186 research outputs found
The bispectrum of matter perturbations from cosmic strings
We present the first calculation of the bispectrum of the matter perturbations induced by cosmic strings. The calculation is performed in two different ways: the first uses the unequal time correlators (UETCs) of the string network - computed using a Gaussian model previously employed for cosmic string power spectra. The second approach uses the wake model, where string density perturbations are concentrated in sheet-like structures whose surface density grows with time. The qualitative and quantitative agreement of the two gives confidence to the results. An essential ingredient in the UETC approach is the inclusion of compensation factors in the integration with the Green's function of the matter and radiation fluids, and we show that these compensation factors must be included in the wake model also. We also present a comparison of the UETCs computed in the Gaussian model, and those computed in the unconnected segment model (USM) used by the standard cosmic string perturbation package CMBACT. We compare numerical estimates for the bispectrum of cosmic strings to those produced by perturbations from an inflationary era, and discover that, despite the intrinsically non-Gaussian nature of string-induced perturbations, the matter bispectrum is unlikely to produce competitive constraints on a population of cosmic strings
Kink-boundary collisions in a two dimensional scalar field theory
In a two-dimensional toy model, motivated from five-dimensional heterotic
M-theory, we study the collision of scalar field kinks with boundaries. By
numerical simulation of the full two-dimensional theory, we find that the kink
is always inelastically reflected with a model-independent fraction of its
kinetic energy converted into radiation. We show that the reflection can be
analytically understood as a fluctuation around the scalar field vacuum. This
picture suggests the possibility of spontaneous emission of kinks from the
boundary due to small perturbations in the bulk. We verify this picture
numerically by showing that the radiation emitted from the collision of an
initial single kink eventually leads to a bulk populated by many kinks.
Consequently, processes changing the boundary charges are practically
unavoidable in this system. We speculate that the system has a universal final
state consisting of a stack of kinks, their number being determined by the
initial energy
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-term Inflation in Superstring Theories
An inflationary stage dominated by a -term avoids the slow-roll problem of
inflation in supergravity and may emerge in theories with a non-anomalous or
anomalous U(1) gauge symmetry. The most intriguing and commonly invoked
possibility is that the Fayet-Iliopoulos -term triggering inflation is the
one emerging in superstring theories. We discuss the complications one has to
face when trying to build up a successful -term inflationary scenario in
superstring models. In particular, we show that the ``vacuum shifting''
phenomenon of string theories is usually very efficient even in the early
Universe, thus preventing inflation from taking place. On the other hand, when
-term inflation is free to occur, the presence of a plethora of fields and
several non-anomalous additional abelian symmetries in string theories may help
in reconciling the value of the Fayet-Iliopoulos -term required by the COBE
normalization with the value predicted by string theories. We also show that in
superstring -term inflation gravitinos are likely to pose no cosmological
problem.Comment: 19 pages, LaTeX fil
Leptogenesis through direct inflaton decay to light particles
We present a scenario of nonthermal leptogenesis following supersymmetric
hybrid inflation, in the case where inflaton decay to both heavy right handed
neutrino and SU(2)_L triplet superfields is kinematically disallowed. Lepton
asymmetry is generated through the decay of the inflaton into light particles
by the interference of one-loop diagrams with right handed neutrino and SU(2)_L
triplet exchange respectively. We require superpotential couplings explicitly
violating a U(1) R-symmetry and R-parity. However, the broken R-parity need not
have currently observable low-energy signatures. Also, the lightest sparticle
can be stable. Some R-parity violating slepton decays may, though, be
detectable in the future colliders. We take into account the constraints from
neutrino masses and mixing and the preservation of the primordial lepton
asymmetry.Comment: 11 pages including 3 figures, uses Revtex, minor corrections,
references adde
F-term strings in the Bogomol'nyi limit are also BPS states
We derive the Bogomol'nyi equations for supersymmetric Abelian F-term cosmic
strings in four-dimensional flat space and show that, contrary to recent
statements in the literature, they are BPS states in the Bogomol'nyi limit, but
the partial breaking of supersymmetry is from N=2. The second supersymmetry is
not obvious in the N=1 formalism, so we give it explicitly in components and in
terms of a different set of N=1 chiral superfields. We also discuss the
appearance of a second supersymmetry in D-term models, and the relation to N=2
F-term models. The analysis sheds light on an apparent paradox raised by the
recent observation that D-term strings remain BPS when coupled to N=1
supergravity, whereas F-term strings break the supersymmetry completely, even
in the Bogomol'nyi limit. Finally, we comment on their semilocal extensions and
their relevance to cosmology.Comment: 11 pages; References added, minor corrections, matches published
versio
Evolution of cosmic string configurations
We extend and develop our previous work on the evolution of a network of
cosmic strings. The new treatment is based on an analysis of the probability
distribution of the end-to-end distance of a randomly chosen segment of
left-moving string of given length. The description involves three distinct
length scales: , related to the overall string density, , the
persistence length along the string, and , describing the small-scale
structure, which is an important feature of the numerical simulations that have
been done of this problem. An evolution equation is derived describing how the
distribution develops in time due to the combined effects of the universal
expansion, of intercommuting and loop formation, and of gravitational
radiation. With plausible assumptions about the unknown parameters in the
model, we confirm the conclusions of our previous study, that if gravitational
radiation and small-scale structure effects are neglected, the two dominant
length scales both scale in proportion to the horizon size. When the extra
effects are included, we find that while and grow,
initially does not. Eventually, however, it does appear to scale, at a much
lower level, due to the effects of gravitational back-reaction.Comment: 61 pages, requires RevTex v3.0, SUSSEX-TH-93/3-4,
IMPERIAL/TP/92-93/4
Estimation of vortex density after superconducting film quench
This paper addresses the problem of vortex formation during a rapid quench in
a superconducting film. It builds on previous work showing that in a local
gauge theory there are two distinct mechanisms of defect formation, based on
fluctuations of the scalar and gauge fields, respectively. We show how vortex
formation in a thin film differs from the fully two-dimensional case, on which
most theoretical studies have focused. We discuss ways of testing theoretical
predictions in superconductor experiments and analyse the results of recent
experiments in this light.Comment: 7 pages, no figure
Five-Branes in Heterotic Brane-World Theories
The effective action for five-dimensional heterotic M-theory in the presence
of five-branes is systematically derived from Horava-Witten theory coupled to
an M5-brane world-volume theory. This leads to a five-dimensional N=1 gauged
supergravity theory on S^1/Z_2 coupled to four-dimensional N=1 theories
residing on the two orbifold fixed planes and an additional bulk three-brane.
We analyse the properties of this action, particularly the four-dimensional
effective theory associated with the domain-wall vacuum state. The moduli
Kahler potential and the gauge-kinetic functions are determined along with the
explicit relations between four-dimensional superfields and five-dimensional
component fields.Comment: 19 pages, Latex, typos corrected, reference adde
Inhomogeneous cosmologies with Q-matter and varying
Starting from the inhomogeneous shear--free Nariai metric we show, by solving
the Einstein--Klein--Gordon field equations, how a self--interacting scalar
field plus a material fluid, a variable cosmological term and a heat flux can
drive the universe to its currently observed state of homogeneous accelerated
expansion. A quintessence scenario where power-law inflation takes place for a
string-motivated potential in the late--time dominated field regime is
proposed.Comment: 11 pages, Revtex. To be published in Physical Review
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