Dark matter swirling into a black hole?

Yousuf Khan wrote:

George Dishman wrote:
Yousuf Khan wrote:
I look upon it in a fluid mechanics point of view. The
intergalactic gas is a gas afterall, therefore loose and easy to
move. The galaxies on the other hand are more like solids, their
stars tightly bound together, harder to tug around except all as a
unit. The air around us is far more massive than we as tiny little
solid-body creatures are, however the air is far more affected by
the Earth's rotation than we are (i.e. wind).

Yes, I was thinking of the galaxies like shot from two guns shooting
at each other simultaneously, the pellets are unlikely to impact as
they are small compared to their separation (for a reasonable range).

I was thinking the gas arrived ahead of the galaxies *and* the dark matter.

But in the picture it is behind. The red, blue and cluster on
the right are moving to the right.

Let me just lay out my mind's eye description for you about what I'm
picturing here, so that we don't dance around the issue too much. What
I'm thinking here is that the Universe is a flowing ocean with islands
embedded inside it. Of course the water in the ocean I'm talking about
is space-time, and the islands are the galaxies and galactic clusters
inside it.

The problem with that picture is that spacetime is traced by
free-falling test particles and they wil accelerate towards a
mass whereas a flow of water approaching an island is slowed.

As the water flows around the islands, it slows down ahead of
it, to its sides, and behind it, creating a boundary layer which is
bigger than the islands themselves, looking at it from a top-down aerial
view. In an archipelago of islands, the boundary layers combine to form
a bigger one. Eventually as you get higher in the aerial view, the
islands themselves might become insignificant, and all you'll see is the
boundary layer. At lower and lower levels, as we go from the galactic
cluster to the individual galaxies, then to the stars, we may find that
space-time is getting ever more slower at each level.

I'm picturing the Dark Matter to be just the boundary layer of a
slowed-down space-time flow. In other words, it doesn't exist, it just
is an illusion of fluid flowing at different rates.

Those two blue clouds seem to be set pretty dead-center upon
the galaxies within those two colliding clusters.
Yes, if the blue cloud on the left was trailing behind the red
cloud and galaxies on the right (and vice versa) it would be a
remarkable coincidence that the large cloud associated with the
small group of galaxies happened to match the location of the
large group of galaxies that is pulling along the small blue
cloud :-o
I'm not sure what you're trying to say here. Are you agreeing with
me?

I may have misunderstod your first comment, you seemed to be
sugesting the blue region on the left was "slowly following" the
small group of galaxies and the red gas on the right and hadn't
reached the other blue cloud yet, and vice versa. Thus the dark
matter on the left and the galaxies and gas on the right would all be
moving from left to right.

If that isn't what you meant then then the cloud on the left is
associated with the galaxies and gas on the left and they are all
moving from right to left. The point of encounter would be half way
between the two groups and the galaxies and dark matter have moved
almost the same distance since then but the hot gas has moved less
far and is consequently displaced from the galaxies. That means any
interaction between the dark matter clouds has to be much smaller
than for normal matter.

What I'm saying is that the dark matter (blue) is always following the
galaxies, not the intergalactic gas (red), despite the fact that the gas
might be more massive than the galaxies. The little blue cloud is
centered atop the little cluster, while the big blue cloud is centered
atop the big cluster; and they are *always* centered atop them. In other
words, the blue clouds (DM) never diverge away from their galaxies.

From the point of view of the group though, they are not moving, it
is the other clusters that are moving, so given a reasonable time
to reach equilibruim, they must all be centred together, the galaxies
gas and dark matter, all in motion about the barycentre of the total
of the three. It is only the past encounter with the other cluster that

has briefly displaced the gas which is now falling back towards the
DM and galaxies.

The
two blue clouds may eventually converge but they will always be atop the
galaxies and not the intergalactic gas as much.

What we see at the moment is the galxies/DM moving apart and
possibly at a speed that means they will not encounter each other
again, with each gas cloud trying to catch up to its home.

Here in the water view,
the intergalactic gas is just the silt and mud of the Universe that gets
pushed around much more easily than the islands.


The gas must originally have been associated with the galaxies and
the clusters must have passed through each other for the gas to be
displaced by the encounter.

Yes, that's a possibility. There was one other possibility that I could
picture, but it's not as good as what you just described. I was thinking
that the other possibility was that the loose gas got drawn out by the
gravity of the opposite cluster ahead of the clusters themselves joining.

Bear in mind gravity accelerates all masses equally, be it a
galaxy or an atom. Tidal forces would tend to stretch the gas
because the edge nearer the other group would be pulled
more strongly while the far edge would be affected less than
the centre, but that's true of both galaxies and ags. You would
expect a slight elongation with almost symmetrical tails of
gas on both sides of the cluster. The tidal effect can be
calculated given the masses and I think it would prove to be
much too small and that doesn't explain the marked asymmetry.

George