Subject: Fang Li Zhi's work on cosmology

From: [email-masked] (Matt Austern)
Date: Fri, 25 Oct 1991 06:20:09 GMT

In article <[email-masked]> [email-masked] (Daniel Fischer) writes:

> The following is translated from an article in the THUERINGER TAGEBLATT of 17
> June 1991 that dealt with the Very Large Array run in Socorro, New Mexico:
 
> ] The well-known Chinese astrophysicist Fang Li Zhi raised the
> ] question of the limits of the universe when he met with 30
> ] scientists in Socorro recently for an exchange of thoughts. Fang,
> ] who as a dissident had to escape from China and is doing research
> ] now at the University of Princeton, said, that the UNIVERSE
> ] PERHAPS ISN'T INFINITE BUT LIMITED TO A DIAMETER OF 2*10^9 LIGHT
> ] YEARS [emphasis mine]. He opined that some stellar clusters at the
> ] end of the visible spectrum needn't to be real objects but could
> ] be 'duplicates', mirror images of other galaxies.
 
> That's it! Anyone knowing details of Fang's claim, data, references
> in the literature? Sounds like an attempted compromise between an
> Arp/van Flandrian and a standard universe to me :-)

I attended a talk by Fang about six months or so ago where he
discussed his work on this question.  Bear in mind that my
recollection is not necessarily accurate, and also bear in mind that
what he said six months ago is not necessarily what he believes now.

Anyway... the point of Fang's talk wasn't so much that he had a
particular model for the universe.  (Nor was his point the possibility
that the universe might be finite; this is one of the standard
possibilities of the Big Bang model, after all.)

What Fang observed is that most discussions of the Big Bang model
assume that the universe is simply connected, in a topological sense.
However, there isn't really much reason for that assumption.
Non-trivial topologies are perfectly consistent with the Einstein
field equations, and most people ignore them just for the sake of
simplicity.  The topology of the universe is probably determined in
the regime in which quantum gravity is important, so we have no way,
at this point, of even thinking about what kind of topology is most
likely.  Once the evolution of the universe proceeds according to
classical gravity, of course, the topology is fixed.

(He waved around a couple of path integrals over different topologies.
You're welcome to believe that sort of thing if you like, but I
don't.)

The spirit of the talk wasn't "I believe that the universe isn't
simply connected," but "in the absence of convincing evidence for a
particular topology, let's think about the alternatives."

The remainder of the talk, then, was mainly devoted to discussing what
the observable consequences of a nontrivial topology would be.  And,
indeed, one of those consequences was that astronomical objects would
be duplicated.  Observing this duplication, however, wouldn't be
simple: you would have to do some rather complicated statistical tests
on large data samples of galaxies, correcting for various kinds of
uncertainties.  At the time of the talk I attended, Fang wasn't very
certain of his results; he did this work by performing numerical
simulations, and he did it when he was holed up in the American
Embassy.  Not surprisingly, the diplomats didn't have computers that
were very useful for large-scale scientific computing.

One point he made that I found interesting was that in a universe with
a nontrivial topology, it is likely that the inhomogeneity in matter
would be much greater than the inhomogeneity in radiation.  (Which,
perhaps, is what we are seeing.)  Although I found this remark
interesting, I also don't understand it, and I didn't have a chance to
ask Fang for an explanation.  Does anyone out there understand why
this would be the case?
X
--
Matthew Austern              I dreamt I was being followed by a roving band of
(415) 644-2618               of young Republicans, all wearing the same suit,
[email-masked]    taunting me and shouting, "Politically correct
[email-masked]       multiculturist scum!"... They were going to make
[email-masked]	     me kiss Jesse Helms's picture when I woke up.