3,393 research outputs found
Enlargeability and index theory
Let M be a closed enlargeable spin manifold. We show non-triviality of the
universal index obstruction in the K-theory of the maximal -algebra of the
fundamental group of M. Our proof is independent from the injectivity of the
Baum-Connes assembly map for the fundamental group of M and relies on the
construction of a certain infinite dimensional flat vector bundle out of a
sequence of finite dimensional vector bundles on M whose curvatures tend to
zero.
Besides the well known fact that M does not carry a metric with positive
scalar curvature, our results imply that the classifying map
sends the fundamental class of M to a nontrivial homology class in H_n(B
\pi_1(M) ; \Q). This answers a question of Burghelea (1983).Comment: 32 pages, final version accepted for publication, added relation to
Gromov's 1-systole, typos corrected; to appear in Journal of Differential
Geometr
The space of metrics of positive scalar curvature
We study the topology of the space of positive scalar curvature metrics on
high dimensional spheres and other spin manifolds. Our main result provides
elements of infinite order in higher homotopy and homology groups of these
spaces, which, in contrast to previous approaches, are of infinite order and
survive in the (observer) moduli space of such metrics.
Along the way we construct smooth fiber bundles over spheres whose total
spaces have non-vanishing A-hat-genera, thus establishing the
non-multiplicativity of the A-hat-genus in fibre bundles with simply connected
base.Comment: 24 pages, v2: minor additions and corrections, based in particular on
comments of referees, v3: minor corrections, final version, to appear in
Publ.Math. IHE
Comparison of different sources for laboratory X-ray microscopy
This paper describes the setup of two different solutions for laboratory
X-ray microscopy working with geometric magnification. One setup uses thin-film
transmission targets with an optimized tungsten-layer thickness and the
electron gun and optics of an electron probe micro analyzer to generate a very
small X-ray source. The other setup is based on a scanning electron microscope
and uses microstructured reflection targets. We also describe the structuring
process for these targets. In both cases we show that resolutions of 100 nm can
be achieved. Also the possibilities of computed tomography for 3D imaging are
explored and we show first imaging examples of high-absorption as well as
low-absorption specimens to demonstrate the capabilities of the setups.Comment: 6 pages, 4 figures, proceedings of the 14th International Workshop on
Radiation Imaging Detector
The strong Novikov conjecture for low degree cohomology
We show that for each discrete group G, the rational assembly map
K_*(BG) \otimes Q \to K_*(C*_{max} G) \otimes \Q is injective on classes dual
to the subring generated by cohomology classes of degree at most 2 (identifying
rational K-homology and homology via the Chern character). Our result implies
homotopy invariance of higher signatures associated to these cohomology
classes. This consequence was first established by Connes-Gromov-Moscovici and
Mathai.
Our approach is based on the construction of flat twisting bundles out of
sequences of almost flat bundles as first described in our previous work. In
contrast to the argument of Mathai, our approach is independent of (and indeed
gives a new proof of) the result of Hilsum-Skandalis on the homotopy invariance
of the index of the signature operator twisted with bundles of small curvature.Comment: 11 page
Neutrino signature of supernova hydrodynamical instabilities in three dimensions
The first full-scale three-dimensional (3D) core-collapse supernova (SN)
simulations with sophisticated neutrino transport show pronounced effects of
the standing accretion shock instability (SASI) for two high-mass progenitors
(20 and 27 M_sun). In a low-mass progenitor (11.2 M_sun), large-scale
convection is the dominant nonradial hydrodynamic instability in the postshock
accretion layer. The SASI-associated modulation of the neutrino signal (80 Hz
in our two examples) will be clearly detectable in IceCube or the future
Hyper-Kamiokande detector, depending on progenitor properties, distance, and
observer location relative to the main SASI sloshing direction. The neutrino
signal from the next galactic SN can therefore diagnose the nature of the
hydrodynamic instability.Comment: 6 pages, including 4 figures. Results unchanged. Matches published
version in PRL. Animated visualization available at:
http://www.mpa-garching.mpg.de/ccsnarchive/data/Hanke2013_movie/index.htm
Core-Collapse Supernovae: Reflections and Directions
Core-collapse supernovae are among the most fascinating phenomena in
astrophysics and provide a formidable challenge for theoretical investigation.
They mark the spectacular end of the lives of massive stars and, in an
explosive eruption, release as much energy as the sun produces during its whole
life. A better understanding of the astrophysical role of supernovae as birth
sites of neutron stars, black holes, and heavy chemical elements, and more
reliable predictions of the observable signals from stellar death events are
tightly linked to the solution of the long-standing puzzle how collapsing stars
achieve to explode. In this article our current knowledge of the processes that
contribute to the success of the explosion mechanism are concisely reviewed.
After a short overview of the sequence of stages of stellar core-collapse
events, the general properties of the progenitor-dependent neutrino emission
will be briefly described. Applying sophisticated neutrino transport in
axisymmetric (2D) simulations with general relativity as well as in simulations
with an approximate treatment of relativistic effects, we could find successful
neutrino-driven explosions for a growing set of progenitor stars. First results
of three-dimensional (3D) models have been obtained, and magnetohydrodynamic
simulations demonstrate that strong initial magnetic fields in the pre-collapse
core can foster the onset of neutrino-powered supernova explosions even in
nonrotating stars. These results are discussed in the context of the present
controversy about the value of 2D simulations for exploring the supernova
mechanism in realistic 3D environments, and they are interpreted against the
background of the current disagreement on the question whether the standing
accretion shock instability (SASI) or neutrino-driven convection is the crucial
agency that supports the onset of the explosion.Comment: 36 pages, 20 figures (43 eps files); submitted to Progress of
Theoretical and Experimental Physics (PTEP
Self-sustained asymmetry of lepton-number emission: A new phenomenon during the supernova shock-accretion phase in three dimensions
During the stalled-shock phase of our 3D hydrodynamical core-collapse
simulations with energy-dependent, 3-flavor neutrino transport, the
lepton-number flux (nue minus antinue) emerges predominantly in one hemisphere.
This novel, spherical-symmetry breaking neutrino-hydrodynamical instability is
termed LESA for "Lepton-number Emission Self-sustained Asymmetry." While the
individual nue and antinue fluxes show a pronounced dipole pattern, the
heavy-flavor neutrino fluxes and the overall luminosity are almost spherically
symmetric. Initially, LESA seems to develop stochastically from convective
fluctuations, it exists for hundreds of milliseconds or more, and it persists
during violent shock sloshing associated with the standing accretion shock
instability. The nue minus antinue flux asymmetry originates mainly below the
neutrinosphere in a region of pronounced proto-neutron star (PNS) convection,
which is stronger in the hemisphere of enhanced lepton-number flux. On this
side of the PNS, the mass-accretion rate of lepton-rich matter is larger,
amplifying the lepton-emission asymmetry, because the spherical stellar infall
deflects on a dipolar deformation of the stalled shock. The increased shock
radius in the hemisphere of less mass accretion and minimal lepton-number flux
(antinue flux maximum) is sustained by stronger convection on this side, which
is boosted by stronger neutrino heating because the average antinue energy is
higher than the average nue energy. Asymmetric heating thus supports the global
deformation despite extremely nonstationary convective overturn behind the
shock. While these different elements of LESA form a consistent picture, a full
understanding remains elusive at present. There may be important implications
for neutrino-flavor oscillations, the neutron-to-proton ratio in the
neutrino-heated supernova ejecta, and neutron-star kicks, which remain to be
explored.Comment: 21 pages, 15 figures; new results and new figure added; accepted by
Ap
Induction motors improvement for a variable speed drive
New improvement way of induction motors for a variable speed drive when changing mass dimension indices has been proposed. It allows for improvement of energy indices and reduction of running costs. The analysis of simulation results has been carried out and calculation results of economic efficiency of the achieved methods of approach to energy effective induction motors design have been suggested
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