20,537 research outputs found
Pair creation of particles and black holes in external fields
It is well known that massive black holes may form through the gravitational
collapse of a massive astrophysical body. Less known is the fact that a black
hole can be produced by the quantum process of pair creation in external
fields. These black holes may have a mass much lower than their astrophysical
counterparts. This mass can be of the order of Planck mass so that quantum
effects may be important. This pair creation process can be investigated
semiclassically using non-perturbative instanton methods, thus it may be used
as a theoretical laboratory to obtain clues for a quantum gravity theory. In
this work, we review briefly the history of pair creation of particles and
black holes in external fields. In order to present some features of the
euclidean instanton method which is used to calculate pair creation rates, we
study a simple model of a scalar field and propose an effective one-loop action
for a two-dimensional soliton pair creation problem. This action is built from
the soliton field itself and the soliton charge is no longer treated as a
topological charge but as a Noether charge. The results are also valid
straightforwardly to the problem of pair creation rate of domain walls in
dimensions greater than 2.Comment: LaTeX file (World Scientific macros), no figures, 9 pages, talk given
at Xth Portuguese Meeting on Astronomy and Astrophysics, (Lisbon, Portugal,
27-28 July 2000), to be published in Proc. Xth A & A meeting, edited by J. P.
S. Lemos, A. Mourao, L. Teodoro, R. Ugoccioni, (World Scientific, 2001
Hairy black holes and the endpoint of AdS charged superradiance
We construct hairy black hole solutions that merge with the anti-de Sitter
(AdS) Reissner-Nordstr\"om black hole at the onset of superradiance. These
hairy black holes have, for a given mass and charge, higher entropy than the
corresponding AdS-Reissner-Nordstr\"om black hole. Therefore, they are
natural candidates for the endpoint of the charged superradiant instability. On
the other hand, hairy black holes never dominate the canonical and
grand-canonical ensembles. The zero-horizon radius of the hairy black holes is
a soliton (i.e. a boson star under a gauge transformation). We construct our
solutions perturbatively, for small mass and charge, so that the properties of
hairy black holes can be used to testify and compare with the endpoint of
initial value simulations. We further discuss the near-horizon scalar
condensation instability which is also present in global
AdS-Reissner-Nordstr\"om black holes. We highlight the different nature of
the near-horizon and superradiant instabilities and that hairy black holes
ultimately exist because of the non-linear instability of AdS.Comment: 41 pages, 6 figures. v2: Minor changes to match published versio
Charged Rotating Black Hole Formation from Thin Shell Collapse in Three Dimensions
The thin shell collapse leading to the formation of charged rotating black
holes in three dimensions is analyzed in the light of a recently developed
Hamiltonian formalism for these systems. It is proposed to demand, as a way to
reconcile the properties of an infinitely extended solenoid in flat space with
a magnetic black hole in three dimensions, that the magnetic field should
vanish just outside the shell. The adoption of this boundary condition results
in an exterior solution with a magnetic field different from zero at a finite
distance from the shell. The interior solution is also found and assigns
another interpretation, in a different context, to the magnetic solution
previously obtained by Cl\'{e}ment and by Hirschmann and Welch.Comment: 15 pages, no figures. Discussion on junction conditions and
conclusions enlarged. Few references added. Final version for MPL
Boundary Conditions for Kerr-AdS Perturbations
The Teukolsky master equation and its associated spin-weighted spheroidal
harmonic decomposition simplify considerably the study of linear gravitational
perturbations of the Kerr(-AdS) black hole. However, the formulation of the
problem is not complete before we assign the physically relevant boundary
conditions. We find a set of two Robin boundary conditions (BCs) that must be
imposed on the Teukolsky master variables to get perturbations that are
asymptotically global AdS, i.e. that asymptotes to the Einstein Static
Universe. In the context of the AdS/CFT correspondence, these BCs allow a
non-zero expectation value for the CFT stress-energy tensor while keeping fixed
the boundary metric. When the rotation vanishes, we also find the gauge
invariant differential map between the Teukolsky and the Kodama-Ishisbashi
(Regge-Wheeler-Zerilli) formalisms. One of our Robin BCs maps to the scalar
sector and the other to the vector sector of the Kodama-Ishisbashi
decomposition. The Robin BCs on the Teukolsky variables will allow for a
quantitative study of instability timescales and quasinormal mode spectrum of
the Kerr-AdS black hole. As a warm-up for this programme, we use the Teukolsky
formalism to recover the quasinormal mode spectrum of global AdS-Schwarzschild,
complementing previous analysis in the literature.Comment: 33 pages, 6 figure
Algebraically special perturbations of the Schwarzschild solution in higher dimensions
We study algebraically special perturbations of a generalized Schwarzschild
solution in any number of dimensions. There are two motivations. First, to
learn whether there exist interesting higher-dimensional algebraically special
solutions beyond the known ones. Second, algebraically special perturbations
present an obstruction to the unique reconstruction of general metric
perturbations from gauge-invariant variables analogous to the Teukolsky scalars
and it is desirable to know the extent of this non-uniqueness. In four
dimensions, our results generalize those of Couch and Newman, who found
infinite families of time-dependent algebraically special perturbations. In
higher dimensions, we find that the only regular algebraically special
perturbations are those corresponding to deformations within the Myers-Perry
family. Our results are relevant for several inequivalent definitions of
"algebraically special".Comment: 23 pages, no figures. v2: references added; discussion improved;
matches published versio
AdS nonlinear instability: moving beyond spherical symmetry
Anti-de Sitter (AdS) is conjectured to be nonlinear unstable to a weakly
turbulent mechanism that develops a cascade towards high frequencies, leading
to black hole formation [1,2]. We give evidence that the gravitational sector
of perturbations behaves differently from the scalar one studied in [2]. In
contrast with [2], we find that not all gravitational normal modes of AdS can
be nonlinearly extended into periodic horizonless smooth solutions of the
Einstein equation. In particular, we show that even seeds with a single normal
mode can develop secular resonances, unlike the spherically symmetric scalar
field collapse studied in [2]. Moreover, if the seed has two normal modes, more
than one resonance can be generated at third order, unlike the spherical
collapse of [2]. We also show that weak turbulent perturbative theory predicts
the existence of direct and inverse cascades, with the former dominating the
latter for equal energy two-mode seeds.Comment: 7 pages, no figures, 2 table
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