1,010 research outputs found
The Nature of Generic Cosmological Singularities
The existence of a singularity by definition implies a preferred scale--the
affine parameter distance from/to the singularity of a causal geodesic that is
used to define it. However, this variable scale is also captured by the
expansion along the geodesic, and this can be used to obtain a regularized
state space picture by means of a conformal transformation that factors out the
expansion. This leads to the conformal `Hubble-normalized' orthonormal frame
approach which allows one to translate methods and results concerning spatially
homogeneous models into the generic inhomogeneous context, which in turn
enables one to derive the dynamical nature of generic cosmological
singularities. Here we describe this approach and outline the derivation of the
`cosmological billiard attractor,' which describes the generic dynamical
asymptotic behavior towards a generic spacelike singularity. We also compare
the `dynamical systems picture' resulting from this approach with other work on
generic spacelike singularities: the metric approach of Belinskii, Lifschitz,
and Khalatnikov, and the recent Iwasawa based Hamiltonian method used by
Damour, Henneaux, and Nicolai; in particular we show that the cosmological
billiards obtained by the latter and the cosmological billiard attractor form
complementary `dual' descriptions of the generic asymptotic dynamics of generic
spacelike singularities.Comment: 14 pages, six figures; invited talk at the 11th Marcel Grossmann
Meeting on Recent Developments in General Relativity, Berlin, Germany, 23-29
July 200
RISK, UNCERTAINTY, AND SPATIAL DISTINCTION: A STUDY OF URBAN PLANNING IN STOCKHOLM
This paper examines urban planning in Stockholm, focusing on the proposal for a new comprehensive plan. It explores the problems urban planning has set out to solve and whether – and if so, how – the concepts of risk and uncertainty form part of the planning discourse. A departure point is that both urban planning ideals and the problems these ideals claim to address are constructed. Explicitly or implicitly, planning creates demarcations that make places and activities appear safe or risky, attractive or problematic, etc. Analysis of the proposal for a new comprehensive plan for Stockholm identifies at least three such boundaries or spatial distinctions: between centre and periphery, green areas and other parts of the city, and risky or unsafe areas and other areas. Likewise, the analysis finds evidence of a tension between rational planning and normative ideas of the “good city” in urban planning.risk; uncertainty; urban planning.
Stationary Bianchi type II perfect fluid models
Einstein's field equations for stationary Bianchi type II models with a
perfect fluid source are investigated. The field equations are rewritten as a
system of autonomous first order differential equations. Dimensionless
variables are subsequently introduced for which the reduced phase space is
compact. The system is then studied qualitatively using the theory of dynamical
systems. It is shown that the locally rotationally symmetric models are not
asymptotically self-similar for small values of the independent , tovariable. A
new exact solution is also given.Comment: 6 pages, 1 figure LaTeX. To appear in JM
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