50 research outputs found
Non-Gaussianity from Symmetry
We point out that a light scalar field fluctuating around a symmetry-enhaced
point can generate large non-Gaussianity in density fluctuations. We name such
a particle as an "ungaussiton", a scalar field dominantly produced by the
quantum fluctuations,generating sizable non-Gaussianity in the density
fluctuations. We derive a consistency relation between the bispectrum and the
trispectrum, tau_NL = 10^3 f_NL^(4/3), which can be extended to arbitrary high
order correlation functions. If such a relation is confirmed by future
observations, it will strongly support this mechanism.Comment: 26 pages, 1 figure;v2 discussion and references added. To appear in
JCA
Non-Gaussianity from isocurvature perturbations
We develop a formalism to study non-Gaussianity in both curvature and
isocurvature perturbations. It is shown that non-Gaussianity in the
isocurvature perturbation between dark matter and photons leaves distinct
signatures in the CMB temperature fluctuations, which may be confirmed in
future experiments, or possibly, even in the currently available observational
data. As an explicit example, we consider the QCD axion and show that it can
actually induce sizable non-Gaussianity for the inflationary scale, H_{inf} =
O(10^9 - 10^{11})GeV.Comment: 24 pages, 6 figures; references added; version to appear in JCA
Higher Order Corrections to the Primordial Gravitational Wave Spectrum and its Impact on Parameter Estimates for Inflation
We study the impact of the use of the power series expression for the
primordial tensor spectrum on parameter estimation from future direct detection
gravitational wave experiments. The spectrum approximated by the power series
expansion may give large deviation from the true (fiducial) value when it is
normalized at CMB scale because of the large separation between CMB and direct
detection scales. We derive the coefficients of the higher order terms of the
expansion up to the sixth order within the framework of the slow-roll
approximation and investigate how well the inclusion of higher order terms
improves the analytic prediction of the spectrum amplitude by comparing with
numerical results. Using the power series expression, we consider future
constraints on inflationary parameters expected from direct detection
experiments of the inflationary gravitational wave background and show that the
truncation of the higher order terms can lead to incorrect evaluation of the
parameters. We present two example models; a quadratic chaotic inflation model
and mixed inflaton and curvaton model with a quartic inflaton potential.Comment: 25 pages, 7 figures, revised version accepted by JCA
Non-linear isocurvature perturbations and non-Gaussianities
We study non-linear primordial adiabatic and isocurvature perturbations and
their non-Gaussianity. After giving a general formulation in the context of an
extended delta N-formalism, we analyse in detail two illustrative examples. The
first is a mixed curvaton-inflaton scenario in which fluctuations of both the
inflaton and a curvaton (a light isocurvature field during inflation)
contribute to the primordial density perturbation. The second example is that
of double inflation involving two decoupled massive scalar fields during
inflation. In the mixed curvaton-inflaton scenario we find that the bispectrum
of primordial isocurvature perturbations may be large and comparable to the
bispectrum of adiabatic curvature perturbations.Comment: 24 pages, typos corrected, references adde
Exact Wave Propagation in a Spacetime with a Cosmic String
We present exact solutions of the massless Klein-Gordon equation in a
spacetime in which an infinite straight cosmic string resides. The first
solution represents a plane wave entering perpendicular to the string
direction. We also present and analyze a solution with a static point-like
source. In the short wavelength limit these solutions approach the results
obtained by using the geometrical optics approximation: magnification occurs if
the observer lies in front of the string within a strip of angular width , where is the string tension. We find that when the distance from
the observer to the string is less than , where is the
wave length, the magnification is significantly reduced compared with the
estimate based on the geometrical optics due to the diffraction effect. For
gravitational waves from neutron star(NS)-NS mergers the several lensing events
per year may be detected by DECIGO/BBO.Comment: 15 pages, 8 figures, reference adde
Curvaton Dynamics and the Non-Linearity Parameters in Curvaton Model
We investigate the curvaton dynamics and the non-linearity parameters in
curvaton model with potential slightly deviating from the quadratic form in
detail. The non-linearity parameter will show up due to the curvaton
self-interaction. We also point out that the leading order of non-quadratic
term in the curvaton potential can be negative, for example in the axion-type
curvaton model. If a large positive is detected, the axion-type
curvaton model will be preferred.Comment: 14 pages, 4 figures; refs adde
Supergravity based inflation models: a review
In this review, we discuss inflation models based on supergravity. After
explaining the difficulties in realizing inflation in the context of
supergravity, we show how to evade such difficulties. Depending on types of
inflation, we give concrete examples, particularly paying attention to chaotic
inflation because the ongoing experiments like Planck might detect the tensor
perturbations in near future. We also discuss inflation models in Jordan frame
supergravity, motivated by Higgs inflation.Comment: 30 pages, invited review for Classical and Quantum Gravity, published
versio
Strongly scale-dependent polyspectra from curvaton self-interactions
We study the scale dependence of the non-linearity parameters f_NL and g_NL
in curvaton models with self-interactions. We show that the spectral indices
n_fNL=d ln|f_NL|/(d ln k) and n_gNL=d ln |g_NL|/(d ln k) can take values much
greater than the slow--roll parameters and the spectral index of the power
spectrum. This means that the scale--dependence of the bi and trispectrum could
be easily observable in this scenario with Planck, which would lead to tight
additional constraints on the model. Inspite of the highly non-trivial
behaviour of f_NL and g_NL in the curvaton models with self-interactions, we
find that the model can be falsified if g_NL(k) is also observed.Comment: 19 pages, many figures. v2: Figure 4 replaced with a corrected
normalisation, conclusions unchanged. Matches version published in JCA
Primordial Non-Gaussianities of General Multiple Field Inflation
We perform a general study of the primordial scalar non-Gaussianities in
multi-field inflationary models in Einstein gravity. We consider models
governed by a Lagrangian which is a general function of the scalar fields and
their first spacetime derivatives. We use formalism to relate scalar
fields and curvature perturbations. We calculate the explicit cubic order
perturbation action and the three-point function of curvature perturbation
evaluated at horizon-crossing. Under reasonable assumptions, in the limit of
small slow-varying parameters and a sound speed close to one, we find
that the non-Gaussianity is completely determined by these slow-varying
parameters and some other parameters determined by the structure of the
inflationary models. Our work generalizes previous results, and would be useful
to study non-Gaussianity in multi-field inflationary models that will be
constructed in the future.Comment: 26 pages, no figure; v2, minor revision; v3 minor misprints
corrected; v4 minor misprints correcte
Curvaton with Polynomial Potential
In general a weakly self-interacting curvaton field is expected and the
curvaton potential takes the polynomial form. The curvaton potential can be
dominated by the self-interaction term during the period of inflation if the
curvaton field stays at a large vacuum expectation value. We use the formalism to calculate the primordial curvature perturbation in the
various possible scenarios which make the curvaton model much richer.Comment: 18 pages; a mistake in sec.2.2.2 corrected, enhancement of
in the mixed scenarion discussed, version for publication in JCA