7,023 research outputs found

### 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

### 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

### 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

### Cosmological string models from Milne spaces and SL(2,Z) orbifold

The $n+1$-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 $p=k \rho$, $k=(4-n)/3n$. 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

### Discrete gravity and and its continuum limit

Recently Gambini and Pullin proposed a new consistent discrete approach to
quantum gravity and applied it to cosmological models. One remarkable result of
this approach is that the cosmological singularity can be avoided in a general
fashion. However, whether the continuum limit of such discretized theories
exists is model dependent. In the case of massless scalar field coupled to
gravity with $\Lambda=0$, the continuum limit can only be achieved by fine
tuning the recurrence constant. We regard this failure as the implication that
cosmological constant should vary with time. For this reason we replace the
massless scalar field by Chaplygin gas which may contribute an effective
cosmological constant term with the evolution of the universe. It turns out
that the continuum limit can be reached in this case indeed.Comment: 16 pages,revised version published in MPL

### Statistical Effects and the Black Hole/D-brane Correspondence

The horizon area and curvature of three-charge BPS black strings are studied
in the D-brane ensemble for the stationary black string. The charge
distributions along the string are used to translate the classical expressions
for the horizon area and curvature of BPS black strings with waves into
operators on the D-brane Hilbert space. Despite the fact that any `wavy' black
string has smaller horizon area and divergent curvature, the typical values of
the horizon area and effects of the horizon curvature in the D-brane ensemble
deviate negligibly from those of the original stationary black string in the
limit of large integer charges. Whether this holds in general will depend on
certain properties of the quantum bound states.Comment: 13 pages, RevTex, small errors corrected, some interpretation changed
in light of new result

### 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

### Microstates of a Neutral Black Hole in M Theory

We consider vacuum solutions in M theory of the form of a five-dimensional
Kaluza-Klein black hole cross T^6. In a certain limit, these include the
five-dimensional neutral rotating black hole (cross T^6). From a IIA
standpoint, these solutions carry D0 and D6 charges. We show that there is a
weakly coupled D-brane description which precisely reproduces the
Hawking-Bekenstein entropy in the extremal limit, even though supersymmetry is
completely broken.Comment: 11 pages. v2: microstate counting extended to generic angular moment

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