1,999 research outputs found
Magnetogenesis and the primordial non-gaussianity
The primordial density fluctuation inevitably couples to all forms of matter
via loop corrections and depends on the ambient conditions while inflation was
ongoing. This gives us an opportunity to observe processes which were in
progress while the universe was inflating, provided they were sufficiently
dramatic to overcome suppression by powers of (H/MP)^2 ~ 10^(-9), where H is
the Hubble scale during inflation and MP is the Planck mass. As an example, if
a primordial magnetic field was synthesized during inflation, as suggested by
some interpretations of the apparently universal 10^(-6) gauss field observed
on galactic scales, then this could leave traces in inflationary observables.
In this paper, I compute corrections to the spectrum and bispectrum generated
by a varying electromagnetic coupling during inflation, assuming that the
variation in this coupling is mediated by interaction with a collection of
light scalar fields. If the mass scale associated with this interaction is too
far below the Planck scale then the stability of perturbation theory can be
upset. For the mass-scale which is relevant in the standard magnetogenesis
scenario, however, the theory is stable and the model is apparently consistent
with observational constraints.Comment: 37 pages, uses feynmp.sty and iopart.cls. v2: minor improvements in
comparison with version submitted to (and accepted by) JCAP. Improves v1 with
a more refined discussion of cutoffs and Lorentz invariance in Sections 4-5,
but calculations are unchanged. Minor textual improvements throughou
Relics of spatial curvature in the primordial non-gaussianity
We study signatures in the Cosmic Microwave Background (CMB) induced by the
presence of strong spatial curvature prior to the epoch of inflation which
generated our present universe. If inflation does not last sufficiently long to
drive the large-scale spatial curvature to zero, then presently observable
scales may have left the horizon while spatial slices could not be approximated
by a flat, Euclidean geometry. We compute corrections to the power spectrum and
non-gaussianity of the CMB temperature anisotropy in this scenario. The power
spectrum does not receive significant corrections and is a weak diagnostic of
the presence of curvature in the initial conditions, unless its running can be
determined with high accuracy. However, the bispectral non-gaussianity
parameter f_NL receives modifications on the largest observable scales. We
estimate that the maximum signal would correspond to f_NL ~ 0.3, which is out
of reach for present-day microwave background experiments.Comment: 23 pages, uses ioplatex.sty. v2: only bibliographic change
The curvature perturbation at second order
We give an explicit relation, up to second-order terms, between scalar-field fluctuations defined on spatially-flat slices and the curvature perturbation on uniform-density slices. This expression is a necessary ingredient for calculating observable quantities at second-order and beyond in multiple-field inflation. We show that traditional cosmological perturbation theory and the `separate universe' approach yield equivalent expressions for superhorizon wavenumbers, and in particular that all nonlocal terms can be eliminated from the perturbation-theory expressions
Primordial non-gaussianities from multiple-field inflation
We calculate the three-point correlation function evaluated at horizon
crossing for a set of interacting scalar fields coupled to gravity during
inflation. This provides the initial condition for the three-point function of
the curvature perturbation in the Sasaki--Stewart \delta N formulation. We find
that the effect is small, of the order of a slow-roll parameter, and that the
non-gaussianity can be determined on large scales once the unperturbed
background evolution is known. As an example of the use of our formalism, we
calculate the primordial non-gaussianity arising in a model of assisted
inflation.Comment: 24 pages, JCAP LaTeX style; replaced with version accepted by JCAP.
Some corrections to Sections 2 and 5, conclusions unchange
Dominance of gauge artifact in the consistency relation for the primordial bispectrum
The conventional cosmological perturbation theory has been performed under
the assumption that we know the whole spatial region of the universe with
infinite volume. This is, however, not the case in the actual observations
because observable portion of the universe is limited. To give a theoretical
prediction to the observable fluctuations, gauge-invariant observables should
be composed of the information in our local observable universe with finite
volume. From this point of view, we reexamine the primordial non-Gaussianity in
single field models, focusing on the bispectrum in the squeezed limit. A
conventional prediction states that the bispectrum in this limit is related to
the power spectrum through the so-called consistency relation. However, it
turns out that, if we adopt a genuine gauge invariant variable which is
naturally composed purely of the information in our local universe, the leading
term for the bispectrum in the squeezed limit predicted by the consistency
relation vanishes.Comment: 12 pages; v2: accepted version in JCA
Transport equations for the inflationary trispectrum
We use transport techniques to calculate the trispectrum produced in
multiple-field inflationary models with canonical kinetic terms. Our method
allows the time evolution of the local trispectrum parameters, tauNL and gNL,
to be tracked throughout the inflationary phase. We illustrate our approach
using examples. We give a simplified method to calculate the superhorizon part
of the relation between field fluctuations on spatially flat hypersurfaces and
the curvature perturbation on uniform density slices, and obtain its
third-order part for the first time. We clarify how the 'backwards' formalism
of Yokoyama et al. relates to our analysis and other recent work. We supply
explicit formulae which enable each inflationary observable to be computed in
any canonical model of interest, using a suitable first-order ODE solver.Comment: 24 pages, plus references and appendix. v2: matches version published
in JCAP; typo fixed in Eq. (54
Primordial Non-Gaussianity and Gravitational Waves: Observational Tests of Brane Inflation in String Theory
We study brane inflation scenarios in a warped throat geometry and show that
there exists a consistency condition between the non-Gaussianity of the
curvature perturbation and the amplitude and scale-dependence of the primordial
gravitational waves. This condition is independent of the warping of the throat
and the form of the inflaton potential. We find that such a relation could be
tested by a future CMB polarization experiment if the Planck satellite is able
to detect both a gravitational wave background and a non-Gaussian statistic. In
models where the observable stage of inflation occurs when the brane is in the
tip region of the throat, we derive a further consistency condition involving
the scalar spectral index, the tensor-scalar ratio and the curvature
perturbation bispectrum. We show that when such a relation is combined with the
WMAP3 results, it leads to a model-independent bound on the gravitational wave
amplitude given by 0.001 < r < 0.01. This corresponds to the range of
sensitivity of the next generation of CMB polarization experiments.Comment: 7 pages, uses RevTeX4. v2, replaced with version accepted by Phys.
Rev.
Primordial non-gaussianities in single field inflation
We calculate the three-point function for primordial scalar fluctuations in a
single field inflationary scenario where the scalar field Lagrangian is a
completely general function of the field and its first derivative. We obtain an
explicit expression for the three-point correlation function in a
self-consistent approximation scheme where the expansion rate varies slowly,
analogous to the slow-roll limit in standard, single-field inflation. The
three-point function can be written in terms of the familiar slow-roll
parameters and three new parameters which measure the non-trivial kinetic
structure of the scalar field, the departure of the sound speed from the speed
of light, and the rate of change of the sound speed.Comment: 26 pages, uses iopart.cls. Updated to match version published in JCA
The δN formula is the dynamical renormalization group
We derive the 'separate universe' method for the inflationary bispectrum,
beginning directly from a field-theory calculation. We work to tree-level in
quantum effects but to all orders in the slow-roll expansion, with masses
accommodated perturbatively. Our method provides a systematic basis to account
for novel sources of time-dependence in inflationary correlation functions, and
has immediate applications. First, we use our result to obtain the correct
matching prescription between the 'quantum' and 'classical' parts of the
separate universe computation. Second, we elaborate on the application of this
method in situations where its validity is not clear. As a by-product of our
calculation we give the leading slow-roll corrections to the three-point
function of field fluctuations on spatially flat hypersurfaces in a canonical,
multiple-field model.Comment: v1: 33 pages, plus appendix and references; 5 figures. v2:
typographical typos fixed, minor changes to the main text and abstract,
reference added; matches version published in JCA
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