60 research outputs found
What can we say about seed fields for galactic dynamos?
We demonstrate that a quasi-uniform cosmological seed field is a much less
suitable seed for a galactic dynamo than has often been believed. The age of
the Universe is insufficient for a conventional galactic dynamo to generate a
contemporary galactic magnetic field starting from such a seed, accepting
conventional estimates for physical quantities. We discuss modifications to the
scenario for the evolution of galactic magnetic fields implied by this result.
We also consider briefly the implications of a dynamo number that is
significantly larger than that given by conventional estimates
Nonlinearity in a dynamo
Using a rotating flat layer heated from below as an example, we consider
effects which lead to stabilizing an exponentially growing magnetic field in
magnetostrophic convection in transition from the kinematic dynamo to the full
non-linear dynamo. We present estimates of the energy redistribution over the
spectrum and helicity quenching by the magnetic field. We also study the
alignment of the velocity and magnetic fields. These regimes are similar to
those in planetary dynamo simulations.Comment: Accepted to Geophys. Astrophys. Fluid Dyna
Yoshizawa's cross-helicity effect and its quenching
A central quantity in mean-field magnetohydrodynamics is the mean
electromotive force EMF, which in general depends on the mean magnetic field.
It may however have a part independent of the mean magnetic field. Here we
study an example of a rotating conducting body of turbulent fluid with non-zero
cross-helicity, in which a contribution to the EMF proportional to the angular
velocity occurs (Yoshizawa 1990). If the forcing is helical, it also leads to
an alpha effect, and large-scale magnetic fields can be generated. For not too
rapid rotation, the field configuration is such that Yoshizawa's contribution
to the EMF is considerably reduced compared to the case without alpha effect.
In that case, large-scale flows are also found to be generated.Comment: 10 pages, 8 figures, compatible with published versio
Atomic Parity Nonconservation and Nuclear Anapole Moments
Anapole moments are parity-odd, time-reversal-even moments of the E1
projection of the electromagnetic current. Although it was recognized, soon
after the discovery of parity violation in the weak interaction, that
elementary particles and composite systems like nuclei must have anapole
moments, it proved difficult to isolate this weak radiative correction. The
first successful measurement, an extraction of the nuclear anapole moment of
133Cs from the hyperfine dependence of the atomic parity violation, was
obtained only recently. An important anapole moment bound in Tl also exists. We
discuss these measurements and their significance as tests of the hadronic weak
interaction, focusing on the mechanisms that operate within the nucleus to
generate the anapole moment. The atomic results place new constraints on weak
meson-nucleon couplings, ones we compare to existing bounds from a variety of
p-p and nuclear tests of parity nonconservation.Comment: 35 pages; 8 figures; late
The Cosmic Microwave Background and Particle Physics
In forthcoming years, connections between cosmology and particle physics will
be made increasingly important with the advent of a new generation of cosmic
microwave background (CMB) experiments. Here, we review a number of these
links. Our primary focus is on new CMB tests of inflation. We explain how the
inflationary predictions for the geometry of the Universe and primordial
density perturbations will be tested by CMB temperature fluctuations, and how
the gravitational waves predicted by inflation can be pursued with the CMB
polarization. The CMB signatures of topological defects and primordial magnetic
fields from cosmological phase transitions are also discussed. Furthermore, we
review current and future CMB constraints on various types of dark matter (e.g.
massive neutrinos, weakly interacting massive particles, axions, vacuum
energy), decaying particles, the baryon asymmetry of the Universe,
ultra-high-energy cosmic rays, exotic cosmological topologies, and other new
physics.Comment: 43 pages. To appear in Annual Reviews of Nuclear and Particle Scienc
Parameters of Pseudo-Random Quantum Circuits
Pseudorandom circuits generate quantum states and unitary operators which are
approximately distributed according to the unitarily invariant Haar measure. We
explore how several design parameters affect the efficiency of pseudo-random
circuits, with the goal of identifying relevant trade-offs and optimizing
convergence. The parameters we explore include the choice of single- and
two-qubit gates, the topology of the underlying physical qubit architecture,
the probabilistic application of two-qubit gates, as well as circuit size,
initialization, and the effect of control constraints. Building on the
equivalence between pseudo-random circuits and approximate -designs, a
Markov matrix approach is employed to analyze asymptotic convergence properties
of pseudo-random second-order moments to a 2-design. Quantitative results on
the convergence rate as a function of the circuit size are presented for qubit
topologies with a sufficient degree of symmetry. Our results may be
theoretically and practically useful to optimize the efficiency of random state
and operator generation.Comment: 17 pages, 14 figures, 2 Appendice
Initial conditions, Discreteness and non-linear structure formation in cosmology
In this lecture we address three different but related aspects of the initial
continuous fluctuation field in standard cosmological models. Firstly we
discuss the properties of the so-called Harrison-Zeldovich like spectra. This
power spectrum is a fundamental feature of all current standard cosmological
models. In a simple classification of all stationary stochastic processes into
three categories, we highlight with the name ``super-homogeneous'' the
properties of the class to which models like this, with , belong. In
statistical physics language they are well described as glass-like. Secondly,
the initial continuous density field with such small amplitude correlated
Gaussian fluctuations must be discretised in order to set up the initial
particle distribution used in gravitational N-body simulations. We discuss the
main issues related to the effects of discretisation, particularly concerning
the effect of particle induced fluctuations on the statistical properties of
the initial conditions and on the dynamical evolution of gravitational
clustering.Comment: 28 pages, 1 figure, to appear in Proceedings of 9th Course on
Astrofundamental Physics, International School D. Chalonge, Kluwer, eds N.G.
Sanchez and Y.M. Pariiski, uses crckapb.st pages, 3 figure, ro appear in
Proceedings of 9th Course on Astrofundamental Physics, International School
D. Chalonge, Kluwer, Eds. N.G. Sanchez and Y.M. Pariiski, uses crckapb.st
Big bang simulation in superfluid 3He-B -- Vortex nucleation in neutron-irradiated superflow
We report the observation of vortex formation upon the absorption of a
thermal neutron in a rotating container of superfluid He-B. The nuclear
reaction n + He = p + H + 0.76MeV heats a cigar shaped region of the
superfluid into the normal phase. The subsequent cooling of this region back
through the superfluid transition results in the nucleation of quantized
vortices. Depending on the superflow velocity, sufficiently large vortex rings
grow under the influence of the Magnus force and escape into the container
volume where they are detected individually with nuclear magnetic resonance.
The larger the superflow velocity the smaller the rings which can expand. Thus
it is possible to obtain information about the morphology of the initial defect
network. We suggest that the nucleation of vortices during the rapid cool-down
into the superfluid phase is similar to the formation of defects during
cosmological phase transitions in the early universe.Comment: 4 pages, LaTeX file, 4 figures are available at
ftp://boojum.hut.fi/pub/publications/lowtemp/LTL-95009.p
Dynamical Mean-Field Theory within an Augmented Plane-Wave Framework: Assessing Electronic Correlations in the Iron Pnictide LaFeAsO
We present an approach that combines the local density approximation (LDA)
and the dynamical mean-field theory (DMFT) in the framework of the
full-potential linear augmented plane waves (FLAPW) method. Wannier-like
functions for the correlated shell are constructed by projecting local orbitals
onto a set of Bloch eigenstates located within a certain energy window. The
screened Coulomb interaction and Hund's coupling are calculated from a
first-principle constrained RPA scheme. We apply this LDA+DMFT implementation,
in conjunction with continuous-time quantum Monte-Carlo, to study the
electronic correlations in LaFeAsO. Our findings support the physical picture
of a metal with intermediate correlations. The average value of the mass
renormalization of the Fe 3d bands is about 1.6, in reasonable agreement with
the picture inferred from photoemission experiments. The discrepancies between
different LDA+DMFT calculations (all technically correct) which have been
reported in the literature are shown to have two causes: i) the specific value
of the interaction parameters used in these calculations and ii) the degree of
localization of the Wannier orbitals chosen to represent the Fe 3d states, to
which many-body terms are applied. The latter is a fundamental issue in the
application of many-body calculations, such as DMFT, in a realistic setting. We
provide strong evidence that the DMFT approximation is more accurate and more
straightforward to implement when well-localized orbitals are constructed from
a large energy window encompassing Fe-3d, As-4p and O-2p, and point out several
difficulties associated with the use of extended Wannier functions associated
with the low-energy iron bands. Some of these issues have important physical
consequences, regarding in particular the sensitivity to the Hund's coupling.Comment: 16 pages, 9 figures, published versio
Did smokefree legislation in England reduce exposure to secondhand smoke among nonsmoking adults? Cotinine analysis from the Health Survey for England.
Background: On 1 July 2007, smokefree legislation was implemented in England, which made virtually all enclosed public places and workplaces smokefree.
Objectives: We examined trends in and predictors of secondhand smoke exposure among nonsmoking adults to determine whether exposure changed after the introduction of smokefree legislation and whether these changes varied by socioeconomic status (SES) and by household smoking status.
Methods: We analyzed salivary cotinine data from the Health Survey for England that were collected in 7 of 11 annual surveys undertaken between 1998 and 2008. We conducted multivariate regression analyses to examine secondhand smoke exposure as measured by the proportion of nonsmokers with undetectable levels of cotinine and by geometric mean cotinine.
Results: Secondhand smoke exposure was higher among those exposed at home and among lower-SES groups. Exposure declined markedly from 1998 to 2008 (the proportion of participants with undetectable cotinine was 2.9 times higher in the last 6 months of 2008 compared with the first 6 months of 1998 and geometric mean cotinine declined by 80%). We observed a significant fall in exposure after legislation was introduced—the odds of having undetectable cotinine were 1.5 times higher [95% confidence interval (CI): 1.3, 1.8] and geometric mean cotinine fell by 27% (95% CI: 17%, 36%) after adjusting for the prelegislative trend and potential confounders. Significant reductions were not, however, seen in those living in lower-social class households or homes where smoking occurs inside on most days.
Conclusions: We found that the impact of England’s smokefree legislation on secondhand smoke exposure was above and beyond the underlying long-term decline in secondhand smoke exposure and demonstrates the positive effect of the legislation. Nevertheless, some population subgroups appear not to have benefitted significantly from the legislation. This finding suggests that these groups should receive more support to reduce their exposure
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