2,146 research outputs found
On the nonlinear stability of symplectic integrators
The modified Hamiltonian is used to study the nonlinear stability of symplectic integrators, especially
for nonlinear oscillators. We give conditions under which an initial condition on a compact
energy surface will remain bounded for exponentially long times for sufficiently small time steps.
For example, the implicit midpoint rule achieves this for the critical energy surface of the H´enon-
Heiles system, while the leapfrog method does not. We construct explicit methods which are
nonlinearly stable for all simple mechanical systems for exponentially long times. We also address
questions of topological stability, finding conditions under which the original and modified energy
surfaces are topologically equivalent
Viscous Dark Energy Models with Variable G and Lambda
We consider a cosmological model with bulk viscosity () and variable
cosmological ) and
gravitational () constants. The model exhibits many interesting cosmological
features. Inflation proceeds du to the presence of bulk viscosity and dark
energy without requiring the equation of state . During the
inflationary era the energy density () does not remain constant, as in
the de-Sitter type. Moreover, the cosmological and gravitational constants
increase exponentially with time, whereas the energy density and viscosity
decrease exponentially with time. The rate of mass creation during inflation is
found to be very huge suggesting that all matter in the universe was created
during inflation.Comment: 6 Latex page
Opportunities for future supernova studies of cosmic acceleration
We investigate the potential of a future supernova dataset, as might be
obtained by the proposed SNAP satellite, to discriminate among different ``dark
energy'' theories that describe an accelerating Universe. We find that many
such models can be distinguished with a fit to the effective
pressure-to-density ratio, , of this energy. More models can be
distinguished when the effective slope, , of a changing is also fit,
but only if our knowledge of the current mass density, , is improved.
We investigate the use of ``fitting functions'' to interpret luminosity
distance data from supernova searches, and argue in favor of a particular
preferred method, which we use in our analysis.Comment: Four pages including figures. Final published version. No significant
changes from v
Phantom Energy Accretion by Stringy Charged Black Hole
We investigate the dynamical behavior of phantom energy near stringy
magnetically charged black hole. For this purpose, we derive equations of
motion for steady-state spherically symmetric flow of phantom energy onto the
stringy magnetically charged black hole. It is found that phantom energy
accreting onto black hole decreases its mass. Further, the location of critical
points of accretion is explored, which yields mass to charge ratio. This ratio
implies that accretion process cannot transform a black hole into an extremal
black hole or a naked singularity, hence cosmic censorship hypothesis remains
valid here.Comment: 7 pages, no figur
Zeldovich flow on cosmic vacuum background: new exact nonlinear analytical solution
A new exact nonlinear Newtonian solution for a plane matter flow superimposed
on the isotropic Hubble expansion is reported. The dynamical effect of cosmic
vacuum is taken into account. The solution describes the evolution of nonlinear
perturbations via gravitational instability of matter and the termination of
the perturbation growth by anti-gravity of vacuum at the epoch of transition
from matter domination to vacuum domination. On this basis, an `approximate' 3D
solution is suggested as an analog of the Zeldovich ansatz.Comment: 9 pages, 1 figure
Positive Selection and Horizontal Gene Transfer in the Genome of a Male-Killing Wolbachia
Wolbachia are a genus of widespread bacterial endosymbionts in which some strains can hijack or manipulate arthropod host reproduction. Male killing is one such manipulation in which these maternally transmitted bacteria benefit surviving daughters in part by removing competition with the sons for scarce resources. Despite previous findings of interesting genome features of microbial sex ratio distorters, the population genomics of male-killers remain largely uncharacterized. Here, we uncover several unique features of the genome and population genomics of four Arizonan populations of a male-killing Wolbachia strain, wInn, that infects mushroom-feeding Drosophila innubila. We first compared the wInn genome with other closely related Wolbachia genomes of Drosophila hosts in terms of genome content and confirm that the wInn genome is largely similar in overall gene content to the wMel strain infecting D. melanogaster. However, it also contains many unique genes and repetitive genetic elements that indicate lateral gene transfers between wInn and non-Drosophila eukaryotes. We also find that, in line with literature precedent, genes in the Wolbachia prophage and Octomom regions are under positive selection. Of all the genes under positive selection, many also show evidence of recent horizontal transfer among Wolbachia symbiont genomes. These dynamics of selection and horizontal gene transfer across the genomes of several Wolbachia strains and diverse host species may be important underlying factors in Wolbachia’s success as a male-killer of divergent host species
Multi-Color Light Curves of Type Ia Supernovae on the Color-Magnitude Diagram: a Novel Step Toward More Precise Distance and Extinction Estimates
We show empirically that fits to the color-magnitude relation of Type Ia
supernovae after optical maximum can provide accurate relative extragalactic
distances. We report the discovery of an empirical color relation for Type Ia
light curves: During much of the first month past maximum, the magnitudes of
Type Ia supernovae defined at a given value of color index have a very small
magnitude dispersion; moreover, during this period the relation between
magnitude and color (or or color) is strikingly linear, to
the accuracy of existing well-measured data. These linear relations can provide
robust distance estimates, in particular, by using the magnitudes when the
supernova reaches a given color. After correction for light curve strech factor
or decline rate, the dispersion of the magnitudes taken at the intercept of the
linear color-magnitude relation are found to be around 0.08 for the
sub-sample of supernovae with \BVm , and around 0.11 for the
sub-sample with \BVm . This small dispersion is consistent with
being mostly due to observational errors. The method presented here and the
conventional light curve fitting methods can be combined to further improve
statistical dispersions of distance estimates. It can be combined with the
magnitude at maximum to deduce dust extinction. The slopes of the
color-magnitude relation may also be used to identify intrinsically different
SN Ia systems. The method provides a tool that is fundamental to using SN Ia to
estimate cosmological parameters such as the Hubble constant and the mass and
dark energy content of the universe.Comment: ApJ, in pres
A single synonymous nucleotide change impacts the male-killing phenotype of prophage WO gene wmk
Wolbachia are the most widespread bacterial endosymbionts in animals. Within arthropods, these maternally transmitted bacteria can selfishly hijack host reproductive processes to increase the relative fitness of their transmitting females. One such form of reproductive parasitism called male killing, or the selective killing of infected males, is recapitulated to degrees by transgenic expression of the prophage WO-mediated killing (wmk) gene. Here, we characterize the genotype-phenotype landscape of wmk-induced male killing in D. melanogaster using transgenic expression. While phylogenetically distant wmk homologs induce no sex-ratio bias, closely-related homologs exhibit complex phenotypes spanning no death, male death, or death of all hosts. We demonstrate that alternative start codons, synonymous codons, and notably a single synonymous nucleotide in wmk can ablate killing. These findings reveal previously unrecognized features of transgenic wmk-induced killing and establish new hypotheses for the impacts of post-transcriptional processes in male killing variation. We conclude that synonymous sequence changes are not necessarily silent in nested endosymbiotic interactions with life-or-death consequences
Geodesic Warps by Conformal Mappings
In recent years there has been considerable interest in methods for
diffeomorphic warping of images, with applications e.g.\ in medical imaging and
evolutionary biology. The original work generally cited is that of the
evolutionary biologist D'Arcy Wentworth Thompson, who demonstrated warps to
deform images of one species into another. However, unlike the deformations in
modern methods, which are drawn from the full set of diffeomorphism, he
deliberately chose lower-dimensional sets of transformations, such as planar
conformal mappings.
In this paper we study warps of such conformal mappings. The approach is to
equip the infinite dimensional manifold of conformal embeddings with a
Riemannian metric, and then use the corresponding geodesic equation in order to
obtain diffeomorphic warps. After deriving the geodesic equation, a numerical
discretisation method is developed. Several examples of geodesic warps are then
given. We also show that the equation admits totally geodesic solutions
corresponding to scaling and translation, but not to affine transformations
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