poly: Open vs closed universe

From: Hal Finney <hal@rain.org>
Date: Thu Feb 26 1998 - 15:46:45 PST

There is an article in this week's Science News magazine about the
possibility that even a negative curvature universe could be topologically


This solution is not generally predicted by general relativity, but it
is not forbidden by it, either. The idea is that the universe could be
"wrapped around" in some way, so that opposite sides of the universe are
mapped onto each other. You travel along in one direction and eventually
get back where you started. The geometry of the universe in terms of the
metric and curvature is still controlled by relativity, but the topology
acts as a sort of boundary condition which can be freely assigned.

If the universe were this way, and the wrap-around points were
sufficiently small, then conceivably some of the distant galaxies we
observe could be our own Milky Way galaxy at an earlier point in its
evolution. It is difficult to notice this with galaxies or quasars
because they change so much over time, but there are proposals to look
at variations in the microwave background and detect patterns which
would indicate wraparound.

I'm somewhat bemused about the motivation for these studies. Astronomers
almost seem to have a phobia about the infinite. They worked so hard to
find missing mass to close the universe, even confusing and conflating
the term to apply both to the missing matter which was known to have
gravitational effects in galaxies, and also the theorized missing matter
which would have to be there for the universe to be closed.

Now that observations seem to be clearly showing the universe to be open,
we have new proposals which would eliminate the threat of an spatially
infinite universe yet again. There is not a shred of evidence or theory
to believe that these models will work, but they seem to be attractive
enough to researchers that people will spend months of work analyzing
the microwave data, hoping to find the signature of a wrapped universe.

When I studied general relativity in college, the wraparound universe
was used as a simple example for exploring how the field equations work.
It imposes a preferred reference frame on the universe when you specify
the spacelike planes where the wraparound occurs. This is philosophically
distasteful to me, and the whole idea seems totally ad hoc. I have never
seen a physical mechanism proposed which would act during cosmogenesis
and give the universe this kind of topology. All in all it seems to be
an intrinsically unattractive idea. I can only assume that the lure of
making the universe finite again is motivating this work.

Received on Fri Feb 27 00:08:40 1998

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