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From: gwyn@brl-vgr.ARPA (Doug Gwyn )
Newsgroups: net.physics,net.astro.expert
Subject: Re: quasars and the history of the universe
Message-ID: <2156@brl-vgr.ARPA>
Date: Fri, 18-May-84 10:27:44 EDT
Article-I.D.: brl-vgr.2156
Posted: Fri May 18 10:27:44 1984
Date-Received: Sat, 19-May-84 01:38:05 EDT
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Thanks for the additional quasar info, Ed.  Is it correct to say that
the observational evidence is:
	The apparent brightness distribution of quasars is much more
	peaked than would be expected if they all had the same intrinsic
	luminosity and were uniformly scattered through a 3-dimensional
	Euclidean space. ?
I.e., are the observed quasars closer to all the same brightness than
one might expect?

I think this is equivalent to what you were saying in your note.  It is
tricky to express these things in purely observational terms...

If I got this stated right, then again there is more than one possible
theoretical interpretation.  The conventional one is that there are a
lot of really luminous quasars far away (i.e. far back in time, toward
the "big bang").  An alternative for those who think quasars are nearby
is that they are all about the same distance away (like, in a sphere
about our galaxy) and have similar intrinsic luminosity (but different
red-shifts, which is admittedly a bit suspect).  Quasars associated with
other galaxies would be hard to detect...

Something like this actually happened with Cepheid variable stars.  I
have seen the distance scale of the universe grow by an order of
magnitude as the result of mistaken assumptions about what was being
seen being corrected.

Two reasons for preferring the nearby theory of quasars to the cosmic
one are:
	(a) The intrinsic luminosity is reasonable if they are nearby,
	    but really hard to explain if they are far away;
	(b) The periods of pulsars are hard to explain for far-away
	    pulsars, but admit of simpler explanations if the objects
	    are nearby.
Neither of these is a decisive argument (unless one can disprove the
existence of neutron stars), but they make the nearby quasar theorists
feel more comfortable with their position.

The trouble with this cosmology business is that it is hard to directly
check what the theories are telling us.  The actual observed evidence
is much more constrained than would be true for an experimental science.
It would be great if a space probe could be sent out of the galaxy to
see how things look from there (would settle the quasar question and
2.7oK radiation question), but it is not going to happen soon (even if
it did, the answers would not be known soon).

Much of what I have been saying in these discussions boils down to
"just because the evidence agrees with a particular theory, that does
not mean that that theory is correct, and it is important to distinguish
between what we observe and our explanations of what we observe based
on a particular theory".  I hope this statement is acceptable...