🚀 go-pugleaf

From article <9364@hoptoad.uucp>, by tim@hoptoad.uucp (Tim Maroney):
> So many scientists seem to take the dark matter for granted, 

I'n not sure this is true, but press reports may make it seem so.

> but I've yet to see any clear reason for
> postulating it other than a sort of religious dogma that the universe
> will eventually recollapse.  Is there really any stronger basis for
> believing that we only see ten percent of the universe, or are people
> letting their aesthetics guide their modeling?

There are actually two separate questions that are often confused.  The
first is best called the "Hidden Mass Problem."  Gravitational
evidence, including rotation curves, the stability of spiral galaxy
disks, and the dynamics of clusters of galaxies imply that galaxies
have about 3-10 times more mass than estimated for the visible stellar
population.  The "hidden mass" is distributed in roughly the same way
as the visible mass; i.e., it accompanies the observed galaxies.  The
debate now is over the form of this matter, not its existence.
Possibilities include "brown dwarfs" or almost anything else (black
holes, "planets") in the mass range between 0.05 and 0.005 solar mass.
There are other possibilities as well.

The second problem is usually called the "Missing Mass Problem."  Even
accounting for the hidden mass, observable galaxies add up to only
0.3 or less of the "critical mass," i.e., the maximum mass allowed
where the Universe expands forever (not recollapses).  As far as I can
tell, there is indeed no observational evidence for existence of the
"missing mass;" indeed, there is considerable evidence that the
"missing" mass, if it exists, cannot be in the form of baryons.  The
"missing mass" must also _not_ accompany the visible galaxies but rather
must be more uniformly distributed through the Universe.

Theorists often (always?) "let their aesthetics guide their modeling."
What's wrong with that?  The relevant questions are whether models are
ruled out by existing data and whether simpler models would explain the
data just as well.  The answer to the first is "maybe" and to the second
"no".  Models with mass lower than critical invoke either coincidence
or some other component just as arbitrary as non-baryonic dark
matter.  Both the "hidden mass" and the "missing mass" questions are at
the forefront of research these days, which means that nobody knows the
right answers. 
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Steve Willner            Phone 617-495-7123         Bitnet:   willner@cfa
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