464 research outputs found
The Autobiography of Dr. Russell R. Rudd (1898-1998) From Memoirs Written in 1989, Chapter I
The Autobiography of Dr. Russell R. Rudd (1898-1998) From Memoirs Written in 1989, Chapter I
Dr. Russell R. Rud
The Autobiography of Dr. Russell R. Rudd (1898-1998): From Memoirs Written in 1989, Chapter II
The Autobiography of Dr. Russell R. Rudd (1898-1998): From Memoirs Written in 1989, Chapter II
Dr. Russell R. Rud
Nutational resonances, transitional precession, and precession-averaged evolution in binary black-hole systems
In the post-Newtonian (PN) regime, the timescale on which the spins of binary
black holes precess is much shorter than the radiation-reaction timescale on
which the black holes inspiral to smaller separations. On the precession
timescale, the angle between the total and orbital angular momenta oscillates
with nutation period , during which the orbital angular momentum
precesses about the total angular momentum by an angle . This defines
two distinct frequencies that vary on the radiation-reaction timescale: the
nutation frequency and the precession frequency
. We use analytic solutions for generic spin
precession at 2PN order to derive Fourier series for the total and orbital
angular momenta in which each term is a sinusoid with frequency for integer . As black holes inspiral, they can pass through
nutational resonances () at which the total angular momentum
tilts. We derive an approximate expression for this tilt angle and show that it
is usually less than radians for nutational resonances at binary
separations . The large tilts occurring during transitional precession
(near zero total angular momentum) are a consequence of such states being
approximate nutational resonances. Our new Fourier series for the total
and orbital angular momenta converge rapidly with providing an intuitive
and computationally efficient approach to understanding generic precession that
may facilitate future calculations of gravitational waveforms in the PN regime.Comment: 18 pages, 9 figures, version published in PR
Evidence for diquarks in lattice QCD
Diquarks may play an important role in hadron spectroscopy, baryon decays and
color superconductivity. We investigate the existence of diquark correlations
in lattice QCD by considering systematically all the lowest energy diquark
channels in a color gauge-invariant setup. We measure mass differences between
the various channels and show that the positive parity scalar diquark is the
lightest. Quark-quark correlations inside the diquark are clearly seen in this
channel, and yield a diquark size of order 1 fm.Comment: Version as published in Phys. Rev. Lett.97, 222002,2006; 4 pages, 5
figure
Precessional Instability in Binary Black Holes with Aligned Spins.
Binary black holes on quasicircular orbits with spins aligned with their orbital angular momentum have been test beds for analytic and numerical relativity for decades, not least because symmetry ensures that such configurations are equilibrium solutions to the spin-precession equations. In this work, we show that these solutions can be unstable when the spin of the higher-mass black hole is aligned with the orbital angular momentum and the spin of the lower-mass black hole is antialigned. Spins in these configurations are unstable to precession to large misalignment when the binary separation r is between the values r(ud±)=(√(χ(1))±√(qχ(2)))(4)(1-q)(-2)M, where M is the total mass, q≡m(2)/m(1) is the mass ratio, and χ(1) (χ(2)) is the dimensionless spin of the more (less) massive black hole. This instability exists for a wide range of spin magnitudes and mass ratios and can occur in the strong-field regime near the merger. We describe the origin and nature of the instability using recently developed analytical techniques to characterize fully generic spin precession. This instability provides a channel to circumvent astrophysical spin alignment at large binary separations, allowing significant spin precession prior to merger affecting both gravitational-wave and electromagnetic signatures of stellar-mass and supermassive binary black holes.D.G. is supported by the UK STFC
and the Isaac Newton Studentship of the Univer-
sity of Cambridge. M.K. is supported by Alfred
P. Sloan Foundation grant FG-2015-65299. R.O'S.
is supported by NSF grants PHY-0970074 and PHY-
1307429. A.K. and E.B. are supported by NSF CA-
REER Grant PHY-1055103. E.B. acknowledges support
from FCT contract IF/00797/2014/CP1214/CT0012
under the IF2014 Programme. U.S. is supported
by FP7-PEOPLE-2011-CIG Grant No. 293412, FP7-
PEOPLE-2011-IRSES Grant No. 295189, H2020-MSCA-
RISE-2015 Grant No. StronGrHEP-690904, SDSC and
TACC through XSEDE Grant No. PHY-090003 by
the NSF, H2020 ERC Consolidator Grant Agree-
ment No. MaGRaTh-646597, STFC Roller Grant No.
ST/L000636/1 and DiRAC's Cosmos Shared Memory
system through BIS Grant No. ST/J005673/1 and STFC
Grant Nos. ST/H008586/1, ST/K00333X/1. D.T. is
partially supported by the NSF awards PHY-1067985
and PHY-1404139.This is the author accepted manuscript. The final version is available from American Physical Society at http://dx.doi.org/10.1103/PhysRevLett.115.141102
Deep Neural Ensemble for Retinal Vessel Segmentation in Fundus Images towards Achieving Label-free Angiography
Automated segmentation of retinal blood vessels in label-free fundus images
entails a pivotal role in computed aided diagnosis of ophthalmic pathologies,
viz., diabetic retinopathy, hypertensive disorders and cardiovascular diseases.
The challenge remains active in medical image analysis research due to varied
distribution of blood vessels, which manifest variations in their dimensions of
physical appearance against a noisy background.
In this paper we formulate the segmentation challenge as a classification
task. Specifically, we employ unsupervised hierarchical feature learning using
ensemble of two level of sparsely trained denoised stacked autoencoder. First
level training with bootstrap samples ensures decoupling and second level
ensemble formed by different network architectures ensures architectural
revision. We show that ensemble training of auto-encoders fosters diversity in
learning dictionary of visual kernels for vessel segmentation. SoftMax
classifier is used for fine tuning each member auto-encoder and multiple
strategies are explored for 2-level fusion of ensemble members. On DRIVE
dataset, we achieve maximum average accuracy of 95.33\% with an impressively
low standard deviation of 0.003 and Kappa agreement coefficient of 0.708 .
Comparison with other major algorithms substantiates the high efficacy of our
model.Comment: Accepted as a conference paper at IEEE EMBC, 201
The geology and geophysics of the Oslo rift
The regional geology and geophysical characteristics of the Oslo graben are reviewed. The graben is part of a Permian age failed continental rift. Alkali olivine, tholefitic, and monzonitic intrusives as well as basaltic lavas outline the extent of the graben. Geophysical evidence indicates that rifting activity covered a much greater area in Skagerrak Sea as well as the Paleozoic time, possibly including the northern Skagerrak Sea as well as the Oslo graben itself. Much of the surficial geologic characteristics in the southern part of the rift have since been eroded or covered by sedimentation. Geophysical data reveal a gravity maximum along the strike of the Oslo graben, local emplacements of magnetic material throughout the Skagerrak and the graben, and a slight mantle upward beneath the rift zone. Petrologic and geophysical maps which depict regional structure are included in the text. An extensive bibliography of pertinent literature published in English between 1960 and 1980 is also provided
Endpoint of the up-down instability in precessing binary black holes
Binary black holes in which both spins are aligned with the binary's orbital
angular momentum do not precess. However, the up-down configuration, in which
the spin of the heavier (lighter) black hole is aligned (anti-aligned) with the
orbital angular momentum, is unstable to spin precession at small orbital
separations. We first cast the spin precession problem in terms of a simple
harmonic oscillator and provide a cleaner derivation of the instability onset.
Surprisingly, we find that following the instability, up-down binaries do not
disperse in the available parameter space but evolve toward precise endpoints.
We then present an analytic scheme to locate these final configurations and
confirm them with numerical integrations. Namely, unstable up-down binaries
approach mergers with the two spins coaligned with each other and equally
misaligned with the orbital angular momentum. Merging up-down binaries relevant
to LIGO/Virgo and LISA may be detected in these endpoint configurations if the
instability onset occurs prior to the sensitivity threshold of the detector. As
a by-product, we obtain new generic results on binary black hole spin-orbit
resonances at 2nd~post-Newtonian order. We finally apply these findings to a
simple astrophysical population of binary black holes where a formation
mechanism aligns the spins without preference for co- or counteralignment, as
might be the case for stellar-mass black holes embedded in the accretion disk
of a supermassive black hole.Comment: Animated versions of Figs. 2, 7 and 13 are available at
https://davidegerosa.com/spinprecessio
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