9,888 research outputs found
Black holes die hard: can one spin-up a black hole past extremality?
A possible process to destroy a black hole consists on throwing point
particles with sufficiently large angular momentum into the black hole. In the
case of Kerr black holes, it was shown by Wald that particles with dangerously
large angular momentum are simply not captured by the hole, and thus the event
horizon is not destroyed. Here we reconsider this gedanken experiment for a
variety of black hole geometries, from black holes in higher dimensions to
black rings. We show that this particular way of destroying a black hole does
not succeed and that Cosmic Censorship is preserved.Comment: 10 pages, 7 figures. RevTex4
General covariance, and supersymmetry without supersymmetry
An unusual four-dimensional generally covariant and supersymmetric SU(2)
gauge theory is described. The theory has propagating degrees of freedom, and
is invariant under a local (left-handed) chiral supersymmetry, which is half
the supersymmetry of supergravity. The Hamiltonian 3+1 decomposition of the
theory reveals the remarkable feature that the local supersymmetry is a
consequence of Yang-Mills symmetry, in a manner reminiscent of how general
coordinate invariance in Chern-Simons theory is a consequence of Yang-Mills
symmetry. It is possible to write down an infinite number of conserved
currents, which strongly suggests that the theory is classically integrable. A
possible scheme for non-perturbative quantization is outlined. This utilizes
ideas that have been developed and applied recently to the problem of
quantizing gravity.Comment: 17 pages, RevTeX, two minor errors correcte
Lorentz violating kinematics: Threshold theorems
Recent tentative experimental indications, and the subsequent theoretical
speculations, regarding possible violations of Lorentz invariance have
attracted a vast amount of attention. An important technical issue that
considerably complicates detailed calculations in any such scenario, is that
once one violates Lorentz invariance the analysis of thresholds in both
scattering and decay processes becomes extremely subtle, with many new and
naively unexpected effects. In the current article we develop several extremely
general threshold theorems that depend only on the existence of some energy
momentum relation E(p), eschewing even assumptions of isotropy or monotonicity.
We shall argue that there are physically interesting situations where such a
level of generality is called for, and that existing (partial) results in the
literature make unnecessary technical assumptions. Even in this most general of
settings, we show that at threshold all final state particles move with the
same 3-velocity, while initial state particles must have 3-velocities
parallel/anti-parallel to the final state particles. In contrast the various
3-momenta can behave in a complicated and counter-intuitive manner.Comment: V1: 32 pages, 6 figures, 3 tables. V2: 5 references adde
Numerical simulations of gravitational collapse in Einstein-aether theory
We study gravitational collapse of a spherically symmetric scalar field in
Einstein-aether theory (general relativity coupled to a dynamical unit timelike
vector field). The initial value formulation is developed, and numerical
simulations are performed. The collapse produces regular, stationary black
holes, as long as the aether coupling constants are not too large. For larger
couplings a finite area singularity occurs. These results are shown to be
consistent with the stationary solutions found previously.Comment: 9 pages, 7 figures; v2: corrected typos, added minor clarifying
remarks, improved discussion of results in conclusio
Hawking radiation without black hole entropy
In this Letter I point out that Hawking radiation is a purely kinematic
effect that is generic to Lorentzian geometries. Hawking radiation arises for
any test field on any Lorentzian geometry containing an event horizon
regardless of whether or not the Lorentzian geometry satisfies the dynamical
Einstein equations of general relativity. On the other hand, the classical laws
of black hole mechanics are intrinsically linked to the Einstein equations of
general relativity (or their perturbative extension into either semiclassical
quantum gravity or string-inspired scenarios). In particular, the laws of black
hole thermodynamics, and the identification of the entropy of a black hole with
its area, are inextricably linked with the dynamical equations satisfied by the
Lorentzian geometry: entropy is proportional to area (plus corrections) if and
only if the dynamical equations are the Einstein equations (plus corrections).
It is quite possible to have Hawking radiation occur in physical situations in
which the laws of black hole mechanics do not apply, and in situations in which
the notion of black hole entropy does not even make any sense. This observation
has important implications for any derivation of black hole entropy that seeks
to deduce black hole entropy from the Hawking radiation.Comment: Uses ReV_TeX 3.0; Five pages in two-column forma
On the degeneracies of the mass-squared differences for three-neutrino oscillations
Using an algebraic formulation, we explore two well-known degeneracies
involving the mass-squared differences for three-neutrino oscillations assuming
CP symmetry is conserved. For vacuum oscillation, we derive the expression for
the mixing angles that permit invariance under the interchange of two
mass-squared differences. This symmetry is most easily expressed in terms of an
ascending mass order. This can be used to reduce the parameter space by one
half in the absence of the MSW effect. For oscillations in matter, we derive
within our formalism the known approximate degeneracy between the standard and
inverted mass hierarchies in the limit of vanishing . This is done
with a mass ordering that permits the map .
Our techniques allow us to translate mixing angles in this mass order
convention into their values for the ascending order convention. Using this
dictionary, we demonstrate that the vacuum symmetry and the approximate
symmetry invoked for oscillations in matter are distinctly different.Comment: 5 pages, revised manuscrip
Ionospheric Power-Spectrum Tomography in Radio Interferometry
A tomographic method is described to quantify the three-dimensional
power-spectrum of the ionospheric electron-density fluctuations based on
radio-interferometric observations by a two-dimensional planar array. The
method is valid to first-order Born approximation and might be applicable to
correct observed visibilities for phase variations due to the imprint of the
full three-dimensional ionosphere. It is shown that not the ionospheric
electron density distribution is the primary structure to model in
interferometry, but its autocorrelation function or equivalent its
power-spectrum. An exact mathematical expression is derived that provides the
three dimensional power-spectrum of the ionospheric electron-density
fluctuations directly from a rescaled scattered intensity field and an incident
intensity field convolved with a complex unit phasor that depends on the w-term
and is defined on the full sky pupil plane. In the limit of a small field of
view, the method reduces to the single phase screen approximation. Tomographic
self-calibration can become important in high-dynamic range observations at low
radio frequencies with wide-field antenna interferometers, because a
three-dimensional ionosphere causes a spatially varying convolution of the sky,
whereas a single phase screen results in a spatially invariant convolution. A
thick ionosphere can therefore not be approximated by a single phase screen
without introducing errors in the calibration process. By applying a Radon
projection and the Fourier projection-slice theorem, it is shown that the
phase-screen approach in three dimensions is identical to the tomographic
method. Finally we suggest that residual speckle can cause a diffuse intensity
halo around sources, due to uncorrectable ionospheric phase fluctuations in the
short integrations, which could pose a fundamental limit on the dynamic range
in long-integration images.Comment: 8 pages; Accepted for publication in Ap
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