442 research outputs found
Supermassive black holes or boson stars? Hair counting with gravitational wave detectors
The evidence for supermassive Kerr black holes in galactic centers is strong
and growing, but only the detection of gravitational waves will convincingly
rule out other possibilities to explain the observations. The Kerr spacetime is
completely specified by the first two multipole moments: mass and angular
momentum. This is usually referred to as the ``no-hair theorem'', but it is
really a ``two-hair'' theorem. If general relativity is the correct theory of
gravity, the most plausible alternative to a supermassive Kerr black hole is a
rotating boson star. Numerical calculations indicate that the spacetime of
rotating boson stars is determined by the first three multipole moments
(``three-hair theorem''). LISA could accurately measure the oscillation
frequencies of these supermassive objects. We propose to use these measurements
to ``count their hair'', unambiguously determining their nature and properties.Comment: 8 pages. This essay received an honorable mention in the Gravity
Research Foundation Essay Competition, 200
A geometric approach to scalar field theories on the supersphere
Following a strictly geometric approach we construct globally supersymmetric
scalar field theories on the supersphere, defined as the quotient space
. We analyze the superspace geometry of the
supersphere, in particular deriving the invariant vielbein and spin connection
from a generalization of the left-invariant Maurer-Cartan form for Lie groups.
Using this information we proceed to construct a superscalar field action on
, which can be decomposed in terms of the component fields, yielding a
supersymmetric action on the ordinary two-sphere. We are able to derive
Lagrange equations and Noether's theorem for the superscalar field itself.Comment: 38 pages, 1 figur
Oscillatons formed by non linear gravity
Oscillatons are solutions of the coupled Einstein-Klein-Gordon (EKG)
equations that are globally regular and asymptotically flat. By means of a
Legendre transformation we are able to visualize the behaviour of the
corresponding objects in non-linear gravity where the scalar field has been
absorbed by means of the conformal mapping.Comment: Revtex file, 6 pages, 3 eps figure; matches version published in PR
Tensor hypercontraction: A universal technique for the resolution of matrix elements of local, finite-range -body potentials in many-body quantum problems
Configuration-space matrix elements of N-body potentials arise naturally and
ubiquitously in the Ritz-Galerkin solution of many-body quantum problems. For
the common specialization of local, finite-range potentials, we develop the
eXact Tensor HyperContraction (X-THC) method, which provides a quantized
renormalization of the coordinate-space form of the N-body potential, allowing
for a highly separable tensor factorization of the configuration-space matrix
elements. This representation allows for substantial computational savings in
chemical, atomic, and nuclear physics simulations, particularly with respect to
difficult "exchange-like" contractions.Comment: Third version of the manuscript after referee's comments. In press in
PRL. Main text: 4 pages, 2 figures, 1 table; Supplemental material (also
included): 14 pages, 2 figures, 2 table
Accretion disc onto a static non-baryonic compact object
We study the emissivity properties of a geometrically thin, optically thick,
steady accretion disc about a static boson star. Starting from a numerical
computation of the metric potentials and the rotational velocities of the
particles in the vicinity of the compact object, we obtain the power per unit
area, the temperature of the disc, and the spectrum of the emitted radiation.
In order to see if different central objects could be actually distinguished,
all these results are compared with the case of a central Schwarzschild black
hole of equal mass. We considered different situations both for the boson star,
assumed with and without self-interactions, and the disc, whose internal
commencement can be closer to the center than in the black hole case. We
finally make some considerations about the Eddington luminosity, which becomes
radially dependent for a transparent object. We found that, particularly at
high energies, differences in the emitted spectrum are notorious. Reasons for
that are discussed.Comment: 16 pages, 8 figures. Accepted for publication in Nuclear Physics
Observation of Feshbach resonances between two different atomic species
We have observed three Feshbach resonances in collisions between lithium-6
and sodium-23 atoms. The resonances were identified as narrow loss features
when the magnetic field was varied. The molecular states causing these
resonances have been identified, and additional lithium-sodium resonances are
predicted. These resonances will allow the study of degenerate Bose-Fermi
mixtures with adjustable interactions, and could be used to generate ultracold
heteronuclear molecules
Rotating Boson Stars in 5 Dimensions
We study rotating boson stars in five spacetime dimensions. The boson fields
consist of a complex doublet scalar field. Considering boson stars rotating in
two orthogonal planes with both angular momenta of equal magnitude, a special
ansatz for the boson field and the metric allows for solutions with nontrivial
dependence on the radial coordinate only. The charge of the scalar field equals
the sum of the angular momenta. The rotating boson stars are globally regular
and asymptotically flat. For our choice of a sixtic potential the rotating
boson star solutions possess a flat spacetime limit. We study the solutions in
flat and curved spacetime.Comment: 17 pages, 6 figure
Formation Time of a Fermion Pair Condensate
The formation time of a condensate of fermionic atom pairs close to a
Feshbach resonance was studied. This was done using a phase-shift method in
which the delayed response of the many-body system to a modulation of the
interaction strength was recorded. The observable was the fraction of condensed
molecules in the cloud after a rapid magnetic field ramp across the Feshbach
resonance. The measured response time was slow compared to the rapid ramp,
which provides final proof that the molecular condensates reflect the presence
of fermion pair condensates before the ramp.Comment: 5 pages, 4 figure
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