24,206 research outputs found
Cosmology with Varying Constants
The idea of possible time or space variations of the `fundamental' constants
of nature, although not new, is only now beginning to be actively considered by
large numbers of researchers in the particle physics, cosmology and
astrophysics communities. This revival is mostly due to the claims of possible
detection of such variations, in various different contexts and by several
groups. Here, I present the current theoretical motivations and expectations
for such variations, review the current observational status, and discuss the
impact of a possible confirmation of these results in our views of cosmology
and physics as a whole.Comment: 14 pages, no figures. Essay to appear in Phil. Trans. Roy. Soc. Lond.
A Triennial Series (Christmas 2002 Issue
Modelling radiation emission in the transition from the classical to the quantum regime
An emissivity formula is derived using the generalised
Fermi-Weizacker-Williams method of virtual photons which accounts for the
recoil the charged particle experiences as it emits radiation. It is found that
through this derivation the formula obtained by Sokolov et al using QED
perturbation theory is recovered. The corrected emissivity formula is applied
to nonlinear Thomson scattering scenarios in the transition from the classical
to the quantum regime, for small values of the nonlinear quantum parameter
\chi. Good agreement is found between this method and a QED probabilistic
approach for scenarios where both are valid. In addition, signatures of the
quantum corrections are identified and explored.Comment: 11 pages, 4 figures, submitted for publicatio
Detecting stars, galaxies, and asteroids with Gaia
(Abridged) Gaia aims to make a 3-dimensional map of 1,000 million stars in
our Milky Way to unravel its kinematical, dynamical, and chemical structure and
evolution. Gaia's on-board detection software discriminates stars from spurious
objects like cosmic rays and Solar protons. For this, parametrised
point-spread-function-shape criteria are used. This study aims to provide an
optimum set of parameters for these filters. We developed an emulation of the
on-board detection software, which has 20 free, so-called rejection parameters
which govern the boundaries between stars on the one hand and sharp or extended
events on the other hand. We evaluate the detection and rejection performance
of the algorithm using catalogues of simulated single stars, double stars,
cosmic rays, Solar protons, unresolved galaxies, and asteroids. We optimised
the rejection parameters, improving - with respect to the functional baseline -
the detection performance of single and double stars, while, at the same time,
improving the rejection performance of cosmic rays and of Solar protons. We
find that the minimum separation to resolve a close, equal-brightness double
star is 0.23 arcsec in the along-scan and 0.70 arcsec in the across-scan
direction, independent of the brightness of the primary. We find that, whereas
the optimised rejection parameters have no significant impact on the
detectability of de Vaucouleurs profiles, they do significantly improve the
detection of exponential-disk profiles. We also find that the optimised
rejection parameters provide detection gains for asteroids fainter than 20 mag
and for fast-moving near-Earth objects fainter than 18 mag, albeit this gain
comes at the expense of a modest detection-probability loss for bright,
fast-moving near-Earth objects. The major side effect of the optimised
parameters is that spurious ghosts in the wings of bright stars essentially
pass unfiltered.Comment: Accepted for publication in A&
Rossby waves in rapidly rotating Bose-Einstein condensates
We predict and describe a new collective mode in rotating Bose-Einstein
condensates, which is very similar to the Rossby waves in geophysics. In the
regime of fast rotation, the Coriolis force dominates the dynamics and acts as
a restoring force for acoustic-drift waves along the condensate. We derive a
nonlinear equation that includes the effects of both the zero-point pressure
and the anharmonicity of the trap. It is shown that such waves have negative
phase speed, propagating in the opposite sense of the rotation. We discuss
different equilibrium configurations and compare with those resulting from the
Thomas-Fermi approximation.Comment: 4 pages, 2 figures (submitted to PRL
Emergence of Hierarchy on a Network of Complementary Agents
Complementarity is one of the main features underlying the interactions in
biological and biochemical systems. Inspired by those systems we propose a
model for the dynamical evolution of a system composed by agents that interact
due to their complementary attributes rather than their similarities. Each
agent is represented by a bit-string and has an activity associated to it; the
coupling among complementary peers depends on their activity. The connectivity
of the system changes in time respecting the constraint of complementarity. We
observe the formation of a network of active agents whose stability depends on
the rate at which activity diffuses in the system. The model exhibits a
non-equilibrium phase transition between the ordered phase, where a stable
network is generated, and a disordered phase characterized by the absence of
correlation among the agents. The ordered phase exhibits multi-modal
distributions of connectivity and activity, indicating a hierarchy of
interaction among different populations characterized by different degrees of
activity. This model may be used to study the hierarchy observed in social
organizations as well as in business and other networks.Comment: 13 pages, 4 figures, submitte
Multicolored Temperley-Lieb lattice models. The ground state
Using inversion relation, we calculate the ground state energy for the
lattice integrable models, based on a recently obtained baxterization of non
trivial multicolored generalization of Temperley-Lieb algebras. The simplest
vertex and IRF models are analyzed and found to have a mass gap.Comment: 15 pages 2 figure
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.
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