale H, at
which scale classical physics is not expected to be a good approximation to the
quantum theory. An explicit calculation of the one loop correction to the two
point function, supports the argument that contributions from loop momenta of
scale are not negligible. Generalization of the arguments for the toy model
to derivative interactions and the curvature perturbation leads to the
conclusion that the leading orders of non-Gaussian effects generated after
horizon exit, can be approximated quite well by classical methods. Furthermore
we compare with a theorem by Weinberg. We find that growing loop corrections
after horizon exit are not excluded, even in single field inflation.Comment: 44 pages, 1 figure; v2: corrected errors, added references,
conclusions unchanged; v3: added section in which we compare with stochastic
approach; this version matches published versio
One-loop corrections to a scalar field during inflation
The leading quantum correction to the power spectrum of a
gravitationally-coupled light scalar field is calculated, assuming that it is
generated during a phase of single-field, slow-roll inflation.Comment: 33 pages, uses feynmp.sty and ioplatex journal style. v2: matches
version published in JCAP. v3: corrects sign error in Eq. (58). Corrects
final coefficient of the logarithm in Eq. (105). Small corrections to
discussion of divergences in 1-point function. Minor improvements to
discussion of UV behaviour in Sec. 4.
Infrared effects in inflationary correlation functions
In this article, I briefly review the status of infrared effects which occur
when using inflationary models to calculate initial conditions for a subsequent
hot, dense plasma phase. Three types of divergence have been identified in the
literature: secular, "time-dependent" logarithms, which grow with time spent
outside the horizon; "box-cutoff" logarithms, which encode a dependence on the
infrared cutoff when calculating in a finite-sized box; and "quantum"
logarithms, which depend on the ratio of a scale characterizing new physics to
the scale of whatever process is under consideration, and whose interpretation
is the same as conventional field theory. I review the calculations in which
these divergences appear, and discuss the methods which have been developed to
deal with them.Comment: Invited review for focus section of Classical & Quantum Gravity on
nonlinear and nongaussian perturbation theory. Some improvements compared to
version which will appear in CQG, especially in Sec. 2.3. 30 pages +
references
One-loop corrections to the curvature perturbation from inflation
An estimate of the one-loop correction to the power spectrum of the
primordial curvature perturbation is given, assuming it is generated during a
phase of single-field, slow-roll inflation. The loop correction splits into two
parts, which can be calculated separately: a purely quantum-mechanical
contribution which is generated from the interference among quantized field
modes around the time when they cross the horizon, and a classical contribution
which comes from integrating the effect of field modes which have already
passed far beyond the horizon. The loop correction contains logarithms which
may invalidate the use of naive perturbation theory for cosmic microwave
background (CMB) predictions when the scale associated with the CMB is
exponentially different from the scale at which the fundamental theory which
governs inflation is formulated.Comment: 28 pages, uses feynmp.sty and ioplatex journal style. v2: supersedes
version published in JCAP. Some corrections and refinements to the discussion
and conclusions. v3: Corrects misidentification of quantum correction with an
IR effect. Improvements to the discussio
Enhanced Non-Gaussianity from Excited Initial States
We use the techniques of effective field theory in an expanding universe to
examine the effect of choosing an excited inflationary initial state built over
the Bunch-Davies state on the CMB bi-spectrum. We find that even for Hadamard
states, there are unexpected enhancements in the bi-spectrum for certain
configurations in momentum space due to interactions of modes in the early
stages of inflation. These enhancements can be parametrically larger than the
standard ones and are potentially observable in current and future data. These
initial state effects have a characteristic signature in -space which
distinguishes them from the usual contributions, with the enhancement being
most pronounced for configurations corresponding to flattened triangles for
which two momenta are collinear.Comment: 33 pages, 1 figure. Refs added and minor addition
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
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
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