6,441 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
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
Neutron star sensitivities in Ho\u159ava gravity after GW170817
Horava gravity breaks boost invariance in the gravitational sector by introducing a preferred time foliation. The dynamics of this preferred slicing is governed, in the low-energy limit suitable for most astrophysical applications, by three dimensionless parameters , and . The first two of these parameters are tightly bound by solar system and gravitational wave propagation experiments, but remains relatively unconstrained (). We restrict here to the parameter space region defined by (with kept generic), which in a previous paper we showed to be the only one where black hole solutions are non-pathological at the universal horizon, and we focus on possible violations of the strong equivalence principle in systems involving neutron stars. We compute neutron star 'sensitivities', which parametrize violations of the strong equivalence principle at the leading post-Newtonian order, and find that they vanish identically, like in the black hole case, for and generic . This implies that no violations of the strong equivalence principle (neither in the conservative sector nor in gravitational wave fluxes) can occur at the leading post-Newtonian order in binaries of compact objects, and that data from binary pulsars and gravitational interferometers are unlikely to further constrain
Trans-Planckian Tail in a Theory with a Cutoff
Trans-planckian frequencies can be mimicked outside a black-hole horizon as a
tail of an exponentially large amplitude wave that is mostly hidden behind the
horizon. The present proposal requires implementing a final state condition.
This condition involves only frequencies below the cutoff scale. It may be
interpreted as a condition on the singularity. Despite the introduction of the
cutoff, the Hawking radiation is restored for static observers. Freely falling
observers see empty space outside the horizon, but are "heated" as they cross
the horizon.Comment: 17 pages, RevTe
Astrophysical Bounds on Planck Suppressed Lorentz Violation
This article reviews many of the observational constraints on Lorentz
symmetry violation (LV). We first describe the GZK cutoff and other phenomena
that are sensitive to LV. After a brief historical sketch of research on LV, we
discuss the effective field theory description of LV and related questions of
principle, technical results, and observational constraints. We focus on
constraints from high energy astrophysics on mass dimension five operators that
contribute to LV electron and photon dispersion relations at order E/M_Planck.
We also briefly discuss constraints on renormalizable operators, and review the
current and future contraints on LV at order (E/M_Planck)^2.Comment: 30 pages, submitted to Lecture Notes in Physics, Quantum Gravity
Phenomenology, eds. G.Amelino-Camelia, J. Kowalski-Glikman (Springer-Verlag
Stochastically Fluctuating Black-Hole Geometry, Hawking Radiation and the Trans-Planckian Problem
We study the propagation of null rays and massless fields in a black hole
fluctuating geometry. The metric fluctuations are induced by a small
oscillating incoming flux of energy. The flux also induces black hole mass
oscillations around its average value. We assume that the metric fluctuations
are described by a statistical ensemble. The stochastic variables are the
phases and the amplitudes of Fourier modes of the fluctuations. By averaging
over these variables, we obtain an effective propagation for massless fields
which is characterized by a critical length defined by the amplitude of the
metric fluctuations: Smooth wave packets with respect to this length are not
significantly affected when they are propagated forward in time. Concomitantly,
we find that the asymptotic properties of Hawking radiation are not severely
modified. However, backward propagated wave packets are dissipated by the
metric fluctuations once their blue shifted frequency reaches the inverse
critical length. All these properties bear many resemblences with those
obtained in models for black hole radiation based on a modified dispersion
relation. This strongly suggests that the physical origin of these models,
which were introduced to confront the trans-Planckian problem, comes from the
fluctuations of the black hole geometry.Comment: 32 page
Reality Conditions and Ashtekar Variables: a Different Perspective
We give in this paper a modified self-dual action that leads to the
-ADM formalism without having to face the difficult second class
constraints present in other approaches (for example if one starts from the
Hilbert-Palatini action). We use the new action principle to gain some new
insights into the problem of the reality conditions that must be imposed in
order to get real formulations from complex general relativity. We derive also
a real formulation for Lorentzian general relativity in the Ashtekar phase
space by using the modified action presented in the paper.Comment: 22 pages, LATEX, Preprint CGPG-94/10-
Inflation and de Sitter Thermodynamics
We consider the quasi-de Sitter geometry of the inflationary universe. We
calculate the energy flux of the slowly rolling background scalar field through
the quasi-de Sitter apparent horizon and set it equal to the change of the
entropy (1/4 of the area) multiplied by the temperature, dE=TdS. Remarkably,
this thermodynamic law reproduces the Friedmann equation for the rolling scalar
field. The flux of the slowly rolling field through the horizon of the quasi-de
Sitter geometry is similar to the accretion of a rolling scalar field onto a
black hole, which we also analyze. Next we add inflaton fluctuations which
generate scalar metric perturbations. Metric perturbations result in a
variation of the area entropy. Again, the equation dE=TdS with fluctuations
reproduces the linearized Einstein equations. In this picture as long as the
Einstein equations hold, holography does not put limits on the quantum field
theory during inflation. Due to the accumulating metric perturbations, the
horizon area during inflation randomly wiggles with dispersion increasing with
time. We discuss this in connection with the stochastic decsription of
inflation. We also address the issue of the instability of inflaton
fluctuations in the ``hot tin can'' picture of de Sitter horizon.Comment: 19 pages, 5 figure
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