Chance and Chandra (and repulsive dark matter)
  A few examples are given of Chandra's work on statistical and stochastic
problems that relate to open questions in astrophysics, in particular his
theory of dynamical relaxation in systems with inverse-square interparticle
forces. The roles of chaos and integrability in this theory require
clarification, especially for systems having a dominant central mass. After
this prelude, a hypothetical form of repulsive bosonic dark matter is
discussed. The repulsion leads to nontrivial thermodynamic behavior, including
superfluidity, and would tend to suppress dynamical friction, greatly reducing
the drag exerted on rotating galactic bars. However, this form of dark matter
can probably be ruled out, at least for parameters that allow halos to reach
thermal equilibria within a Hubble time. One combination of the particle mass
and interparticle repulsion determines the minimum core radius of dark halos.
Bounds on dark-matter collisionality inferred from the Bullet Cluster constrain
a second combination. It is possible to satisfy both constraints only for
parameters that predict unacceptable rotation curves outside the halo core.
