520 research outputs found
Black objects in the Einstein-Gauss-Bonnet theory with negative cosmological constant and the boundary counterterm method
We propose to compute the action and global charges of the asymptotically
anti-de Sitter solutions in Einstein-Gauss-Bonnet theory by adding boundary
counterterms to the gravitational action. The general expression of the
counterterms and the boundary stress tensor is presented for spacetimes of
dimension . We apply this tehnique for several different types of
black objects. Apart from static and rotating black holes, we consider also
Einstein-Gauss-Bonnet black string solutions with negative cosmological
constant.Comment: 27 pages, 6 figures, references added, discussion extende
Non-perturbative spinning black holes in dynamical Chern-Simons gravity
Spinning black holes in dynamical Einstein-Chern-Simons gravity are
constructed by directly solving the field equations, without resorting to any
perturbative expansion. This model is obtained by adding to the
Einstein-Hilbert action a particular higher-curvature correction: the
Pontryagin density, linearly coupled to a scalar field. The spinning black
holes are stationary, axi-symmetric, asymptotically flat generalisations of the
Kerr solution of Einstein's gravity, but they possess a non-trivial
(odd-parity) scalar field. They are regular on and outside the horizon and
satisfy a generalized Smarr relation. We discuss the deviations from Kerr at
the level of the spin and mass distribution, the horizon angular velocity, the
ergo-region and some basic properties of geodesic motion. For sufficiently
small values of the Chern-Simons coupling our results match those previously
obtained using a perturbative approach.Comment: 14 pages, 5 figure
Spherical electro-vacuum black holes with resonant, scalar -hair
The asymptotically flat, spherical, electro-vacuum black holes (BHs) are
shown to support static, spherical configurations of a gauged,
self-interacting, scalar field, minimally coupled to the geometry. Considering
a -ball type potential for the scalar field, we dub these configurations
-clouds, in the test field approximation. The clouds exist under a resonance
condition, at the threshold of (charged) superradiance. This is similar to the
stationary clouds supported by Kerr BHs, which exist for a synchronisation
condition, at the threshold of (rotational) superradiance. In contrast with the
rotating case, however, -clouds require the scalar field to be massive and
self-interacting; no similar clouds exist for massive but free scalar fields.
First, considering a decoupling limit, we construct -clouds around
Schwarzschild and Reissner-Nordstr\"om BHs, showing there is always a mass gap.
Then, we make the -clouds backreact, and construct fully non-linear
solutions of the Einstein-Maxwell-gauged scalar system describing spherical,
charged BHs with resonant, scalar -hair. Amongst other properties, we
observe there is non-uniqueness of charged BHs in this model and the -hairy
BHs can be entropically preferred over Reissner-Nordstr\"om, for the same
charge to mass ratio; some -hairy BH solutions can be overcharged. We also
discuss how some well known no-hair theorems in the literature, applying to
electro-vacuum plus minimally coupled scalar fields, are circumvented by this
new type of BHs.Comment: 18 pages, 5 figures; v2. typos corrected, matches published versio
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