101 research outputs found
Dynamical friction in slab geometries and accretion disks
The evolution of planets, stars and even galaxies is driven, to a large
extent, by dynamical friction of gravitational origin. There is now a good
understanding of the friction produced by extended media, either collisionless
of fluid-like. However, the physics of accretion or protoplanetary disks, for
instance, is described by slab-like geometries instead, compact in one spatial
direction. Here, we find, for the first time, the gravitational wake due to a
massive perturber moving through a slab-like medium, describing e.g. accretion
disks with sharp transitions. We show that dynamical friction in such
environments can be substantially reduced relatively to spatially extended
profiles. Finally, we provide simple and accurate expressions for the
gravitational drag force felt by the perturber, in both the subsonic and
supersonic regime.Comment: 10 pages, 8 figure
Mathisson's helical motions demystified
The motion of spinning test particles in general relativity is described by
Mathisson-Papapetrou-Dixon equations, which are undetermined up to a spin
supplementary condition, the latter being today still an open question. The
Mathisson-Pirani (MP) condition is known to lead to rather mysterious helical
motions which have been deemed unphysical, and for this reason discarded. We
show that these assessments are unfounded and originate from a subtle (but
crucial) misconception. We discuss the kinematical explanation of the helical
motions, and dynamically interpret them through the concept of hidden momentum,
which has an electromagnetic analogue. We also show that, contrary to previous
claims, the frequency of the helical motions coincides exactly with the
zitterbewegung frequency of the Dirac equation for the electron.Comment: To appear in the Proceedings of the Spanish Relativity Meeting 2011
(ERE2011), "Towards new paradigms", Madrid 29 August - 2 September 201
Collisions of charged black holes
We perform fully nonlinear numerical simulations of charged-black-hole collisions, described by the Einstein-Maxwell equations, and contrast the results against analytic expectations. We focus on head-on collisions of nonspinning black holes, starting from rest and with the same charge-to-mass ratio, Q/M. The addition of charge to black holes introduces a new interesting channel of radiation and dynamics, most of which seem to be captured by Newtonian dynamics and flat-space intuition. The waveforms can be qualitatively described in terms of three stages: (i) an infall phase prior to the formation of a common apparent horizon; (ii) a nonlinear merger phase that corresponds to a peak in gravitational and electromagnetic energy; (iii) the ringdown marked by an oscillatory pattern with exponentially decaying amplitude and characteristic frequencies that are in good agreement with perturbative predictions. We observe that the amount of gravitational-wave energy generated throughout the collision decreases by about 3 orders of magnitude as the charge-to-mass ratio Q/M is increased from 0 to 0.98. We interpret this decrease as a consequence of the smaller accelerations present for larger values of the charge. In contrast, the ratio of energy carried by electromagnetic to gravitational radiation increases, reaching about 22% for the maximum Q/M ratio explored, which is in good agreement with analytic predictions
Collisions of oppositely charged black holes
The first fully non-linear numerical simulations of colliding charged black
holes in D=4 Einstein-Maxwell theory were recently reported arXiv:1205.1063.
These collisions were performed for black holes with equal charge-to-mass
ratio, for which initial data can be found in closed analytic form. Here we
generalize the study of collisions of charged black holes to the case of
unequal charge-to-mass ratios. We focus on oppositely charged black holes, as
to maximize acceleration-dependent effects. As |Q|/M increases from 0 to 0.99,
we observe that the gravitational radiation emitted increases by a factor of ~
2.7; the electromagnetic radiation emission becomes dominant for |Q|/M >~ 0.37
and at |Q|/M=0.99 is larger, by a factor of ~ 5.8, than its gravitational
counterpart. We observe that these numerical results exhibit a precise and
simple scaling with the charge. Furthermore, we show that the results from the
numerical simulations are qualitatively captured by a simple analytic model
that computes the electromagnetic dipolar radiation and the gravitational
quadrupolar radiation of two non-relativistic interacting particles in
Minkowski spacetime.Comment: 11 pages, 4 figures. Matches version to appear in PR
Strong cosmic censorship: The nonlinear story
A satisfactory formulation of the laws of physics entails that the future
evolution of a physical system should be determined from appropriate initial
conditions. The existence of Cauchy horizons in solutions of the Einstein field
equations is therefore problematic, and expected to be an unstable artifact of
General Relativity. This is asserted by the Strong Cosmic Censorship
Conjecture, which was recently put into question by an analysis of the
linearized equations in the exterior of charged black holes in an expanding
universe. Here, we numerically evolve the nonlinear Einstein-Maxwell-scalar
field equations with a positive cosmological constant, under spherical
symmetry, and provide strong evidence that mass inflation indeed does not occur
in the near extremal regime. This shows that nonlinear effects might not
suffice to save the Strong Cosmic Censorship Conjecture.Comment: 9 pages, 8 figures. v2: Matches published versio
Dynamics of black holes in de Sitter spacetimes
Nonlinear dynamics in cosmological backgrounds has the potential to teach us immensely about our Universe, and also to serve as prototype for nonlinear processes in generic curved spacetimes. Here we report on dynamical evolutions of black holes in asymptotically de Sitter spacetimes. We focus on the head-on collision of equal mass binaries and for the first time compare analytical and perturbative methods with full blown nonlinear simulations. Our results include an accurate determination of the merger/scatter transition (consequence of an expanding background) for small mass binaries and a test of the cosmic censorship conjecture, for large mass binaries. We observe that, even starting from small separations, black holes in large mass binaries eventually lose causal contact, in agreement with the conjecture
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