518 research outputs found
Newtonian and Post-Newtonian approximations of the k = 0 Friedmann Robertson Walker Cosmology
In a previous paper we derived a post-Newtonian approximation to cosmology
which, in contrast to former Newtonian and post-Newtonian cosmological
theories, has a well-posed initial value problem. In this paper, this new
post-Newtonian theory is compared with the fully general relativistic theory,
in the context of the k = 0 Friedmann Robertson Walker cosmologies. It is found
that the post-Newtonian theory reproduces the results of its general
relativistic counterpart, whilst the Newtonian theory does not.Comment: 11 pages, Latex, corrected typo
Post-Newtonian Cosmology
Newtonian Cosmology is commonly used in astrophysical problems, because of
its obvious simplicity when compared with general relativity. However it has
inherent difficulties, the most obvious of which is the non-existence of a
well-posed initial value problem. In this paper we investigate how far these
problems are met by using the post-Newtonian approximation in cosmology.Comment: 12 pages, Late
Integrability of irrotational silent cosmological models
We revisit the issue of integrability conditions for the irrotational silent
cosmological models. We formulate the problem both in 1+3 covariant and 1+3
orthonormal frame notation, and show there exists a series of constraint
equations that need to be satisfied. These conditions hold identically for
FLRW-linearised silent models, but not in the general exact non-linear case.
Thus there is a linearisation instability, and it is highly unlikely that there
is a large class of silent models. We conjecture that there are no spatially
inhomogeneous solutions with Weyl curvature of Petrov type I, and indicate
further issues that await clarification.Comment: Minor corrections and improvements; 1 new reference; to appear Class.
Quantum Grav.; 16 pages Ioplpp
Maximally extended, explicit and regular coverings of the Schwarzschild - de Sitter vacua in arbitrary dimension
Maximally extended, explicit and regular coverings of the Schwarzschild - de
Sitter family of vacua are given, first in spacetime (generalizing a result due
to Israel) and then for all dimensions (assuming a sphere). It is
shown that these coordinates offer important advantages over the well known
Kruskal - Szekeres procedure.Comment: 12 pages revtex4 5 figures in color. Higher resolution version at
http://www.astro.queensu.ca/~lake/regularcoordinates.pd
How to measure spatial distances?
The use of time--like geodesics to measure temporal distances is better
justified than the use of space--like geodesics for a measurement of spatial
distances. We give examples where a ''spatial distance'' cannot be
appropriately determined by the length of a space--like geodesic.Comment: 4 pages, latex, no figure
Collisions of Einstein-Conformal Scalar Waves
A large class of solutions of the Einstein-conformal scalar equations in
D=2+1 and D=3+1 is identified. They describe the collisions of asymptotic
conformal scalar waves and are generated from Einstein-minimally coupled scalar
spacetimes via a (generalized) Bekenstein transformation. Particular emphasis
is given to the study of the global properties and the singularity structure of
the obtained solutions. It is shown, that in the case of the absence of pure
gravitational radiation in the initial data, the formation of the final
singularity is not only generic, but is even inevitable.Comment: 17 pages, LaTe
Nonperturbative gravito-magnetic fields
In a cold matter universe, the linearized gravito-magnetic tensor field
satisfies a transverse condition (vanishing divergence) when it is purely
radiative. We show that in the nonlinear theory, it is no longer possible to
maintain the transverse condition, since it leads to a non-terminating chain of
integrability conditions. These conditions are highly restrictive, and are
likely to hold only in models with special symmetries, such as the known
Bianchi and examples. In models with realistic inhomogeneity, the
gravito-magnetic field is necessarily non-transverse at second and higher
order.Comment: Minor changes to match published version; to appear in Phys. Rev.
Ideally embedded space-times
Due to the growing interest in embeddings of space-time in higher-dimensional
spaces we consider a specific type of embedding. After proving an inequality
between intrinsically defined curvature invariants and the squared mean
curvature, we extend the notion of ideal embeddings from Riemannian geometry to
the indefinite case. Ideal embeddings are such that the embedded manifold
receives the least amount of tension from the surrounding space. Then it is
shown that the de Sitter spaces, a Robertson-Walker space-time and some
anisotropic perfect fluid metrics can be ideally embedded in a five-dimensional
pseudo-Euclidean space.Comment: layout changed and typos corrected; uses revtex
Moderate deviations for the determinant of Wigner matrices
We establish a moderate deviations principle (MDP) for the log-determinant
of a Wigner matrix matching four moments with
either the GUE or GOE ensemble. Further we establish Cram\'er--type moderate
deviations and Berry-Esseen bounds for the log-determinant for the GUE and GOE
ensembles as well as for non-symmetric and non-Hermitian Gaussian random
matrices (Ginibre ensembles), respectively.Comment: 20 pages, one missing reference added; Limit Theorems in Probability,
Statistics and Number Theory, Springer Proceedings in Mathematics and
Statistics, 201
Inter-layer spin diffusion and electric conductivity in the organic conductors {\kappa}-ET2-Cl and {\kappa}-ET2-Br
A high frequency (111.2-420 GHz) electron spin resonance study of the
inter-layer (perpendicular) spin diffusion as a function of pressure and
temperature is presented in the conducting phases of the layered organic
compounds, {\kappa}-(BEDT-TTF)2-Cu[N(CN)2]X ({\kappa}-ET2-X), X=Cl or Br. The
resolved ESR lines of adjacent layers at high temperatures and high frequencies
allows for the determination of the inter-layer cross spin relaxation time, Tx
and the intrinsic spin relaxation time, T2 of single layers. In the bad metal
phase spin diffusion is two-dimensional, i.e. spins are not hopping to adjacent
layers within T2. Tx is proportional to the perpendicular resistivity at least
approximately, as predicted in models where spin and charge excitations are
tied together. In {\kappa}-ET2-Cl, at zero pressure Tx increases as the bad
metal-insulator transition is approached. On the other hand, Tx decreases as
the normal metal and superconducting phases are approached with increasing
pressure and/or decreasing temperature.Comment: 18 pages, 11 figure
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