Re: Dark matter swirling into a black hole?


Yousuf Khan <bbbl67@xxxxxxxxx> writes:
If Dark Matter exists, and the only way normal matter interacts with
it is through gravity, then shouldn't there be Dark Matter vortex
swirling into one of the biggest sources of normal matter mass around,
i.e. a black hole? In fact, shouldn't the black hole be getting bigger
by several fold, just by fattening up with Dark Matter, which is
supposed to be an order of magnitude more prevalent than Normal
Matter? A black hole would then be growing more massive even if no
sources of normal matter are nearby to feed it.

Getting a black hole to "suck up" even normal matter is actually
rather difficult. Of course there will be a very small fraction of
gas and stars that free-fall directly into the black hole. However,
most will not. Most normal matter in our galaxy is not gravitational
bound to the black hole in the center. The tiny fraction that is
bound probably got that way via tidal interactions and frictional
dissipation, which then forms an accretion disk. Even then, it is
fairly difficult to push matter into the black hole, since a large
amount of potential energy and angular momentum must be removed.
However, accretion disks are highly viscuous and turbulent, and a
small amount of mass transport does occur.

Looking at the mechanisms quoted above for binding matter to a black
hole and pushing it in: "tidal interactions" - "frictional
dissipation" - "accretion disk" - "viscuous" - "turbulent".
None of these effects is known to apply to dark matter since then
all require normal matter (electromagnetic) interactions.

Thus, most dark matter near a black hole probably spends its time in
bound orbits quasi-forever, without dissipation. And probably an even
larger fraction of far away dark matter is not even bound to the black
hole and does whatever it does.

CM
.

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