3,315 research outputs found
The Friedmann-Lemaitre-Robertson-Walker Big Bang singularities are well behaved
We show that the Big Bang singularity of the
Friedmann-Lemaitre-Robertson-Walker model does not raise major problems to
General Relativity. We prove a theorem showing that the Einstein equation can
be written in a non-singular form, which allows the extension of the spacetime
before the Big Bang. The physical interpretation of the fields used is
discussed. These results follow from our research on singular semi-Riemannian
geometry and singular General Relativity.Comment: 10 pages, 5 figure
Short-range nuclear effects on axion emissivities by nucleon-nucleon bremsstrahlung
The rates of axion emission by nucleon-nucleon (NN) bremsstrahlung are
reconsidered by taking into account the NN short range correlations. The
analytical formulas for the neutron-neutron (nn), proton-proton (pp) and
neutron-proton (np) processes with the inclusion of the full momentum
dependence of an one- and two- pion exchange nuclear potentials, in the
non-degenerate limit, are explicitly given. We find that the two-pion exchange
(short range) effects can give a significant contribution to the emission
rates, and are temperature dependent. Other short range nuclear effects like
effective nucleon mass, polarization effects and use of correlated wave
functions, are discused as well. The trend of all these nuclear effects is to
diminish the corresponding axion emission rates. Further, we estimate that the
values of the emission rates calculated with the inclusion of all these effects
can differ from the corresponding ones derived with constant nuclear matrix
elements by a factor of . This leads to an uncertainty factor of when extracting bounds of the axion parametersComment: 11 pages, 4 figure
No-Core shell model for A = 47 and A = 49
We apply an {\it ab-initio} approach to the nuclear structure of odd-mass
nuclei straddling . Starting with the NN interaction, that fits
two-body scattering and bound state data we evaluate the nuclear properties of
and nuclei in a no-core approach. Due to model space
limitations and the absence of 3-body interactions, we incorporate
phenomenological terms determined by fits to nuclei in a previous
effort. Our modified Hamiltonian produces reasonable spectra for these odd mass
nuclei. In addition to the differences in single-particle basis states, the
absence of a single-particle Hamiltonian in our no-core approach obscures
direct comparisons with valence effective NN interactions. Nevertheless, we
compare the fp-shell matrix elements of our initial and modified Hamiltonians
in the harmonic oscillator basis with a recent model fp-shell interaction, the
GXPF1 interaction of Honma, Otsuka, Brown and Mizusaki. Notable differences
emerge from these comparisons. In particular, our diagonal two-body
matrix elements are, on average, about 800-900keV more attractive. Furthermore,
while our initial and modified NN Hamiltonian fp-shell matrix elements are
strongly correlated, there is much less correlation with the GXPF1 matrix
elements.Comment: 17 pages including 14 figure
Understanding the Nature of ICT-based Innovation Processes in Education - A Theoretical Framework for Informing Policy, Research and Action
Model Order Selection Rules For Covariance Structure Classification
The adaptive classification of the interference covariance matrix structure
for radar signal processing applications is addressed in this paper. This
represents a key issue because many detection architectures are synthesized
assuming a specific covariance structure which may not necessarily coincide
with the actual one due to the joint action of the system and environment
uncertainties. The considered classification problem is cast in terms of a
multiple hypotheses test with some nested alternatives and the theory of Model
Order Selection (MOS) is exploited to devise suitable decision rules. Several
MOS techniques, such as the Akaike, Takeuchi, and Bayesian information criteria
are adopted and the corresponding merits and drawbacks are discussed. At the
analysis stage, illustrating examples for the probability of correct model
selection are presented showing the effectiveness of the proposed rules
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