54 research outputs found
Violation of Universal Lower Bound for the Shear Viscosity to Entropy Density Ratio in Dark Energy Dominated Accretion
The universal lower bound of the ratio of shear viscosity to entropy density
is suggested by the string theory and gauge duality for any matter. We examined
the ratio of shear viscosity to entropy density for viscous accretion flow
towards a central gravitating object in the presence of dark energy. The ratio
appears close to the universal lower bound for certain optically thin, hot
accretion flows as they are embedded by strong magnetic field. Dark energy is a
kind of exotic matter which has negative pressure. So dark energy creates
repulsive force between the accreting particles, which indicates that shear
viscosity of the flow becomes very low. Dark energy as accreting fluid has very
high entropy density. The ratio should reach near to the lowest value for dark
energy accretion. We wish to study what happens to the shear viscosity to
entropy density ratio for viscous dark energy accretion flow.Comment: 9 pages, 12 figure
Fate of an Accretion Disc around a Black Hole when both the Viscosity and Dark Energy is Effecting
This paper deals with the viscous accretion flow of modified Chaplygin gas
towards a black hole as the central gravitating object. Modified Chaplygin gas
is a particular type of dark energy model which mimics of radiation era to
phantom era depending on the different values of its parameters. We compare the
drak energy accretion with the flow of adiabatic gas. An accretion disc flow
around a black hole is an example of a transonic flow. To make the model, we
consider three components of Navier Stokes' equation, the equation of
continuity and the modified Chaplygin gas's equation of state. As a transonic
flow passes through the sonic point, the velocity gradient being apparently
singular there, gives rise to two flow branches : one infalling, the accretion
and the other outgoing, the wind. We show that the wind curve is stronger and
wind speed reaches to that of light at a finite distance from the black hole
when dark energy is considered. Besides, if we increase the viscosity,
accretion disc is being shortened. This two process acting together deviates
much from adiabatic accretion case. It shows a weakening process for the
accretion procedure by the works of viscous system influenced angular momentum
transport and the repulsive force of the modified Chaplygin gas together.Comment: 12 pages, 18 figure
Bayes Model Selection with Path Sampling: Factor Models and Other Examples
We prove a theorem justifying the regularity conditions which are needed for
Path Sampling in Factor Models. We then show that the remaining ingredient,
namely, MCMC for calculating the integrand at each point in the path, may be
seriously flawed, leading to wrong estimates of Bayes factors. We provide a new
method of Path Sampling (with Small Change) that works much better than
standard Path Sampling in the sense of estimating the Bayes factor better and
choosing the correct model more often. When the more complex factor model is
true, PS-SC is substantially more accurate. New MCMC diagnostics is provided
for these problems in support of our conclusions and recommendations. Some of
our ideas for diagnostics and improvement in computation through small changes
should apply to other methods of computation of the Bayes factor for model
selection.Comment: Published in at http://dx.doi.org/10.1214/12-STS403 the Statistical
Science (http://www.imstat.org/sts/) by the Institute of Mathematical
Statistics (http://www.imstat.org
- …