🚀 go-pugleaf

In article <1d78l7INNhu2@agate.berkeley.edu> ted@physics1 (Emory F. Bunn) writes:

> I believe that the effect Matt is referring to is the following:
> If you measure the orbital speeds of objects in our galaxy, and
> use the laws of mechanics to determine the amount of matter in the
> galaxy, you find a need for some dark matter.  If you do a similar
> experiment over a larger distance scale, say by measuring the orbital
> properties of a pair of binary galaxies, you need an even larger
> density of dark matter.  If you keep going to still larger scales,
> you find an even greater required density.

Yes, that's more or less what I meant.

I just want to emphasize something that maybe I didn't stress enough
in my original post: these measurements are difficult.  In particular,
these largest-scale measurements involve analyzing the motion of
galactic clusters, picking out that part of the motion that doesn't
seem to be from the overall expansion of the universe, deducing what
the concentrations of matter might be that would produce those motions
(this is done with rather complicated numerical programs), and then
comparing it to the visible matter that we see in the galaxies.

There's an enormous potential for error here.  In other words: I think
that this trend (more dark matter at larger scales) is interesting,
but shouldn't necessarily be taken seriously until we have a better
idea of how to control these uncertainties.


--
Matthew Austern                   Just keep yelling until you attract a
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