Science: Journal Articles
http://hdl.handle.net/10092/24
Fri, 27 Nov 2015 03:33:00 GMT2015-11-27T03:33:00ZSpacetime as a membrane in higher dimensions
http://hdl.handle.net/10092/11430
Spacetime as a membrane in higher dimensions
Gibbons, G.W.; Wiltshire, D.L.
By means of a simple model we investigate the possibility that spacetime
is a membrane embedded in higher dimensions. We present cosmological solutions
of d-dimensional Einstein-Maxwell theory which compactify to two dimensions.
These solutions are analytically continued to obtain dual solutions
in which a (d−2)-dimensional Einstein spacetime “membrane” is embedded in
d-dimensions. The membrane solutions generalise Melvin’s 4-dimensional flux
tube solution. The flat membrane is shown to be classically stable. It is shown
that there are zero mode solutions of the d-dimensional Dirac equation which
are confined to a neighbourhood of the membrane and move within it like massless
chiral (d − 2)-dimensional fermions. An investigation of the spectrum of
scalar perturbations shows that a well-defined mass gap between the zero modes
and massive modes can be obtained if there is a positive cosmological term in
(d − 2) dimensions or a negative cosmological term in d dimensions.
Thu, 01 Jan 1987 00:00:00 GMThttp://hdl.handle.net/10092/114301987-01-01T00:00:00ZGravitational energy and cosmic acceleration
http://hdl.handle.net/10092/11429
Gravitational energy and cosmic acceleration
Wiltshire, D.L.
Cosmic acceleration is explained quantitatively, as an apparent effect due to gravitational energy differences that arise in the decoupling of bound systems from the global expansion of the universe. "Dark energy" is a misidentification of those aspects of gravitational energy which by virtue of the equivalence principle cannot be localized, namely gradients in the energy due to the expansion of space and spatial curvature variations in an inhomogeneous universe. A new scheme for cosmological averaging is proposed which solves the Sandage – de Vaucouleurs paradox. Concordance parameters fit supernova luminosity distances, the angular scale of the sound horizon in the CMB anisotropies, and the effective comoving baryon acoustic oscillation scale seen in galaxy clustering statistics. Key observational anomalies are potentially resolved, and unique predictions made, including a quantifiable variance in the Hubble flow below the scale of apparent homogeneity.
Publisher listing says "This essay received an "honorable mention" in the 2007 Essay Competition of the Gravity Research Foundation."
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/10092/114292008-01-01T00:00:00ZGravitational energy as dark energy: Concordance of cosmological tests
http://hdl.handle.net/10092/11428
Gravitational energy as dark energy: Concordance of cosmological tests
Leith, B.M.; Ng, S.C.C.; Wiltshire, D.L.
We provide preliminary quantitative evidence that a new solution to averaging the observed inhomogeneous structure of matter in the universe may lead to an observationally viable cosmology without exotic dark energy. We find parameters which simultaneously satisfy three independent tests: the match to the angular scale of the sound horizon detected in the cosmic microwave background anisotropy spectrum; the effective comoving baryon acoustic oscillation scale detected in galaxy clustering statistics; and Type Ia supernova luminosity distances. Independently of the supernova data, concordance is obtained for a value of the Hubble constant which agrees with the measurement of the Hubble Key team of Sandage and coworkers. Best-fit parameters include a global average Hubble constant km s-1 Mpc-1, a present epoch void volume fraction of , and an age of the universe of billion years as measured by observers in galaxies. The mass ratio of nonbaryonic dark matter to baryonic matter is , computed with a baryon-to-photon ratio that is in concordance with primordial lithium abundances.
Tue, 01 Jan 2008 00:00:00 GMThttp://hdl.handle.net/10092/114282008-01-01T00:00:00ZStable gravastars - an alternative to black holes?
http://hdl.handle.net/10092/11295
Stable gravastars - an alternative to black holes?
Visser, M.; Wiltshire, D.L.
The “gravastar” picture developed by Mazur and Mottola is one of a very
small number of serious challenges to our usual conception of a “black hole”. In the
gravastar picture there is effectively a phase transition at
/ near where the event horizon
would have been expected to form, and the interior of what would have been the black
hole is replaced by a segment of de Sitter space. While Mazur and Mottola were
able to argue for the thermodynamic stability of their configuration, the question of
dynamic stability against spherically symmetric perturbations of the matter or gravity
fields remains somewhat obscure. In this article we construct a model that shares the
key features of the Mazur–Mottola scenario, and which is sufficiently simple for a full
dynamical analysis. We find that there are
some
physically reasonable equations of
state for the transition layer that lead to stability
Available online via the Institute of Physics site by subscription.
Thu, 01 Jan 2004 00:00:00 GMThttp://hdl.handle.net/10092/112952004-01-01T00:00:00Z