678,896 research outputs found

### Numerical analysis of a spontaneous collapse model for a two-level system

We study a spontaneous collapse model for a two-level (spin) system, in which
the Hamiltonian and the stochastic terms do not commute. The numerical solution
of the equations of motions allows to give precise estimates on the regime at
which the collapse of the state vector occurs, the reduction and delocalization
times, and the reduction probabilities; it also allows to quantify the effect
that an Hamiltonian which does not commute with the reducing terms has on the
collapse mechanism. We also give a clear picture of the transition from the
"microscopic" regime (when the noise terms are weak and the Hamiltonian
prevents the state vector to collapse) to the "macroscopic" regime (when the
noise terms are dominant and the collapse becomes effective for very long
times). Finally, we clarify the distinction between decoherence and collapse.Comment: 7 pages, RevTeX. Significative improvements made. To appear on Phys.
Rev.

### Quasi-spherical collapse with cosmological constant

The junction conditions between static and non-static space-times are studied
for analyzing gravitational collapse in the presence of a cosmological
constant. We have discussed about the apparent horizon and their physical
significance. We also show the effect of cosmological constant in the collapse
and it has been shown that cosmological constant slows down the collapse of
matter.Comment: 7 pages, No figures, RevTeX styl

### Experimental Animal Decompressions to a Near-Vacuum Environment

Rapid decompression of dogs to near vacuum environment to estimate times of consciousness, collapse, and surviva

### Collapsing Bose-Einstein condensates beyond the Gross-Pitaevskii approximation

We analyse quantum field models of the bosenova experiment, in which
$^{85}$Rb Bose-Einstein condensates were made to collapse by switching their
atomic interactions from repulsive to attractive. Specifically, we couple the
lowest order quantum field correlation functions to the Gross-Pitaevskii
function, and solve the resulting dynamical system numerically. Comparing the
computed collapse times with the experimental measurements, we find that the
calculated times are much larger than the measured values. The addition of
quantum field corrections does not noticeably improve the agreement compared to
a pure Gross-Pitaevskii theory.Comment: 8 pages, 4 figure

### A Lagrangian Dynamical Theory for the Mass Function of Cosmic Structures: I Dynamics

A new theory for determining the mass function of cosmic structures is
presented. It relies on a realistic treatment of collapse dynamics.
Gravitational collapse is analyzed in the Lagrangian perturbative framework.
Lagrangian perturbations provide an approximation of truncated type, i.e.
small-scale structure is filtered out. The collapse time is suitably defined as
the instant at which orbit crossing takes place. The convergence of the
Lagrangian series in predicting the collapse time of a homogeneous ellipsoid is
demonstrated; it is also shown that third-order calculations are necessary in
predicting collapse. Then, the Lagrangian prediction, with a correction for
quasi-spherical perturbations, can be used to determine the collapse time of a
homogeneous ellipsoid in a fast and precise way. Furthermore, ellipsoidal
collapse can be considered as a particular truncation of the Lagrangian series.
Gaussian fields with scale-free power spectra are then considered. The
Lagrangian series for the collapse time is found to converge when the collapse
time is not large. In this case, ellipsoidal collapse gives a fast and accurate
approximation of the collapse time; spherical collapse is found to poorly
reproduce the collapse time, even in a statistical sense. Analytical fits of
the distribution functions of the inverse collapse times, as predicted by the
ellipsoid model and by third-order Lagrangian theory, are given. These will be
necessary for a determination of the mass function, which will be given in
paper II.Comment: 18 pages, Latex, uses mn.sty and psfig, 7 postscript figures (fig. 2
and 3 not complete). Revised version, stylistic changes. MNRAS, in pres

### Collapse and Fragmentation of Molecular Cloud Cores. X. Magnetic Braking of Prolate and Oblate Cores

The collapse and fragmentation of initially prolate and oblate, magnetic
molecular clouds is calculated in three dimensions with a gravitational,
radiative hydrodynamics code. The code includes magnetic field effects in an
approximate manner: magnetic pressure, tension, braking, and ambipolar
diffusion are all modelled. The parameters varied for both the initially
prolate and oblate clouds are the initial degree of central concentration of
the radial density profile, the initial angular velocity, and the efficiency of
magnetic braking (represented by a factor $f_{mb} = 10^{-4}$ or $10^{-3}$). The
oblate cores all collapse to form rings that might be susceptible to
fragmentation into multiple systems. The outcome of the collapse of the prolate
cores depends strongly on the initial density profile. Prolate cores with
central densities 20 times higher than their boundary densities collapse and
fragment into binary or quadruple systems, whereas cores with central densities
100 times higher collapse to form single protostars embedded in bars. The
inclusion of magnetic braking is able to stifle protostellar fragmentation in
the latter set of models, as when identical models were calculated without
magnetic braking (Boss 2002), those cores fragmented into binary protostars.
These models demonstrate the importance of including magnetic fields in studies
of protostellar collapse and fragmentation, and suggest that even when magnetic
fields are included, fragmentation into binary and multiple systems remains as
a possible outcome of protostellar collapse.Comment: 20 pages, 8 figures. Astrophysical Journal, in pres

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