1,193 research outputs found

### Virtual Black Holes

One would expect spacetime to have a foam-like structure on the Planck scale
with a very high topology. If spacetime is simply connected (which is assumed
in this paper), the non-trivial homology occurs in dimension two, and spacetime
can be regarded as being essentially the topological sum of $S^2\times S^2$ and
$K3$ bubbles. Comparison with the instantons for pair creation of black holes
shows that the $S^2\times S^2$ bubbles can be interpreted as closed loops of
virtual black holes. It is shown that scattering in such topological
fluctuations leads to loss of quantum coherence, or in other words, to a
superscattering matrix $\$$ that does not factorise into an $S$ matrix and its
adjoint. This loss of quantum coherence is very small at low energies for
everything except scalar fields, leading to the prediction that we may never
observe the Higgs particle. Another possible observational consequence may be
that the $\theta$ angle of QCD is zero without having to invoke the
problematical existence of a light axion. The picture of virtual black holes
given here also suggests that macroscopic black holes will evaporate down to
the Planck size and then disappear in the sea of virtual black holes.Comment: 24p, LaTeX, 3 postscript figures included with epsf sent in a
seperate uuencoded fil

### Open Inflation, the Four Form and the Cosmological Constant

Fundamental theories of quantum gravity such as supergravity include a four
form field strength which contributes to the cosmological constant. The
inclusion of such a field into our theory of open inflation (hep-th/9802030)
allows an anthropic solution to the cosmological constant problem in which the
cosmological constant gives a small but non-negligible contribution to the
density of today's universe. We include a discussion of the role of the
singularity in our solution and a reply to Vilenkin's recent criticism
(hep-th/9803084).Comment: 11 pages, RevTex file. Replaced version contains major corrections,
including a crucial new surface term, and important additions. A connection
to eleven dimensional supergravity is made. The anthropic solution of the
cosmological constant problem now holds with a real four form in the
Lorentzian region. The previously claimed solution to the empty universe
problem is shown to be incorrect, but a new solution is suggeste

### No-Go Theorem in Spacetimes with Two Commuting Spacelike Killing Vectors

Four-dimensional Riemannian spacetimes with two commuting spacelike Killing
vectors are studied in Einstein's theory of gravity, and found that no outer
apparent horizons exist, provided that the dominant energy condition holds.Comment: latex, 1 figure, version published in Gen. Relativ. Grav., 37,
1919-1926 (2005

### Comment on "The black hole final state"

Horowitz and Maldacena have suggested that the unitarity of the black hole
S-matrix can be reconciled with Hawking's semiclassical arguments if a
final-state boundary condition is imposed at the spacelike singularity inside
the black hole. We point out that, in this scenario, departures from unitarity
can arise due to interactions between the collapsing body and the infalling
Hawking radiation inside the event horizon. The amount of information lost when
a black hole evaporates depends on the extent to which these interactions are
entangling.Comment: 4 pages, REVTe

### Why Does Inflation Start at the Top of the Hill?

We show why the universe started in an unstable de Sitter state. The quantum
origin of our universe implies one must take a `top down' approach to the
problem of initial conditions in cosmology, in which the histories that
contribute to the path integral, depend on the observable being measured. Using
the no boundary proposal to specify the class of histories, we study the
quantum cosmological origin of an inflationary universe in theories like trace
anomaly driven inflation in which the effective potential has a local maximum.
We find that an expanding universe is most likely to emerge in an unstable de
Sitter state, by semiclassical tunneling via a Hawking-Moss instanton. Since
the top down view is forced upon us by the quantum nature of the universe, we
argue that the approach developed here should still apply when the framework of
quantum cosmology will be based on M-Theory.Comment: 21 pages, 1 figur

### Entropy in the RST Model

The RST Model is given boundary term and Z-field so that it is well-posed and
local. The Euclidean method is described for general theory and used to
calculate the RST intrinsic entropy. The evolution of this entropy for the
shockwave solutions is found and obeys a second law.Comment: 10 pages, minor revisions, published version in Late

### Classical and Thermodynamic Stability of Black Branes

It is argued that many non-extremal black branes exhibit a classical
Gregory-Laflamme instability if, and only if, they are locally
thermodynamically unstable. For some black branes, the Gregory-Laflamme
instability must therefore disappear near extremality. For the black $p$-branes
of the type II supergravity theories, the Gregory-Laflamme instability
disappears near extremality for $p=1,2,4$ but persists all the way down to
extremality for $p=5,6$ (the black D3-brane is not covered by the analysis of
this paper). This implies that the instability also vanishes for the
near-extremal black M2 and M5-brane solutions.Comment: 21 pages, LaTeX. v2: Various points clarified, typos corrected and
reference adde

### Open Inflation With Scalar-tensor Gravity

The open inflation model recently proposed by Hawking and Turok is
investigated in scalar-tensor gravity context. If the dilaton-like field has no
potential, the instanton of our model is singular but has a finite action. The
Gibbons-Hawking surface term vanishes and hence, can not be used to make
$\Omega_0$ nonzero. To obtain a successful open inflation one should introduce
other matter fields or a potential for the dilaton-like fields.Comment: 10 pages.1 figure. Some comments and references are improved. to be
published in PR

### Black Hole Entropy is Noether Charge

We consider a general, classical theory of gravity in $n$ dimensions, arising
from a diffeomorphism invariant Lagrangian. In any such theory, to each vector
field, $\xi^a$, on spacetime one can associate a local symmetry and, hence, a
Noether current $(n-1)$-form, ${\bf j}$, and (for solutions to the field
equations) a Noether charge $(n-2)$-form, ${\bf Q}$. Assuming only that the
theory admits stationary black hole solutions with a bifurcate Killing horizon,
and that the canonical mass and angular momentum of solutions are well defined
at infinity, we show that the first law of black hole mechanics always holds
for perturbations to nearby stationary black hole solutions. The quantity
playing the role of black hole entropy in this formula is simply $2 \pi$ times
the integral over $\Sigma$ of the Noether charge $(n-2)$-form associated with
the horizon Killing field, normalized so as to have unit surface gravity.
Furthermore, we show that this black hole entropy always is given by a local
geometrical expression on the horizon of the black hole. We thereby obtain a
natural candidate for the entropy of a dynamical black hole in a general theory
of gravity. Our results show that the validity of the ``second law" of black
hole mechanics in dynamical evolution from an initially stationary black hole
to a final stationary state is equivalent to the positivity of a total Noether
flux, and thus may be intimately related to the positive energy properties of
the theory. The relationship between the derivation of our formula for black
hole entropy and the derivation via ``Euclidean methods" also is explained.Comment: 16 pages, EFI 93-4

### Naked and Thunderbolt Singularities in Black Hole Evaporation

If an evaporating black hole does not settle down to a non radiating remnant,
a description by a semi classical Lorentz metric must contain either a naked
singularity or what we call a thunderbolt, a singularity that spreads out to
infinity on a spacelike or null path. We investigate this question in the
context of various two dimensional models that have been proposed. We find that
if the semi classical equations have an extra symmetry that make them solvable
in closed form, they seem to predict naked singularities but numerical
calculations indicate that more general semi classical equations, such as the
original CGHS ones give rise to thunderbolts. We therefore expect that the semi
classical approximation in four dimensions will lead to thunderbolts. We
interpret the prediction of thunderbolts as indicating that the semi classical
approximation breaks down at the end point of black hole evaporation, and we
would expect that a full quantum treatment would replace the thunderbolt with a
burst of high energy particles. The energy in such a burst would be too small
to account for the observed gamma ray bursts.Comment: 21 pages (10 diagrams available on request

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