13,881 research outputs found
Effects of Bose-Einstein Condensation on forces among bodies sitting in a boson heat bath
We explore the consequences of Bose-Einstein condensation on
two-scalar-exchange mediated forces among bodies that sit in a boson gas. We
find that below the condensation temperature the range of the forces becomes
infinite while it is finite at temperatures above condensation.Comment: 10 pages, 2 figure
Collinear versus non-collinear magnetic order in Pd atomic clusters: ab-initio calculations
We present a thorough theoretical assessment of the stability of
non-collinear spin arrangements in small palladium clusters. We generally find
that ferromagnetic order is always preferred, but that antiferromagnetic and
non-collinear configurations of different sorts exist and compete for the first
excited isomers. We also show that the ground state is insensitive to the
choice of atomic configuration for the pseudopotential used and to the
approximation taken for the exchange and correlation potential. Moreover, the
existence and relative stability of the different excited configurations also
depends weakly on the approximations employed. These results provide strong
evidence on the transferability of pseudopotential and exchange and correlation
functionals for palladium clusters as opposed to the situation found for the
bulk phases of palladium.Comment: 5 pages, 4 figure
A "Baedecker" for the Dark Matter Annihilation Signal
We provide a ``Baedecker'' or travel guide to the directions on the sky where
the dark matter annihilation signal may be expected. We calculate the flux of
high energy gamma-rays from annihilation of neutralino dark matter in the
centre of the Milky Way and the three nearest dwarf spheroidals (Sagittarius,
Draco and Canis Major), using realistic models of the dark matter distribution.
Other investigators have used cusped dark halo profiles (such as the
Navarro-Frenk-White) to claim a significant signal. This ignores the
substantial astrophysical evidence that the Milky Way is not dark-matter
dominated in the inner regions. We show that the annihilation signal from the
Galactic Centre falls by two orders of magnitude on substituting a cored dark
matter density profile for a cusped one. The present and future generation of
high energy gamma-ray detectors, whether atmospheric Cerenkov telescopes or
space missions like GLAST, lack the sensitivity to detect any of the
monochromatic gamma-ray annihilation lines. The continuum gamma-ray signal
above 1 GeV and above 50 GeV may however be detectable either from the dwarf
spheroidals or from the Milky Way itself. If the density profiles of the dwarf
spheroidals are cusped, then the best prospects are for detecting Sagittarius
and Canis Major. However, if the dwarf spheroidals have milder, cored profiles,
then the annihilation signal is not detectable. For GLAST, an attractive
strategy is to exploit the wide field of view and observe the Milky Way at
medium latitudes, as suggested by Stoehr et al. This is reasonably robust
against changes in the density profile.Comment: 13 pages, 3 figures, version in press at The Physical Review
Casimir-Polder forces in the presence of the cosmic photon heat bath
We study the effect of a photon background at finite temperature on the
Van der Waals interactions among neutral bodies. It turns out that the
long-range Casimir-Polder force is unaffected for distances much less than
and strongly enhanced for distances much above .Comment: 8 pages, 2 figure
Suppressing Super-Horizon Curvature Perturbations?
We consider the possibility of suppressing superhorizon curvature
perturbations after the end of the ordinary slow-roll inflationary stage. This
is the opposite of the curvaton limit. We assume that large curvature
perturbations are created by the inflaton and investigate if they can be
diluted or suppressed by a second very homogeneous field which starts to
dominate the energy density of the universe shortly after the end of inflation.
We show explicit that the gravitational sourcing of inhomogeneities from the
more inhomogeneous fluid to the more homogeneous fluid makes the suppression
difficult if not impossible to achieve.Comment: 10 pages, 1 figure. Important revision. Conclusions more negativ
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