41,569 research outputs found
Self-Interacting Dark Matter Halos and the Gravothermal Catastrophe
We study the evolution of an isolated, spherical halo of self-interacting
dark matter (SIDM) in the gravothermal fluid formalism. We show that the
thermal relaxation time, , of a SIDM halo with a central density and
velocity dispersion of a typical dwarf galaxy is significantly shorter than its
age. We find a self-similar solution for the evolution of a SIDM halo in the
limit where the mean free path between collisions, , is everywhere
longer than the gravitational scale height, . Typical halos formed in this
long mean free path regime relax to a quasistationary gravothermal density
profile characterized by a nearly homogeneous core and a power-law halo where
. We solve the more general time-dependent problem and
show that the contracting core evolves to sufficiently high density that
inevitably becomes smaller than in the innermost region. The core
undergoes secular collapse to a singular state (the ``gravothermal
catastrophe'') in a time , which is longer than the
Hubble time for a typical dark matter-dominated galaxy core at the present
epoch. Our model calculations are consistent with previous, more detailed,
N-body simulations for SIDM, providing a simple physical interpretation of
their results and extending them to higher spatial resolution and longer
evolution times. At late times, mass loss from the contracting, dense inner
core to the ambient halo is significantly moderated, so that the final mass of
the inner core may be appreciable when it becomes relativistic and radially
unstable to dynamical collapse to a black hole.Comment: ApJ in press (to appear in April), 12 pages. Extremely minor changes
to agree with published versio
Fluctuating hydrodynamics of multi-species, non-reactive mixtures
In this paper we discuss the formulation of the fuctuating Navier-Stokes
(FNS) equations for multi-species, non-reactive fluids. In particular, we
establish a form suitable for numerical solution of the resulting stochastic
partial differential equations. An accurate and efficient numerical scheme,
based on our previous methods for single species and binary mixtures, is
presented and tested at equilibrium as well as for a variety of non-equilibrium
problems. These include the study of giant nonequilibrium concentration
fluctuations in a ternary mixture in the presence of a diffusion barrier, the
triggering of a Rayleigh-Taylor instability by diffusion in a four-species
mixture, as well as reverse diffusion in a ternary mixture. Good agreement with
theory and experiment demonstrates that the formulation is robust and can serve
as a useful tool in the study of thermal fluctuations for multi-species fluids.
The extension to include chemical reactions will be treated in a sequel paper
How much larger quantum correlations are than classical ones
Considering as distance between two two-party correlations the minimum number
of half local results one party must toggle in order to turn one correlation
into the other, we show that the volume of the set of physically obtainable
correlations in the Einstein-Podolsky-Rosen-Bell scenario is (3 pi/8)^2 = 1.388
larger than the volume of the set of correlations obtainable in local
deterministic or probabilistic hidden-variable theories, but is only 3 pi^2/32
= 0.925 of the volume allowed by arbitrary causal (i.e., no-signaling)
theories.Comment: REVTeX4, 6 page
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