🚀 go-pugleaf

In article <MATT.92Aug4141039@physics16.berkeley.edu> matt@physics.berkeley.edu writes:
>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.  

You are absolutely right.  There is an extremely strong case for the
existence of substantial quantities of "dark matter" (That is, stuff
whose gravitational effects we observe but which we can't see directly)
from dynamical observations of the sort you describe.  You get information
about the gravitational force on something from the way it moves, and
figure out how much mass must be present to produce that force.  The
answers consistently show that galaxies and clusters contain several times
more matter than is visible.

What I should have said is that the theoretical arguments that the density
of the Universe is equal to the critical density provide evidence for
a lot more dark matter.  Even if you take into account the well-established
"dynamical dark matter" of the sort you're talking about, you still only
get Omega=0.2 or so.  To get Omega = 1 (density=critical density)
you need more dark matter.  

I agree with you that the theoretical arguments that Omega = 1 are
much less convincing than the evidence for the dynamical dark matter.

Another thing:  There's pretty good evidence that you can't get Omega bigger
than about 0.2 or so without having some of the matter be made of things
other than protons, neutrons, and electrons.  So while it's possible for
the "dynamical dark matter" to be ordinary matter, the "Omega = 1"
dark matter, if it exists, is probably something else, something we've
never seen before.

-Ted
(ted@physics.berkeley.edu)