248 research outputs found
Radiation constraints from cosmic strings
We show that it is possible to evolve a network of global strings numerically
including the effects of radiative backreaction, using the renormalised
equations for the Kalb-Ramond action. We calculate radiative corrections to the
equations of motion and deduce the effect on a network of global strings. We
also discuss the implications of this work for the cosmological axion density.Comment: 4 Pages, UUencoded postscript file, to appear in 'Trends in
Astro-Particle Physics - Nuclear Physics B, Proceedings Supplement
Spectrum of radiation from axion strings
In the wide variety of axion cosmologies in which axion strings form, their
radiative decay is the dominant mechanism for the production of axions,
imposing a tight constraint on the axion mass. Here, we focus on the mechanism
by which axions are produced in this scenario and, in particular, the key issue
of the axion spectrum emitted by an evolving network of strings.Comment: to be published in the proceedings of the 5th IFT Workshop on Axion
Searching for Signatures of Cosmic Superstrings in the CMB
Because cosmic superstrings generically form junctions and gauge theoretic
strings typically do not, junctions may provide a signature to distinguish
between cosmic superstrings and gauge theoretic cosmic strings. In cosmic
microwave background anisotropy maps, cosmic strings lead to distinctive line
discontinuities. String junctions lead to junctions in these line
discontinuities. In turn, edge detection algorithms such as the Canny algorithm
can be used to search for signatures of strings in anisotropy maps. We apply
the Canny algorithm to simulated maps which contain the effects of cosmic
strings with and without string junctions. The Canny algorithm produces edge
maps. To distinguish between edge maps from string simulations with and without
junctions, we examine the density distribution of edges and pixels crossed by
edges. We find that in string simulations without Gaussian noise (such as
produced by the dominant inflationary fluctuations) our analysis of the output
data from the Canny algorithm can clearly distinguish between simulations with
and without string junctions. In the presence of Gaussian noise at the level
expected from the current bounds on the contribution of cosmic strings to the
total power spectrum of density fluctuations, the distinction between models
with and without junctions is more difficult. However, by carefully analyzing
the data the models can still be differentiated.Comment: 15 page
Cosmic Superstring Scattering in Backgrounds
We generalize the calculation of cosmic superstring reconnection probability
to non-trivial backgrounds. This is done by modeling cosmic strings as wound
tachyon modes in the 0B theory, and the spacetime effective action is then used
to couple this to background fields. Simple examples are given including
trivial and warped compactifications. Generalization to strings is
discussed.Comment: 12 pages, 2 figures; v2: references adde
Universality and Critical Phenomena in String Defect Statistics
The idea of biased symmetries to avoid or alleviate cosmological problems
caused by the appearance of some topological defects is familiar in the context
of domain walls, where the defect statistics lend themselves naturally to a
percolation theory description, and for cosmic strings, where the proportion of
infinite strings can be varied or disappear entirely depending on the bias in
the symmetry. In this paper we measure the initial configurational statistics
of a network of string defects after a symmetry-breaking phase transition with
initial bias in the symmetry of the ground state. Using an improved algorithm,
which is useful for a more general class of self-interacting walks on an
infinite lattice, we extend the work in \cite{MHKS} to better statistics and a
different ground state manifold, namely , and explore various different
discretisations. Within the statistical errors, the critical exponents of the
Hagedorn transition are found to be quite possibly universal and identical to
the critical exponents of three-dimensional bond or site percolation. This
improves our understanding of the percolation theory description of defect
statistics after a biased phase transition, as proposed in \cite{MHKS}. We also
find strong evidence that the existence of infinite strings in the Vachaspati
Vilenkin algorithm is generic to all (string-bearing) vacuum manifolds, all
discretisations thereof, and all regular three-dimensional lattices.Comment: 62 pages, plain LaTeX, macro mathsymb.sty included, figures included.
also available on
http://starsky.pcss.maps.susx.ac.uk/groups/pt/preprints/96/96011.ps.g
Patients' inability to perform a preoperative cardiopulmonary exercise test or demonstrate an anaerobic threshold is associated with inferior outcomes after major colorectal surgery.
BACKGROUND: Surgical patients with poor functional capacity, determined by oxygen consumption at anaerobic threshold (AT) during cardiopulmonary exercise testing (CPET), experience longer hospital stays and worse short- and medium-term survival. However, previous studies excluded patients who were unable to perform a CPET or who failed to demonstrate an AT. We hypothesized that such patients are at risk of inferior outcomes after elective surgery. METHODS: All patients undergoing major colorectal surgery attempted CPET to assist in the planning of care. Patients were stratified by their test results into Fit (AT ≥ 11.0 ml O2 kg(-1) min(-1)), Unfit (AT < 11.0 ml O2 kg(-1) min(-1)), or Unable to CPET groups (failed to pedal or demonstrate an AT). For each group, we determined hospital stay and mortality. RESULTS: Between March 2009 and April 2010, 269 consecutive patients were screened, and proceeded to bowel resection. Median hospital stay was 8 days (IQR 5.1-13.4) and there were 44 deaths (16%) at 2 yr; 26 (9.7%) patients were categorized as Unable to CPET, 69 (25.7%) Unfit and 174 (64.7%) Fit. There were statistically significant differences between the three groups in hospital stay [median (IQR) 14.0 (10.5-23.8) vs 9.9 (5.5-15) vs 7.1 (4.9-10.8) days, P < 0.01] and mortality at 2 yr [11/26 (42%) vs 14/69 (20%) vs 19/174 (11%), respectively (P < 0.01)] although the differences between Unable and Unfit were not statistically different. CONCLUSIONS: Patients' inability to perform CPET is associated with inferior outcomes after major colorectal surgery. Future studies evaluating CPET in risk assessment for major surgery should report outcomes for this subgroup
D-Branes in Field Theory
Certain gauge theories in four dimensions are known to admit semi-classical
D-brane solitons. These are domain walls on which vortex flux tubes may end.
The purpose of this paper is to develop an open-string description of these
D-branes. The dynamics of the domain walls is shown to be governed by a
Chern-Simons-Higgs theory which, at the quantum level, captures the classical
"closed string" scattering of domain wall solitons.Comment: 23 Pages, 3 figures. v2: reference adde
Reconnection of Colliding Cosmic Strings
For vortex strings in the Abelian Higgs model and D-strings in superstring
theory, both of which can be regarded as cosmic strings, we give analytical
study of reconnection (recombination, inter-commutation) when they collide, by
using effective field theories on the strings. First, for the vortex strings,
via a string sigma model, we verify analytically that the reconnection is
classically inevitable for small collision velocity and small relative angle.
Evolution of the shape of the reconnected strings provides an upper bound on
the collision velocity in order for the reconnection to occur. These analytical
results are in agreement with previous numerical results. On the other hand,
reconnection of the D-strings is not classical but probabilistic. We show that
a quantum calculation of the reconnection probability using a D-string action
reproduces the nonperturbative nature of the worldsheet results by Jackson,
Jones and Polchinski. The difference on the reconnection -- classically
inevitable for the vortex strings while quantum mechanical for the D-strings --
is suggested to originate from the difference between the effective field
theories on the strings.Comment: 29 pages, 14 eps figures, JHEP style; references added, typos
correcte
Cosmological scaling solutions in generalised Gauss-Bonnet gravity theories
The conditions for the existence and stability of cosmological power-law
scaling solutions are established when the Einstein-Hilbert action is modified
by the inclusion of a function of the Gauss-Bonnet curvature invariant. The
general form of the action that leads to such solutions is determined for the
case where the universe is sourced by a barotropic perfect fluid. It is shown
by employing an equivalence between the Gauss-Bonnet action and a scalar-tensor
theory of gravity that the cosmological field equations can be written as a
plane autonomous system. It is found that stable scaling solutions exist when
the parameters of the model take appropriate values.Comment: 10 pages and 5 figure
Collisions of Cosmic F- and D-strings
Recent work suggests that fundamental and Dirichlet strings, and their (p,q)
bound states, may be observed as cosmic strings. The evolution of cosmic string
networks, and therefore their observational signals, depends on what happens
when two strings collide. We study this in string perturbation theory for
collisions between all possible pairs of strings; different cases involve
sphere, disk, and annulus amplitudes. The result also depends on the details of
compactification; the dependence on ratios of scales is only logarithmic, but
this is still numerically important. We study a range of models and parameters,
and find that in most cases these strings can be distinguished from cosmic
strings that arise as gauge theory solitons.Comment: 42 pages, 7 figures; v.2: added references, expanded discussion of
reconnection in field theor
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