2,848 research outputs found
Numerical evaluation of one-loop QCD amplitudes
We present the publicly available program NGluon allowing the numerical
evaluation of primitive amplitudes at one-loop order in massless QCD. The
program allows the computation of one-loop amplitudes for an arbitrary number
of gluons. The focus of the present article is the extension to one-loop
amplitudes including an arbitrary number of massless quark pairs. We discuss in
detail the algorithmic differences to the pure gluonic case and present cross
checks to validate our implementation. The numerical accuracy is investigated
in detail.Comment: Talk given at ACAT 2011 conference in London, 5-9 Septembe
On Shear-Free perturbations of FLRW Universes
A surprising exact result for the Einstein Field Equations is that if
pressure-free matter is moving in a shear-free way, then it must be either
expansion-free or rotation-free. It has been suggested this result is also true
for any barotropic perfect fluid, but a proof has remained elusive. We consider
the case of barotropic perfect fluid solutions linearized about a
Robertson-Walker geometry, and prove that the result remains true except for
the case of a specific highly non-linear equation of state. We argue that this
equation of state is non-physical, and hence the result is true in the
linearized case for all physically realistic barotropic perfect fluids. This
result, which is not true in Newtonian cosmology, demonstrates that the
linearized solutions, believed to result in standard local Newtonian theory, do
not always give the usual behaviour of Newtonian solutions
Constraints on Inflationary Solutions in the Presence of Shear and Bulk Viscosity
Inflationary models and their claim to solve many of the outstanding problems
in cosmology have been the subject of a great deal of debate over the last few
years. A major sticking point has been the lack of both good observational and
theoretical arguments to single out one particular model out of the many that
solve these problems. Here we examine the degree of restrictiveness on the
dynamical relationship between the cosmological scale factor and the inflation
driving self-interaction potential of a minimally coupled scalar field, imposed
by the condition that the scalar field is required to be real during a
classical regime (the reality condition). We systema\-tically look at the
effects of this constraint on many of the inflationary models found in the
literature within the FLRW framework, and also look at what happens when
physically motivated perturbations such as shear and bulk viscosity are
introduced. We find that in many cases, either the models are totally excluded
or the reality condition gives rise to constraints on the scale factor and on
the various parameters of the model.Comment: 21 pages, LaTe
Community Perspective on Consultation on Urban Stormwater Management: Lessons from Brownhill Creek, South Australia
This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).There are salutary lessons from contrasting community consultation efforts in 2011 and
2015 to develop and gain support for an urban stormwater management plan for the Brownhill Creek
catchment in Adelaide, South Australia. The 2011 process was a failure in the human dimension,
precipitating loss of community confidence, unnecessarily entrained thousands of hours of time of
residents who initiated a community action group for environmental conservation and caused a
three-year delay to decision making. By contrast, the 2015 process was vastly improved, resulted in a
landslide level of support for an obvious option not previously offered, achieved the required level of
flood protection, saved Aus$5 million (14%) on the previously proposed option and protected a highly
valued natural environment from an unnecessary dam. This paper presents a rarely heard perspective
on these community consultation processes from a participating community environmental and
heritage conservation action group (the Brownhill Creek Association) that was deeply engaged in
reforming the Draft Brown Hill Keswick Creek Stormwater Management Plan. This reveals that the
community needs to see that all options are considered and to have access to accurate information
with which to assess them. It is also necessary that the proposed plan is consistent with existing
agreed plans and policies developed through public consultation. Community concerns need to
be heard, acknowledged and acted upon or responded to, and the consultation process needs to
be transparently fair and democratic to win community support. A major contributor to success in
the second consultation was that all community action groups were invited to meetings to discuss
the purpose of the consultation and the methods to be used. Feedback was subsequently received
before the process commenced to show what had changed and why any suggestions concerning the
consultation process were not being adopted. This openness helped to mend the distrust of the first
consultation process and is recommended as an essential early step in any public consultation process
Weak lensing B-modes on all scales as a probe of local isotropy
This article derives a multipolar hierarchy for the propagation of the
weak-lensing shear and convergence in a general spacetime. The origin of
B-modes, in particular on large angular scales, is related to the local
isotropy of space. Known results assuming a Friedmann-Lema\^itre background are
naturally recovered. The example of a Bianchi I spacetime illustrates our
formalism and its implications for future observations are stressed.Comment: 10 pages, 2 figures. Replaced to match published versio
A fully covariant description of CMB anisotropies
Starting from the exact non-linear description of matter and radiation, a
fully covariant and gauge-invariant formula for the observed temperature
anisotropy of the cosmic microwave background (CBR) radiation, expressed in
terms of the electric () and magnetic () parts of the Weyl
tensor, is obtained by integrating photon geodesics from last scattering to the
point of observation today. This improves and extends earlier work by Russ et
al where a similar formula was obtained by taking first order variations of the
redshift. In the case of scalar (density) perturbations, is related to
the harmonic components of the gravitational potential and the usual
dominant Sachs-Wolfe contribution to the
temperature anisotropy is recovered, together with contributions due to the
time variation of the potential (Rees-Sciama effect), entropy and velocity
perturbations at last scattering and a pressure suppression term important in
low density universes. We also explicitly demonstrate the validity of assuming
that the perturbations are adiabatic at decoupling and show that if the surface
of last scattering is correctly placed and the background universe model is
taken to be a flat dust dominated Friedmann-Robertson-Walker model (FRW), then
the large scale temperature anisotropy can be interpreted as being due to the
motion of the matter relative to the surface of constant temperature which
defines the surface of last scattering on those scales.Comment: 18 pages LaTeX, 1 figure. Submitted to Classical and Quantum Gravity.
Also available at http://shiva.mth.uct.ac.za/preprints/9705.htm
Report by the ESA-ESO Working Group on Fundamental Cosmology
ESO and ESA agreed to establish a number of Working Groups to explore
possible synergies between these two major European astronomical institutions.
This Working Group's mandate was to concentrate on fundamental questions in
cosmology, and the scope for tackling these in Europe over the next ~15 years.
One major resulting recommendation concerns the provision of new generations of
imaging survey, where the image quality and near-IR sensitivity that can be
attained only in space are naturally matched by ground-based imaging and
spectroscopy to yield massive datasets with well-understood photometric
redshifts (photo-z's). Such information is essential for a range of new
cosmological tests using gravitational lensing, large-scale structure, clusters
of galaxies, and supernovae. Great scope in future cosmology also exists for
ELT studies of the intergalactic medium and space-based studies of the CMB and
gravitational waves; here the synergy is less direct, but these areas will
remain of the highest mutual interest to the agencies. All these recommended
facilities will produce vast datasets of general applicability, which will have
a tremendous impact on broad areas of astronomy.Comment: ESA-ESO Working Groups Report No. 3, 125 pages, 28 figures. A PDF
version including the cover is available from
http://www.stecf.org/coordination/esa_eso/cosmology/report_cover.pdf and a
printed version (A5 booklet) is available in limited numbers from the Space
Telescope-European Coordinating Facility (ST-ECF): [email protected]
Cosmic Electromagnetic Fields due to Perturbations in the Gravitational Field
We use non-linear gauge-invariant perturbation theory to study the
interaction of an inflation produced seed magnetic field with density and
gravitational wave perturbations in an almost
Friedmann-Lema\^itre-Robertson-Walker (FLRW) spacetime. We compare the effects
of this coupling under the assumptions of poor conductivity, infinite
conductivity and the case where the electric field is sourced via the coupling
of velocity perturbations to the seed field in the ideal magnetohydrodynamic
(MHD) regime, thus generalizing, improving on and correcting previous results.
We solve our equations for long wavelength limits and numerically integrate the
resulting equations to generate power spectra for the electromagnetic field
variables, showing where the modes cross the horizon. We find that the rotation
of the electric field dominates the power spectrum on small scales, in
agreement with previous arguments.Comment: 16 pages, 3 figures, published in PR
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