25 research outputs found
Detection of gravitational waves from black holes: Is there a window for alternative theories?
Recently LIGO and VIRGO collaborations reported about observation of
gravitational-wave signal corresponding to the inspiral and merger of two black
holes, resulting into formation of the final black hole. It was shown that the
observations are consistent with the Einstein theory of gravity with high
accuracy limited mainly by the statistical error. Angular momentum and mass of
the final black hole were determined with rather large allowance of tens of
percents. Here we shall show that this indeterminacy in the range of the
black-hole parameters allows for some not negligible deformations of the Kerr
spacetime leading to the same frequencies of black-hole ringing. This means
that at the current precision of the experiment there remain some possibilities
for alternative theories of gravity.Comment: 5 pages, 1 figur
Massive scalar field quasi-normal modes of higher dimensional black holes
We study quasinormal spectrum of massive scalar field in the -dimensional
black hole background. We found the qualitatively different dependence on the
field mass of the fundamental modes for . The behaviour of higher modes
is qualitatively the same for all . Thus for some particular values of mass
(of the field and of the black hole) the spectrum has two dominating
oscillations with a very long lifetime. Also we show that the asymptotically
high overtones do not depend on the field mass. In addition, we present the
generalisation of the Nollert improvement of the continued fraction technique
for the numerical calculation of quasi-normal frequencies of -dimensional
black holes.Comment: 8 pages, 4 figures, misprints corrected, version to appear in Phys.
Rev.
Gravitational instability of simply rotating AdS black holes in higher dimensions
We study the stability of AdS black hole holes rotating in a single two plane
for tensor-type gravitational perturbations in space-time dimensions.
First, by an analytic method, we show that there exists no unstable mode when
the magnitude of the angular momentum is smaller than where
is the horizon radius, and is the AdS curvature radius. Then, by numerical
calculations of quasinormal modes, using the separability of the relevant
perturbation equations, we show that an instability occurs for rapidly rotating
black holes with , although the growth rate is tiny (of order
of the inverse horizon radius). We give numerical evidences
indicating that this instability is caused by superradiance.Comment: 17 page
Quasi-normal modes of the scalar hairy black hole
We calculate QNMs of the scalar hairy black hole in the AdS background using
Horowitz-Hubeny method for the potential that is not known in analytical form.
For some black hole parameters we found pure imaginary frequencies. Increasing
of the scalar field mass does not cause the imaginary part to vanish, it
reaches some minimum and then increases, thus in the case under consideration
the infinitely long living modes (quasi-resonances) do not appear.Comment: 17 pages, 17 figures, LaTe
Gravitational Lensing by Black Holes
We review the theoretical aspects of gravitational lensing by black holes,
and discuss the perspectives for realistic observations. We will first treat
lensing by spherically symmetric black holes, in which the formation of
infinite sequences of higher order images emerges in the clearest way. We will
then consider the effects of the spin of the black hole, with the formation of
giant higher order caustics and multiple images. Finally, we will consider the
perspectives for observations of black hole lensing, from the detection of
secondary images of stellar sources and spots on the accretion disk to the
interpretation of iron K-lines and direct imaging of the shadow of the black
hole.Comment: Invited article for the GRG special issue on lensing (P. Jetzer, Y.
Mellier and V. Perlick Eds.). 31 pages, 12 figure