9,545 research outputs found
Local Operator Multipliers and Positivity
We establish an unbounded version of Stinespring's Theorem and a lifting
result for Stinespring representations of completely positive modular maps
defined on the space of all compact operators. We apply these results to study
positivity for Schur multipliers. We characterise positive local Schur
multipliers, and provide a description of positive local Schur multipliers of
Toeplitz type. We introduce local operator multipliers as a non-commutative
analogue of local Schur multipliers, and obtain a characterisation that extends
earlier results concerning operator multipliers and local Schur multipliers. We
provide a description of the positive local operator multipliers in terms of
approximation by elements of canonical positive cones.Comment: 31 page
Housing subsidisation in the Netherlands; measuring its distortionary and distributional effects
This paper measures the distortionary and distributional effects of housing subsidies. The results are discussed in the light of the main justifications for subsidising housing, i.e. the merit-good argument, external effects and the distribution motive. Our measurements reveal some patterns of subsidisation that seem difficult to justify on these grounds. This applies especially to the differences between subsidisation of rental and owner-occupied housing, the differences between mortgage- and equity-financed ownership and the increase of relative subsidisation above a certain income level.
Constraints on the sound speed of dark energy
We have studied constraints on the equation of state, , and speed of
sound, c_s, of the dark energy from a joint analysis of data from the cosmic
microwave background, large scale structure and type-Ia supernovae. We find
that current observations have no significant sensitivity to c_s. However,
there is a slight difference between models in which there are no dark energy
perturbations and models in which dark energy behaves as a fluid. Assuming that
there are no dark energy perturbations shifts the allowed region for to
slightly higher values. At present models with and without dark energy
perturbations provide roughly equally good fits to observations, but the
difference is potentially important for future parameter estimations. Finally,
we have also performed error forecasts for future measurements of c_s.Comment: 9 pages, 6 figures, Revte
Supernova Neutrino Opacity from Nucleon-Nucleon Bremsstrahlung and Related Processes
Elastic scattering on nucleons, \nu N -> N \nu, is the dominant supernova
(SN) opacity source for \mu and \tau neutrinos. The dominant energy- and
number-changing processes were thought to be \nu e^- -> e^- \nu and \nu\bar \nu
e^+ e^- until Suzuki (1993) showed that the bremsstrahlung process \nu\bar
\nu NN NN was actually more important. We find that for energy exchange,
the related ``inelastic scattering process'' \nu NN NN \nu is even more
effective by about a factor of 10. A simple estimate implies that the \nu_\mu
and \nu_\tau spectra emitted during the Kelvin-Helmholtz cooling phase are much
closer to that of \nu\bar_e than had been thought previously. To facilitate a
numerical study of the spectra formation we derive a scattering kernel which
governs both bremsstrahlung and inelastic scattering and give an analytic
approximation formula. We consider only neutron-neutron interactions, we use a
one-pion exchange potential in Born approximation, nonrelativistic neutrons,
and the long-wavelength limit, simplifications which appear justified for the
surface layers of a SN core. We include the pion mass in the potential and we
allow for an arbitrary degree of neutron degeneracy. Our treatment does not
include the neutron-proton process and does not include nucleon-nucleon
correlations. Our perturbative approach applies only to the SN surface layers,
i.e. to densities below about 10^{14} g cm^{-3}.Comment: 36 pages, LaTeX, 6 postscript figs included, matches version accepted
for publication in Astrophysical Journa
Cosmology seeking friendship with sterile neutrinos
Precision cosmology and big-bang nucleosynthesis mildly favor extra radiation
in the universe beyond photons and ordinary neutrinos, lending support to the
existence of low-mass sterile neutrinos. We use the WMAP 7-year data,
small-scale CMB observations from ACBAR, BICEP and QuAD, the SDSS 7th data
release, and measurement of the Hubble parameter from HST observations to
derive credible regions for the assumed common mass scale m_s and effective
number N_s of thermally excited sterile neutrino states. Our results are
compatible with the existence of one or perhaps two sterile neutrinos, as
suggested by LSND and MiniBooNE, if m_s is in the sub-eV range.Comment: 4 pages, 1 figure, matches version published in PR
Cosmological axion bounds
We discuss current cosmological constraints on axions, as well as future
sensitivities. Bounds on axion hot dark matter are discussed first, and
subsequently we discuss both current and future sensitivity to models in which
axions play the role as cold dark matter, but where the Peccei-Quinn symmetry
is not restored during reheating.Comment: 4 pages, 2 figures, To appear in the proceedings of 5th Patras
Workshop on Axions, WIMPs and WISPs, Durham 13-17 July 200
Achieving scalability in hierarchical location services
Services for locating mobile objects are often organized as a distributed search tree. A potential problem with this organization is that high-level nodes may become a bottleneck, affecting the scalability of the service. A traditional approach to handle such problems is to also distribute the location information managed by a single node across multiple machines
Pragmatics in legal interpretation
How should pragmatic dimensions of meaning be understood when they occur in the frequently normative use of language in law? In this chapter, referring principally to common law systems, we outline how pragmatic issues arise in the interpretation of legal texts; we describe how legal interpretation has treated indirect and implied meaning; and we ask how far dialogue between linguistic pragmatics and law can enrich thinking and practice in each of these fields
A Cosmological Three Level Neutrino Laser
We present a calculation of a neutrino decay scenario in the early Universe.
The specific decay is \nu_{2} \to \nu_{1} + \phi, where \phi is a boson. If
there is a neutrino mass hierarchy, m_{\nu_{e}} < m_{\nu_{\mu}} <
m_{\nu_{\tau}}, we show that it is possible to generate stimulated decay and
effects similar to atomic lasing without invoking new neutrinos, even starting
from identical neutrino distributions. Under the right circumstances the decay
can be to very low momentum boson states thereby producing something similar to
a Bose condensate, with possible consequences for structure formation. Finally,
we argue that this type of decay may also be important other places in early
Universe physics.Comment: 7 pages, RevTex, due for publication in Phys. Rev. D, April 15 issu
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