82,539 research outputs found
Stability of self-gravitating magnetic monopoles
The stability of a spherically symmetric self-gravitating magnetic monopole
is examined in the thin wall approximation: modeling the interior false vacuum
as a region of de Sitter space; the exterior as an asymptotically flat region
of the Reissner-Nordstr\"om geometry; and the boundary separating the two as a
charged domain wall. There remains only to determine how the wall gets embedded
in these two geometries. In this approximation, the ratio of the false
vacuum to surface energy densities is a measure of the symmetry breaking scale
. Solutions are characterized by this ratio, the charge on the wall ,
and the value of the conserved total energy . We find that for each fixed
and up to some critical value, there exists a unique globally static
solution, with ; any stable radial excitation has bounded
above by , the value assumed in an extremal Reissner-Nordstr\"om geometry
and these are the only solutions with . As is raised above a black
hole forms in the exterior: (i) for low or , the wall is crushed; (ii)
for higher values, it oscillates inside the black hole. If the mass is not too
high these `collapsing' solutions co-exist with an inflating bounce; (iii) for
, or outside the above regimes, there is a unique inflating
solution. In case (i) the course of the bounce lies within a single
asymptotically flat region (AFR) and it resembles closely the bounce exhibited
by a false vacuum bubble (with Q=0). In cases (ii) and (iii) the course of the
bounce spans two consecutive AFRs.Comment: 19 pages, RevTex two cols., 11 eps figs. Submitted to Phys. Rev.
Refractive Distortions of Two-Particle Correlations from Classical Trajectory Calculations
Calculations of two-particle correlations usually assume particles interact
only pair-wise after their final collisions with third bodies. By considering
classical trajectories, we show that interactions with the mean field can alter
the spatial dimensions of the outgoing phase-space-density profiles by tens of
percent, consistent with more complicated quantum complications.Comment: 10 pages, 5 figure
Modelling the dynamics of global monopoles
A thin wall approximation is exploited to describe a global monopole coupled
to gravity. The core is modelled by de Sitter space; its boundary by a thin
wall with a constant energy density; its exterior by the asymptotic
Schwarzschild solution with negative gravitational mass and solid angle
deficit, , where is the symmetry
breaking scale. The deficit angle equals when . We find that: (1) if , there exists a unique globally
static non-singular solution with a well defined mass, . provides
a lower bound on . If , the solution oscillates. There are no
inflating solutions in this symmetry breaking regime. (2) if ,
non-singular solutions with an inflating core and an asymptotically
cosmological exterior will exist for all . (3) if is not too large,
there exists a finite range of values of where a non-inflating monopole
will also exist. These solutions appear to be metastable towards inflation. If
is positive all solutions are singular. We provide a detailed description
of the configuration space of the model for each point in the space of
parameters, and trace the wall trajectories on both the interior
and the exterior spacetimes. Our results support the proposal that topological
defects can undergo inflation.Comment: 44 pages, REVTeX, 11 PostScript figures, submitted to the Physical
Review D. Abstract's correcte
Braneworld stars and black holes
We look for spherically symmetric star or black hole solutions on a
Randall-Sundrum brane from the perspective of the bulk. We take a known bulk
solution, and analyse possible braneworld trajectories within it that
correspond, from the braneworld point of view, to solutions of the brane
Tolman-Oppenheimer-Volkoff equations. Our solutions are therefore embedded
consistently into a full bulk solution. We find the full set of static
gravitating matter sources on a brane in a range of bulk spacetimes, analyzing
which can correspond to physically sensible sources. Finally, we look at
time-dependent trajectories in a Schwarzschild--anti de Sitter spacetime as
possible descriptions of time-dependent braneworld black holes, highlighting
some of the general features one might expect, as well as some of the
difficulties involved in getting a full solution to the question.Comment: 39 pages, 15 figure
Properties of cosmologies with dynamical pseudo Nambu-Goldstone bosons
We study observational constraints on cosmological models with a quintessence
field in the form of a dynamical pseudo Nambu-Goldstone boson. After reviewing
the properties of the solutions, from a dynamical systems phase space analysis,
we consider the constraints on parameter values imposed by luminosity distances
from the 60 Type Ia supernovae published by Perlmutter et al., and also from
gravitational lensing statistics of distant quasars. In the case of the Type Ia
supernovae we explicitly allow for the possibility of evolution of the peak
luminosities of the supernovae sources, using simple empirical models which
have been recently discussed in the literature. We find weak evidence to
suggest that the models with supernovae evolution fit the data better in the
context of the quintessence models in question. If source evolution is a
reality then the greatest challenge facing these models is the tension between
current value of the expansion age, H_0 t_0, and the fraction of the critical
energy density, Omega_{phi0}, corresponding to the scalar field. Nonetheless
there are ranges of the free parameters which fit all available cosmological
data.Comment: 22 pages, RevTeX, 13 figures, epsf. v3: References added, plus a few
sentences to clarify some small points; v4: Typos fixe
Local re-acceleration and a modified thick target model of solar flare electrons
The collisional thick target model (CTTM) of solar hard X-ray (HXR) bursts
has become an almost 'Standard Model' of flare impulsive phase energy transport
and radiation. However, it faces various problems in the light of recent data,
particularly the high electron beam density and anisotropy it involves.} {We
consider how photon yield per electron can be increased, and hence fast
electron beam intensity requirements reduced, by local re-acceleration of fast
electrons throughout the HXR source itself, after injection.} {We show
parametrically that, if net re-acceleration rates due to e.g. waves or local
current sheet electric () fields are a significant fraction of
collisional loss rates, electron lifetimes, and hence the net radiative HXR
output per electron can be substantially increased over the CTTM values. In
this local re-acceleration thick target model (LRTTM) fast electron number
requirements and anisotropy are thus reduced. One specific possible scenario
involving such re-acceleration is discussed, viz, a current sheet cascade (CSC)
in a randomly stressed magnetic loop.} {Combined MHD and test particle
simulations show that local fields in CSCs can efficiently
accelerate electrons in the corona and and re-accelerate them after injection
into the chromosphere. In this HXR source scenario, rapid synchronisation and
variability of impulsive footpoint emissions can still occur since primary
electron acceleration is in the high Alfv\'{e}n speed corona with fast
re-acceleration in chromospheric CSCs. It is also consistent with the
energy-dependent time-of-flight delays in HXR features.Comment: 8 pages, 2 figure
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