794,333 research outputs found
Cosmological perturbations in extended electromagnetism. General gauge invariant approach
A certain vector-tensor (VT) theory is revisited. It was proposed and
analyzed as a theory of electromagnetism without the standard gauge invariance.
Our attention is first focused on a detailed variational formulation of the
theory, which leads to both a modified Lorentz force and the true energy
momentum tensor of the vector field. The theory is then applied to cosmology. A
complete gauge invariant treatment of the scalar perturbations is presented.
For appropriate gauge invariant variables describing the scalar modes of the
vector field (A-modes), it is proved that the evolution equations of these
modes do not involve the scalar modes appearing in General Relativity
(GR-modes), which are associated to the metric and the energy momentum tensor
of the cosmological fluids. However, the A-modes modify the standard gauge
invariant equations describing the GR-modes. By using the new formalism, the
evolution equations of the A-perturbations are derived and separately solved
and, then, the correction terms --due to the A-perturbations-- appearing in the
evolution equations of the GR-modes are estimated. The evolution of these
correction terms is studied for an appropriate scale. The relevance of these
terms depends on both the spectra and the values of the normalization constants
involved in extended electromagnetism. Further applications of the new
formalism will be presented elsewhere.Comment: 25 pages, 3 figures, published in Physical Review
A Hamiltonian system for interacting Benjamin-Feir resonances
In this paper, we present a model describing the time evolution of two
dimensional surface waves in gravity and infinite depth. The model of six
interacting modes derives from the normal form of the system describing the
dynamics of surface waves and is governed by a Hamiltonian system of equations
of cubic order in the amplitudes of the waves. We derive a Hamiltonian system
with two degrees of freedom from this Hamiltonian using conserved quantities.
The interactions are those of two coupled Benjamin-Feir resonances. The
temporal evolution of the amplitude of the different modes is described
according to the parameters of the system. In particular, we study the energy
exchange produced by the modulations of the amplitudes of the modes. The
evolution of the modes reveals a chaotic dynamics
Exotic modes of excitation in atomic nuclei far from stability
We review recent studies of the evolution of collective excitations in atomic
nuclei far from the valley of -stability. Collective degrees of freedom
govern essential aspects of nuclear structure, and for several decades the
study of collective modes such as rotations and vibrations has played a vital
role in our understanding of complex properties of nuclei. The multipole
response of unstable nuclei and the possible occurrence of new exotic modes of
excitation in weakly-bound nuclear systems, present a rapidly growing field of
research, but only few experimental studies of these phenomena have been
reported so far. Valuable data on the evolution of the low-energy dipole
response in unstable neutron-rich nuclei have been gathered in recent
experiments, but the available information is not sufficient to determine the
nature of observed excitations. Even in stable nuclei various modes of giant
collective oscillations had been predicted by theory years before they were
observed, and for that reason it is very important to perform detailed
theoretical studies of the evolution of collective modes of excitation in
nuclei far from stability. We therefore discuss the modern theoretical tools
that have been developed in recent years for the description of collective
excitations in weakly-bound nuclei. The review focuses on the applications of
these models to studies of the evolution of low-energy dipole modes from stable
nuclei to systems near the particle emission threshold, to analyses of various
isoscalar modes, those for which data are already available, as well as those
that could be observed in future experiments, to a description of
charge-exchange modes and their evolution in neutron-rich nuclei, and to
studies of the role of exotic low-energy modes in astrophysical processes.Comment: 123 pages, 59 figures, submitted to Reports on Progress in Physic
Solitons and diffusive modes in the noiseless Burgers equation: Stability analysis
The noiseless Burgers equation in one spatial dimension is analyzed from the
point of view of a diffusive evolution equation in terms of nonlinear soliton
modes and linear diffusive modes. The transient evolution of the profile is
interpreted as a gas of right hand solitons connected by ramp solutions with
superposed linear diffusive modes. This picture is supported by a linear
stability analysis of the soliton mode. The spectrum and phase shift of the
diffusive modes are determined. In the presence of the soliton the diffusive
modes develop a gap in the spectrum and are phase-shifted in accordance with
Levinson's theorem. The spectrum also exhibits a zero-frequency translation or
Goldstone mode associated with the broken translational symmetry.Comment: 9 pages, Revtex file, 5 figures, to be submitted to Phys. Rev.
Towards an understanding of the stability properties of the 3+1 evolution equations in general relativity
We study the stability properties of the standard ADM formulation of the 3+1
evolution equations of general relativity through linear perturbations of flat
spacetime. We focus attention on modes with zero speed of propagation and
conjecture that they are responsible for instabilities encountered in numerical
evolutions of the ADM formulation. These zero speed modes are of two kinds:
pure gauge modes and constraint violating modes. We show how the decoupling of
the gauge by a conformal rescaling can eliminate the problem with the gauge
modes. The zero speed constraint violating modes can be dealt with by using the
momentum constraints to give them a finite speed of propagation. This analysis
sheds some light on the question of why some recent reformulations of the 3+1
evolution equations have better stability properties than the standard ADM
formulation.Comment: 15 pages, 9 figures. Added a new section, plus incorporated many
comments made by refere
The asexual genome of Drosophila
The rate of recombination affects the mode of molecular evolution. In
high-recombining sequence, the targets of selection are individual genetic
loci; under low recombination, selection collectively acts on large,
genetically linked genomic segments. Selection under linkage can induce clonal
interference, a specific mode of evolution by competition of genetic clades
within a population. This mode is well known in asexually evolving microbes,
but has not been traced systematically in an obligate sexual organism. Here we
show that the Drosophila genome is partitioned into two modes of evolution: a
local interference regime with limited effects of genetic linkage, and an
interference condensate with clonal competition. We map these modes by
differences in mutation frequency spectra, and we show that the transition
between them occurs at a threshold recombination rate that is predictable from
genomic summary statistics. We find the interference condensate in segments of
low-recombining sequence that are located primarily in chromosomal regions
flanking the centromeres and cover about 20% of the Drosophila genome.
Condensate regions have characteristics of asexual evolution that impact gene
function: the efficacy of selection and the speed of evolution are lower and
the genetic load is higher than in regions of local interference. Our results
suggest that multicellular eukaryotes can harbor heterogeneous modes and tempi
of evolution within one genome. We argue that this variation generates
selection on genome architecture
Theoretical power spectra of mixed modes in low mass red giant stars
CoRoT and Kepler observations of red giant stars revealed very rich spectra
of non-radial solar-like oscillations. Of particular interest was the detection
of mixed modes that exhibit significant amplitude, both in the core and at the
surface of the stars. It opens the possibility of probing the internal
structure from their inner-most layers up to their surface along their
evolution on the red giant branch as well as on the red-clump. Our objective is
primarily to provide physical insight into the physical mechanism responsible
for mixed-modes amplitudes and lifetimes. Subsequently, we aim at understanding
the evolution and structure of red giants spectra along with their evolution.
The study of energetic aspects of these oscillations is also of great
importance to predict the mode parameters in the power spectrum. Non-adiabatic
computations, including a time-dependent treatment of convection, are performed
and provide the lifetimes of radial and non-radial mixed modes. We then combine
these mode lifetimes and inertias with a stochastic excitation model that gives
us their heights in the power spectra. For stars representative of CoRoT and
Kepler observations, we show under which circumstances mixed modes have heights
comparable to radial ones. We stress the importance of the radiative damping in
the determination of the height of mixed modes. Finally, we derive an estimate
for the height ratio between a g-type and a p-type mode. This can thus be used
as a first estimate of the detectability of mixed-modes
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