Detecting dark matter

In article , greywolf42 wrote:

Actually, it is not motion of galaxies relative to each other that requires
the "additional" dark matter, it is the the fact that cosmologists "need"
about 100 times more matter than is observed to "make" omega = 1. (Omega is
the ratio of matter in the universe to that needed to "close" the universe.)

I don't think that's right -- a lot of dark matter is detected in
galaxy clusters by the need to keep galaxies bound to the cluster,
given their motions relative to the cluster. So Ed is right.
X-ray-emitting gas in galaxy clusters also allows measuring
the cluster's total mass, and again shows lots more dark than visible matter.

This is sometimes confusing, because the internal rotation of spiral
galaxies needs about 9 parts in 10 dark matter to "normal" matter to make
pure gravitation work. So the omega "flatness" problem needs ANOTHER factor
of 10 (above the dark matter "needed" for internal spiral galaxy modelling).

Observations of motion in
galaxies have been done by measuring the motion
of gas halos with spectrometers as individual stars
cannot be resolved.

This is also true inside spiral galaxies. We measure ONLY the motions of
gas*, not the motions of stars. Then we ASSUME that the stars move the same
as gas -- because we ASSUME that ONLY gravity is at work.

This is quite false too. Of course we can and do measure the
bulk orbital motions of stars in *disk* galaxies, in visible light,
even without detecting the stars individually. Motions of gas clouds
(HI and CO) are also measured, in radio frequencies. I haven't heard
any reason to think they're inconsistent.

A new scope, the Planetary
Nebula Spectrograph, can measure the motion
of individual novas in elliptical galaxies that do
not contain gas. The unexpectedly close mass to
light ratio needed to explain the motion within these
galaxies shows that they do not contain a lot of
invisible matter.

Translation: No dark matter is needed in galaxies if we focus on stellar
motions.

I don't know about ellipticals, but that's certainly not true
for most disk galaxies. They sure do show evidence of dark matter
(or something like MOND, though I'm told that's hard to support now).

Even minimal fact-checking would have prevented your posting these errors!

Stuart Levy