2,497 research outputs found
On the Weyl Curvature Hypothesis
The Weyl curvature hypothesis of Penrose attempts to explain the high
homogeneity and isotropy, and the very low entropy of the early universe, by
conjecturing the vanishing of the Weyl tensor at the Big-Bang singularity.
In previous papers it has been proposed an equivalent form of Einstein's
equation, which extends it and remains valid at an important class of
singularities (including in particular the Schwarzschild, FLRW, and isotropic
singularities). Here it is shown that if the Big-Bang singularity is from this
class, it also satisfies the Weyl curvature hypothesis.
As an application, we study a very general example of cosmological models,
which generalizes the FLRW model by dropping the isotropy and homogeneity
constraints. This model also generalizes isotropic singularities, and a class
of singularities occurring in Bianchi cosmologies. We show that the Big-Bang
singularity of this model is of the type under consideration, and satisfies
therefore the Weyl curvature hypothesis.Comment: 10 pages, slides at
http://www.sciencedirect.com/science/article/pii/S000349161300171
Spin-Isospin Excitations and Muon Capture by Nuclei
By analyzing the energy-weighted moments of the strength function calculated
in RPA and beyond it is shown that the explanation of the effect of missing
strength of Gamow-Teller transitions requires that residual interaction produce
high-excited particle-hole collective states. The example of this
interaction is presented. The manifestations of spin-isospin nuclear response
in nuclear muon capture are discussed.Comment: 16 pages, 5 figures, 2 tables. The talk at the XVI International
School on Nuclear Physics, Neutron Physics and Nuclear Energy, September
19-26, Varna, Bulgari
Emergent Quantum Mechanics and Emergent Symmetries
Quantum mechanics is 'emergent' if a statistical treatment of large scale
phenomena in a locally deterministic theory requires the use of quantum
operators. These quantum operators may allow for symmetry transformations that
are not present in the underlying deterministic system. Such theories allow for
a natural explanation of the existence of gauge equivalence classes (gauge
orbits), including the equivalence classes generated by general coordinate
transformations. Thus, local gauge symmetries and general coordinate invariance
could be emergent symmetries, and this might lead to new alleys towards
understanding the flatness problem of the Universe.Comment: 10 pages, 1 figure. Presented at PASCOS 13, Imperial College, London,
July 6, 200
Neutrino-less Double Electron Capture - a tool to research for Majorana neutrinos
The possibility to observe the neutrino-less double decay and thus
to prove the Majorana nature of neutrino as well as provide a sensitive measure
of its mass is a major challenge of to-day's neutrino physics. As an attractive
alternative we propose to study the inverse process, the radiative
neutrino-less double electron capture . The associated monoenergetic
photon provides a convenient experimental signature. Other advantages include
the favourable ratio of the to the competing capture
rates and, very importantly, the existence of coincidence trigger to suppress
the random background. These advantages partly offset the expected longer
lifetimes. Rates for the process are calculated. High Z atoms are
strongly favoured. A resonance enhancement of the capture rates is predicted at
energy release comparable to the atomic level difference. The resonance
conditions are likely to be met for decays to excited states in final nuclei.
Candidates for such studies are considered. The experimental feasibility is
estimated and found highly encouraging.Comment: New figure added, table updated, physical background discusse
Recommended from our members
The influence of Mg doping on the nucleation of self-induced GaN nanowires
GaN nanowires were grown without any catalyst by plasma-assisted molecular beam
epitaxy. Under supply of Mg, nanowire nucleation is faster, the areal density of
wires increases to a higher value, and nanowire coalescence is more pronounced
than without Mg. During nanowire nucleation the Ga desorption was monitored insitu
by line-of-sight quadrupolemass spectrometry for various substrate temperatures.
Nucleation energies of 4.0±0.3 eV and 3.2±0.3 eV without and with Mg supply were
deduced, respectively. This effect has to be taken into account for the fabrication of
nanowire devices and could be employed to tune the NW areal density
Fully-Renormalized QRPA fulfills Ikeda sum rule exactly
The renormalized quasiparticle-RPA is reformulated for even-even nuclei using
restrictions imposed by the commutativity of the phonon creation operator with
the total particle number operator. This new version, Fully-Renormalized QRPA
(FR-QRPA), is free from the spurious low-energy solutions. Analytical proof is
given that the Ikeda sum rule is fullfiled within the FR-QRPA.Comment: 9 page
GaN and InN nanowires grown by MBE: a comparison
Morphological, optical and transport properties of GaN and InN nanowires
grown by molecular beam epitaxy (MBE) have been studied. The differences
between the two materials in respect to growth parameters and optimization
procedure was stressed. The nanowires crystalline quality has been investigated
by means of their optical properties. A comparison of the transport
characteristics was given. For each material a band schema was shown, which
takes into account transport and optical features and is based on Fermi level
pinning at the surface.Comment: 5 pages, 5 figure
- …