454 research outputs found
On the Obstructions to non-Cliffordian Pin Structures
We derive the topological obstructions to the existence of non-Cliffordian
pin structures on four-dimensional spacetimes. We apply these obstructions to
the study of non-Cliffordian pin-Lorentz cobordism. We note that our method of
derivation applies equally well in any dimension and in any signature, and we
present a general format for calculating obstructions in these situations.
Finally, we interpret the breakdown of pin structure and discuss the relevance
of this to aspects of physics.Comment: 31 pages, latex, published in Comm. Math. Phys. 164, No. 1, pages
65-87 (1994
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 and
bubbles. Comparison with the instantons for pair creation of black holes
shows that the 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 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 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
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 -branes
of the type II supergravity theories, the Gregory-Laflamme instability
disappears near extremality for but persists all the way down to
extremality for (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
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
Gravitational multi-NUT solitons, Komar masses and charges
Generalising expressions given by Komar, we give precise definitions of
gravitational mass and solitonic NUT charge and we apply these to the
description of a class of Minkowski-signature multi-Taub-NUT solutions without
rod singularities. A Wick rotation then yields the corresponding class of
Euclidean-signature gravitational multi-instantons.Comment: Some references adde
Thermodynamics of a Kerr Newman de Sitter Black Hole
We compute the conserved quantities of the four-dimensional Kerr-Newman-dS
(KNdS) black hole through the use of the counterterm renormalization method,
and obtain a generalized Smarr formula for the mass as a function of the
entropy, the angular momentum and the electric charge. The first law of
thermodynamics associated to the cosmological horizon of KNdS is also
investigated. Using the minimal number of intrinsic boundary counterterms, we
consider the quasilocal thermodynamics of asymptotic de Sitter
Reissner-Nordstrom black hole, and find that the temperature is equal to the
product of the surface gravity (divided by ) and the Tolman redshift
factor. We also perform a quasilocal stability analysis by computing the
determinant of Hessian matrix of the energy with respect to its thermodynamic
variables in both the canonical and the grand-canonical ensembles and obtain a
complete set of phase diagrams. We then turn to the quasilocal thermodynamics
of four-dimensional Kerr-Newman-de Sitter black hole for virtually all possible
values of the mass, the rotation and the charge parameters that leave the
quasilocal boundary inside the cosmological event horizon, and perform a
quasilocal stability analysis of KNdS black hole.Comment: REVTEX4, 12 pages, 12 figures, references added and some points in
Sec II have been clarified, version to appear in Can. J. Phy
Black hole solutions with dilatonic hair in higher curvature gravity
A new numerical integration method for examining a black hole structure was
realized. Black hole solutions with dilatonic hair of 4D low energy effective
SuperString Theory action with Gauss-Bonnet quadratic curvature contribution
were studied, using this method, inside and outside the event horizon.
Thermodynamical properties of this solution were also studied.Comment: 10 pages, 6 figures, RevTeX, figures in LaTeX or PostScript are
avaible upon request via e-mail address: [email protected], Submitted
to Phys.Rev.
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
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
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