On 14 Jan 2018 19:29:55 +0000 (GMT), root wrote:
super-massive black holes could have been created in the
early universe by accretion of dark matter?

"Dark matter" does not "accrete" because accretion is baryonic
behaviour and dark matter, whatever it is, is known not to be baryonic
and acts more as a halo. How's that for a one-sentence summary?

[[Mod. note -- Excellent.

The underlying physics of matter accreting onto a BH is that we imagine
a cloud of matter (whether dark or otherwise, baryonic or non-baryonic)
around the BH, the matter far from the BH having some random distribution
of velocities with respect to the BH.

In order to accrete the matter must physically reach the BH's event
horizon, which is very small, i.e., the matter must be an an almost
radial orbit with respect to the BH, i.e., the matter must be on a very
low-angular-momentum orbit about the BH. For any reasonable velocity
distribution of the matter far from the BH, most of the matter in the
cloud will have far too much angular momentum to reach the event horizon.

So, matter needs to lose a significant amount of angular momentum
before it can accrete. This requires friction, a magnetic field, and/or
some similar process, to transpare the angular momentum out away from
the BH as the matter moves inward (typicaly in an accretion disk).

Dark matter is uncharged (and hence unaffected by magnetic fields) and
essentially collisionless (and hence unaffected by friction), so there's
no easy (quick) way for it to lose large amounts of angular momentum
and accrete onto the BH.

In other words, only a tiny fraction of the ambient dark matter (that
fraction which happens to be on a trajectory that takes it to the event
horizon) will accrete onto the BH.
-- jt]]