253 research outputs found
Effects of Nonlinear Dispersion Relations on Non-Gaussianities
We investigate the effect of non-linear dispersion relations on the
bispectrum. In particular, we study the case were the modified relations do not
violate the WKB condition at early times, focusing on a particular example
which is exactly solvable: the Jacobson-Corley dispersion relation with quartic
correction with positive coefficient to the squared linear relation. We find
that the corrections to the standard result for the bispectrum are suppressed
by a factor where is the scale where the modification
to the dispersion relation becomes relevant. The modification is {\it mildly}
configuration-dependent and equilateral configurations are more suppressed with
respect to the local ones, by a factor of one percent. There is no
configuration leading to enhancements. We then analyze the results in the
framework of particle creation using the approximate gluing method of
Brandenberger and Martin, which relates more directly to the modeling of the
trans-Planckian physics via modifications of the vacuum at a certain cutoff
scale. We show that the gluing method overestimates the leading order
correction to the spectrum and bispectrum by one and two orders, respectively,
in . We discuss the various approximation and conclude that for
dispersion relations not violating WKB at early times the particle creation is
small and does not lead to enhanced contributions to the bispectrum. We also
show that in many cases enhancements do not occur when modeling the
trans-Planckian physics via modifications of the vacuum at a certain cutoff
scale. Most notably they are only of order O(1) when the Bogolyubov
coefficients accounting for particle creation are determined by the Wronskian
condition and the minimization of the uncertainty between the field and its
conjugate momentum.Comment: v1: 11 pages, 2 figures; v2: references update
The distribution of transit durations for Kepler planet candidates and implications for their orbital eccentricities
‘In these times, during the rise in the popularity of institutional repositories, the Society does not forbid authors from depositing their work in such repositories. However, the AAS regards the deposit of scholarly work in such repositories to be a decision of the individual scholar, as long as the individual's actions respect the diligence of the journals and their reviewers.’ Original article can be found at : http://iopscience.iop.org/ Copyright American Astronomical SocietyDoppler planet searches have discovered that giant planets follow orbits with a wide range of orbital eccentricities, revolutionizing theories of planet formation. The discovery of hundreds of exoplanet candidates by NASA's Kepler mission enables astronomers to characterize the eccentricity distribution of small exoplanets. Measuring the eccentricity of individual planets is only practical in favorable cases that are amenable to complementary techniques (e.g., radial velocities, transit timing variations, occultation photometry). Yet even in the absence of individual eccentricities, it is possible to study the distribution of eccentricities based on the distribution of transit durations (relative to the maximum transit duration for a circular orbit). We analyze the transit duration distribution of Kepler planet candidates. We find that for host stars with T > 5100 K we cannot invert this to infer the eccentricity distribution at this time due to uncertainties and possible systematics in the host star densities. With this limitation in mind, we compare the observed transit duration distribution with models to rule out extreme distributions. If we assume a Rayleigh eccentricity distribution for Kepler planet candidates, then we find best fits with a mean eccentricity of 0.1-0.25 for host stars with T ≤ 5100 K. We compare the transit duration distribution for different subsets of Kepler planet candidates and discuss tentative trends with planetary radius and multiplicity. High-precision spectroscopic follow-up observations for a large sample of host stars will be required to confirm which trends are real and which are the results of systematic errors in stellar radii. Finally, we identify planet candidates that must be eccentric or have a significantly underestimated stellar radius.Peer reviewedFinal Accepted Versio
Conditions for Successful Extended Inflation
We investigate, in a model-independent way, the conditions required to obtain
a satisfactory model of extended inflation in which inflation is brought to an
end by a first-order phase transition. The constraints are that the correct
present strength of the gravitational coupling is obtained, that the present
theory of gravity is satisfactorily close to general relativity, that the
perturbation spectra from inflation are compatible with large scale structure
observations and that the bubble spectrum produced at the phase transition
doesn't conflict with the observed level of microwave background anisotropies.
We demonstrate that these constraints can be summarized in terms of the
behaviour in the conformally related Einstein frame, and can be compactly
illustrated graphically. We confirm the failure of existing models including
the original extended inflation model, and construct models, albeit rather
contrived ones, which satisfy all existing constraints.Comment: 8 pages RevTeX file with one figure incorporated (uses RevTeX and
epsf). Also available by e-mailing ARL, or by WWW at
http://star-www.maps.susx.ac.uk/papers/infcos_papers.html; Revised to include
extra references, results unchanged, to appear Phys Rev
The CMB Bispectrum
We use a separable mode expansion estimator with WMAP data to estimate the
bispectrum for all the primary families of non-Gaussian models. We review the
late-time mode expansion estimator methodology which can be applied to any
non-separable primordial and CMB bispectrum model, and we demonstrate how the
method can be used to reconstruct the CMB bispectrum from an observational map.
We extend the previous validation of the general estimator using local map
simulations. We apply the estimator to the coadded WMAP 5-year data,
reconstructing the WMAP bispectrum using multipoles and
orthonormal 3D eigenmodes. We constrain all popular nearly scale-invariant
models, ensuring that the theoretical bispectrum is well-described by a
convergent mode expansion. Constraints from the local model \fnl=54.4\pm
29.4 and the equilateral model \fnl=143.5\pm 151.2 (\Fnl = 25.1\pm 26.4)
are consistent with previously published results. (Here, we use a nonlinearity
parameter \Fnl normalised to the local case, to allow more direct comparison
between different models.) Notable new constraints from our method include
those for the constant model \Fnl = 35.1 \pm 27.4 , the flattened model \Fnl
= 35.4\pm 29.2, and warm inflation \Fnl = 10.3\pm 27.2. We investigate
feature models surveying a wide parameter range in both the scale and phase,
and we find no significant evidence of non-Gaussianity in the models surveyed.
We propose a measure \barFnl for the total integrated bispectrum and find
that the measured value is consistent with the null hypothesis that CMB
anisotropies obey Gaussian statistics. We argue that this general bispectrum
survey with the WMAP data represents the best evidence for Gaussianity to date
and we discuss future prospects, notably from the Planck satellite
RS1 Cosmology as Brane Dynamics in an AdS/Schwarzschild Bulk
We explore various facets of the cosmology of the Randall-Sundrum scenario
with two branes by considering the dynamics of the branes moving in a bulk
AdS/Schwarzschild geometry. This approach allows us both to understand in more
detail and from a different perspective the role of the stabilization of the
hierarchy in the brane cosmology, as well as to extend to the situation where
the metric contains a horizon. In particular, we explicitly determine how the
Goldberger-Wise stabilization mechanism perturbs the background bulk geometry
to produce a realistic cosmology.Comment: 9 pages, uses ReVTeX, no figure
Enhanced local-type inflationary trispectrum from a non-vacuum initial state
We compute the primordial trispectrum for curvature perturbations produced
during cosmic inflation in models with standard kinetic terms, when the initial
quantum state is not necessarily the vacuum state. The presence of initial
perturbations enhances the trispectrum amplitude for configuration in which one
of the momenta, say , is much smaller than the others, . For those squeezed configurations the trispectrum acquires the
so-called local form, with a scale dependent amplitude that can get values of
order . This amplitude can be larger than the
prediction of the so-called Maldacena consistency relation by a factor ,
and can reach the sensitivity of forthcoming observations, even for
single-field inflationary models.Comment: 11 pages, 1 figure. References added, typos corrected, minor change
N-body simulations with generic non-Gaussian initial conditions I: Power Spectrum and halo mass function
We address the issue of setting up generic non-Gaussian initial conditions
for N-body simulations. We consider inflationary-motivated primordial
non-Gaussianity where the perturbations in the Bardeen potential are given by a
dominant Gaussian part plus a non-Gaussian part specified by its bispectrum.
The approach we explore here is suitable for any bispectrum, i.e. it does not
have to be of the so-called separable or factorizable form. The procedure of
generating a non-Gaussian field with a given bispectrum (and a given power
spectrum for the Gaussian component) is not univocal, and care must be taken so
that higher-order corrections do not leave a too large signature on the power
spectrum. This is so far a limiting factor of our approach. We then run N-body
simulations for the most popular inflationary-motivated non-Gaussian shapes.
The halo mass function and the non-linear power spectrum agree with theoretical
analytical approximations proposed in the literature, even if they were so far
developed and tested only for a particular shape (the local one). We plan to
make the simulations outputs available to the community via the non-Gaussian
simulations comparison project web site
http://icc.ub.edu/~liciaverde/NGSCP.html.Comment: 23 pages, 10 figure
Signatures of very high energy physics in the squeezed limit of the bispectrum (violation of Maldacena's condition)
We investigate the signatures in the squeezed limit of the primordial scalar
bispectrum due to modifications of the standard theory at high energy. In
particular, we consider the cases of modified dispersion relations and/or
modified initial quantum state (both in the Boundary Effective Field Theory and
in the New Physics Hyper-Surface formulations). Using the in-in formalism we
study in details the squeezed limit of the contributions to the bispectrum from
all possible cubic couplings in the effective theory of single-field inflation.
We find general features such as enhancements and/or non-local shape of the
non-Gaussianities, which are relevant, for example, for measurements of the
halo bias and which distinguish these scenarios from the standard one (with
Bunch-Davies vacuum as initial state and standard kinetic terms). We find that
the signatures change according to the magnitude of the scale of new physics,
and therefore several pieces of information regarding high energy physics could
be obtained in case of detection of these signals, especially bounds on the
scales of new physics.Comment: 37 pages plus bibliography, version matching the one accepted for
publication by JCAP. Increased pedagogical comments, improved presentation
and text, added reference
Non-Gaussianity in the Cosmic Microwave Background Anisotropies at Recombination in the Squeezed limit
We estimate analytically the second-order cosmic microwave background
temperature anisotropies at the recombination epoch in the squeezed limit and
we deduce the contamination to the primordial local non-Gaussianity. We find
that the level of contamination corresponds to f_NL^{con}=O(1) which is below
the sensitivity of present experiments and smaller than the value O(5) recently
claimed in the literature.Comment: LaTeX file; 15 pages. Slightly revised version. Main result unchange
N=1 Supergravity Chaotic Inflation in the Braneworld Scenario
We study a N=1 Supergravity chaotic inflationary model, in the context of the
braneworld scenario. It is shown that successful inflation and reheating
consistent with phenomenological constraints can be achieved via the new terms
in the Friedmann equation arising from brane physics. Interestingly, the model
satisfies observational bounds with sub-Planckian field values, implying that
chaotic inflation on the brane is free from the well known difficulties
associated with the presence of higher order non-renormalizable terms in the
superpotential. A bound on the mass scale of the fifth dimension, M_5 \gsim
1.3 \times 10^{-6} M_P, is obtained from the requirement that the reheating
temperature be higher than the temperature of the electroweak phase transition.Comment: 5 pages, 1 Table, Revtex
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