19,488 research outputs found
The Milky Way Galaxy as a Strong Gravitational Lens
We study the gravitational lensing effects of spiral galaxies by taking a
model of the Milky Way and computing its lensing properties. The model is
composed of a spherical Hernquist bulge, a Miyamoto-Nagai disc and an
isothermal halo. As a strong lens, a spiral galaxy like the Milky Way can give
rise to four different imaging geometries. They are (i) three images on one
side of the galaxy centre (`disc triplets'), (ii) three images with one close
to the centre (`core triplets'), (iii) five images and (iv) seven images.
Neglecting magnification bias, we show that the core triplets, disc triplets
and fivefold imaging are roughly equally likely. Even though our models contain
edge-on discs, their image multiplicities are not dominated by disc triplets.
The halo has a small effect on the caustic structure, the time delays and
brightnesses of the images. The Milky Way model has a maximum disc (i.e., the
halo is not dynamically important in the inner parts). Strong lensing by nearly
edge-on disc galaxies breaks the degeneracy between the relative contribution
of the disc and halo to the overall rotation curve. If a spiral galaxy has a
sub-maximum disc, then the astroid caustic shrinks dramatically in size, whilst
the radial caustic shrinks more modestly. This causes changes in the relative
likelihood of the image geometries, specifically (i) core triplets are now 9/2
times more likely than disc triplets, (ii) the cross section for threefold
imaging is reduced by a factor of 2/3, whilst (iii) the cross section for
fivefold imaging is reduced by 1/2. Although multiple imaging is less likely
(the cross sections are smaller), the average total magnification is greater.Comment: MNRAS, in pres
Atom interferometry with Bose-Einstein condensates in a double-well potential
A trapped-atom interferometer was demonstrated using gaseous Bose-Einstein
condensates coherently split by deforming an optical single-well potential into
a double-well potential. The relative phase between the two condensates was
determined from the spatial phase of the matter wave interference pattern
formed upon releasing the condensates from the separated potential wells.
Coherent phase evolution was observed for condensates held separated by 13
m for up to 5 ms and was controlled by applying ac Stark shift potentials
to either of the two separated condensates.Comment: 4 pages, 4 figure
Cohomology of toric line bundles via simplicial Alexander duality
We give a rigorous mathematical proof for the validity of the toric sheaf
cohomology algorithm conjectured in the recent paper by R. Blumenhagen, B.
Jurke, T. Rahn, and H. Roschy (arXiv:1003.5217). We actually prove not only the
original algorithm but also a speed-up version of it. Our proof is independent
from (in fact appeared earlier on the arXiv than) the proof by H. Roschy and T.
Rahn (arXiv:1006.2392), and has several advantages such as being shorter and
cleaner and can also settle the additional conjecture on "Serre duality for
Betti numbers" which was raised but unresolved in arXiv:1006.2392.Comment: 9 pages. Theorem 1.1 and Corollary 1.2 improved; Abstract and
Introduction modified; References updated. To appear in Journal of
Mathematical Physic
Distillation of Bose-Einstein condensates in a double-well potential
Bose-Einstein condensates of sodium atoms, prepared in an optical dipole
trap, were distilled into a second empty dipole trap adjacent to the first one.
The distillation was driven by thermal atoms spilling over the potential
barrier separating the two wells and then forming a new condensate. This
process serves as a model system for metastability in condensates, provides a
test for quantum kinetic theories of condensate formation, and also represents
a novel technique for creating or replenishing condensates in new locations
Thermalization of quark-gluon matter by 2-to-2 and 3-to-3 elastic scatterings
Thermalization of quark-gluon matter is studied with a transport equation
that includes contributions of 2-to-2 and 3-to-3 elastic scatterings.
Thermalization time is related to the squared amplitudes for the elastic
scatterings that are calculated in perturbative QCD.Comment: LaTex, 6 pages, 3 figures, talk presented at the 19th international
conference on ultra-relativistic nucleus-nucleus collisions, Shanghai, China,
Nov. 200
Optical Weak Link between Two Spatially Separate Bose-Einstein Condensates
Two spatially separate Bose-Einstein condensates were prepared in an optical
double-well potential. A bidirectional coupling between the two condensates was
established by two pairs of Bragg beams which continuously outcoupled atoms in
opposite directions. The atomic currents induced by the optical coupling depend
on the relative phase of the two condensates and on an additional controllable
coupling phase. This was observed through symmetric and antisymmetric
correlations between the two outcoupled atom fluxes. A Josephson optical
coupling of two condensates in a ring geometry is proposed. The continuous
outcoupling method was used to monitor slow relative motions of two elongated
condensates and characterize the trapping potential.Comment: 4 pages, 5 figure
Degeneration and impaired regeneration of gray matter oligodendrocytes in amyotrophic lateral sclerosis.
Oligodendrocytes associate with axons to establish myelin and provide metabolic support to neurons. In the spinal cord of amyotrophic lateral sclerosis (ALS) mice, oligodendrocytes downregulate transporters that transfer glycolytic substrates to neurons and oligodendrocyte progenitors (NG2(+) cells) exhibit enhanced proliferation and differentiation, although the cause of these changes in oligodendroglia is unknown. We found extensive degeneration of gray matter oligodendrocytes in the spinal cord of SOD1 (G93A) ALS mice prior to disease onset. Although new oligodendrocytes were formed, they failed to mature, resulting in progressive demyelination. Oligodendrocyte dysfunction was also prevalent in human ALS, as gray matter demyelination and reactive changes in NG2(+) cells were observed in motor cortex and spinal cord of ALS patients. Selective removal of mutant SOD1 from oligodendroglia substantially delayed disease onset and prolonged survival in ALS mice, suggesting that ALS-linked genes enhance the vulnerability of motor neurons and accelerate disease by directly impairing the function of oligodendrocytes
Numerical Regularization of Electromagnetic Quantum Fluctuations in Inhomogeneous Dielectric Media
Electromagnetic Casimir stresses are of relevance to many technologies based
on mesoscopic devices such as MEMS embedded in dielectric media, Casimir
induced friction in nano-machinery, micro-fluidics and molecular electronics.
Computation of such stresses based on cavity QED generally require numerical
analysis based on a regularization process. A new scheme is described that has
the potential for wide applicability to systems involving realistic
inhomogeneous media. From a knowledge of the spectrum of the stationary modes
of the electromagnetic field the scheme is illustrated by estimating
numerically the Casimir stress on opposite faces of a pair of perfectly
conducting planes separated by a vacuum and the change in this result when the
region between the plates is filled with an incompressible inhomogeneous
non-dispersive dielectric.Comment: 5 pages, 2 figures, submitted to PR
Quantum reflection of atoms from a solid surface at normal incidence
We observed quantum reflection of ultracold atoms from the attractive
potential of a solid surface. Extremely dilute Bose-Einstein condensates of
^{23}Na, with peak density 10^{11}-10^{12}atoms/cm^3, confined in a weak
gravito-magnetic trap were normally incident on a silicon surface. Reflection
probabilities of up to 20 % were observed for incident velocities of 1-8 mm/s.
The velocity dependence agrees qualitatively with the prediction for quantum
reflection from the attractive Casimir-Polder potential. Atoms confined in a
harmonic trap divided in half by a solid surface exhibited extended lifetime
due to quantum reflection from the surface, implying a reflection probability
above 50 %.Comment: To appear in Phys. Rev. Lett. (December 2004)5 pages, 4 figure
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