Re: poly: Opne and baby universes

Anders Sandberg writes, quoting Mitchell Porter, <mitch@thehub.com.au>:
> It's hard to see how a spatially infinite universe can be
> produced by the straightforward version of "baby universe"
> reproduction - i.e. a collapsing finite region pinches off and
> becomes an expanding finite hypersphere - since a spatially
> infinite universe has to start infinite in extent.

The John Gribbin article I referred to earlier discusses this issue,
<http://epunix.biols.susx.ac.uk/Home/John_Gribbin/Cosmology.html>.
He lists some variations on inflation theory that can lead to a universe
which appears to have less than the critical density. Such universes
are not infinite in size, unlike in simple Friedmann models.

As I read it, we could be in a baby universe which has lower than
critical density if it formed out of a universe that was earlier in its
inflation stage and had not yet "fully inflated". (Inflation pushes
the universe up to critical density.)

I'm not sure what the metric is in the baby universe. Conceivably it is
locally hyperbolic but globally spherical, curving back around at the
edges to join the parent universe. However this does not sound right
to me. I think instead that it is probably spherical geometry everywhere
(triangle angles add to > 180 degrees), despite the fact that the density
is less than critical density.

> Incidentally, if space really were Euclidean-in-the-large, it
> could still be finite, by having a torus (T^3) topology.

This is ad hoc unless you can propose a physical mechanism which sets the
wrap-around size of the universe.

Hal
Received on Sat Jan 17 16:49:19 1998