1,997 research outputs found
Non-Perturbative One-Loop Effective Action for Electrodynamics in Curved Spacetime
In this paper we explicitly evaluate the one-loop effective action in four
dimensions for scalar and spinor fields under the influence of a strong,
covariantly constant, magnetic field in curved spacetime. In the framework of
zeta function regularization, we find the one-loop effective action to all
orders in the magnetic field up to linear terms in the Riemannian curvature. As
a particular case, we also obtain the one-loop effective action for massless
scalar and spinor fields. In this setting, we found that the vacuum energy of
charged spinors with small mass becomes very large due entirely by the
gravitational correction.Comment: LaTeX, 23 page
An investigation of tape spring fold curvature
Tape springs are being used with increasing frequency in today’s space industry to deploy small satellite aerials and array areas. However, to accurately model the deployment of an appendage mounted with tape spring hinges, it is necessary to accurately model the opening moments produced from the material strains in the tape spring fold. These moments are primarily a function of curvature. This paper uses a photographic method to study the post buckling curvatures at the fold location for both two and three dimensional tape spring folds. The results are finally compared to determine the overall data trends
Low-Energy Effective Action in Non-Perturbative Electrodynamics in Curved Spacetime
We study the heat kernel for the Laplace type partial differential operator
acting on smooth sections of a complex spin-tensor bundle over a generic
-dimensional Riemannian manifold. Assuming that the curvature of the U(1)
connection (that we call the electromagnetic field) is constant we compute the
first two coefficients of the non-perturbative asymptotic expansion of the heat
kernel which are of zero and the first order in Riemannian curvature and of
arbitrary order in the electromagnetic field. We apply these results to the
study of the effective action in non-perturbative electrodynamics in four
dimensions and derive a generalization of the Schwinger's result for the
creation of scalar and spinor particles in electromagnetic field induced by the
gravitational field. We discover a new infrared divergence in the imaginary
part of the effective action due to the gravitational corrections, which seems
to be a new physical effect.Comment: LaTeX, 42 page
Maximum black-hole spin from quasi-circular binary mergers
Black holes of mass M must have a spin angular momentum S below the Kerr
limit chi = S/M^2 < 1, but whether astrophysical black holes can attain this
limiting spin depends on their accretion history. Gas accretion from a thin
disk limits the black-hole spin to chi_gas < 0.9980 +- 0.0002, as
electromagnetic radiation from this disk with retrograde angular momentum is
preferentially absorbed by the black hole. Extrapolation of
numerical-relativity simulations of equal-mass binary black-hole mergers to
maximum initial spins suggests these mergers yield a maximum spin chi_eq <
0.95. Here we show that for smaller mass ratios q = m/M << 1, the superradiant
extraction of angular momentum from the larger black hole imposes a fundamental
limit chi_lim < 0.9979 +- 0.0001 on the final black-hole spin even in the
test-particle limit q -> 0 of binary black-hole mergers. The nearly equal
values of chi_gas and chi_lim imply that measurement of supermassive black-hole
spins cannot distinguish a black hole built by gas accretion from one assembled
by the gravitational inspiral of a disk of compact stellar remnants. We also
show how superradiant scattering alters the mass and spin predicted by models
derived from extrapolating test-particle mergers to finite mass ratios.Comment: final version accepted in PRD, new Fig.4 and discussio
Morphological characterization, germination ecology and conservation of narrow endemic plant species of the Aeolian Islands (Sicily)
This stusy is focused on four narrow endemic species of the Aeolian fora: Anthemis aeolica, Erysimum brulloi, Cytisus aeolicus and Silene hicesiae. Morphological characterization of seeds, seed germination and dormancy regulation have been investigated for these species
Seed morphology and seed coat structure in Mediterranean plants: implication for evolution, taxonomy and germinative performance
A better understanding of morphological, ultra-structural and anatomical seed coat characteristics can contribute to explain and predict seed performance under certain environmental conditions and select the most suitable treatments promoting seed germination. The Seed Banks at the Botanic Gardens of Catania and Palermo (Sicily) are particularly involved in research studies aimed to detect and describe macro- and micro-morphological features of seeds from different plant families and genera, using both light and scanning electron microscopy. These studies mainly aim to 1) recognize and compare inter- and intra-specific variation of seed coat traits, with particular attention on rare and threatened species; 2) evaluate the usefulness of these traits in taxonomic and phylogenetic studies; 3) establish correlations between seed coat structures and seed germination; 4) understand the seed coat adaptive potential and sensitivity to particular ecological conditions and changing environments
Corrigendum to ‘Population structure analysis provides insights into the infection biology and invasion strategies of Kretzschmaria deusta in trees’
Molecular dynamics simulations reveal the determinants of cyclin-dependent kinase 2 inhibition by 5-nitrosopyrimidine derivatives
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