21,028 research outputs found
Vorton Formation
In this paper we present the first analytic model for vorton formation. We
start by deriving the microscopic string equations of motion in Witten's
superconducting model, and show that in the relevant chiral limit these
coincide with the ones obtained from the supersonic elastic models of Carter
and Peter. We then numerically study a number of solutions of these equations
of motion and thereby suggest criteria for deciding whether a given
superconducting loop configuration can form a vorton. Finally, using a recently
developed model for the evolution of currents in superconducting strings we
conjecture, by comparison with these criteria, that string networks formed at
the GUT phase transition should produce no vortons. On the other hand, a
network formed at the electroweak scale can produce vortons accounting for up
to 6% of the critical density. Some consequences of our results are discussed.Comment: 41 pages; color figures 3-6 not included, but available from authors.
To appear in Phys. Rev.
Topological Defects in Contracting Universes
We study the behaviour and consequences of cosmic string networks in
contracting universes. They approximately behave during the collapse phase as a
radiation fluids. Scaling solutions describing this are derived and tested
against high-resolution numerical simulations. A string network in a
contracting universe, together with the gravitational radiation it generates,
can affect the dynamics of the universe both locally and globally, and be an
important source of radiation, entropy and inhomogeneity. We discuss possible
implications for bouncing and cyclic models.Comment: Shorter version of astro-ph/0206287. To appear in Phys. Rev. Let
Differential mesenteric fat deposition in bovines fed on silage or concentrate is independent of glycerol membrane permeability
© The Animal Consortium 2011In the meat industry, the manipulation of fat deposition in cattle is of pivotal importance to improve production efficiency, carcass composition and ultimately meat quality. There is an increasing interest in the identification of key factors and molecular mechanisms responsible for the development of specific fat depots. This study aimed at elucidating the influence of breed and diet on adipose tissue membrane permeability and fluidity and their interplay on fat deposition in bovines. Two Portuguese autochthonous breeds, Alentejana and Barrosã, recognized as late- and early-maturing breeds, respectively, were chosen to examine the effects of breed and diet on fat deposition and on adipose membrane composition and permeability. Twenty-four male bovines from these breeds were fed on silage-based or concentrate-based diets for 11 months. Animals were slaughtered to determine their live slaughter and hot carcass weights, as well as weights of subcutaneous and visceral adipose depots. Mesenteric fat depots were excised and used to isolate adipocyte membrane vesicles where cholesterol content, fatty acid profile as well as permeability and fluidity were determined. Total accumulation of neither subcutaneous nor visceral fat was influenced by breed. In contrast, mesenteric and omental fat depots weights were higher in concentrate-fed bulls relative to silage-fed animals. Membrane fluidity and permeability to water and glycerol in mesenteric adipose tissue were found to be independent of breed and diet. Moreover, the deposition of cholesterol and unsaturated fatty acids, which may influence membrane properties, were unchanged among experimental groups. Adipose membrane lipids from the mesenteric fat depot of ruminants were rich in saturated fatty acids, and unaffected by polyunsaturated fatty acids dietary levels. Our results provide evidence against the involvement of cellular membrane permeability to glycerol on fat accumulation in mesenteric fat tissue of concentrate-fed bovines, which is consistent with the unchanged membrane lipid profile found among experimental groups.This study was supported by Fundação para a Ciência e a Tecnologia (FCT) through grant PTDC/CVT/2006/66114 and individual fellowships to Ana P. Martins (SFRH/BD/2009/65046), Ana S. H. Costa (SFRH/BD/2009/61068) and Susana V. Martins (SFRH/BPD/2009/63019). Paula A. Lopes is a researcher from the program ‘‘Ciência 2008’’ from FC
Non-commutative fermion mass matrix and gravity
The first part is an introductory description of a small cross-section of the
literature on algebraic methods in non-perturbative quantum gravity with a
specific focus on viewing algebra as a laboratory in which to deepen
understanding of the nature of geometry. This helps to set the context for the
second part, in which we describe a new algebraic characterisation of the Dirac
operator in non-commutative geometry and then use it in a calculation on the
form of the fermion mass matrix. Assimilating and building on the various ideas
described in the first part, the final part consists of an outline of a
speculative perspective on (non-commutative) quantum spectral gravity. This is
the second of a pair of papers so far on this project.Comment: To appear in Int. J. Mod. Phys. A Previous title: An outlook on
quantum gravity from an algebraic perspective. 39 pages, 1 xy-pic figure,
LaTex Reasons for new version: added references, change of title and some
comments more up-to-dat
On the Stability of Fundamental Couplings in the Galaxy
Astrophysical tests of the stability of Nature's fundamental couplings are a
key probe of the standard paradigms in fundamental physics and cosmology. In
this report we discuss updated constraints on the stability of the
fine-structure constant and the proton-to-electron mass ratio
within the Galaxy. We revisit and improve upon the analysis by
Truppe {\it et al.} by allowing for the possibility of simultaneous variations
of both couplings and also by combining them with the recent measurements by
Levshakov {\it et al.} By considering representative unification scenarios we
find no evidence for variations of at the 0.4 ppm level, and of
at the 0.6 ppm level; if one uses the Levshakov bound on as a prior,
the bound is improved to 0.1 ppm. We also highlight how these
measurements can constrain (and discriminate among) several fundamental physics
paradigms.Comment: 7 pages, 1 figur
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