3,321 research outputs found
Nongaussianity from Particle Production During Inflation
In a variety of models the motion of the inflaton may trigger the production
of some non-inflaton particles during inflation, for example via parametric
resonance or a phase transition. Such models have attracted interest recently
for a variety of reasons, including the possibility of slowing the motion of
the inflaton on a steep potential. In this review we show that interactions
between the produced particles and the inflaton condensate can lead to a
qualitatively new mechanism for generating cosmological fluctuations from
inflation. We illustrate this effect using a simple prototype model g^2
(\phi-\phi_0)^2\chi^2 for the interaction between the inflaton, \phi, and
iso-inflaton, \chi. Such interactions are quite natural in a variety of
inflation models from supersymmetry and string theory. Using both lattice field
theory simulations and analytical calculations, we study the quantum production
of \chi particles and their subsequent rescatterings off the condensate
\phi(t), which generates bremsstrahlung radiation of light inflaton
fluctuations \delta\phi. This mechanism leads to observable features in the
primordial power spectrum. We derive observational constraints on such features
and discuss their implications for popular models of inflation, including
brane/axion monodromy. Inflationary particle production also leads to a very
novel kind of nongaussian signature which may be observable in future missions.
We argue that this mechanism provides a simple and well-motivated option to
generate large nongaussianity, without fine-tuning the inflationary trajectory
or appealing to re-summation of an infinite series of high dimension operators.Comment: 53 pages, 15 figures. Invited review article, published in Advances
in Astronom
Improving PSF modelling for weak gravitational lensing using new methods in model selection
A simple theoretical framework for the description and interpretation of
spatially correlated modelling residuals is presented, and the resulting tools
are found to provide a useful aid to model selection in the context of weak
gravitational lensing. The description is focused upon the specific problem of
modelling the spatial variation of a telescope point spread function (PSF)
across the instrument field of view, a crucial stage in lensing data analysis,
but the technique may be used to rank competing models wherever data are
described empirically. As such it may, with further development, provide useful
extra information when used in combination with existing model selection
techniques such as the Akaike and Bayesian Information Criteria, or the
Bayesian evidence. Two independent diagnostic correlation functions are
described and the interpretation of these functions demonstrated using a
simulated PSF anisotropy field. The efficacy of these diagnostic functions as
an aid to the correct choice of empirical model is then demonstrated by
analyzing results for a suite of Monte Carlo simulations of random PSF fields
with varying degrees of spatial structure, and it is shown how the diagnostic
functions can be related to requirements for precision cosmic shear
measurement. The limitations of the technique, and opportunities for
improvements and applications to fields other than weak gravitational lensing,
are discussed.Comment: 18 pages, 12 figures. Modified to match version accepted for
publication in MNRA
QCSP on partially reflexive forests
We study the (non-uniform) quantified constraint satisfaction problem QCSP(H)
as H ranges over partially reflexive forests. We obtain a complexity-theoretic
dichotomy: QCSP(H) is either in NL or is NP-hard. The separating condition is
related firstly to connectivity, and thereafter to accessibility from all
vertices of H to connected reflexive subgraphs. In the case of partially
reflexive paths, we give a refinement of our dichotomy: QCSP(H) is either in NL
or is Pspace-complete
A New Formulation of the Initial Value Problem for Nonlocal Theories
There are a number of reasons to entertain the possibility that locality is
violated on microscopic scales, for example through the presence of an infinite
series of higher derivatives in the fundamental equations of motion. This type
of nonlocality leads to improved UV behaviour, novel cosmological dynamics and
is a generic prediction of string theory. On the other hand, fundamentally
nonlocal models are fraught with complications, including instabilities and
complications in setting up the initial value problem. We study the structure
of the initial value problem in an interesting class of nonlocal models. We
advocate a novel new formulation wherein the Cauchy surface is "smeared out"
over the underlying scale of nonlocality, so that the the usual notion of
initial data at t=0 is replaced with an "initial function" defined over -M^{-1}
\leq t \leq 0 where M is the underlying scale of nonlocality. Focusing on some
specific examples from string theory and cosmology, we show that this
mathematical re-formulation has surprising implications for the well-known
stability problem. For D-brane decay in a linear dilaton background, we are
able to show that the unstable directions in phase space cannot be accessed
starting from a physically sensible initial function. Previous examples of
unstable solutions in this model therefore correspond to unphysical initial
conditions, an observation which is obfuscated in the old formulation of the
initial value problem. We also discuss implication of this approach for
nonlocal cosmological models.Comment: 36 pages, 9 figures. Accepted for publication in Nuclear Physics
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