1,300 research outputs found
The Physics of 2-d Stringy Spacetimes
We examine the two-dimensional spacetimes that emerge from string theory. We
find all the solutions with no tachyons, and show that the only non-trivial
solution is the black hole spacetime. We examine the role of duality in this
picture. We then explore the thermodynamics of these solutions which is
complicated by the fact that only in two spacetime dimensions is it impossible
to redefine the dilaton field in terms of a canonical scalar field. Finally, we
extend our analysis to the heterotic string, and briefly comment on exact, as
opposed to perturbative, solutions
Discrete Newtonian Cosmology
In this paper we lay down the foundations for a purely Newtonian theory of
cosmology, valid at scales small compared with the Hubble radius, using only
Newtonian point particles acted on by gravity and a possible cosmological term.
We describe the cosmological background which is given by an exact solution of
the equations of motion in which the particles expand homothetically with their
comoving positions constituting a central configuration. We point out, using
previous work, that an important class of central configurations are
homogeneous and isotropic, thus justifying the usual assumptions of elementary
treatments. The scale factor is shown to satisfy the standard Raychaudhuri and
Friedmann equations without making any fluid dynamic or continuum
approximations. Since we make no commitment as to the identity of the point
particles, our results are valid for cold dark matter, galaxies, or clusters of
galaxies. In future publications we plan to discuss perturbations of our
cosmological background from the point particle viewpoint laid down in this
paper and show consistency with much standard theory usually obtained by more
complicated and conceptually less clear continuum methods. Apart from its
potential use in large scale structure studies, we believe that out approach
has great pedagogic advantages over existing elementary treatments of the
expanding universe, since it requires no use of general relativity or continuum
mechanics but concentrates on the basic physics: Newton's laws for
gravitationally interacting particles.Comment: 33 pages; typos fixed, references added, some clarification
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
Properties of some five dimensional Einstein metrics
The volumes, spectra and geodesics of a recently constructed infinite family
of five-dimensional inhomogeneous Einstein metrics on the two bundles
over are examined. The metrics are in general of cohomogeneity one but
they contain the infinite family of homogeneous metrics . The geodesic
flow is shown to be completely integrable, in fact both the Hamilton-Jacobi and
the Laplace equation separate. As an application of these results, we compute
the zeta function of the Laplace operator on for large . We
discuss the spectrum of the Lichnerowicz operator on symmetric transverse
tracefree second rank tensor fields, with application to the stability of
Freund-Rubin compactifications and generalised black holes.Comment: 1+43 pages, 2 figures, LaTeX. Minor typos correcte
Measures on Mixing Angles
We address the problem of the apparently very small magnitude of CP violation
in the standard model, measured by the Jarlskog invariant J. In order to make
statements about probabilities for certain values of J, we seek to find a
natural measure on the space of Kobayashi-Maskawa matrices, the double quotient
U(1)^2\SU(3)/U(1)^2. We review several possible, geometrically motivated
choices of the measure, and compute expectation values for powers of J for
these measures. We find that different choices of the measure generically make
the observed magnitude of CP violation appear finely tuned. Since the quark
masses and the mixing angles are determined by the same set of Yukawa
couplings, we then do a second calculation in which we take the known quark
mass hierarchy into account. We construct the simplest measure on the space of
3 x 3 Hermitian matrices which reproduces this known hierarchy. Calculating
expectation values for powers of J in this second approach, we find that values
of J close to the observed value are now rather likely, and there does not seem
to be any fine tuning. Our results suggest that the choice of Kobayashi-Maskawa
angles is closely linked to the observed mass hierarchy. We close by discussing
the corresponding case of neutrinos.Comment: 40 pages, 3 figures, corrected references, cited figures etc., no
substantial changes in conten
On de-Sitter Geometry in Cosmic Void Statistics
Starting from the geometrical concept of a 4-dimensional de-Sitter
configuration of spheres in Euclidean 3-space and modelling voids in the
Universe as spheres, we show that a uniform distribution over this
configuration space implies a power-law for the void number density which is
consistent with results from the excursion set formalism and with data, for an
intermediate range of void volumes. The scaling dimension of the large scale
structure can be estimated as well. We also discuss the effect of restricting
the survey geometry on the void statistics. This work is a new application of
de-Sitter geometry to cosmology and also provides a new geometrical perspective
on self-similarity in cosmology.Comment: 8 pages, 4 figures, accepted by MNRAS. Minor changes, appendix adde
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