69,720 research outputs found
Fingers of God
Very long wavelength universal gravitational waves cannot now produce in
clusters of galaxies velocity dispersions greater than that which these systems
would possess if they were expanding with the Universe, if the Universe is not
younger than yr and Hubble's constant is not less than 50 km/sec/
Mpc. A diagram shows that actual velocity dispersions are significantly greater
than this limit.Comment: Published long before the advent of large-scale redshift surveys, as
"A Critique of Rees's Theory of Primordial Gravitational Radiation", this
paper includes the first presentation of what has come to be known as the
fingers-of-god effect. The effect is mentioned several hundred times in
arXive papers, rarely with a wrong attribution, usually with none at al
Deceleration without dark matter
In homogeneous isotropic cosmological models the angular size theta of a
standard measuring rod changes with redshift z in a manner that depends upon
the parameters of the model. It has been argued that as a population
ultracompact (milliarcsecond) radio sources measured by very long-baseline
interferometry (VLBI) do not evolve with cosmic epoch, and thus comprise a set
of standard objects, at least in a statistical sense. Here we examine the
angular-size/redshift relation for 256 ultracompact sources with z in the range
0.5 to 3.8 for cosmological models with two degrees of freedom (Omega_0 and
Lambda_0). The canonical inflationary cold dark matter model(Omega_0=1,
Lambda_0=0) appears to be ruled out by the observed relationship, whereas
low-density models with a cosmological constant of either sign are favoured.Comment: Although published (MNRAS 285, 806, 1997, submitted 1996 May 3), this
paper has not previously appeared on the arXive. Despite its title, a
prominent conclusion is that if the Universe is spatially flat, then the best
cosmological parameters are Omega_m=0.2, Omega_Lambda=0.8, with probable
range 0.1<Omega_m<0.3. It is the first in a series, the second being JCAP
0411(2004)007, astro-ph/0309390; the third is a recent preprint,
astro-ph/060506
Decelerating universes older than their Hubble times
Recent observations suggest that Hubble's constant is large, and hence that
the Universe appears to be younger than some of its constituents. The
traditional escape route, which assumes that the expansion is accelerating,
appears to be blocked by observations of Type 1a supernovae, which suggest(ed)
that the Universe is decelerating. These observations are reconciled in a model
in which the Universe has experienced an inflationary phase in the recent past,
driven by an ultra-light inflaton whose Compton wavelength is of the same order
as the Hubble radius.Comment: This paper, which predates the famous quintessence paper, has not
previously appeared on the e-Print archive. It is essentially a quintessence
model without eternal acceleration. Such models are now very topical in the
context of string theory. 9 pages, 5 figure
Radically solvable graphs
A 2-dimensional framework is a straight line realisation of a graph in the
Euclidean plane. It is radically solvable if the set of vertex coordinates is
contained in a radical extension of the field of rationals extended by the
squared edge lengths. We show that the radical solvability of a generic
framework depends only on its underlying graph and characterise which planar
graphs give rise to radically solvable generic frameworks. We conjecture that
our characterisation extends to all graphs
Prediction of gas leakage of environmental control systems
Mathematical models of leakage configurations and various flow theories are presented with the substantive experimental test data to provide background material for future design and failure analysis. Normal-rate leakage and emergency, high-rate leakage are considered
Extragalactic radio source evolution under the dual-population unification scheme
We show that a dual-population unification scheme provides a successful
paradigm with which to describe the evolution and beaming of all bright
extragalactic radio sources. The paradigm consists of two intrinsic
radio-source populations, based on the two distinct radio-galaxy morphologies
of Fanaroff-Riley classes I and II. These represent the `unbeamed' or `side-on'
parent populations of steep radio spectra; the `beamed' source types including
flat-spectrum quasars and BL Lac objects, arise through the random alignment of
their radio-axis to our line-of-sight where Doppler-beaming of the relativistic
radio jets produces highly anisotropic radio emission.Comment: 18 pages & 18 postscript figures, accepted for publication in MNRA
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