16,047 research outputs found
Quasar H II Regions During Cosmic Reionization
Cosmic reionization progresses as HII regions form around sources of ionizing
radiation. Their average size grows continuously until they percolate and
complete reionization. We demonstrate how this typical growth can be calculated
around the largest, biased sources of UV emission, such as quasars, by further
developing an analytical model based on the excursion set formalism. This
approach allows us to calculate the sizes and growth of the HII regions created
by the progenitors of any dark matter halo of given mass and redshift with a
minimum of free parameters. Statistical variations in the size of these
pre-existing HII regions are an additional source of uncertainty in the
determination of very high redshift quasar properties from their observed HII
region sizes. We use this model to demonstrate that the transmission gaps seen
in very high redshift quasars can be understood from the radiation of only
their progenitors and associated clustered small galaxies. The fit sets a lower
limit on the redshift of overlap at z = 5.8 +/- 0.1. This interpretation makes
the transmission gaps independent of the age of the quasars observed. If this
interpretation were correct it would raise the prospects of using radio
interferometers currently under construction to detect the epoch of
reionization.Comment: 6 pages, 3 figures, accepted by MNRAS, revised to match published
versio
Quasi-Fixed Points and Charge and Colour Breaking in Low Scale Models
We show that the current LEP2 lower bound upon the minimal supersymmetric
standard model (MSSM) lightest Higgs mass rules out quasi-fixed scenarios for
string scales between 10^6 and 10^{11} GeV unless the heaviest stop mass is
more than 2 TeV. We consider the implications of the low string scale for
charge and colour breaking (CCB) bounds in the MSSM, and demonstrate that CCB
bounds from F and D-flat directions are significantly weakened. For scales less
than 10^{10} GeV these bounds become merely that degenerate scalar mass squared
values are positive at the string scale.Comment: 17 pages, 4 figures. Replacement has added discussion on errors due
to alpha_s(MZ) errors, as well as deviations from the quasi-fixed point. Text
has been clarifie
Front propagation in laminar flows
The problem of front propagation in flowing media is addressed for laminar
velocity fields in two dimensions. Three representative cases are discussed:
stationary cellular flow, stationary shear flow, and percolating flow.
Production terms of Fisher-Kolmogorov-Petrovskii-Piskunov type and of Arrhenius
type are considered under the assumption of no feedback of the concentration on
the velocity. Numerical simulations of advection-reaction-diffusion equations
have been performed by an algorithm based on discrete-time maps. The results
show a generic enhancement of the speed of front propagation by the underlying
flow. For small molecular diffusivity, the front speed depends on the
typical flow velocity as a power law with an exponent depending on the
topological properties of the flow, and on the ratio of reactive and advective
time-scales. For open-streamline flows we find always , whereas for
cellular flows we observe for fast advection, and for slow advection.Comment: Enlarged, revised version, 37 pages, 14 figure
Brane Gases on K3 and Calabi-Yau Manifolds
We initiate the study of Brane Gas Cosmology (BGC) on manifolds with
non-trivial holonomy. Such compactifications are required within the context of
superstring theory in order to make connections with realistic particle
physics. We study the dynamics of brane gases constructed from various string
theories on background spaces having a K3 submanifold. The K3 compactifications
provide a stepping stone for generalising the model to the case of a full
Calabi-Yau three-fold. Duality symmetries are discussed within a cosmological
context. Using a duality, we arrive at an N=2 theory in four-dimensions
compactified on a Calabi-Yau manifold with SU(3) holonomy. We argue that the
Brane Gas model compactified on such spaces maintains the successes of the
trivial toroidal compactification while greatly enhancing its connection to
particle physics. The initial state of the universe is taken to be a small, hot
and dense gas of p-branes near thermal equilibrium. The universe has no initial
singularity and the dynamics of string winding modes allow three spatial
dimensions to grow large, providing a possible solution to the dimensionality
problem of string theory.Comment: 26 pages; Significant revisions: review material truncated;
presentation improve
Cosmology of the Next-to-Minimal Supersymmetric Standard Model
We discuss the domain wall problem in the Next-to-Minimal Supersymmetric
Standard Model, with particular attention to the usual solution of explicit
breaking of the discrete symmetry by non-renormalisable operators. This
``solution'' leads to a contradiction between the requirements of cosmology and
those of avoiding the destabilisation of the hierarchy.Comment: 6 pages LaTeX, needs sprocl.sty (included at end) Talk presented by
P.L. White at Valencia 9
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