11,625 research outputs found
Comments on Black Holes in String Theory
A very brief review is given of some of the developments leading to our
current understanding of black holes in string theory. This is followed by a
discussion of two possible misconceptions in this subject - one involving the
stability of small black holes and the other involving scale radius duality.
Finally, I describe some recent results concerning quasinormal modes of black
holes in anti de Sitter spacetime, and their implications for strongly coupled
conformal field theories (in various dimensions).Comment: 13 pages. Talk given at Strings '99, Potsdam, German
Black Strings and Classical Hair
We examine the geometry near the event horizon of a family of black string
solutions with traveling waves. It has previously been shown that the metric is
continuous there. Contrary to expectations, we find that the geometry is not
smooth, and the horizon becomes singular whenever a wave is present. Both five
dimensional and six dimensional black strings are considered with similar
results.Comment: 14 pages, harvma
Where is the Information Stored in Black Holes?
It is shown that many modes of the gravitational field exist only inside the
horizon of an extreme black hole in string theory. At least in certain cases,
the number of such modes is sufficient to account for the Bekenstein-Hawking
entropy. These modes are associated with sources which carry Ramond-Ramond
charge, and so may be viewed as the strong coupling limit of D-branes. Although
these sources naturally live at the singularity, they are well defined and
generate modes which extend out to the horizon. This suggests that the
information in an extreme black hole is not localized near the singularity or
the horizon, but extends between them.Comment: 21 pages, reference corrected and comment adde
Black Holes with Multiple Charges and the Correspondence Principle
We consider the entropy of near extremal black holes with multiple charges in
the context of the recently proposed correspondence principle of Horowitz and
Polchinski, including black holes with two, three and four Ramond-Ramond
charges. We find that at the matching point the black hole entropy can be
accounted for by massless open strings ending on the D-branes for all cases
except a black hole with four Ramond-Ramond charges, in which case a possible
resolution in terms of brane-antibrane excitations is considered.Comment: 26 pages, harvmac, minor correction
Focusing versus defocusing properties of truly naked black holes
We study the properties of the congruence of null geodesics propagating near
the so-called truly naked horizons (TNH) - objects having finite Kretschmann
scalar but with diverging tidal acceleration for freely falling observers. The
expansion of outgoing rays near the future horizon always tends to vanish for
the non-extremal case but may be non-zero for the distorted (ultra)extremal
one. It tends to diverge for the ingoing ones if the the null energy condition
(NEC) is satisfied in the vicinity of the horizon outside. However, it also
tends to zero for NEC violating cases except the remote horizons. We also
discuss the validity of test particle approximation for TNHs and find the
sufficient condition for backreaction remaining small.Comment: 16 pages. To appear in IJMP
Black holes with non-minimal derivative coupling
We study the gravitational field equations in the presence of a coupling
between the derivative of a massless scalar field and the Einstein tensor. This
configuration is motivated by Galileon gravity as it preserves shift invariance
in the scalar sector. We analytically obtain solutions with static and
spherically symmetric geometry, which also include black holes with a single
regular horizon. We examine the thermodynamical properties of these solutions,
and we reveal the non-perturbative nature of the Galileon coupling constant. We
also find a phase transition, similar to the one described by Hawking and Page,
which occurs at a critical temperature determined by both the black hole mass
and by the strength of the coupling.Comment: Matches the published versio
Atmospheric densities from Explorer 17 density gauges and a comparison with satellite drag data
Atmospheric density data from Explorer XVII GAUGES and satellite drag dat
Cosmological string models from Milne spaces and SL(2,Z) orbifold
The -dimensional Milne Universe with extra free directions is used to
construct simple FRW cosmological string models in four dimensions, describing
expansion in the presence of matter with , . We then
consider the n=2 case and make SL(2,Z) orbifold identifications. The model is
surprisingly related to the null orbifold with an extra reflection generator.
The study of the string spectrum involves the theory of harmonic functions in
the fundamental domain of SL(2,Z). In particular, from this theory one can
deduce a bound for the energy gap and the fact that there are an infinite
number of excitations with a finite degeneracy. We discuss the structure of
wave functions and give examples of physical winding states becoming light near
the singularity.Comment: 14 pages, harvma
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