🚀 go-pugleaf

In article <1992Nov4.203930.20410@murdoch.acc.Virginia.EDU> crb7q@kelvin.seas.Virginia.EDU (Cameron Randale Bass) writes:
>     This has always sounded to me like a perfectly good explanation
>     of how pixies push galaxies around.  Lemme see, lets postulate
>     a whole bunch of matter that we cannot see (or alternatively,
>     invent it at an earlier time via various hypothetical processes) and
>     has completely different properties from all the matter that 
>     seems to be in our neighborhood.  I prefer pixies.

The big difference between the two hypotheses is that the dark matter one is
testable and the pixie one isn't.  There are at least two tests of the
dark matter hypothesis that would distinguish it from the pixie hypothesis.
One is internal consistency:  Only certain distributions of densities and
velocities are consistent with the model in which structure grew from
small initial perturbations by gravitational instability.  While no conclusive
answer is out yet, these calculations are constantly being done, and
so far the model seems to be withstanding the test.

The other way of testing the dark matter hypothesis is to go out and
look for the dark matter.  There are lots of experiments going on that
should be sensitive to the various kinds of dark matter that people have
proposed.  If the experiments get sensitive enough, and fail to find anything,
then the theory will have to be abandoned.

>
>     If the dark matter is basically not in gravitational 'equilibrium'
>     with the baryonic matter in some sense, why do we apparently see no 
>     extremely local inhomogenities in our neighborhood?  After all, you're 
>     going to need one buttload of non-baryonic matter, and even tiny
>     local inhomogeneities would have a rather disconcerting effect
>     on solar system orbits.
>
Whether you agree with the standard cosmological model or not, you should
at least give the people who made it up credit for not being complete 
fools.  The evolution of the distribution of dark matter in the theory
has been carefully calculated by dozens of different people under
a variety of different versions of the model.  You simply don't get large
inhomogeneities on local distance scales in any of these models.  (If
this did happen, then these models would have been summarily rejected as
disagreeing with observations.)  The remarkable thing about the dark-matter
gravitational-instability theory of the formation of structure in the 
Universe is that it produces more or less the kinds of gravitational fields
you need to explain how galaxies and clusters of galaxies formed, without
producing local gravitational fields that conflict with observations.
This doesn't mean that the theory is right, but it does mean that you're
going to have to work harder than that to convince people it's wrong.

(Stuff deleted.) 
I said:
>>Dark matter, in contrast, seems qualitatively to explain what's observed
>>without any completely outrageous new physics.
>
To which Dale replied:
>     Except, of course, for the completely outrageous production of 
>     unknown matter.
>

Whether you think the hypothesis that much of the matter in the Universe
is of a kind that has so far escaped detection is an outrageous one
is your business.  I certainly agree that it's a big one, and needs to
be supported by substantial evidence.  It's very possible that a more
economical theory will be discovered, and the postulate of dark matter
will be abandoned.  However, it hasn't happened yet.  If you've got one,
I'd love to hear it.

The hypothesis becomes a little bit more believable if you notice that
virtually every extension of the standard model of particle physics predicts
the existence of weakly interacting particles that have masses, interactions,
and cosmological densities in roughly the right range to be cold dark matter,
and predicts that neutrinos have nonzero mass, which may be in the right
range to be hot dark matter.

For the moment, the theory that the Universe is dominated by dark matter
is far from proven.  There are, however, a large number of otherwise 
problematic phenomena that it explains.  I know of no other theory
that can do so without introducing a much larger number of ad hoc
hypotheses.

-Ted