24,907 research outputs found
Anomalous character of the axion-photon coupling in a magnetic field distorted by a pp-wave gravitational background
We study the problem of axion-photon coupling in the magnetic field
influenced by gravitational radiation. We focus on exact solutions to the
equations for axion electrodynamics in the pp-wave gravitational background for
two models with initially constant magnetic field. The first model describes
the response of an initially constant magnetic field in a gravitational-wave
vacuum with unit refraction index; the second model is characterized by a
non-unit refraction index prescribed to the presence of ordinary and/or dark
matter. We show that both models demonstrate anomalous behavior of the
electromagnetic field generated by the axion-photon coupling in the presence of
magnetic field, evolving in the gravitational wave background. The role of
axionic dark matter in the formation of the anomalous response of this
electrodynamic system is discussed.Comment: 26 pages, no figure
Electron-positron pair creation in a vacuum by an electromagnetic field in 3+1 and lower dimensions
We calculate the probability of electron-positron pair creation in vacuum in
3+1 dimensions by an external electromagnetic field composed of a constant
uniform electric field and a constant uniform magnetic field, both of arbitrary
magnitudes and directions. The same problem is also studied in 2+1 and 1+1
dimensions in appropriate external fields and similar results are obtained.Comment: REVTeX, 10 pages, no figure, a brief note and some more references
added in the proo
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Effective video multicast over wireless internet
With the rapid growth of wireless networks and great success of Internet video, wireless video services are expected to be widely deployed in the near future. As different types of wireless networks are converging into all IP networks, i.e., the Internet, it is important to study video delivery over the wireless Internet. This paper proposes a novel end-system based adaptation protocol calledWireless Hybrid Adaptation Layered Multicast (WHALM) protocol for layered video multicast over wireless Internet. In WHALM the sender dynamically collects bandwidth distribution from the receivers and uses an optimal layer rate allocation mechanism to reduce the mismatches between the coarse-grained layer subscription levels and the heterogeneous and dynamic rate requirements from the receivers, thus maximizing the degree of satisfaction of all the receivers in a multicast session. Based on sampling theory and theory of probability, we reduce the required number of bandwidth feedbacks to a reasonable degree and use a scalable feedback mechanism to control the feedback process practically. WHALM is also tuned to perform well in wireless networks by integrating an end-to-end loss differentiation algorithm (LDA) to differentiate error losses from congestion losses at the receiver side. With a series of simulation experiments over NS platform, WHALM has been proved to be able to greatly improve the degree of satisfaction of all the receivers while avoiding congestion collapse on the wireless Internet
The effect of the motion of the Sun on the light-time in interplanetary relativistic experiments
In 2002 a measurement of the effect of solar gravity upon the phase of
coherent microwave beams passing near the Sun has been carried out with the
Cassini mission, allowing a very accurate measurement of the PPN parameter
. The data have been analyzed with NASA's Orbit Determination Program
(ODP) in the Barycentric Celestial Reference System, in which the Sun moves
around the centre of mass of the solar system with a velocity of
about 10 m/sec; the question arises, what correction this implies for the
predicted phase shift. After a review of the way the ODP works, we set the
problem in the framework of Lorentz (and Galilean) transformations and evaluate
the correction; it is several orders of magnitude below our experimental
accuracy. We also discuss a recent paper \cite{kopeikin07}, which claims wrong
and much larger corrections, and clarify the reasons for the discrepancy.Comment: Final version accepted by Classical and Quantum Gravity (8 Jan. 2008
Pseudoscalar-photon Interactions, Axions, Non-Minimal Extensions, and Their Empirical Constraints from Observations
Pseudoscalar-photon interactions were proposed in the study of the relations
among equivalence principles. The interaction of pseudoscalar axion with gluons
was proposed as a way to solve the strong CP problem. Subsequent proposal of
axion as a dark matter candidate has been a focus of search. Motivation from
superstring theories add to its importance. After a brief introduction and
historical review, we present (i) the current status of our optical experiment
using high-finesse Fabry-Perot resonant cavity - Q & A experiment - to detect
pseudoscalar-photon interactions, (ii) the constraints on pseudoscalar-photon
interactions from astrophysical and cosmological observations on cosmic
polarization rotation, and (iii) theoretical models of non-minimal interactions
of gravitational, electromagnetic and pseudoscalar (axion) fields, and their
relevance to cosmology.Comment: 8 page
An optimal gap theorem
By solving the Cauchy problem for the Hodge-Laplace heat equation for
-closed, positive -forms, we prove an optimal gap theorem for
K\"ahler manifolds with nonnegative bisectional curvature which asserts that
the manifold is flat if the average of the scalar curvature over balls of
radius centered at any fixed point is a function of .
Furthermore via a relative monotonicity estimate we obtain a stronger
statement, namely a `positive mass' type result, asserting that if is
not flat, then for any
Non-minimal coupling of photons and axions
We establish a new self-consistent system of equations accounting for a
non-minimal interaction of gravitational, electromagnetic and axion fields. The
procedure is based on a non-minimal extension of the standard
Einstein-Maxwell-axion action. The general properties of a ten-parameter family
of non-minimal linear models are discussed. We apply this theory to the models
with pp-wave symmetry and consider propagation of electromagnetic waves
non-minimally coupled to the gravitational and axion fields. We focus on exact
solutions of electrodynamic equations, which describe quasi-minimal and
non-minimal optical activity induced by the axion field. We also discuss
empirical constraints on coupling parameters from astrophysical birefringence
and polarization rotation observations.Comment: 31 pages, 2 Tables; replaced with the final version published in
Classical and Quantum Gravit
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