15,700 research outputs found
Noncommutative scalar field minimally coupled to gravity
A model for noncommutative scalar fields coupled to gravity based on the
generalization of the Moyal product is proposed. Solutions compatible with
homogeneous and isotropic flat Robertson-Walker spaces to first non-trivial
order in the perturbation of the star-product are presented. It is shown that
in the context of a typical chaotic inflationary scenario, at least in the
slow-roll regime, noncommutativity yields no observable effect.Comment: Talk presented at the Workshop on Quantum Gravity and Noncommutative
Geometry, 20-23 July 2004, Universidade Lus\'ofona, Lisbon, Portugal. To
appear at Int. J. Mod. Phys.
Discovery of a 3.6-hr Eclipsing Luminous X-Ray Binary in the Galaxy NGC 4214
We report the discovery of an eclipsing X-ray binary with a 3.62-hr period
within 24" of the center of the dwarf starburst galaxy NGC 4214. The orbital
period places interesting constraints on the nature of the binary, and allows
for a few very different interpretations. The most likely possibility is that
the source lies within NGC 4214 and has an X-ray luminosity of up to 7 e38
ergs/s. In this case the binary may well be comprised of a naked He-burning
donor star with a neutron-star accretor, though a stellar-mass black-hole
accretor cannot be completely excluded. There is no obvious evidence for a
strong stellar wind in the X-ray orbital light curve that would be expected
from a massive He star; thus, the mass of the He star should be <3-4 solar
masses. If correct, this would represent a new class of very luminous X-ray
binary -- perhaps related to Cyg X-3. Other less likely possibilities include a
conventional low-mass X-ray binary that somehow manages to produce such a high
X-ray luminosity and is apparently persistent over an interval of years; or a
foreground AM Her binary of much lower luminosity that fortuitously lies in the
direction of NGC 4214. Any model for this system must accommodate the lack of
an optical counterpart down to a limiting magnitude of 22.6 in the visible.Comment: 7 pages, ApJ accepted versio
Interaction of Close-in Planets with the Magnetosphere of their Host Stars I: Diffusion, Ohmic Dissipation of Time Dependent Field, Planetary Inflation, and Mass Loss
The unanticipated discovery of the first close-in planet around 51 Peg has
rekindled the notion that shortly after their formation outside the snow line,
some planets may have migrated to the proximity of their host stars because of
their tidal interaction with their nascent disks. If these planets indeed
migrated to their present-day location, their survival would require a halting
mechanism in the proximity of their host stars. Most T Tauri stars have strong
magnetic fields which can clear out a cavity in the innermost regions of their
circumstellar disks and impose magnetic induction on the nearby young planets.
Here we consider the possibility that a magnetic coupling between young stars
and planets could quench the planet's orbital evolution. After a brief
discussion of the complexity of the full problem, we focus our discussion on
evaluating the permeation and ohmic dissipation of the time dependent component
of the stellar magnetic field in the planet's interior. Adopting a model first
introduced by C. G. Campbell for interacting binary stars, we determine the
modulation of the planetary response to the tilted magnetic field of a
non-synchronously spinning star. We first compute the conductivity in the young
planets, which indicates that the stellar field can penetrate well into the
planet's envelope in a synodic period. For various orbital configurations, we
show that the energy dissipation rate inside the planet is sufficient to induce
short-period planets to inflate. This process results in mass loss via Roche
lobe overflow and in the halting of the planet's orbital migration.Comment: 47 pages, 12 figure
Testing Spatial Noncommutativity via Rydberg Atoms
The possibility of testing spatial noncommutativity via Rydberg atoms is
explored. An atomic dipole of a cold Rydberg atom is arranged in appropriate
electric and magnetic field, so that the motion of the dipole is constrained to
be planar and rotationally symmetric. Spatial noncommutativity leads to that
the canonical angular momentum possesses fractional values. In the limit of
vanishing kinetic energy, the dominate value of the lowest canonical angular
momentum takes . Furthermore, in the limit of eliminating magnetic
field, the dominate value of the lowest canonical angular momentum changes from
to . This result is a clear signal of spatial
noncommutativity. An experimental verification of this prediction is suggested.Comment: 10 pages. Physical Review Letters (in press
Reverse geometric engineering of singularities
One can geometrically engineer supersymmetric field theories theories by
placing D-branes at or near singularities. The opposite process is described,
where one can reconstruct the singularities from quiver theories. The
description is in terms of a noncommutative quiver algebra which is constructed
from the quiver diagram and the superpotential. The center of this
noncommutative algebra is a commutative algebra, which is the ring of
holomorphic functions on a variety V. If certain algebraic conditions are met,
then the reverse geometric engineering produces V as the geometry that D-branes
probe. It is also argued that the identification of V is invariant under
Seiberg dualities.Comment: 17 pages, Latex. v2: updates reference
Solving matrix models using holomorphy
We investigate the relationship between supersymmetric gauge theories with
moduli spaces and matrix models. Particular attention is given to situations
where the moduli space gets quantum corrected. These corrections are controlled
by holomorphy. It is argued that these quantum deformations give rise to
non-trivial relations for generalized resolvents that must hold in the
associated matrix model. These relations allow to solve a sector of the
associated matrix model in a similar way to a one-matrix model, by studying a
curve that encodes the generalized resolvents. At the level of loop equations
for the matrix model, the situations with a moduli space can sometimes be
considered as a degeneration of an infinite set of linear equations, and the
quantum moduli space encodes the consistency conditions for these equations to
have a solution.Comment: 38 pages, JHEP style, 1 figur
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