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From: ethan@utastro.UUCP (Ethan Vishniac)
Newsgroups: net.physics,net.astro.expert
Subject: simplicity and symmetry in cosmology
Message-ID: <294@utastro.UUCP>
Date: Thu, 17-May-84 10:23:57 EDT
Article-I.D.: utastro.294
Posted: Thu May 17 10:23:57 1984
Date-Received: Fri, 18-May-84 06:00:54 EDT
Organization: UTexas Astronomy Dept., Austin, Texas
Lines: 63

[Eat this header]

There is a recurring thread which runs through Mr. Gwyn's arguments
on cosmology to which I take strong exception.  I am *not* referring
to his favorite unified theory (although I am not sure how it is
supposed to relate to the current experimental and theoretical work
to particle physics).  What bothers me are his comments about symmetry
and simplicity in a physical theory.  Any one of a number of quotes
from him would serve to illustrate my point.  Let me confine myself to
one. ( I don't think Mr. Gwyn would regard this one as unfair or taken
out of context.)

>The closed, bounded, perfect-cosmological-principle cosmology is
>actually simpler in any measurable sense of which I am aware than
>the expanding "big bang" universe.

Now, Mr. Gwyn is clearly referring to simplicity in the sense that a
mathematician would i.e. devoid of complications, maximizing the internal
symmetries of the universe.  This has nothing to do with simplicity in
the sense of being the most economical explanation of the facts.  The
fundamental laws of physics may have many symmetries, none of which are
realized exactly, or even approximately, in any real situation.  Given
that the laws of physics are rotationally invariant do we conclude
that ferromagnets do not exist?  As for translational invariance, the
same laws of physics apply everywhere, but if you think that translates
to translational invariance of our environment, then I invite you to test
this by casting yourself off the nearest roof.  I could go on, but I think
that you all must see my point.

A successful model of the universe should account, as simply as possible,
for our observations.  I have discussed elsewhere what these are.  Mr.
Gwyn seems to be willing to take only one of them seriously, the Hubble
law.  I think this is completely unjustified, but even on this one point
his comments leave me unsatisfied.  I have been left with the impression
that he regards any ad hoc explanation as "simple" if it allows him to
continue with his prejudices intact.  I would prefer to base any explanation
of this (or any other) observation on the laws of physics as we know them, or
at least some recognizable expansion of them.  I cannot say if the E-S theory
falls into this category or not (anybody out there wish to comment?), but
I haven't heard Mr. Gwyn say that the E-S theory can explain the  Hubble
law in any natural way.

One final comment, the most symmetric possible metric (i.e. structure
of space-time) is the DeSitter space mentioned by Mr. Gwyn.  It has
the interesting property of satisfying the perfect cosmological principal
(homogeneity in space and time).  For this reason it was used as the
basis for the steady-state theory of Bondi, Gold, and Hoyle.  It does
indeed account for the Hubble flow, *because* it is an expanding universe.
Particles following geodesics (i.e. not being influenced by nongravitational
forces) diverge exponentially from one another at late times.  The theory
has been discarded within the astrophysical community because of its complete
failure to address the other observational points I have mentioned.  Even
before these points became generally accepted it was already necessary to
allow matter to be spontaneously produced from the vacuum in order to make
the model logically consistent.

                     "Just another Cosmic Cowboy"
                         
                         Ethan Vishniac
                         {ut-sally,ut-ngp,kpno}!utastro!ethan
                         Department of Astronomy
                         University of Texas
                         Austin, Texas 78712