"XeNO" wrote in message news:mhhec.951$fS6.831@okepread01...
Okay I understand that current thinking leans towards hidden matter in the
spiral arms of our galaxy. And that a black hole is at the center of the
galaxy. Couldn't the problem of the spiral arms not "flying off" be due to
the possibility that they are in the Event Horizon of the galactic center?
Would we still need hidden matter even then? It just seems kind of "extra"
and not fitting quite right with Occam's Razor to me.

There are a few problems with this hypotheses.

First, a black hole with a Schwarzschild radius
(event horizon) large enough to encompass the
spiral arms of the galaxy would need to have a
truly prodigious mass.

The Schwarzschild radius Rs for a mass M is:

Rs = 2*M*G/c^2

If the galaxy disk has a radius of about 50,000 LY,
then the mass of the galaxy would have to be on the
order of 1.6x10^17 solar masses. Estimates for the
number of stars in the galaxy have an upper limit of
about 2x10^11 stars. No one would credit an amount
of dark matter some 10^5 times that of ordinary
matter.

Next, inside an event horizon, all matter must follow
trajectories that end in the central singularity.
That means that there would be no stable spiral arms
(they would collapse into the center), and no stars
could follow a path that leads it outwards from the
center. Yet we observe stars within our galaxy with
velocities that take them outwards.

Further, the "problem" of flattened velocity curves
exists for other galaxies as well. Since we can
observe their stars, they can't be hidden behind an
event horizon.