🚀 go-pugleaf

In article <15mqt8INNlp@agate.berkeley.edu> ted@physics2 (Emory F. Bunn) writes:

> There are theoretical reasons for suspecting that Omega=1, but
> direct measurements of the density (which is a tricky thing to do)
> imply that Omega is about 0.1 or 0.2.  That's why people talk about 
> the existence of large amounts of "dark matter," to bring up the
> density to the critical density.

Well, really this is only one of the reasons why people talk about
dark matter, and, in my opinion, not the most compelling.  (Why, after
all, should we believe these theoretical reasons?  Just because some
particle physicist likes inflation, that doesn't make it true!)

A more compelling reason is that by analyzing the motion of objects
that we see, we can learn about the kinds of gravitational fields that
they are moving in; it turns out that the amount of matter which we
observe is insufficient to produce these fields.  (I am being
intentionally vague here, because the principle holds in many
different cases---from stars within globular clusters, to the
rotations of galaxies, to the motion of galactic clusters with respect
to each other.  On all of these distance scales, it appears that there
is more matter than we are directly observing.)

There are two ways to interpret this.  One is that there is "dark
matter" which we observe only gravitationally, and the other is that
our theories of gravitation are inadequate to describe distance scales
of more than a few light-years.  I find the first explanation much
more plausible.

Incidentally, I should warn people that I am not an expert on this
subject (or any other subject in astrophysics), merely an interested
observer.  Essentially everything I know is from Kolb and Turner,
which is a very nice book if you are a grad student who wants to learn
a little bit of cosmology.
--
Matthew Austern              I dreamt I was being followed by a roving band of
(510) 644-2618               of young Republicans, all wearing the same suit,
matt@physics.berkeley.edu    taunting me and shouting, "Politically correct
austern@theorm.lbl.gov       multiculturist scum!"... They were going to make
austern@lbl.bitnet	     me kiss Jesse Helms's picture when I woke up.