2,217 research outputs found
Wick's Theorem for non-symmetric normal ordered products and contractions
We consider arbitrary splits of field operators into two parts, and use the
corresponding definition of normal ordering introduced by Evans and Steer. In
this case the normal ordered products and contractions have none of the special
symmetry properties assumed in existing proofs of Wick's theorem. Despite this,
we prove that Wick's theorem still holds in its usual form as long as the
contraction is a c-number. Wick's theorem is thus shown to be much more general
than existing derivations suggest, and we discuss possible simplifying
applications of this result.Comment: 17 page
Gravitational wave signatures from kink proliferation on cosmic (super-) strings
Junctions on cosmic string loops give rise to the proliferation of sharp
kinks. We study the effect of this proliferation on the gravitational wave (GW)
signals emitted from string networks with junctions, assuming a scaling
solution. We calculate the rate of occurrence and the distribution in amplitude
of the GW bursts emitted at cusps and kinks in the frequency bands of LIGO and
LISA as a function of the string tension, the number of sharp kinks on loops
with junctions and the fraction of loops in the cosmological network which have
junctions. Combining our results with current observational constraints, we
find that pulsar data rule out a significant number of kinks on loops for
strings with tensions G\mu > 10^{-12}. By contrast, for smaller tensions
current observations allow for a large number of kinks on loops. If this is the
case, the incoherent superposition of small bursts emitted at kink-kink
encounters leads to an enhanced GW background that hides the strong individual
bursts from kinks and cusps.Comment: 32 pages, 13 figure
On Normal ordering and Canonical transformations in Thermal Field Theory
We look at a real scalar field in thermal equilibrium in the context of the
new normal ordering and field split defined by Evans and Steer. We show that
the field split defines a natural canonical transformation, but that this
transformation differs from others known in thermal field theory.Comment: 13 pages, LaTeX. (Revisions made to discussion and various small
errors in equations corrected
Constraints on string networks with junctions
We consider the constraints on string networks with junctions in which the
strings may all be different, as may be found for example in a network of
cosmic superstrings. We concentrate on three aspects of junction
dynamics. First we consider the propagation of small amplitude waves across a
static three-string junction. Then, generalizing our earlier work, we determine
the kinematic constraints on two colliding strings with different tensions. As
before, the important conclusion is that strings do not always reconnect with a
third string; they can pass straight through one another (or in the case of
non-abelian strings become stuck in an X configuration), the constraint
depending on the angle at which the strings meet, on their relative velocity,
and on the ratios of the string tensions. For example, if the two colliding
strings have equal tensions, then for ultra-relativistic initial velocities
they pass through one another. However, if their tensions are sufficiently
different they can reconnect. Finally, we consider the global properties of
junctions and strings in a network. Assuming that, in a network, the incoming
waves at a junction are independently randomly distributed, we determine the
r.m.s. velocities of strings and calculate the average speed at which a
junction moves along each of the three strings from which it is formed. Our
findings suggest that junction dynamics may be such as to preferentially remove
the heavy strings from the network leaving a network of predominantly light
strings. Furthermore the r.m.s. velocity of strings in a network with junctions
is smaller than 1/\sqrt{2}, the result for conventional Nambu-Goto strings
without junctions in Minkowski spacetime.Comment: 12 pages, 6 figures. Version to appear in PRD. (2 new references and
slightly extended discussion in section VII
Gravitational Wave Bursts from Cosmic Superstrings with Y-junctions
Cosmic superstring loops generically contain strings of different tensions
that meet at Y-junctions. These loops evolve non-periodically in time, and have
cusps and kinks that interact with the junctions. We study the effect of
junctions on the gravitational wave signal emanating from cosmic string cusps
and kinks. We find that earlier results on the strength of individual bursts
from cusps and kinks on strings without junctions remain largely unchanged, but
junctions give rise to additional contributions to the gravitational wave
signal coming from strings expanding at the speed of light at a junction and
kinks passing through a junction.Comment: 20 pages, 5 figure
Racial variation in the number of spontaneous abortions before a first successful pregnancy, and effects on subsequent pregnancies.
OBJECTIVE: To assess the relationship between race and spontaneous abortion, whether the relationship varies by risk factors, and the effect of spontaneous abortions on subsequent pregnancies. METHODS: A retrospective study was undertaken using data collected in London, UK, between 1988 and 2000. Logistic regression evaluated histories of spontaneous abortion and associations with small-for-gestational-age and preterm births in black African, black Caribbean, and South Asian women relative to white European women. Interactions with risk factors were assessed. RESULTS: Overall, 196 040 women were included. Compared with white Europeans, the odds of a previous spontaneous abortion were increased in black African (adjusted odds ratio [aOR] 1.20; 95% confidence interval [CI] 1.12-1.29) and black Caribbean women (aOR 1.31; 95% CI 1.21-1.41). The strength of the association with black African race increased with age (P=0.002), and the association with South Asian race increased with age and body mass index (P<0.001 for both). Spontaneous abortion was associated with preterm birth in all races, but was strongest in black African women (aOR 1.47; 95% CI 1.29-1.67). CONCLUSION: The greater incidence of spontaneous abortion in black African and black Caribbean women should prompt further study of risk factors in relation to race. The interaction with age in black African and South Asian women could be important for counseling in relation to timing of pregnancy.This work was supported by a Medical Research Council
PhD Studentship.This is the final published version. It first appeared at http://www.sciencedirect.com/science/article/pii/S002072921500123X
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