3,056 research outputs found
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
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 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
Recommended from our members
Economies of Scale and Scope in Network Industries: Lessons for the UK water and sewerage sectors
Many studies of the water and sewerage industries place significant importance on the benefits of economies of scale and scope and how these relate to vertically integrated firms. We have re-examined the early literature on scale and scope economies and find that these studies do not implicitly preclude non-integrated firms from fully utilising assets for multi-product activities; they can still reap scope economies through trading in the open market. Further, we demonstrate difficulties in separating these two types of economies when assessing firm performance. Trade-offs between governance and production costs are a key issue in determining the value of vertically integrated firms that may undergo divestiture. We discuss the present policy debate regarding the optimal structure of the water and sewerage markets in England and Wales in the light of this, and recommend avenues of research that will help determine the optimal policy
Local energy policy and managing low carbon transition: the case of Leicester, UK
National and local energy policies are implemented within a complex energy landscape that makes any evaluation of their impacts far from straightforward. Drawing upon a case study of Leicester this paper argues that the ability of local authorities to deliver significant energy savings within this landscape is questionable, albeit with other additional benefits being realised (e.g. job creation, community engagement). It examines existing domestic energy demand and multiple deprivation data for Leicester and neighbouring cities and combines this with a qualitative description of the transition process. The paper identifies the need for a more systematic analysis of how national energy policy translates to the local level and concludes that it is problematic even for a leading, pro-active and innovative local authority to have a statistically meaningful energy policy. Even where energy policies are favourable, carbon reduction is less easy to realise than other – more local - co-benefits and that in the light of significant financial and co-ordination constraints more attention needs to be given to how local communities can be more effectively supported in their desire to meet (or exceed) national targets.We acknowledge the financial assistance of the EPSRC Flexnet award (EP/E0411X/1) and all of the Leicester based contributors to the project, as well as the very helpful comments of two anonymous referees; the usual disclaimers apply
Scaling configurations of cosmic superstring networks and their cosmological implications
We study the cosmic microwave background temperature and polarisation spectra
sourced by multi-tension cosmic superstring networks. First we obtain solutions
for the characteristic length scales and velocities associated with the
evolution of a network of F-D strings, allowing for the formation of junctions
between strings of different tensions. We find two distinct regimes describing
the resulting scaling distributions for the relative densities of the different
types of strings, depending on the magnitude of the fundamental string coupling
g_s. In one of them, corresponding to the value of the coupling being of order
unity, the network's stress-energy power spectrum is dominated by populous
light F and D strings, while the other regime, at smaller values of g_s, has
the spectrum dominated by rare heavy D strings. These regimes are seen in the
CMB anisotropies associated with the network. We focus on the dependence of the
shape of the B-mode polarisation spectrum on g_s and show that measuring the
peak position of the B-mode spectrum can point to a particular value of the
string coupling. Finally, we assess how this result, along with pulsar bounds
on the production of gravitational waves from strings, can be used to constrain
a combination of g_s and the fundamental string tension mu_F. Since CMB and
pulsar bounds constrain different combinations of the string tensions and
densities, they result in distinct shapes of bounding contours in the (mu_F,
g_s) parameter plane, thus providing complementary constraints on the
properties of cosmic superstrings.Comment: 23 pages, 8 figures, 3 tables; V2: matches published version (PRD
Microsatellite analysis reveals the spatial dynamics of Bombus humilis and Bombus sylvarum
1. Substantial reductions in the distributional ranges of several species of bumblebee have been recorded in the UK. Loss and fragmentation of suitable foraging and nesting habitat to agricultural intensification is thought to be the main driving force behind declines.
2. Limited knowledge of species ecology means that effective conservation management prescriptions cannot be put into place.
3. Here we investigated the spatial dynamics of two UK Biodiversity Action Plan bumblebee species, Bombus sylvarum and Bombus humilis. For the first time, microsatellite DNA analysis was used to estimate foraging distances in rare bumblebees.
4. Sisterhoods were identified between bees sampled at discrete forage patches within a fragmented landscape. Using these sisterhoods, minimum estimates of maximum and mean foraging distances were calculated from distances separating sister bees.
5. Minimum mean foraging distances were calculated as 475 ± 97m for B. humilis and 231 ± 58m for B. sylvarum. Mean distances were significantly greater for B. humilis than B. sylvarum (P<0.001).
6. The differences between the spatial scales that the species were recorded over highlighted a need for further research into the spatial dynamics of rare and more ubiquitous foraging bumblebees
- …