4,433 research outputs found
On a Classical, Geometric Origin of Magnetic Moments, Spin-Angular Momentum and the Dirac Gyromagnetic Ratio
By treating the real Maxwell Field and real linearized Einstein equations as
being imbedded in complex Minkowski space, one can interpret magnetic moments
and spin-angular momentum as arising from a charge and mass monopole source
moving along a complex world line in the complex Minkowski space. In the
circumstances where the complex center of mass world-line coincides with the
complex center of charge world-line, the gyromagnetic ratio is that of the
Dirac electron.Comment: 17 page
Edge modes in band topological insulators
We characterize gapless edge modes in translation invariant topological
insulators. We show that the edge mode spectrum is a continuous deformation of
the spectrum of a certain gluing function defining the occupied state bundle
over the Brillouin zone (BZ). Topologically non-trivial gluing functions,
corresponding to non-trivial bundles, then yield edge modes exhibiting spectral
flow. We illustrate our results for the case of chiral edge states in two
dimensional Chern insulators, as well as helical edges in quantum spin Hall
states.Comment: 4 pages, 2 figures; v4 minor change
On the Transfer of Metric Fluctuations when Extra Dimensions Bounce or Stabilize
In this report, we study within the context of general relativity with one
extra dimension compactified either on a circle or an orbifold, how radion
fluctuations interact with metric fluctuations in the three non-compact
directions. The background is non-singular and can either describe an extra
dimension on its way to stabilization, or immediately before and after a series
of non-singular bounces. We find that the metric fluctuations transfer
undisturbed through the bounces or through the transients of the
pre-stabilization epoch. Our background is obtained by considering the effects
of a gas of massless string modes in the context of a consistent 'massless
background' (or low energy effective theory) limit of string theory. We discuss
applications to various approaches to early universe cosmology, including the
ekpyrotic/cyclic universe scenario and string gas cosmology.Comment: V2. Minor Clarifications V3. appendix and 2 figures added, typos
corrected, conclusions unchanged 12 pages, 6 figure
Discovery of Bright Variable X-ray Sources in NGC 1569 with Chandra
From the analysis of a ~100 ks Chandra observation of the dwarf starburst
galaxy NGC 1569, we have found that the X-ray point sources, CXOU
043048.1+645050 and CXOU 043048.6+645058, showed significant time variability.
During this observation, the X-ray flux of CXOU 043048.1+645050 increased by 10
times in only 2 x 10^4 s. Since the spectrum in its bright phase was fitted
with a disk blackbody model with kT_in ~0.43 keV and the bolometric luminosity
is L_bol ~10^38 ergs s^-1, this source is an X-ray binary with a stellar mass
black-hole. Since the spectrum in its faint phase was also fitted with a disk
blackbody model, the time variability can be explained by a change of the
accretion rate onto the black hole. The other variable source, CXOU
043048.6+645058, had a flat spectrum with a photon index of ~1.6. This source
may be an X-ray binary with an X-ray luminosity of several x 10^37 ergs s^-1.
In addition, three other weak sources showed possible time variability. Taking
all of the variability into account may suggest an abundant population of
compact X-ray sources in NGC 1569.Comment: 15 pages including 4 Postscript figures; accepted for publication in
ApJ
Volume Expansion of Swiss-Cheese Universe
In order to investigate the effect of inhomogeneities on the volume expansion
of the universe, we study modified Swiss-Cheese universe model. Since this
model is an exact solution of Einstein equations, we can get an insight into
non-linear dynamics of inhomogeneous universe from it. We find that
inhomogeneities make the volume expansion slower than that of the background
Einstein-de Sitter universe when those can be regarded as small fluctuations in
the background universe. This result is consistent with the previous studies
based on the second order perturbation analysis. On the other hand, if the
inhomogeneities can not be treated as small perturbations, the volume expansion
of the universe depends on the type of fluctuations. Although the volume
expansion rate approaches to the background value asymptotically, the volume
itself can be finally arbitrarily smaller than the background one and can be
larger than that of the background but there is an upper bound on it.Comment: 22 pages, 7 figures, to be submitted to Phys. Rev.
H_2 Absorption and Fluorescence for Gamma Ray Bursts in Molecular Clouds
If a gamma ray burst with strong UV emission occurs in a molecular cloud,
there will be observable consequences resulting from excitation of the
surrounding H2. The UV pulse from the GRB will pump H2 into
vibrationally-excited levels which produce strong absorption at wavelengths <
1650 A. As a result, both the prompt flash and later afterglow will exhibit
strong absorption shortward of 1650 A, with specific spectroscopic features.
Such a cutoff in the emission from GRB 980329 may already have been observed by
Fruchter et al.; if so, GRB 980329 was at redshift 3.0 < z < 4.4 . BVRI
photometry of GRB 990510 could also be explained by H2 absorption if GRB 990510
is at redshift 1.6 < z < 2.3. The fluorescence accompanying the UV pumping of
the H2 will result in UV emission from the GRB which can extend over days or
months, depending on parameters of the ambient medium and beaming of the GRB
flash. The 7.5-13.6 eV fluorescent luminosity is \sim 10^{41.7} erg/s for
standard estimates of the parameters of the GRB and the ambient medium.
Spectroscopy can distinguish this fluorescent emission from other possible
sources of transient optical emission, such as a supernova.Comment: 13 pages, including 4 figures. submitted to Ap.J.(Letters
Anisotropic domain walls
We find an anisotropic, non-supersymmetric generalization of the extreme
supersymmetric domain walls of simple non-dilatonic supergravity theory. As
opposed to the isotropic non- and ultra-extreme domain walls, the anisotropic
non-extreme wall has the \emph{same} spatial topology as the extreme wall. The
solution has naked singularities which vanish in the extreme limit. Since the
Hawking temperature on the two sides is different, the generic solution is
unstable to Hawking decay.Comment: 11 pages, LaTeX, 3 PostScript figures, uses amstex and epsfi
Gauss-Bonnet brane gravity with a confining potential
A brane scenario is envisaged in which the -dimensional bulk is endowed
with a Gauss-Bonnet term and localization of matter on the brane is achieved by
means of a confining potential. The resulting Friedmann equations on the brane
are modified by various extra terms that may be interpreted as the X-matter,
providing a possible phenomenological explanation for the accelerated expansion
of the universe. The age of the universe in this scenario is studied and shown
to be consistent with the present observational data.Comment: 14 pages, 4 figures, to appear in PR
Entropy Identity and Material-Independent Equilibrium Conditions in Relativistic Thermodynamics
On the basis of the balance equations for energy-momentum, spin, particle and
entropy density, an approach is considered which represents a comparatively
general framework for special- and general-relativistic continuum
thermodynamics. In the first part of the paper, a general entropy density
4-vector, containing particle, energy-momentum, and spin density contributions,
is introduced which makes it possible, firstly, to judge special assumptions
for the entropy density 4-vector made by other authors with respect to their
generality and validity and, secondly, to determine entropy supply and entropy
production. Using this entropy density 4-vector, in the second part,
material-independent equilibrium conditions are discussed. While in literature,
at least if one works in the theory of irreversible thermodynamics assuming a
Riemann space-time structure, generally thermodynamic equilibrium is determined
by introducing a variety of conditions by hand, the present approach proceeds
as follows: For a comparatively wide class of space-time geometries the
necessary equilibrium conditions of vanishing entropy supply and entropy
production are exploited and, afterwards, supplementary conditions are assumed
which are motivated by the requirement that thermodynamic equilibrium
quantities have to be determined uniquely.Comment: Research Paper, 30 page
Stability of Chaplygin gas thin-shell wormholes
In this paper we construct spherical thin-shell wormholes supported by a
Chaplygin gas. For a rather general class of geometries we introduce a new
approach for the stability analysis of static solutions under perturbations
preserving the symmetry. We apply this to wormholes constructed from
Schwarzschild, Schwarzschild-de Sitter, Schwarzschild-anti de Sitter and
Reissner-Nordstrom metrics. In the last two cases, we find that there are
values of the parameters for which stable static solutions exist.Comment: 14 pages, 5 figures; v2: minor changes and new references added.
Accepted for publication in Physical Review
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