Xref: utzoo sci.space:16253 sci.astro:5968
Newsgroups: sci.space,sci.astro
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Big Bang:  Did it happen?
Message-ID: <1989Dec20.173711.18688@utzoo.uucp>
Organization: U of Toronto Zoology
References: <963@YaleVM.YCC.Yale.Edu> <263@cfa.HARVARD.EDU> <9364@hoptoad.uucp>
Date: Wed, 20 Dec 89 17:37:11 GMT

In article <9364@hoptoad.uucp> tim@hoptoad.UUCP (Tim Maroney) writes:
>I've often wondered about this.  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...

It is very difficult to explain either the motions of stars in our galaxy
or the motions of galaxies in clusters without assuming that there is
quite a bit of extra mass present somewhere.  Galactic velocities are
too high for clusters to survive unless there is extra mass present
to raise the clusters' escape velocities.  The distribution of star
velocities in our galaxy is grossly wrong if you assume no dark mass;
it doesn't fall off nearly quickly enough as you go out from the center,
and the only way to account for it is to assume considerable extra mass
which is much more spread out than the visible mass.

Now, whether this extra mass is enough to close the universe is a different
question.

As I understand it, one reason for suspecting an Omega of 1 (universe flat,
precisely balanced between closed and open) is that the current Omega is
within an order of magnitude or so of 1, and differences from 1 get
magnified with time.  That is, it must have been *very* close to 1 in
the beginning to be this close now.  This is considered a suspicious
coincidence; pre-inflationary theories offer no reason for it.
-- 
1755 EST, Dec 14, 1972:  human |     Henry Spencer at U of Toronto Zoology
exploration of space terminates| uunet!attcan!utzoo!henry henry@zoo.toronto.edu