🚀 go-pugleaf

Matt Austern writes:
>(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.  ...
>
>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!)
>...it turns out that the amount of matter which we
>observe is insufficient to produce these [gravitational] fields.

Agreed - if I believed everything that some particle physicist liked
 ... [stereotypes deleted].  Seriously, I think the amount of luminous
matter observed gives \Omega < 0.03, assuming an everyday mass-to-
light ratio, which is almost certainly a bad assumption - there are
probably a lot of brown dwarfs et al in the dark matter.  Rotation
curves of galaxies imply significant dark matter, but you have to get
to group/cluster scales before an \Omega ~ 0.2 is required.  IRAS
galaxy clustering supposedly suggests \Omega > 0.4 or something like
that.  

Primordial nucleosynthesis limits \Omega_baryons to ~0.2 or less,
so to have inflation weird particles (and weird particle physicists)
are necessary.  A baryonic universe with \Omega ~ 0.2 is a faint
possibility, but the pros seem to dislike it because the large scale
structure comes out wrong, plus you still have the horizon and
flatness problems that inflation is supposed to solve.

Good reviews of the dark matter evidence are in Tremaine's article
in Physics Today (Feb 92) and the last chapter of Binney and
Tremaine's _Galactic Dynamics_ (quite detailed).