100 research outputs found
Towards the entropy of gravity time-dependent models via the Cardy-Verlinde formula
For models with several time-dependent components generalized entropies can
be defined. This is shown for the Bianchi type IX model. We first derive the
Cardy-Verlinde formula under the assumption that the first law of
thermodynamics is valid. This leads to an explicit expression of the total
entropy associated with this type of universes. Assuming the validity of the
Cardy entropy formula, we obtain expressions for the corresponding Bekenstein,
Bekenstein-Hawking and Hubble entropies. We discuss the validity of the
Cardy-Verlinde formula and possible extensions of the outlined procedure to
other time-dependent models.Comment: 13 page
Supersymmetric AdS5 black holes
The first examples of supersymmetric, asymptotically AdS5, black hole
solutions are presented. They form a 1-parameter family of solutions of minimal
five-dimensional gauged supergravity. Their angular momentum can never vanish.
The solutions are obtained by a systematic analysis of supersymmetric solutions
with Killing horizons. Other new examples of such solutions are obtained. These
include solutions for which the horizon is a homogeneous Nil or SL(2,R)
manifold.Comment: 31 pages. v2: References and calculation of holographic stress tensor
added. v3: Solutions preserve 2 supersymmetries. Our original claim that they
preserve 4 supersymmetries was based on Ref. [30], which contains a mistake
(the general timelike solution preserves 2, not 4, supersymmetries). Nothing
else affecte
New Five Dimensional Black Holes Classified by Horizon Geometry, and a Bianchi VI Braneworld
We introduce two new families of solutions to the vacuum Einstein equations
with negative cosmological constant in 5 dimensions. These solutions are static
black holes whose horizons are modelled on the 3-geometries nilgeometry and
solvegeometry. Thus the horizons (and the exterior spacetimes) can be foliated
by compact 3-manifolds that are neither spherical, toroidal, hyperbolic, nor
product manifolds, and therefore are of a topological type not previously
encountered in black hole solutions. As an application, we use the
solvegeometry solutions to construct Bianchi VI braneworld cosmologies.Comment: LaTeX, 20 pages, 2 figures Typographical errors corrected, and
references to printed matter added in favour of preprints where possibl
Six-dimensional localized black holes: numerical solutions
To test the strong-gravity regime in Randall-Sundrum braneworlds, we consider
black holes bound to a brane. In a previous paper, we studied numerical
solutions of localized black holes whose horizon radii are smaller than the AdS
curvature radius. In this paper, we improve the numerical method and discuss
properties of the six dimensional (6D) localized black holes whose horizon
radii are larger than the AdS curvature radius. At a horizon temperature
, the thermodynamics of the localized black
hole undergo a transition with its character changing from a 6D Schwarzschild
black hole type to a 6D black string type. The specific heat of the localized
black holes is negative, and the entropy is greater than or nearly equal to
that of the 6D black strings with the same thermodynamic mass. The large
localized black holes show flattened horizon geometries, and the intrinsic
curvature of the horizon four-geometry becomes negative near the brane. Our
results indicate that the recovery mechanism of lower-dimensional Einstein
gravity on the brane works even in the presence of the black holes.Comment: 17 pages, 9 figures, RevTeX4, typos correcte
Anti-de Sitter Quotients: When Are They Black Holes?
We point out that the BTZ black holes, and their relatives, can be defined in
a cleaner way than they originally were. The covering space can be taken to be
anti-de Sitter space, period, while scri splits up into components due to
Misner singularities. Our definition permits us to choose between two
conflicting claims concerning BTZ black holes in 3+1 dimensions.Comment: 16 pages, 4 figures; minor polish adde
Quest for Localized 4-D Black Holes in Brane Worlds
We investigate the possibility of obtaining localized black hole solutions in
brane worlds by introducing a dependence of the four-dimensional line--element
on the extra dimension. An analysis, performed for the cases of an empty bulk
and of a bulk containing either a scalar or a gauge field, reveals that no
conventional type of matter can support such a dependence. Considering a
particular ansatz for the five-dimensional line--element that corresponds to a
black hole solution with a ``decaying'' horizon, we determine the bulk
energy--momentum tensor capable of sustaining such a behaviour. It turns out
that an exotic, shell-like distribution of matter is required. For such
solutions, the black hole singularity is indeed localized near the brane and
the spacetime is well defined near the AdS horizon, in contrast to the
behaviour found in black string type solutions.Comment: 17 pages, RevTex, 3 figures, version to appear in Physical Review D,
comments and references added, typos correcte
The pulse profile and spin evolution of the accreting pulsar in Terzan 5, IGR J17480-2446, during its 2010 outburst
(abridged) We analyse the spectral and pulse properties of the 11 Hz
transient accreting pulsar, IGR J17480-2446, in the globular cluster Terzan 5,
considering all the available RXTE, Swift and INTEGRAL observations performed
between October and November, 2010.
By measuring the pulse phase evolution we conclude that the NS spun up at an
average rate of =1.48(2)E-12 Hz/s, compatible with the accretion of the
Keplerian angular momentum of matter at the inner disc boundary. Similar to
other accreting pulsars, the stability of the pulse phases determined by using
the second harmonic component is higher than that of the phases based on the
fundamental frequency. Under the assumption that the second harmonic is a good
tracer of the neutron star spin frequency, we successfully model its evolution
in terms of a luminosity dependent accretion torque. If the NS accretes the
specific Keplerian angular momentum of the in-flowing matter, we estimate the
inner disc radius to lie between 47 and 93 km when the luminosity attains its
peak value. Smaller values are obtained if the interaction between the magnetic
field lines and the plasma in the disc is considered.
The phase-averaged spectrum is described by thermal Comptonization of photons
with energy of ~1 keV. A hard to soft state transition is observed during the
outburst rise. The Comptonized spectrum evolves from a Comptonizing cloud at an
electron temperature of ~20 keV towards an optically denser cloud at kT_e~3
keV. At the same time, the pulse amplitude decreases from 27% to few per cent
and becomes strongly energy dependent. We discuss various possibilities to
explain such a behaviour, proposing that at large accretion luminosities a
significant fraction of the in-falling matter is not channelled towards the
magnetic poles, but rather accretes more evenly onto the NS surface.Comment: To appear in MNRA
Anti-de Sitter no-hair, AdS/CFT and the brane-world
We study the asymptotic behavior of the bulk spacetimes with the negative
cosmological constant in the context of the brane-world scenario. We show that,
in Euclidean bulk, or in Lorentzian static bulk, some sequences of
hypersurfaces with the positive Ricci scalar evolve to the warped geometries
like the anti-de Sitter spacetime. Based on the AdS/CFT correspondence, we
discuss that the positivity of the Ricci scalar is related to the stability of
CFT on the brane. In addition, the brane-world is described from the
holographic point of view. The asymptotic local structure of the conformal
infinity is also investigated.Comment: 6 pages, minor changes, to appear in Phys. Rev.
Small localized black holes in a braneworld: Formulation and numerical method
No realistic black holes localized on a 3-brane in the Randall-Sundrum
infinite braneworld have been found so far. The problem of finding a static
black hole solution is reduced to a boundary value problem. We solve it by
means of a numerical method, and show numerical examples of a localized black
hole whose horizon radius is small compared to the bulk curvature scale. The
sequence of small localized black holes exhibits a smooth transition from a
five-dimensional Schwarzschild black hole, which is a solution in the limit of
small horizon radius. The localized black hole tends to flatten as its horizon
radius increases. However, it becomes difficult to find black hole solutions as
its horizon radius increases.Comment: RevTeX, 13 pages, 6 figures, references corrected, typos corrected;
to appear in Phys.Rev.
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