492 research outputs found
A Naturally Large Four-Point Function in Single Field Inflation
Non-Gaussianities of the primordial density perturbations have emerged as a
very powerful possible signal to test the dynamics that drove the period of
inflation. While in general the most sensitive observable is the three-point
function in this paper we show that there are technically natural inflationary
models where the leading source of non-Gaussianity is the four-point function.
Using the recently developed Effective Field Theory of Inflation, we are able
to show that it is possible to impose an approximate parity symmetry and an
approximate continuos shift symmetry on the inflaton fluctuations that allow,
when the dispersion relation is of the form , for a unique
quartic operator, while approximately forbidding all the cubic ones. The
resulting shape for the four-point function is unique. In the models where the
dispersion relation is of the form a similar construction
can be carried out and additional shapes are possible.Comment: 13 pages, 1 figure. v2: extended discussion on near-de-Sitter model
Brane-Antibrane Backreaction in Axion Monodromy Inflation
We calculate the interaction potential between D5 and anti-D5 branes wrapping
distant but homologous 2-cycles. The interaction potential is logarithmic in
the separation radius and does not decouple at infinity. We show that
logarithmic backreaction is generic for 5-branes wrapping distant but
homologous 2-cycles, and we argue that this destabilises models of axion
monodromy inflation involving NS5 brane-antibrane pairs in separate warped
throats towards an uncontrolled region.Comment: 12 page
Oscillations in the bispectrum
There exist several models of inflation that produce primordial bispectra
that contain a large number of oscillations. In this paper we discuss these
models, and aim at finding a method of detecting such bispectra in the data. We
explain how the recently proposed method of mode expansion of bispectra might
be able to reconstruct these spectra from separable basis functions. Extracting
these basis functions from the data might then lead to observational
constraints on these models.Comment: 6 pages, 2 figures, submitted to JOP: Conference Series, PASCOS 201
Constraining holographic inflation with WMAP
In a class of recently proposed models, the early universe is strongly
coupled and described holographically by a three-dimensional, weakly coupled,
super-renormalizable quantum field theory. This scenario leads to a power
spectrum of scalar perturbations that differs from the usual empirical LCDM
form and the predictions of generic models of single field, slow roll
inflation. This spectrum is characterized by two parameters: an amplitude, and
a parameter g related to the coupling constant of the dual theory. We estimate
these parameters, using WMAP and other astrophysical data. We compute Bayesian
evidence for both the holographic model and standard LCDM and find that their
difference is not significant, although LCDM provides a somewhat better fit to
the data. However, it appears that Planck will permit a definitive test of this
holographic scenario.Comment: 24 pages, 9 figs, published versio
(Small) Resonant non-Gaussianities: Signatures of a Discrete Shift Symmetry in the Effective Field Theory of Inflation
We apply the Effective Field Theory of Inflation to study the case where the
continuous shift symmetry of the Goldstone boson \pi is softly broken to a
discrete subgroup. This case includes and generalizes recently proposed String
Theory inspired models of Inflation based on Axion Monodromy. The models we
study have the property that the 2-point function oscillates as a function of
the wavenumber, leading to oscillations in the CMB power spectrum. The
non-linear realization of time diffeomorphisms induces some self-interactions
for the Goldstone boson that lead to a peculiar non-Gaussianity whose shape
oscillates as a function of the wavenumber. We find that in the regime of
validity of the effective theory, the oscillatory signal contained in the
n-point correlation functions, with n>2, is smaller than the one contained in
the 2-point function, implying that the signature of oscillations, if ever
detected, will be easier to find first in the 2-point function, and only then
in the higher order correlation functions. Still the signal contained in
higher-order correlation functions, that we study here in generality, could be
detected at a subleading level, providing a very compelling consistency check
for an approximate discrete shift symmetry being realized during inflation.Comment: v2 minor revisions; 39 pages, 5 figure
Subleading effects and the field range in axion inflation
An attractive candidate for the inflaton is an axion slowly rolling down a
flat potential protected by a perturbative shift symmetry. Realisations of this
idea within large field, natural and monomial inflation have been disfavoured
by observations and are difficult to embed in string theory. We show that
subleading, but significant non-perturbative corrections can superimpose sharp
cliffs and gentle plateaus into the potential, whose overall effect is to
enhance the number of e-folds of inflation. Sufficient e-folds are therefore
achieved for smaller field ranges compared to the potential without such
corrections. Thus, both single-field natural and monomial inflation in UV
complete theories like string theory, can be restored into the favour of
current observations, with distinctive signatures. Tensor modes result
un-observably small, but there is a large negative running of the spectral
index. Remarkably, natural inflation can be achieved with a single field whose
axion decay constant is sub-Planckian.Comment: 18 pages, 15 figures; v2 references improve
On Slow-roll Moduli Inflation in Massive IIA Supergravity with Metric Fluxes
We derive several no-go theorems in the context of massive type IIA string
theory compactified to four dimensions in a way that, in the absence of fluxes,
preserves N=1 supersymmetry. Our derivation is based on the dilaton, Kaehler
and complex structure moduli dependence of the potential of the
four-dimensional effective field theory, that is generated by the presence of
D6-branes, O6-planes, RR-fluxes, NSNS 3-form flux, and geometric fluxes. To
demonstrate the usefulness of our theorems, we apply them to the most commonly
studied class of toroidal orientifolds. We show that for all but two of the
models in this class the slow-roll parameter \epsilon is bounded from below by
numbers of order unity as long as the fluxes satisfy the Bianchi identities,
ruling out slow-roll inflation and even the existence of de Sitter extrema in
these models. For the two cases that avoid the no-go theorems, we provide some
details of our numerical studies, demonstrating that small \epsilon can indeed
be achieved. We stress that there seems to be an \eta-problem, however,
suggesting that none of the models in this class are viable from a cosmological
point of view at least at large volume, small string coupling, and leading
order in the \alpha'-expansion.Comment: 34 pages, v3: summary table added, comments added, accepted for
publication in PR
Stability Constraints on Classical de Sitter Vacua
We present further no-go theorems for classical de Sitter vacua in Type II
string theory, i.e., de Sitter constructions that do not invoke
non-perturbative effects or explicit supersymmetry breaking localized sources.
By analyzing the stability of the 4D potential arising from compactification on
manfiolds with curvature, fluxes, and orientifold planes, we found that
additional ingredients, beyond the minimal ones presented so far, are necessary
to avoid the presence of unstable modes. We enumerate the minimal setups for
(meta)stable de Sitter vacua to arise in this context.Comment: 18 pages; v2: argument improved, references adde
BINGO: A code for the efficient computation of the scalar bi-spectrum
We present a new and accurate Fortran code, the BI-spectra and
Non-Gaussianity Operator (BINGO), for the efficient numerical computation of
the scalar bi-spectrum and the non-Gaussianity parameter f_{NL} in single field
inflationary models involving the canonical scalar field. The code can
calculate all the different contributions to the bi-spectrum and the parameter
f_{NL} for an arbitrary triangular configuration of the wavevectors. Focusing
firstly on the equilateral limit, we illustrate the accuracy of BINGO by
comparing the results from the code with the spectral dependence of the
bi-spectrum expected in power law inflation. Then, considering an arbitrary
triangular configuration, we contrast the numerical results with the analytical
expression available in the slow roll limit, for, say, the case of the
conventional quadratic potential. Considering a non-trivial scenario involving
deviations from slow roll, we compare the results from the code with the
analytical results that have recently been obtained in the case of the
Starobinsky model in the equilateral limit. As an immediate application, we
utilize BINGO to examine of the power of the non-Gaussianity parameter f_{NL}
to discriminate between various inflationary models that admit departures from
slow roll and lead to similar features in the scalar power spectrum. We close
with a summary and discussion on the implications of the results we obtain.Comment: v1: 5 pages, 5 figures; v2: 35 pages, 11 figures, title changed,
extensively revised; v3: 36 pages, 11 figures, to appear in JCAP. The BINGO
code is available online at
http://www.physics.iitm.ac.in/~sriram/bingo/bingo.htm
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