8,430 research outputs found
Cosmological parameter inference with Bayesian statistics
Bayesian statistics and Markov Chain Monte Carlo (MCMC) algorithms have found
their place in the field of Cosmology. They have become important mathematical
and numerical tools, especially in parameter estimation and model comparison.
In this paper, we review some fundamental concepts to understand Bayesian
statistics and then introduce MCMC algorithms and samplers that allow us to
perform the parameter inference procedure. We also introduce a general
description of the standard cosmological model, known as the CDM
model, along with several alternatives, and current datasets coming from
astrophysical and cosmological observations. Finally, with the tools acquired,
we use an MCMC algorithm implemented in python to test several cosmological
models and find out the combination of parameters that best describes the
Universe.Comment: 30 pages, 17 figures, 5 tables; accepted for publication in Universe;
references adde
Gauss-Bonnet brane-world cosmology without -symmetry
We consider a single 3-brane situated between two bulk spacetimes that posses
the same cosmological constant, but whose metrics do not posses a
-symmetry. On each side of the brane, the bulk is a solution to
Gauss-Bonnet gravity. This asymmetry modifies junction conditions, and so new
terms arise in the Friedmann equation. If these terms become dominant, these
behave cosmological constant at early times for some case, and might remove the
initial singularity for other case. However, we show that these new terms can
not become dominant ones under usual conditions when our brane is outside an
event horizon. We also show that any brane-world scenarios of this type revert
to a -symmetric form at late times, and hence rule out certain proposed
scenarios.Comment: 8 pages, 3 figures; Minor typos corrected. References added. V3:
Numerical errors are corrected. Fig.1 and Fig.3 are replaced. V4: published
versio
What’s new in the YRRS? A look at new questionnaire items with a focus on sexual minority status.
Presented at: 2015 Annual Conference of the New Mexico Public Health Association; March 31-April 1, 2015; Albuquerque, NM.https://digitalrepository.unm.edu/prc-posters-presentations/1039/thumbnail.jp
Stealth Acceleration and Modified Gravity
We show how to construct consistent braneworld models which exhibit late time
acceleration. Unlike self-acceleration, which has a de Sitter vacuum state, our
models have the standard Minkowski vacuum and accelerate only in the presence
of matter, which we dub ``stealth-acceleration''. We use an effective action
for the brane which includes an induced gravity term, and allow for an
asymmetric set-up. We study the linear stability of flat brane vacua and find
the regions of parameter space where the set-up is stable. The 4-dimensional
graviton is only quasi-localised in this set-up and as a result gravity is
modified at late times. One of the two regions is strongly coupled and the
scalar mode is eaten up by an extra symmetry that arises in this limit. Having
filtered the well-defined theories we then focus on their cosmology. When the
graviton is quasi-localised we find two main examples of acceleration. In each
case, we provide an illustrative model and compare it to LambdaCDM.Comment: 32 pages, 5 figure
Braneworld holography in Gauss-Bonnet gravity
We investigate holography on an (n-1)-dimensional brane embedded in a
background of AdS black holes, in n-dimensional Gauss-Bonnet gravity. We
demonstrate that for a critical brane near the AdS boundary, the Friedmann
equation corresponds to that of the standard cosmology driven by a CFT dual to
the AdS bulk. We show that there is no holographic description for non-critical
branes, or when the brane is further away from the AdS boundary. We then derive
a Cardy-Verlinde formula for the dual CFT on the critical brane near the
boundary. This gives us insight into the remarkable correspondence between
Cardy-Verlinde formulae and Friedmann equations in Einstein gravity.Comment: 24 pages, no figures; references added, minor changes, version to
appear in CQ
First-principles investigation of 180-degree domain walls in BaTiO_3
We present a first-principles study of 180-degree ferroelectric domain walls
in tetragonal barium titanate. The theory is based on an effective Hamiltonian
that has previously been determined from first-principles
ultrasoft-pseudopotential calculations. Statistical properties are investigated
using Monte Carlo simulations. We compute the domain-wall energy, free energy,
and thickness, analyze the behavior of the ferroelectric order parameter in the
interior of the domain wall, and study its spatial fluctuations. An abrupt
reversal of the polarization is found, unlike the gradual rotation typical of
the ferromagnetic case.Comment: Revtex (preprint style, 13 pages) + 3 postscript figures. A version
in two-column article style with embedded figures is available at
http://electron.rutgers.edu/~dhv/preprints/index.html#pad_wal
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