5,227 research outputs found
Anomalous hydrodynamics and "normal" fluids in rapidly rotating BECs
In rapidly rotating bose systems we show that there is a region of anomalous
hydrodynamics whilst the system is still condensed, which coincides with the
mean field quantum Hall regime. An immediate consequence is the absence of a
normal fluid in any conventional sense. However, even the superfluid
hydrodynamics is not described by conventional Bernoulli and continuity
equations. We show there are kinematic constraints which connect spatial
variations of density and phase, that the positions of vortices are not the
simplest description of the dynamics of such a fluid (despite their utility in
describing the instantaneous state of the condensate) and that the most compact
description allows solution of some illuminating examples of motion. We
demonstrate, inter alia, a very simple relation between vortices and surface
waves. We show the surface waves can form a "normal fluid" which absorbs energy
and angular momentum from vortex motion in the trap. The time scale of this
process is sensitive to the initial configuration of the vortices, which can
lead to long-lived vortex patches - perhaps related to those observed at JILA.Comment: 4 pages; 1 sentence and references modifie
Issues for the Next Generation of Galaxy Surveys
I argue that the weight of the available evidence favours the conclusions
that galaxies are unbiased tracers of mass, the mean mass density (excluding a
cosmological constant or its equivalent) is less than the critical Einstein-de
Sitter value, and an isocurvature model for structure formation offers a viable
and arguably attractive model for the early assembly of galaxies. If valid
these conclusions complicate our work of adding structure formation to the
standard model for cosmology, but it seems sensible to pay attention to
evidence.Comment: 14 pages, 3 postscript figures, uses rspublic.st
The evolution of clustering and bias in the galaxy distribution
This paper reviews the measurements of galaxy correlations at high redshifts,
and discusses how these may be understood in models of hierarchical
gravitational collapse. The clustering of galaxies at redshift one is much
weaker than at present, and this is consistent with the rate of growth of
structure expected in an open universe. If , this observation would
imply that bias increases at high redshift, in conflict with observed
values for known high- clusters. At redshift 3, the population of
Lyman-limit galaxies displays clustering which is of similar amplitude to that
seen today. This is most naturally understood if the Lyman-limit population is
a set of rare recently-formed objects. Knowing both the clustering and the
abundance of these objects, it is possible to deduce empirically the
fluctuation spectrum required on scales which cannot be measured today owing to
gravitational nonlinearities. Of existing physical models for the fluctuation
spectrum, the results are most closely matched by a low-density spatially flat
universe. This conclusion is reinforced by an empirical analysis of CMB
anisotropies, in which the present-day fluctuation spectrum is forced to have
the observed form. Open models are strongly disfavoured, leaving CDM
as the most successful simple model for structure formation.Comment: Invited review at the Royal Society Meeting `Large-scale structure in
the universe', London, March 1998. 20 Pages LaTe
Exact vortex nucleation and cooperative vortex tunneling in dilute BECs
With the imminent advent of mesoscopic rotating BECs in the lowest Landau
level (LLL) regime, we explore LLL vortex nucleation. An exact many-body
analysis is presented in a weakly elliptical trap for up to 400 particles.
Striking non-mean field features are exposed at filling factors >>1 . Eg near
the critical rotation frequency pairs of energy levels approach each other with
exponential accuracy. A physical interpretation is provided by requantising a
mean field theory, where 1/N plays the role of Planck's constant, revealing two
vortices cooperatively tunneling between classically degenerate energy minima.
The tunnel splitting variation is described in terms of frequency, particle
number and ellipticity.Comment: 4 pages,4 figure
The velocity peaks in the cold dark matter spectrum on Earth
The cold dark matter spectrum on earth is expected to have peaks in velocity
space. We obtain estimates for the sizes and locations of these peaks. To this
end we have generalized the secondary infall model of galactic halo formation
to include angular momentum of the dark matter particles. This new model is
still spherically symmetric and it has self-similar solutions. Our results are
relevant to direct dark matter search experiments.Comment: 12 pages including 1 table and 4 figures, LaTeX, REVTEX 3.0 versio
Deep ROSAT Surveys & the contribution of AGNs to the soft X-ray background
The ROSAT Deep Surveys in the Lockman Hole have revealed that AGNs are the main contributors (~75%) to the soft X-ray background in the 1–2 keV band. Using new optical/infrared and radio observations we have obtained a nearly complete identification (93%) of the 91 X-ray sources down to a limiting flux of 1.2·10^(–15) erg cm^(–2) s^(–1) in the 0.5–2.0 keV band. We present the optical colors and the emission line properties of our AGNs in comparison with other X-ray selected AGN samples. Furthermore we discuss the fraction of red AGNs found in the ROSAT Deep Surveys. From the ROSAT Deep Surveys we see no evidence for a new class of X-ray bright galaxies, which significantly contributes to the soft X-ray background
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