4,020 research outputs found
Constraints on a mixed inflaton and curvaton scenario for the generation of the curvature perturbation
We consider a supersymmetric grand unified model which naturally solves the
strong CP and mu problems via a Peccei-Quinn symmetry and leads to the standard
realization of hybrid inflation. We show that the Peccei-Quinn field of this
model can act as curvaton. In contrast to the standard curvaton hypothesis,
both the inflaton and the curvaton contribute to the total curvature
perturbation. The model predicts an isocurvature perturbation too which has
mixed correlation with the adiabatic one. The cold dark matter of the universe
is mostly constituted by axions plus a small amount of lightest sparticles. The
predictions of the model are confronted with the Wilkinson microwave anisotropy
probe and other cosmic microwave background radiation data. We analyze two
representative choices of parameters and derive bounds on the curvaton
contribution to the adiabatic perturbation. We find that, for the choice which
provides the best fitting of the data, the curvaton contribution to the
adiabatic amplitude must be smaller than about 67% (at 95% confidence level).
The best-fit power spectra are dominated by the adiabatic part of the inflaton
contribution. We use Bayesian model comparison to show that this choice of
parameters is disfavored with respect to the pure inflaton scale-invariant case
with odds of 50 to 1. For the second choice of parameters, the adiabatic mode
is dominated by the curvaton, but this choice is strongly disfavored relative
to the pure inflaton scale-invariant case (with odds of 10^7 to 1). We conclude
that in the present framework the perturbations must be dominated by the
adiabatic component from the inflaton.Comment: 27 pages including 16 figures, uses Revte
Naturalness of the relaxion mechanism
The relaxion mechanism is a novel solution to the hierarchy problem. In this
first statistical analysis of the relaxion mechanism, we quantify the relative
plausibility of a QCD and a non-QCD relaxion model versus the Standard Model
with Bayesian statistics, which includes an automatic penalty for fine-tuning.
We find that in light of the hierarchy between the weak and Planck scales,
relaxion models are favoured by colossal Bayes-factors. Constraints upon \eg
the vacuum energy during relaxation, however, shrink the Bayes-factors such
that relaxion models are only slightly favoured. Including the bounds on
shatters the plausibility of the QCD relaxion
model as it typically yields . Finally,
we augment our models with scalar-field inflation and consider measurements of
inflationary observables from BICEP/Planck. We find that, all told, the
Standard Model is favoured by huge Bayes-factors as the relaxion models require
fine-tuning such that the Hubble parameter is less than the height of the
periodic barriers. Thus, whilst we confirm that relaxion models could solve the
hierarchy problem, we find that their unconventional cosmology demolishes their
plausibility
Single-field inflation constraints from CMB and SDSS data
We present constraints on canonical single-field inflation derived from WMAP
five year, ACBAR, QUAD, BICEP data combined with the halo power spectrum from
SDSS LRG7. Models with a non-scale-invariant spectrum and a red tilt n_s < 1
are now preferred over the Harrison-Zel'dovich model (n_s = 1, tensor-to-scalar
ratio r = 0) at high significance. Assuming no running of the spectral indices,
we derive constraints on the parameters (n_s, r) and compare our results with
the predictions of simple inflationary models. The marginalised credible
intervals read n_s = 0.962^{+0.028}_{-0.026} and r < 0.17 (at 95% confidence
level). Interestingly, the 68% c.l. contours favour mainly models with a convex
potential in the observable region, but the quadratic potential model remains
inside the 95% c.l. contours. We demonstrate that these results are robust to
changes in the datasets considered and in the theoretical assumptions made. We
then consider a non-vanishing running of the spectral indices by employing
different methods, non-parametric but approximate, or parametric but exact.
With our combination of CMB and LSS data, running models are preferred over
power-law models only by a Delta chi^2 ~ 5.8, allowing inflationary stages
producing a sizable negative running -0.063^{+0.061}_{-0.049} and larger
tensor-scalar ratio r < 0.33 at the 95% c.l. This requires large values of the
third derivative of the inflaton potential within the observable range. We
derive bounds on this derivative under the assumption that the inflaton
potential can be approximated as a third order polynomial within the observable
range.Comment: 32 pages, 7 figures. v2: additional references, some typos corrected,
passed to JCAP style. v3: minor changes, matches published versio
Computational statistics using the Bayesian Inference Engine
This paper introduces the Bayesian Inference Engine (BIE), a general
parallel, optimised software package for parameter inference and model
selection. This package is motivated by the analysis needs of modern
astronomical surveys and the need to organise and reuse expensive derived data.
The BIE is the first platform for computational statistics designed explicitly
to enable Bayesian update and model comparison for astronomical problems.
Bayesian update is based on the representation of high-dimensional posterior
distributions using metric-ball-tree based kernel density estimation. Among its
algorithmic offerings, the BIE emphasises hybrid tempered MCMC schemes that
robustly sample multimodal posterior distributions in high-dimensional
parameter spaces. Moreover, the BIE is implements a full persistence or
serialisation system that stores the full byte-level image of the running
inference and previously characterised posterior distributions for later use.
Two new algorithms to compute the marginal likelihood from the posterior
distribution, developed for and implemented in the BIE, enable model comparison
for complex models and data sets. Finally, the BIE was designed to be a
collaborative platform for applying Bayesian methodology to astronomy. It
includes an extensible object-oriented and easily extended framework that
implements every aspect of the Bayesian inference. By providing a variety of
statistical algorithms for all phases of the inference problem, a scientist may
explore a variety of approaches with a single model and data implementation.
Additional technical details and download details are available from
http://www.astro.umass.edu/bie. The BIE is distributed under the GNU GPL.Comment: Resubmitted version. Additional technical details and download
details are available from http://www.astro.umass.edu/bie. The BIE is
distributed under the GNU GP
Extracting information from informal communication
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.Includes bibliographical references (leaves 89-93).This thesis focuses on the problem of extracting information from informal communication. Textual informal communication, such as e-mail, bulletin boards and blogs, has become a vast information resource. However, such information is poorly organized and difficult for a computer to understand due to lack of editing and structure. Thus, techniques which work well for formal text, such as newspaper articles, may be considered insufficient on informal text. One focus of ours is to attempt to advance the state-of-the-art for sub-problems of the information extraction task. We make contributions to the problems of named entity extraction, co-reference resolution and context tracking. We channel our efforts toward methods which are particularly applicable to informal communication. We also consider a type of information which is somewhat unique to informal communication: preferences and opinions. Individuals often expression their opinions on products and services in such communication. Others' may read these "reviews" to try to predict their own experiences. However, humans do a poor job of aggregating and generalizing large sets of data. We develop techniques that can perform the job of predicting unobserved opinions.(cont.) We address both the single-user case where information about the items is known, and the multi-user case where we can generalize opinions without external information. Experiments on large-scale rating data sets validate our approach.by Jason D.M. Rennie.Ph.D
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