600 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
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 , 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
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
Black Holes and Strings: the Polymer Link
Quantum aspects of black holes represent an important testing ground for a
theory of quantum gravity. The recent success of string theory in reproducing
the Bekenstein-Hawking black hole entropy formula provides a link between
general relativity and quantum mechanics via thermodynamics and statistical
mechanics. Here we speculate on the existence of new and unexpected links
between black holes and polymers and other soft-matter systems.Comment: 8 pages, harvmac, references adde
The ground-state of General Relativity, Topological Theories and Dark Matter
We suggest a limit of Einstein equations incorporating the state
as a solution. The large scale behavior of this theory has
interesting properties. For a spherical source, the velocity profile for
circular motions is of the form observed in galaxies (approximately flat). For
FRW cosmologies, the Friedman equation contains an additional contribution in
the matter sector.Comment: More clarifications on the interpretation of the limits. Shorter
version. 4 pages, two column, no figure
A variational principle for stationary, axisymmetric solutions of Einstein's equations
Stationary, axisymmetric, vacuum, solutions of Einstein's equations are
obtained as critical points of the total mass among all axisymmetric and
symmetric initial data with fixed angular momentum. In this
variational principle the mass is written as a positive definite integral over
a spacelike hypersurface. It is also proved that if absolute minimum exists
then it is equal to the absolute minimum of the mass among all maximal,
axisymmetric, vacuum, initial data with fixed angular momentum. Arguments are
given to support the conjecture that this minimum exists and is the extreme
Kerr initial data.Comment: 21 page
Quasi-normal modes of Schwarzschild-de Sitter black holes
The low-laying frequencies of characteristic quasi-normal modes (QNM) of
Schwarzschild-de Sitter (SdS) black holes have been calculated for fields of
different spin using the 6th-order WKB approximation and the approximation by
the P\"{o}shl-Teller potential. The well-known asymptotic formula for large
is generalized here on a case of the Schwarzchild-de Sitter black hole. In the
limit of the near extreme term the results given by both methods are
in a very good agreement, and in this limit fields of different spin decay with
the same rate.Comment: 9 pages, 1 ancillary Mathematica(R) noteboo
Compactification, topology change and surgery theory
We study the process of compactification as a topology change. It is shown
how the mediating spacetime topology, or cobordism, may be simplified through
surgery. Within the causal Lorentzian approach to quantum gravity, it is shown
that any topology change in dimensions may be achieved via a causally
continuous cobordism. This extends the known result for 4 dimensions.
Therefore, there is no selection rule for compactification at the level of
causal continuity. Theorems from surgery theory and handle theory are seen to
be very relevant for understanding topology change in higher dimensions.
Compactification via parallelisable cobordisms is particularly amenable to
study with these tools.Comment: 1+19 pages. LaTeX. 9 associated eps files. Discussion of disconnected
case adde
Black Hole Entropy, Topological Entropy and the Baum-Connes Conjecture in K-Theory
We shall try to exhibit a relation between black hole entropy and topological
entropy using the famous Baum-Connes conjecture for foliated manifolds which
are particular examples of noncommutative spaces. Our argument is qualitative
and it is based on the microscopic origin of the Beckenstein-Hawking
area-entropy formula for black holes, provided by superstring theory, in the
more general noncommutative geometric context of M-Theory following the Connes-
Douglas-Schwarz article.Comment: 17 pages, Latex, contains an important paragraph in section 2 which
gives a better understandin
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