596 research outputs found
The classification and prediction of macroinvertebrate communities in British rivers
This article describes the progress of the River Communities Project which commenced in 1977. This project aimed to develop a sensitive and practical system for river site classification using macroinvertebrates as an objective means of appraising the status of British rivers. The relationship between physical and chemical features of sites and their biological communities were examined. Sampling was undertaken on 41 British rivers. Ordination techniques were used to analyze data and the sites were classified into 16 groups using multiple discrimination analysis. The potential for using the environmental data to predict to which group a site belonged and the fauna likely to be present was investigated
Gravitons in One-Loop Quantum Cosmology: Correspondence Between Covariant and Non-Covariant Formalisms
The discrepancy between the results of covariant and non-covariant one-loop
calculations for higher-spin fields in quantum cosmology is analyzed. A
detailed mode-by-mode study of perturbative quantum gravity about a flat
Euclidean background bounded by two concentric 3-spheres, including
non-physical degrees of freedom and ghost modes, leads to one-loop amplitudes
in agreement with the covariant Schwinger-DeWitt method. This calculation
provides the generalization of a previous analysis of fermionic fields and
electromagnetic fields at one-loop about flat Euclidean backgrounds admitting a
well-defined 3+1 decomposition.Comment: 29 pages, latex, recently appearing in Physical Review D, volume 50,
pages 6329-6337, November 1994. The authors apologize for the delay in
circulating the paper, due to technical problems now fixe
Relativistic Gauge Conditions in Quantum Cosmology
This paper studies the quantization of the electromagnetic field on a flat
Euclidean background with boundaries. One-loop scaling factors are evaluated
for the one-boundary and two-boundary backgrounds. The mode-by-mode analysis of
Faddeev-Popov quantum amplitudes is performed by using zeta-function
regularization, and is compared with the space-time covariant evaluation of the
same amplitudes. It is shown that a particular gauge condition exists for which
the corresponding operator matrix acting on gauge modes is in diagonal form
from the beginning. Moreover, various relativistic gauge conditions are studied
in detail, to investigate the gauge invariance of the perturbative quantum
theory.Comment: 26 pages, plain TeX, no figure
The CMB Bispectrum
We use a separable mode expansion estimator with WMAP data to estimate the
bispectrum for all the primary families of non-Gaussian models. We review the
late-time mode expansion estimator methodology which can be applied to any
non-separable primordial and CMB bispectrum model, and we demonstrate how the
method can be used to reconstruct the CMB bispectrum from an observational map.
We extend the previous validation of the general estimator using local map
simulations. We apply the estimator to the coadded WMAP 5-year data,
reconstructing the WMAP bispectrum using multipoles and
orthonormal 3D eigenmodes. We constrain all popular nearly scale-invariant
models, ensuring that the theoretical bispectrum is well-described by a
convergent mode expansion. Constraints from the local model \fnl=54.4\pm
29.4 and the equilateral model \fnl=143.5\pm 151.2 (\Fnl = 25.1\pm 26.4)
are consistent with previously published results. (Here, we use a nonlinearity
parameter \Fnl normalised to the local case, to allow more direct comparison
between different models.) Notable new constraints from our method include
those for the constant model \Fnl = 35.1 \pm 27.4 , the flattened model \Fnl
= 35.4\pm 29.2, and warm inflation \Fnl = 10.3\pm 27.2. We investigate
feature models surveying a wide parameter range in both the scale and phase,
and we find no significant evidence of non-Gaussianity in the models surveyed.
We propose a measure \barFnl for the total integrated bispectrum and find
that the measured value is consistent with the null hypothesis that CMB
anisotropies obey Gaussian statistics. We argue that this general bispectrum
survey with the WMAP data represents the best evidence for Gaussianity to date
and we discuss future prospects, notably from the Planck satellite
A Non-Singular One-Loop Wave Function of the Universe From a New Eigenvalue Asymptotics in Quantum Gravity
Recent work on Euclidean quantum gravity on the four-ball has proved
regularity at the origin of the generalized zeta-function built from
eigenvalues for metric and ghost modes, when diffeomorphism-invariant boundary
conditions are imposed in the de Donder gauge. The hardest part of the analysis
involves one of the four sectors for scalar-type perturbations, the eigenvalues
of which are obtained by squaring up roots of a linear combination of Bessel
functions of integer adjacent orders, with a coefficient of linear combination
depending on the unknown roots. This paper obtains, first, approximate analytic
formulae for such roots for all values of the order of Bessel functions. For
this purpose, both the descending series for Bessel functions and their uniform
asymptotic expansion at large order are used. The resulting generalized
zeta-function is also built, and another check of regularity at the origin is
obtained. For the first time in the literature on quantum gravity on manifolds
with boundary, a vanishing one-loop wave function of the Universe is found in
the limit of small three-geometry, which suggests a quantum avoidance of the
cosmological singularity driven by full diffeomorphism invariance of the
boundary-value problem for one-loop quantum theory.Comment: 21 Pages, Latex and .eps files with JHEP3 style. The discussion in
Section 5 has been improved, and Ref. 26 has been adde
Zeta function determinant of the Laplace operator on the -dimensional ball
We present a direct approach for the calculation of functional determinants
of the Laplace operator on balls. Dirichlet and Robin boundary conditions are
considered. Using this approach, formulas for any value of the dimension, ,
of the ball, can be obtained quite easily. Explicit results are presented here
for dimensions and .Comment: 22 pages, one figure appended as uuencoded postscript fil
The Cosmic Microwave Background in an Inhomogeneous Universe - why void models of dark energy are only weakly constrained by the CMB
The dimming of Type Ia supernovae could be the result of Hubble-scale
inhomogeneity in the matter and spatial curvature, rather than signaling the
presence of a dark energy component. A key challenge for such models is to fit
the detailed spectrum of the cosmic microwave background (CMB). We present a
detailed discussion of the small-scale CMB in an inhomogeneous universe,
focusing on spherically symmetric `void' models. We allow for the dynamical
effects of radiation while analyzing the problem, in contrast to other work
which inadvertently fine tunes its spatial profile. This is a surprisingly
important effect and we reach substantially different conclusions. Models which
are open at CMB distances fit the CMB power spectrum without fine tuning; these
models also fit the supernovae and local Hubble rate data which favours a high
expansion rate. Asymptotically flat models may fit the CMB, but require some
extra assumptions. We argue that a full treatment of the radiation in these
models is necessary if we are to understand the correct constraints from the
CMB, as well as other observations which rely on it, such as spectral
distortions of the black body spectrum, the kinematic Sunyaev-Zeldovich effect
or the Baryon Acoustic Oscillations.Comment: 23 pages with 14 figures. v2 has considerably extended discussion and
analysis, but the basic results are unchanged. v3 is the final versio
Dimensionless cosmology
Although it is well known that any consideration of the variations of
fundamental constants should be restricted to their dimensionless combinations,
the literature on variations of the gravitational constant is entirely
dimensionful. To illustrate applications of this to cosmology, we explicitly
give a dimensionless version of the parameters of the standard cosmological
model, and describe the physics of Big Bang Neucleosynthesis and recombination
in a dimensionless manner. The issue that appears to have been missed in many
studies is that in cosmology the strength of gravity is bound up in the
cosmological equations, and the epoch at which we live is a crucial part of the
model. We argue that it is useful to consider the hypothetical situation of
communicating with another civilization (with entirely different units),
comparing only dimensionless constants, in order to decide if we live in a
Universe governed by precisely the same physical laws. In this thought
experiment, we would also have to compare epochs, which can be defined by
giving the value of any {\it one} of the evolving cosmological parameters. By
setting things up carefully in this way one can avoid inconsistent results when
considering variable constants, caused by effectively fixing more than one
parameter today. We show examples of this effect by considering microwave
background anisotropies, being careful to maintain dimensionlessness
throughout. We present Fisher matrix calculations to estimate how well the fine
structure constants for electromagnetism and gravity can be determined with
future microwave background experiments. We highlight how one can be misled by
simply adding to the usual cosmological parameter set
Come back Marshall, all is forgiven? : Complexity, evolution, mathematics and Marshallian exceptionalism
Marshall was the great synthesiser of neoclassical economics. Yet with his qualified assumption of self-interest, his emphasis on variation in economic evolution and his cautious attitude to the use of mathematics, Marshall differs fundamentally from other leading neoclassical contemporaries. Metaphors inspire more specific analogies and ontological assumptions, and Marshall used the guiding metaphor of Spencerian evolution. But unfortunately, the further development of a Marshallian evolutionary approach was undermined in part by theoretical problems within Spencer's theory. Yet some things can be salvaged from the Marshallian evolutionary vision. They may even be placed in a more viable Darwinian framework.Peer reviewedFinal Accepted Versio
Magnetic Field Generation in Stars
Enormous progress has been made on observing stellar magnetism in stars from
the main sequence through to compact objects. Recent data have thrown into
sharper relief the vexed question of the origin of stellar magnetic fields,
which remains one of the main unanswered questions in astrophysics. In this
chapter we review recent work in this area of research. In particular, we look
at the fossil field hypothesis which links magnetism in compact stars to
magnetism in main sequence and pre-main sequence stars and we consider why its
feasibility has now been questioned particularly in the context of highly
magnetic white dwarfs. We also review the fossil versus dynamo debate in the
context of neutron stars and the roles played by key physical processes such as
buoyancy, helicity, and superfluid turbulence,in the generation and stability
of neutron star fields.
Independent information on the internal magnetic field of neutron stars will
come from future gravitational wave detections. Thus we maybe at the dawn of a
new era of exciting discoveries in compact star magnetism driven by the opening
of a new, non-electromagnetic observational window.
We also review recent advances in the theory and computation of
magnetohydrodynamic turbulence as it applies to stellar magnetism and dynamo
theory. These advances offer insight into the action of stellar dynamos as well
as processes whichcontrol the diffusive magnetic flux transport in stars.Comment: 41 pages, 7 figures. Invited review chapter on on magnetic field
generation in stars to appear in Space Science Reviews, Springe
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