12,683 research outputs found
Branes as BIons
A BIon may be defined as a finite energy solution of a non-linear field
theory with distributional sources. By contrast a soliton is usually defined to
have no sources. I show how harmonic coordinates map the exteriors of the
topologically and causally non-trivial spacetimes of extreme p-branes to BIonic
solutions of the Einstein equations in a topologically trivial spacetime in
which the combined gravitational and matter energy momentum is located on
distributional sources. As a consequence the tension of BPS p-branes is
classically unrenormalized. The result holds equally for spacetimes with
singularities and for those, like the M-5-brane, which are everywhere
singularity free.Comment: Latex, 9 pages, no figure
Flux-Confinement in Dilatonic Cosmic Strings
We study dilaton-electrodynamics in flat spacetime and exhibit a set of
global cosmic string like solutions in which the magnetic flux is confined.
These solutions continue to exist for a small enough dilaton mass but cease to
do so above a critcal value depending on the magnetic flux. There also exist
domain wall and Dirac monopole solutions. We discuss a mechanism whereby
magnetic monopolesmight have been confined by dilaton cosmic strings during an
epoch in the early universe during which the dilaton was massless.Comment: 8 pages, DAMTP R93/3
Time-Dependent Multi-Centre Solutions from New Metrics with Holonomy Sim(n-2)
The classifications of holonomy groups in Lorentzian and in Euclidean
signature are quite different. A group of interest in Lorentzian signature in n
dimensions is the maximal proper subgroup of the Lorentz group, SIM(n-2).
Ricci-flat metrics with SIM(2) holonomy were constructed by Kerr and Goldberg,
and a single four-dimensional example with a non-zero cosmological constant was
exhibited by Ghanam and Thompson. Here we reduce the problem of finding the
general -dimensional Einstein metric of SIM(n-2) holonomy, with and without
a cosmological constant, to solving a set linear generalised Laplace and
Poisson equations on an (n-2)-dimensional Einstein base manifold. Explicit
examples may be constructed in terms of generalised harmonic functions. A
dimensional reduction of these multi-centre solutions gives new time-dependent
Kaluza-Klein black holes and monopoles, including time-dependent black holes in
a cosmological background whose spatial sections have non-vanishing curvature.Comment: Typos corrected; 29 page
The Action of Instantons with Nut Charge
We examine the effect of a non-trivial nut charge on the action of
non-compact four-dimensional instantons with a U(1) isometry. If the instanton
action is calculated by dimensionally reducing along the isometry, then the nut
charge is found to make an explicit non-zero contribution. For metrics
satisfying AF, ALF or ALE boundary conditions, the action can be expressed
entirely in terms of quantities (including the nut charge) defined on the fixed
point set of the isometry. A source (or sink) of nut charge also implies the
presence of a Misner string coordinate singularity, which will have an
important effect on the Hamiltonian of the instanton.Comment: 25 page
Moduli, Scalar Charges, and the First Law of Black Hole Thermodynamics
We show that under variation of moduli fields the first law of black
hole thermodynamics becomes , where are the scalar charges. We also show
that the ADM mass is extremized at fixed , , when the moduli
fields take the fixed value which depend only on electric
and magnetic charges. It follows that the least mass of any black hole with
fixed conserved electric and magnetic charges is given by the mass of the
double-extreme black hole with these charges. Our work allows us to interpret
the previously established result that for all extreme black holes the moduli
fields at the horizon take a value depending only
on the electric and magnetic conserved charges: is such
that the scalar charges .Comment: 3 pages, no figures, more detailed versio
Some Comments on Gravitational Entropy and the Inverse Mean Curvature Flow
The Geroch-Wald-Jang-Huisken-Ilmanen approach to the positive energy problem
to may be extended to give a negative lower bound for the mass of
asymptotically Anti-de-Sitter spacetimes containing horizons with exotic
topologies having ends or infinities of the form , in
terms of the cosmological constant. We also show how the method gives a lower
bound for for the mass of time-symmetric initial data sets for black holes with
vectors and scalars in terms of the mass, of the double extreme
black hole with the same charges. I also give a lower bound for the area of an
apparent horizon, and hence a lower bound for the entropy in terms of the same
function . This shows that the so-called attractor behaviour extends
beyond the static spherically symmetric case. and underscores the general
importance of the function . There are hints that higher dimensional
generalizations may involve the Yamabe conjectures.Comment: 13pp. late
Black Hole Solutions of Kaluza-Klein Supergravity Theories and String Theory
We find U(1)_{E} \times U(1)_{M} non-extremal black hole solutions of
6-dimensional Kaluza-Klein supergravity theories. Extremal solutions were found
by Cveti\v{c} and Youm\cite{C-Y}. Multi black hole solutions are also
presented. After electro-magnetic duality transformation is performed, these
multi black hole solutions are mapped into the the exact solutions found by
Horowitz and Tseytlin\cite{H-T} in 5-dimensional string theory compactified
into 4-dimensions. The massless fields of this theory can be embedded into the
heterotic string theory compactified on a 6-torus. Rotating black hole
solutions can be read off those of the heterotic string theory found by
Sen\cite{Sen3}.Comment: 23 pages text(latex), a figure upon reques
Gravitational Instantons, Confocal Quadrics and Separability of the Schr\"odinger and Hamilton-Jacobi equations
A hyperk\"ahler 4-metric with a triholomorphic SU(2) action gives rise to a
family of confocal quadrics in Euclidean 3-space when cast in the canonical
form of a hyperk\"ahler 4-metric metric with a triholomorphic circle action.
Moreover, at least in the case of geodesics orthogonal to the U(1) fibres, both
the covariant Schr\"odinger and the Hamilton-Jacobi equation is separable and
the system integrable.Comment: 10 pages Late
Dark Energy, Inflation and Extra Dimensions
We consider how accelerated expansion, whether due to inflation or dark
energy, imposes strong constraints on fundamental theories obtained by
compactification from higher dimensions. For theories that obey the null energy
condition (NEC), we find that inflationary cosmology is impossible for a wide
range of compactifications; and a dark energy phase consistent with
observations is only possible if both Newton's gravitational constant and the
dark energy equation-of-state vary with time. If the theory violates the NEC,
inflation and dark energy are only possible if the NEC-violating elements are
inhomogeneously distributed in thecompact dimensions and vary with time in
precise synchrony with the matter and energy density in the non-compact
dimensions. Although our proofs are derived assuming general relativity applies
in both four and higher dimensions and certain forms of metrics, we argue that
similar constraints must apply for more general compactifications.Comment: 26 pages, 1 figure. v2: reference added, typos correcte
Written evidence to the House of Commons Business and Skills Committee (ed) Local enterprise partnerships and the Regional Growth Fund
The Business, Innovation and Skills Committee announced an inquiry looking into the Local Enterprise Partnerships and the Regional Growth Fund. In particular, the Committee examined how the proposed new structures would work, alongside issues such as distribution of funding, value for money, accountability, timing, transitional arrangements and required legislation. A Report on the Local Enterprise Partnerships was published on 26 April 2013
- …