160,483 research outputs found

### Instrumentation for nondestructive testing of composite honeycomb materials

Program develops instrumentation for nondestructive testing of adhesive-bond strength in honeycomb materials and air coupled inspection methods suitable for large tankage

### Spin Connection and Renormalization of Teleparallel Action

In general relativity, inertia and gravitation are both included in the
Levi-Civita connection. As a consequence, the gravitational action, as well as
the corresponding energy-momentum density, are in general contaminated by
spurious contributions coming from inertial effects. In teleparallel gravity,
on the other hand, because the spin connection represents inertial effects
only, it is possible to separate inertia from gravitation. Relying on this
property, it is shown that to each tetrad there is naturally associated a spin
connection that locally removes the inertial effects from the action. The use
of the appropriate spin connection can be viewed as a renormalization process
in the sense that the computation of energy and momentum naturally yields the
physically relevant values. A self-consistent method for solving field
equations and determining the appropriate spin connection is presented.Comment: v2: 13 pages, minor corrections, new section about solving the field
equations include

### On the Margulis constant for Kleinian groups, I curvature

The Margulis constant for Kleinian groups is the smallest constant $c$ such
that for each discrete group $G$ and each point $x$ in the upper half space
${\bold H}^3$, the group generated by the elements in $G$ which move $x$ less
than distance c is elementary. We take a first step towards determining this
constant by proving that if $\langle f,g \rangle$ is nonelementary and discrete
with $f$ parabolic or elliptic of order $n \geq 3$, then every point $x$ in
${\bold H}^3$ is moved at least distance $c$ by $f$ or $g$ where
$c=.1829\ldots$. This bound is sharp

### Quantum stochastic convolution cocycles II

Schuermann's theory of quantum Levy processes, and more generally the theory
of quantum stochastic convolution cocycles, is extended to the topological
context of compact quantum groups and operator space coalgebras. Quantum
stochastic convolution cocycles on a C*-hyperbialgebra, which are
Markov-regular, completely positive and contractive, are shown to satisfy
coalgebraic quantum stochastic differential equations with completely bounded
coefficients, and the structure of their stochastic generators is obtained.
Automatic complete boundedness of a class of derivations is established,
leading to a characterisation of the stochastic generators of *-homomorphic
convolution cocycles on a C*-bialgebra. Two tentative definitions of quantum
Levy process on a compact quantum group are given and, with respect to both of
these, it is shown that an equivalent process on Fock space may be
reconstructed from the generator of the quantum Levy process. In the examples
presented, connection to the algebraic theory is emphasised by a focus on full
compact quantum groups.Comment: 32 pages, expanded introduction and updated references. The revised
version will appear in Communications in Mathematical Physic

### Probing the growth of supermassive black holes at z>6 with LOFAR

HII regions surrounding supermassive black holes (BHs) in an otherwise still
neutral intergalactic medium (IGM) are likely to be the most easily detectable
sources by future 21cm experiments like LOFAR. We have made predictions for the
size distribution of such HII regions for several physically motivated models
for BH growth at high redshift and compared this to the expected LOFAR
sensitivity to these sources. The number of potentially detectable HII regions
does not only depend on the ionisation state of the intergalactic medium and
the decoupling of the spin temperature of the neutral hydrogen from the cosmic
microwave background (CMB) temperature, but is also strongly sensitive to the
rate of growth of BHs at high redshift. If the supermassive BHs at redshift 6
were built up via continuous Eddington-limited accretion from low mass seed BHs
at high redshift, then LOFAR is not expected to detect isolated QSO HII regions
at redshifts much larger than 6, and only if the IGM is still significantly
neutral. If the high-redshift growth of BHs starts with massive seed BHs and is
driven by short-lived accretion events following the merging of BH hosting
galaxies then the detection of HII regions surrounding supermassive BHs may
extend to redshifts as large as 8-9 but is still very sensitive to the redshift
to which the IGM remains significantly neutral. The most optimistic predictions
are for a model where the supermassive BHs at z>6 have grown slowly. HII
regions around supermassive BHs may then be detected to significantly larger
redshifts.Comment: 11 pages, 6 figures, accepted for publication in MNRA

### Probing gaseous halos of galaxies with radio jets

Reproduced with permission from Astronomy & Astrophysics. © 2019 ESOContext. Gaseous halos play a key role in understanding inflow, feedback, and the overall baryon budget in galaxies. Literature models predict transitions of the state of the gaseous halo between cold and hot accretion, winds, fountains, and hydrostatic halos at certain galaxy masses. Since luminosities of radio AGN are sensitive to halo densities, any significant transition would be expected to show up in the radio luminosities of large samples of galaxies. The LOw Frequency ARray (LOFAR) Two-Metre Sky Survey (LoTSS) has identified a galaxy stellar mass scale, 10 11 M ⊙, above which the radio luminosities increase disproportionately. Aims. We investigate if radio luminosities of galaxies, especially the marked rise at galaxy masses around 10 11 M ⊙, can be explained with standard assumptions regarding jet powers, scaling between black hole mass and galaxy mass, and gaseous halos. Methods. Based on observational data and theoretical constraints, we developed models for the radio luminosity of radio AGN in halos under infall, galactic wind, and hydrostatic conditions. We compared these models to LoTSS data for a large sample of galaxies in the mass range between 10 8.5 M ⊙ and 10 12 M ⊙. Results. Under the assumption that the same characteristic upper limit to jet powers known from high galaxy masses holds at all masses, we find the maximum radio luminosities for the hydrostatic gas halos to lie close to the upper envelope of the distribution of the LOFAR data. The marked rise in radio luminosity at 10 11 M ⊙ is matched in our model and is related to a significant change in halo gas density around this galaxy mass, which is a consequence of lower cooling rates at a higher virial temperature. Wind and infall models overpredict the radio luminosities for small galaxy masses and have no particular steepening of the run of the radio luminosities predicted at any galaxy mass. Conclusions. Radio AGN could have the same characteristic Eddington-scaled upper limit to jet powers in galaxies of all masses in the sample if the galaxies have hydrostatic gas halos in phases when radio AGN are active. We find no evidence of a change of the type of galaxy halo with the galaxy mass. Galactic winds and quasi-spherical cosmological inflow phases cannot frequently occur at the same time as powerful jet episodes unless the jet properties in these phases are significantly different from what we assumed in our model.Peer reviewedFinal Accepted Versio

### Quantum stochastic convolution cocycles III

Every Markov-regular quantum Levy process on a multiplier C*-bialgebra is
shown to be equivalent to one governed by a quantum stochastic differential
equation, and the generating functionals of norm-continuous convolution
semigroups on a multiplier C*-bialgebra are then completely characterised.
These results are achieved by extending the theory of quantum Levy processes on
a compact quantum group, and more generally quantum stochastic convolution
cocycles on a C*-bialgebra, to locally compact quantum groups and multiplier
C*-bialgebras. Strict extension results obtained by Kustermans, together with
automatic strictness properties developed here, are exploited to obtain
existence and uniqueness for coalgebraic quantum stochastic differential
equations in this setting. Then, working in the universal enveloping von
Neumann bialgebra, we characterise the stochastic generators of Markov-regular,
*-homomorphic (respectively completely positive and contractive), quantum
stochastic convolution cocycles.Comment: 20 pages; v2 corrects some typos and no longer contains a section on
quantum random walk approximations, which will now appear as a separate
submission. The article will appear in the Mathematische Annale

- …