1,271 research outputs found
Quasi-Black Holes from Extremal Charged Dust
One can construct families of static solutions that can be viewed as
interpolating between nonsingular spacetimes and those containing black holes.
Although everywhere nonsingular, these solutions come arbitrarily close to
having a horizon. To an observer in the exterior region, it becomes
increasingly difficulty to distinguish these from a true black hole as the
critical limiting solution is approached. In this paper we use the
Majumdar-Papapetrou formalism to construct such quasi-black hole solutions from
extremal charged dust. We study the gravitational properties of these
solutions, comparing them with the the quasi-black hole solutions based on
magnetic monopoles. As in the latter case, we find that solutions can be
constructed with or without hair.Comment: 18 page
Phase and group velocity tracing analysis of projected wave packet motion along oblique radar beams ? qualitative analysis of QP echoes
International audienceThe wave packets of atmospheric gravity waves were numerically generated, with a given characteristic wave period, horizontal wave length and projection mean wind along the horizontal wave vector. Their projection phase and group velocities along the oblique radar beam (vpr and vgr), with different zenith angle ? and azimuth angle ?, were analyzed by the method of phase- and group-velocity tracing. The results were consistent with the theoretical calculations derived by the dispersion relation, reconfirming the accuracy of the method of analysis. The RTI plot of the numerical wave packets were similar to the striation patterns of the QP echoes from the FAI irregularity region. We propose that the striation range rate of the QP echo is equal to the radial phase velocity vpr, and the slope of the energy line across the neighboring striations is equal to the radial group velocity vgr of the wave packet; the horizontal distance between two neighboring striations is equal to the characteristic wave period ?. Then, one can inversely calculate all the properties of the gravity wave responsible for the appearance of the QP echoes. We found that the possibility of some QP echoes being generated by the gravity waves originated from lower altitudes cannot be ruled out
Chern-Simons Modification of General Relativity
General relativity is extended by promoting the three-dimensional
gravitational Chern-Simons term to four dimensions. This entails choosing an
embedding coordinate v_\mu -- an external quantity, which we fix to be a
non-vanishing constant in its time component. The theory is identical to one in
which the embedding coordinate is itself a dynamical variable, rather than a
fixed, external quantity. Consequently diffeomorphism symmetry breaking is
hidden in the modified theory: the Schwarzschild metric is a solution;
gravitational waves possess two polarizations, each traveling at the velocity
of light; a conserved energy-momentum (pseudo-) tensor can be constructed. The
modification is visible in the intensity of gravitational radiation: the two
polarizations of a gravity wave carry intensities that are suppressed/enchanced
by the extension.Comment: 19 pages, added references, an addition in section
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