553 research outputs found
A note on acoustic black holes in neo-Newtonian theory
Newtonian fluid dynamics allows the construction of acoustic metrics from
which black hole configurations can be studied. However, relativistic pressure
effects are neglected within Newtonian theory. We study acoustic black holes in
the framework of neo-Newtonian hydrodynamics, which is designed to take into
account relativistic inertial effects of the pressure . Within this new
hydrodynamical context we show how can influence the formation of the
acoustic horizons.Comment: Latex file, 10 pages. Some discussions extended. Accepted for
publication in MPL
Viscous Cosmology
We discuss the possibility to implement a viscous cosmological model,
attributing to the dark matter component a behaviour described by bulk
viscosity. Since bulk viscosity implies negative pressure, this rises the
possibility to unify the dark sector. At the same time, the presence of
dissipative effects may alleviate the so called small scale problems in the
CDM model. While the unified viscous description for the dark sector
does not lead to consistent results, the non-linear behaviour indeed improves
the situation with respect to the standard cosmological model.Comment: Latex file, 7 pages, 6 figures. To appear in the proceedings of the
XIIth International Conference on Gravitation, Astrophysics and Cosmology,
June 28-July 5, 2015, PFUR, Moscow, Russi
Aspects of the cosmological "coincidence problem"
The observational fact that the present values of the densities of dark
energy and dark matter are of the same order of magnitude,
, seems to indicate that we are
currently living in a very special period of the cosmic history. Within the
standard model, a density ratio of the order of one just at the present epoch
can be seen as coincidental since it requires very special initial conditions
in the early Universe. The corresponding "why now" question constitutes the
cosmological "coincidence problem". According to the standard model the
equality took place "recently" at a redshift . The meaning of "recently" is, however, parameter dependent. In terms of
the cosmic time the situation looks different. We discuss several aspects of
the "coincidence problem", also in its relation to the cosmological constant
problem, to issues of structure formation and to cosmic age considerations.Comment: 8 pages, 7 figure
A new beamline for laser spin-polarization at ISOLDE
A beamline dedicated to the production of laser-polarized radioactive beams
has been constructed at ISOLDE, CERN. We present here different simulations
leading to the design and construction of the setup, as well as technical
details of the full setup and examples of the achieved polarizations for
several radioisotopes. Beamline simulations show a good transmission through
the entire line, in agreement with observations. Simulations of the induced
nuclear spin-polarization as a function of atom-laser interaction length are
presented for Na, [1] and for Ar, which is studied in this
work. Adiabatic spin rotation of the spin-polarized ensemble of atoms, and how
this influences the observed nuclear ensemble polarization, are also performed
for the same nuclei. For Ar, we show that multiple-frequency pumping
enhances the ensemble polarization by a factor 1.85, in agreement with
predictions from a rate equations model.
[1] J. Phys. G: Nucl. Part. Phys./174408400
Paul trapping of radioactive 6He+ions and direct observation of their beta-decay
We demonstrate that abundant quantities of short-lived beta unstable ions can
be trapped in a novel transparent Paul trap and that their decay products can
directly be detected in coincidence. Low energy 6He+ (807 ms half-life) ions
were extracted from the SPIRAL source at GANIL, then decelerated, cooled and
bunched by means of the buffer gas cooling technique. More than 10^8 ions have
been stored over a measuring period of six days and about 10^5 decay
coincidences between the beta particles and the 6Li^{++} recoiling ions have
been recorded. The technique can be extended to other short-lived species,
opening new possibilities for trap assisted decay experiments.Comment: 4 pages, 4 figures, submitted to Phys.Rev.Let
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