Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!utastro!terry
From: terry@utastro.UUCP (Terry Hancock)
Newsgroups: sci.space
Subject: Re: Cosmic Dark Matter
Message-ID: <4738@utastro.UUCP>
Date: 17 Jan 90 21:01:39 GMT
References: <9364@hoptoad.uucp> <284@cfa.HARVARD.EDU>
Reply-To: terry@astro.UUCP (Terry Hancock)
Organization: UT AUSTIN Astronomy Department/McDonald Observatory
Lines: 74

In article <284@cfa.HARVARD.EDU> willner@cfa.HARVARD.EDU (Steve Willner, OIR) writes:
>From article <9364@hoptoad.uucp>, by tim@hoptoad.uucp (Tim Maroney):
>> So many scientists seem to take the dark matter for granted, 
>
>
>> but I've yet to see any clear reason for
>> postulating it [a Closed Universe] 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?
>
>
>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.
>
	Well, there is ONE sort of evidence that I've seen -- the redshift
curve (i.e. the curve relating distance to redshift)  has a different 
shape for the open, flat, and closed models.  Unfortunately, this is a
difficult measurement, since it requires an alternate form of distance
measurement -- AND the difference between the curves is only significant
towards extreme distances.  The only distance method I have heard of
being applied to this problem involves the measurement of relative
brightness and the assumption of statistically homogenous galaxy-brightness
both over space and over the history of the universe.   Needless to
say this is a risky assumption.

	HOWEVER, despite all these caveats, the resulting curves that I
have seen loosely support a closed universe.   This (presumably) implies
that we are missing some of the mass which is responsible for the
closure.

	Of course, the observation could be prey to a number of systematic
errors, and I wasn't all that convinced.


>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. 

	The whole problem with this area is that there's too little
"existing data" to support the theories.  I'm taking all of it with
a grain (maybe a tonne) of salt until a LOT more observations are
made (and possible).  I don't honestly expect to live long enough to
see a day when I take any cosmological theories seriously.
	Just remember that Ptolemy's universe was not "ruled out by
existing data" for a very long time.  Which, of course, is not all
bad -- a lot of good astronomy was done with the Ptolemaic assumptions.
And, of course, cosmology can be fun, if not correct -- which is probably
why there's so much interest in it (aside from the desire to obtain
a "God's Eye" view of the world, perhaps).

>-------------------------------------------------------------------------
>Steve Willner            Phone 617-495-7123         Bitnet:   willner@cfa
>60 Garden St.            FTS:      830-7123           UUCP:   willner@cfa
>Cambridge, MA 02138 USA                 Internet: willner@cfa.harvard.edu

*********************************
Terry Hancock
terry@astro.as.utexas.edu
*********************************