3,764 research outputs found
Stable Isotropic Cosmological Singularities in Quadratic Gravity
We show that, in quadratic lagrangian theories of gravity, isotropic
cosmological singularities are stable to the presence of small scalar, vector
and tensor inhomogeneities. Unlike in general relativity, a particular exact
isotropic solution is shown to be the stable attractor on approach to the
initial cosmological singularity. This solution is also known to act as an
attractor in Bianchi universes of types I, II and IX, and the results of this
paper reinforce the hypothesis that small inhomogeneous and anisotropic
perturbations of this attractor form part of the general cosmological solution
to the field equations of quadratic gravity. Implications for the existence of
a 'gravitational entropy' are also discussed.Comment: 18 pages, no figure
Cosmological Bounds on Spatial Variations of Physical Constants
We derive strong observational limits on any possible large-scale spatial
variation in the values of physical 'constants' whose space-time evolution is
driven by a scalar field. The limits are imposed by the isotropy of the
microwave background on large angular scales in theories which describe space
and time variations in the fine structure constant, the electron-proton mass
ratio, and the Newtonian gravitational constant, G. Large-scale spatial
fluctuations in the fine structure constant are bounded by 2x10^-9 and
1.2x10^-8 in the BSBM and VSL theories respectively, fluctuations in the
electron-proton mass ratio by 9x10^-5 in the BM theory and fluctuations in G by
3.6x10^-10 in Brans-Dicke theory. These derived bounds are significantly
stronger than any obtainable by direct observations of astrophysical objects at
the present time.Comment: 13 pages, 1 table, typos corrected, refs added. Published versio
Cosmological Co-evolution of Yang-Mills Fields and Perfect Fluids
We study the co-evolution of Yang-Mills fields and perfect fluids in Bianchi
type I universes. We investigate numerically the evolution of the universe and
the Yang-Mills fields during the radiation and dust eras of a universe that is
almost isotropic. The Yang-Mills field undergoes small amplitude chaotic
oscillations, which are also displayed by the expansion scale factors of the
universe. The results of the numerical simulations are interpreted analytically
and compared with past studies of the cosmological evolution of magnetic fields
in radiation and dust universes. We find that, whereas magnetic universes are
strongly constrained by the microwave background anisotropy, Yang-Mills
universes are principally constrained by primordial nucleosynthesis and the
bound is comparatively weak, and Omega_YM < 0.105 Omega_rad.Comment: 13 pages, 5 figures, submitted to PR
Spherical Curvature Inhomogeneities in String Cosmology
We study the evolution of non-linear spherically symmetric inhomogeneities in
string cosmology. Friedmann solutions of different spatial curvature are
matched to produce solutions which describe the evolution of non-linear density
and curvature inhomogeneities. The evolution of bound and unbound
inhomogeneities are studied. The problem of primordial black hole formation is
discussed in the string cosmological context and the pattern of evolution is
determined in the pre- and post-big-bang phases of evolution.Comment: 19 pages, Latex, 4 figure
Diseases of winter linseed : occurrence, effects and importance
In 1998, a survey of the incidence and severity of diseases was carried out on 30 crops of winter linseed at early flowering and again at crop maturity. Five crops each were selected in south west, east, east Midlands, west Midlands and north of England and from Scotland. Crops were predominantly cv. Oliver (90% crops), grown from certified seed (83%) and sown in September (97%). Pasmo (Mycosphaerella) was the most important disease, affecting leaves of 73% crops at early flowering and 90% crops at maturity. Powdery mildew (70% crops), Alternaria (30% crops) on leaves and Botrytis on capsules (70% crops) were also common. Regional differences were apparent for powdery mildew, which was present in all regions except the southwest, whilst Alternaria predominated in the Midlands. Half of the crops surveyed had received fungicide sprays, but this appeared to have made limited impact on disease severity. Pasmo is a new threat to UK linseed crops and this raises concerns about the threat it poses to spring linsee
Bouncing Universes with Varying Constants
We investigate the behaviour of exact closed bouncing Friedmann universes in
theories with varying constants. We show that the simplest BSBM varying-alpha
theory leads to a bouncing universe. The value of alpha increases
monotonically, remaining approximately constant during most of each cycle, but
increasing significantly around each bounce. When dissipation is introduced we
show that in each new cycle the universe expands for longer and to a larger
size. We find a similar effect for closed bouncing universes in Brans-Dicke
theory, where also varies monotonically in time from cycle to cycle.
Similar behaviour occurs also in varying speed of light theories
Dynamics of Logamediate Inflation
A computation of the inflationary observables n_{s} and r is made for
`logamediate' inflation where the cosmological scale factor expands as , and is compared to their predicted values in the
intermediate inflationary theory, where . Both versions prove
to be consistent with observational measurements of the cosmic background
radiation. It is shown that the dynamics of a single inflaton field can be
mimicked by a system of several fields in an analogous manner to that created
by the joint evolution of the fields in assisted power-law inflation.Comment: 7 pages, 5 figures. Extended introductio
The Andante Regime of Scalar Field Dynamics
The andante regime of scalar field dynamics in the chaotic inflationary
Universe is defined as the epoch when the field is rolling moderately slowly
down its interaction potential, but at such a rate that first-order corrections
to the slow-roll approximation become important. These conditions should apply
towards the end of inflation as the field approaches the global minimum of the
potential. Solutions to the Einstein-scalar field equations for the class of
power law potentials are found in this regime in
terms of the inverse error function.Comment: 11 pages of plain Latex, FNAL-Pub-94/226-
Intermediate inflation in light of the three-year WMAP observations
The three-year observations from the Wilkinson Microwave Anisotropy Probe
have been hailed as giving the first clear indication of a spectral index
n_s<1. We point out that the data are equally well explained by retaining the
assumption n_s=1 and allowing the tensor-to-scalar ratio r to be non-zero. The
combination n_s=1 and r>0 is given (within the slow-roll approximation) by a
version of the intermediate inflation model with expansion rate H(t) \propto
t^{-1/3}. We assess the status of this model in light of the WMAP3 data.Comment: 4 pages RevTeX4 with one figure. Minor changes to match PRD accepted
versio
The Isotropy of Compact Universes
We discuss the problem of the stability of the isotropy of the universe in
the space of ever-expanding spatially homogeneous universes with a compact
spatial topology. The anisotropic modes which prevent isotropy being
asymptotically stable in Bianchi-type universes with non-compact
topologies are excluded by topological compactness. Bianchi type and type
universes with compact topologies must be exactly isotropic. In the
flat case we calculate the dynamical degrees of freedom of Bianchi-type and
universes with compact 3-spaces and show that type solutions
are more general than type solutions for systems with perfect fluid,
although the type models are more general than type in the vacuum
case. For particular topologies the 4-velocity of any perfect fluid is required
to be non-tilted. Various consequences for the problems of the isotropy,
homogeneity, and flatness of the universe are discussed.Comment: 22 pages in LaTeX2e with the amsmath packag
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