24,175 research outputs found
On the Running of the Cosmological Constant in Quantum General Relativity
We present arguments that show what the running of the cosmological constant
means when quantum general relativity is formulated following the prescription
developed by Feynman.Comment: 5 page
Resummed Quantum Gravity
We present the current status of the a new approach to quantum general
relativity based on the exact resummation of its perturbative series as that
series was formulated by Feynman. We show that the resummed theory is UV finite
and we present some phenomenological applications as well.Comment: 4 pages, 1 figure; presented at ICHEP0
On The Orbital Evolution of Jupiter Mass Protoplanet Embedded in A Self-Gravity Disk
We performed a series of hydro-dynamic simulations to investigate the orbital
migration of a Jovian planet embedded in a proto-stellar disk. In order to take
into account of the effect of the disk's self gravity, we developed and adopted
an \textbf{Antares} code which is based on a 2-D Godunov scheme to obtain the
exact Reimann solution for isothermal or polytropic gas, with non-reflecting
boundary conditions. Our simulations indicate that in the study of the runaway
(type III) migration, it is important to carry out a fully self consistent
treatment of the gravitational interaction between the disk and the embedded
planet. Through a series of convergence tests, we show that adequate numerical
resolution, especially within the planet's Roche lobe, critically determines
the outcome of the simulations. We consider a variety of initial conditions and
show that isolated, non eccentric protoplanet planets do not undergo type III
migration. We attribute the difference between our and previous simulations to
the contribution of a self consistent representation of the disk's self
gravity. Nevertheless, type III migration cannot be completely suppressed and
its onset requires finite amplitude perturbations such as that induced by
planet-planet interaction. We determine the radial extent of type III migration
as a function of the disk's self gravity.Comment: 19 pages, 13 figure
Evolution of Migrating Planets Undergoing Gas Accretion
We analyze the orbital and mass evolution of planets that undergo run-away
gas accretion by means of 2D and 3D hydrodynamic simulations. The disk torque
distribution per unit disk mass as a function of radius provides an important
diagnostic for the nature of the disk-planet interactions. We first consider
torque distributions for nonmigrating planets of fixed mass and show that there
is general agreement with the expectations of resonance theory. We then present
results of simulations for mass-gaining, migrating planets. For planets with an
initial mass of 5 Earth masses, which are embedded in disks with standard
parameters and which undergo run-away gas accretion to one Jupiter mass (Mjup),
the torque distributions per unit disk mass are largely unaffected by migration
and accretion for a given planet mass. The migration rates for these planets
are in agreement with the predictions of the standard theory for planet
migration (Type I and Type II migration). The planet mass growth occurs through
gas capture within the planet's Bondi radius at lower planet masses, the Hill
radius at intermediate planet masses, and through reduced accretion at higher
planet masses due to gap formation. During run-away mass growth, a planet
migrates inwards by only about 20% in radius before achieving a mass of ~1
Mjup. For the above models, we find no evidence of fast migration driven by
coorbital torques, known as Type III migration. We do find evidence of Type III
migration for a fixed mass planet of Saturn's mass that is immersed in a cold
and massive disk. In this case the planet migration is assumed to begin before
gap formation completes. The migration is understood through a model in which
the torque is due to an asymmetry in density between trapped gas on the leading
side of the planet and ambient gas on the trailing side of the planet.Comment: 26 pages, 29 figures. To appear in The Astrophysical Journal vol.684
(September 20, 2008 issue
Sperm survival in the female reproductive tract in the fly Scathophaga stercoraria (L.)
While sperm competition risk favours males transferring many sperm to secure fertilizations, females of a variety of species actively reduce sperm numbers reaching their reproductive tract, e.g. by extrusion or killing. Potential benefits of spermicide to females include nutritional gains, influence over sperm storage and paternity, and the elimination of sperm bearing somatic mutations that would lower zygote fitness.We investigated changes in sperm viability after in vivo and in vitro exposure to the female tract in the polyandrous fly, Scathophaga stercoraria. Sperm viability was significantly lower in the females' spermathecae immediately after mating than in the experimental males' testes. Males also varied significantly in the proportion of live sperm found in storage in vivo. However, the exact mechanism of sperm degradation remains to be clarified. In vitro exposure to extracts of the female reproductive tract, including female accessory glands, failed to significantly lower sperm viability compared to controls. These results are consistent either with postcopulatory sperm mortality in vivo depending entirely on the male (with individual differences in sperm viability, motility or longevity) or with postcopulatory sperm mortality being subtly affected by female effects which were not detected by the in vitro experimental conditions. Importantly, we found no evidence in support of the hypothesis that female accessory glands contribute to sexual conflict via spermicide. Therefore, female muscular control remains to date the only ascertained mechanism of female influence on sperm storage in this species
k-Dirac operator and parabolic geometries
The principal group of a Klein geometry has canonical left action on the
homogeneous space of the geometry and this action induces action on the spaces
of sections of vector bundles over the homogeneous space. This paper is about
construction of differential operators invariant with respect to the induced
action of the principal group of a particular type of parabolic geometry. These
operators form sequences which are related to the minimal resolutions of the
k-Dirac operators studied in Clifford analysis
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