720 research outputs found
Foundations of multiple black hole evolutions
We present techniques for long-term, stable, and accurate evolutions of
multiple-black-hole spacetimes using the `moving puncture' approach with
fourth- and eighth-order finite difference stencils. We use these techniques to
explore configurations of three black holes in a hierarchical system consisting
of a third black hole approaching a quasi-circular black-hole binary, and find
that, depending on the size of the binary, the resulting encounter may lead to
a prompt merger of all three black holes, production of a highly elliptical
binary (with the third black hole remaining unbound), or disruption of the
binary (leading to three free black holes). We also analyze the classical
Burrau three-body problem using full numerical evolutions. In both cases, we
find behaviors distinctly different from Newtonian predictions, which has
important implications for N-body black-hole simulations. For our simulations
we use analytic approximate data. We find that the eighth-order stencils
significantly reduce the numerical errors for our choice of grid sizes, and
that the approximate initial data produces the expected waveforms (after a
rescaling of the puncture masses) for black-hole binaries with modest initial
separations.Comment: Revtex 4, 13 pages, 15 figure
Leaded tin bronzes: the effects of casting method on dry sliding behaviour
In metal-to-metal sliding bearing applications, leaded tin bronzes are widely known as materials with excellent seizure resistance. In conditions of boundary or dry lubrication, lead may smear across the sliding surface, preventing surface contact and catastrophic seizure. The aim of this study was to determine the effects of casting method on the dry sliding behaviour of leaded tin bronzes. Continuous cast, centrifugally cast, and sand cast leaded tin bronze samples with varying lead contents were subjected to pin-on-disk- testing. It was found that casting method has a significant effect on the wear behaviour of leaded tin bronzes in dry sliding conditions. With continuous cast samples, the dominant wear mode was rapid, stable microcracking along copper/lead interfacial boundaries. With centrifugally and sand cast samples, wear occurred more slowly and erratically through the formation of transfer layers. The dominant wear mode was found to be connected to the coarseness of the distribution of lead particles in the copper matrix
Capture of dark matter by the Solar System
We study the capture of galactic dark matter by the Solar System. The effect
is due to the gravitational three-body interaction between the Sun, one of the
planets, and a dark matter particle. The analytical estimate for the capture
cross-section is derived and the upper and lower bounds for the total mass of
the captured dark matter particles are found. The estimates for their density
are less reliable. The most optimistic of them give an enhancement of dark
matter density by about three orders of magnitudes compared to its value in our
Galaxy. However, even this optimistic value remains below the best present
observational upper limits by about two orders of magnitude.Comment: 5 pages, 3 tables; Refs. updated and discussion extende
Approximate action-angle variables for the figure-eight and other periodic three-body orbits
We use the maximally permutation symmetric set of three-body coordinates,
that consist of the "hyper-radius" , the
"rescaled area of the triangle" ) and the (braiding) hyper-angle , to analyze the "figure-eight"
choreographic three-body motion discovered by Moore \cite{Moore1993} in the
Newtonian three-body problem. Here are the two
Jacobi relative coordinate vectors. We show that the periodicity of this motion
is closely related to the braiding hyper-angle . We construct an
approximate integral of motion that together with the hyper-angle
forms the action-angle pair of variables for this problem and show that
it is the underlying cause of figure-eight motion's stability. We construct
figure-eight orbits in two other attractive permutation-symmetric three-body
potentials. We compare the figure-eight orbits in these three potentials and
discuss their generic features, as well as their differences. We apply these
variables to two new periodic, but non-choreographic orbits: One has a
continuously rising in time , just like the figure-eight motion, but
with a different, more complex periodicity, whereas the other one has an
oscillating temporal behavior.Comment: 11 pages, 19 figure
Dark energy domination in the Virgocentric flow
The standard \LambdaCDM cosmological model implies that all celestial bodies
are embedded in a perfectly uniform dark energy background, represented by
Einstein's cosmological constant, and experience its repulsive antigravity
action. Can dark energy have strong dynamical effects on small cosmic scales as
well as globally? Continuing our efforts to clarify this question, we focus now
on the Virgo Cluster and the flow of expansion around it. We interpret the
Hubble diagram, from a new database of velocities and distances of galaxies in
the cluster and its environment, using a nonlinear analytical model which
incorporates the antigravity force in terms of Newtonian mechanics. The key
parameter is the zero-gravity radius, the distance at which gravity and
antigravity are in balance. Our conclusions are: 1. The interplay between the
gravity of the cluster and the antigravity of the dark energy background
determines the kinematical structure of the system and controls its evolution.
2. The gravity dominates the quasi-stationary bound cluster, while the
antigravity controls the Virgocentric flow, bringing order and regularity to
the flow, which reaches linearity and the global Hubble rate at distances \ga
15 Mpc. 3. The cluster and the flow form a system similar to the Local Group
and its outflow. In the velocity-distance diagram, the cluster-flow structure
reproduces the group-flow structure with a scaling factor of about 10; the
zero-gravity radius for the cluster system is also 10 times larger. The phase
and dynamical similarity of the systems on the scales of 1-30 Mpc suggests that
a two-component pattern may be universal for groups and clusters: a
quasi-stationary bound central component and an expanding outflow around it,
due to the nonlinear gravity-antigravity interplay with the dark energy
dominating in the flow component.Comment: 7 pages, 2 figures, Astronomy and Astrophysics (accepted
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