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From: goldader@uhccux.uhcc.hawaii.edu (Jeff Goldader)
Newsgroups: sci.space,sci.astro
Subject: Re: Big Bang:  Did it happen? (long)
Summary: Dark matter
Message-ID: <5771@uhccux.uhcc.hawaii.edu>
Date: 20 Dec 89 18:51:33 GMT
References: <963@YaleVM.YCC.Yale.Edu> <263@cfa.HARVARD.EDU> <9364@hoptoad.uucp>
Reply-To: goldader@uhccux.UUCP (Jeff Goldader)
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Organization: University of Hawaii
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In article <9364@hoptoad.uucp> tim@hoptoad.UUCP (Tim Maroney) writes:
>
>So many scientists seem to take the
>dark matter for granted, 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?

Yes, there is in fact strong observational evidence for dark matter.

For a long time, it was generally assumed by the astronomical community
that the distribution of *mass* in galaxies was traced by the distribution
of *light*; that is, where things are brighter, there is more mass than
where things are relatively dim.  However, when astronomers began to measure
the rotation curves of galaxies, things became confused.

The rotation curves of spiral galaxies, that is, a plot with distance 
from the nucleus on the x-axis and velocity on the y-axis, allow us to
trace the REAL mass distribution, to some extent.  Because of Kepler and
Newton, if a galaxy were mostly a point mass in the nucleus, we'd expect
the rotational velocity to fall off rapidly with distance from the nucleus.
Instead, we find that the rotational velocities stay mostly constant out to
large radii.  This means there is some extra matter whose distribution is
not traced by the light distribution, since the light distribution falls 
off exponentially the farther out you get in the disk and the higher you 
get above the disk.  This matter is necessary to explain the extra 
gravitational force needed to keep the stuff at large radii moving
as quickly as the stuff at smaller radii.  The unseen matter is generally 
believed to be in the form of a "dark halo", roughly spherically symmetric,
surrounding the galaxy.  This evidence for dark matter is very persuasive,
and no one has yet come up with a better explanation for the observed
rotation curves.

No one has been able to directly observe dark matter, although there are many 
theories as to its nature.  Some say massive neutrinos, others say very heavy
exotic particles (axions, wimps, etc.), many like the idea of lots of large
planets (brown dwarfs), and one astronomer has jokingly suggested that the
dark matter is nothing more than a very, very large number of basketballs.
Physicists have calculated the properties of the odd particles (if they
exist) and are devising detectors to sense them, astronomers are frantically
searching for brown dwarfs, and lots of people play basketball, so we may
learn something of the true nature of dark matter in the next few years.

Jeff Goldader                        University of Hawaii
goldader@uhccux.uhcc.hawaii.edu      Institute for Astronomy

"Bill, strange things are afoot at the Circle-K..."
-Ted, "Bill and Ted's Excellent Adventure"

Disclaimer: The University of Hawaii and the Institute for Astronomy neither 
support nor are in *any way* responsible for these opinions.