332 research outputs found
Quantitative bispectra from multifield inflation
After simplifying and improving the non-Gaussian formalism we developed in
previous work, we derive a quantitative expression for the three-point
correlator (bispectrum) of the curvature perturbation in general multiple-field
inflation models. Our result describes the evolution of non-Gaussianity on
superhorizon scales caused by the nonlinear influence of isocurvature
perturbations on the adiabatic perturbation during inflation. We then study a
simple quadratic two-field potential and find that when slow roll breaks down
and the field trajectory changes direction in field space, the non-Gaussianity
can become large. However, for the simple models studied to date, the magnitude
of this non-Gaussianity decays away after the isocurvature mode is converted
into the adiabatic mode.Comment: 7 pages, 1 figure. v4: Added remarks on momentum dependence, minor
textual changes, matches published versio
Simple route to non-Gaussianity in inflation
We present a simple way to calculate non-Gaussianity in inflation using fully
non-linear equations on long wavelengths with stochastic sources to take into
account the short-wavelength quantum fluctuations. Our formalism includes both
scalar metric and matter perturbations, combining them into variables which are
invariant under changes of time slicing in the long-wavelength limit. We
illustrate this method with a perturbative calculation in the single-field
slow-roll case. We also introduce a convenient choice of variables to
graphically present the full momentum dependence of the three-point correlator.Comment: 6 pages, 2 figures. v2: Updated formalism to version described in
astro-ph/0504508, leading to dropping of one unnecessary approximation. Final
results not significantly changed. Extended discussion of calculation and
added graphical presentation of full momentum dependence. References
corrected and added. v3: Final version, only small textual change
Non-linear inflationary perturbations
We present a method by which cosmological perturbations can be quantitatively
studied in single and multi-field inflationary models beyond linear
perturbation theory. A non-linear generalization of the gauge-invariant
Sasaki-Mukhanov variables is used in a long-wavelength approximation. These
generalized variables remain invariant under time slicing changes on long
wavelengths. The equations they obey are relatively simple and can be
formulated for a number of time slicing choices. Initial conditions are set
after horizon crossing and the subsequent evolution is fully non-linear. We
briefly discuss how these methods can be implemented numerically in the study
of non-Gaussian signatures from specific inflationary models.Comment: 10 pages, replaced to match JCAP versio
Quantum inflaton, primordial metric perturbations and CMB fluctuations
We compute the primordial scalar, vector and tensor metric perturbations
arising from quantum field inflation. Quantum field inflation takes into
account the nonperturbative quantum dynamics of the inflaton consistently
coupled to the dynamics of the (classical) cosmological metric. For chaotic
inflation, the quantum treatment avoids the unnatural requirements of an
initial state with all the energy in the zero mode. For new inflation it allows
a consistent treatment of the explosive particle production due to spinodal
instabilities. Quantum field inflation (under conditions that are the quantum
analog of slow roll) leads, upon evolution, to the formation of a condensate
starting a regime of effective classical inflation. We compute the primordial
perturbations taking the dominant quantum effects into account. The results for
the scalar, vector and tensor primordial perturbations are expressed in terms
of the classical inflation results. For a N-component field in a O(N) symmetric
model, adiabatic fluctuations dominate while isocurvature or entropy
fluctuations are negligible. The results agree with the current WMAP
observations and predict corrections to the power spectrum in classical
inflation. Such corrections are estimated to be of the order of m^2/H^2 where m
is the inflaton mass and H the Hubble constant at horizon crossing. This turns
to be about 4% for the cosmologically relevant scales. This quantum field
treatment of inflation provides the foundations to the classical inflation and
permits to compute quantum corrections to it.Comment: LaTeX, 8 pages, no figures. To appear in the Proceedings of the ERE
2006 Meeting, Journal of Physics: Conference Serie
Cosmic Acceleration Driven by Mirage Inhomogeneities
A cosmological model based on an inhomogeneous D3-brane moving in an AdS_5 X
S_5 bulk is introduced. Although there is no special points in the bulk, the
brane Universe has a center and is isotropic around it. The model has an
accelerating expansion and its effective cosmological constant is inversely
proportional to the distance from the center, giving a possible geometrical
origin for the smallness of a present-day cosmological constant. Besides, if
our model is considered as an alternative of early time acceleration, it is
shown that the early stage accelerating phase ends in a dust dominated FRW
homogeneous Universe. Mirage-driven acceleration thus provides a dark matter
component for the brane Universe final state. We finally show that the model
fulfills the current constraints on inhomogeneities.Comment: 14 pages, 1 figure, IOP style. v2, changed style, minor corrections,
references added, version accepted in Class. Quant. Gra
Large non-Gaussianity in multiple-field inflation
We investigate non-Gaussianity in general multiple-field inflation using the
formalism we developed in earlier papers. We use a perturbative expansion of
the non-linear equations to calculate the three-point correlator of the
curvature perturbation analytically. We derive a general expression that
involves only a time integral over background and linear perturbation
quantities. We work out this expression explicitly for the two-field slow-roll
case, and find that non-Gaussianity can be orders of magnitude larger than in
the single-field case. In particular, the bispectrum divided by the square of
the power spectrum can easily be of O(1-10), depending on the model. Our result
also shows the explicit momentum dependence of the bispectrum. This conclusion
of large non-Gaussianity is confirmed in a semi-analytic slow-roll
investigation of a simple quadratic two-field model.Comment: 21 pages, 9 figures. v4: Minor textual changes to match published
version. In addition, and superseding the published version, a small error in
X and X-bar has been corrected; no significant changes to the final results.
Note that an extended (no slow roll) numerical treatment superseding section
V.D is available in astro-ph/051104
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