Re: Universe: finite/infinite, closed/open

[Jack Uretsky <jlu@HEP.ANL.GOV>]

Hi Jim-
        I like Ferris.  His earlier book <Coming of Age in the Milky Way>
is sprightlier reading.
        There are two different issues, that are getting mixed up here.
1. Is the observable universe finite?  Definitely.  That's because we
        are looking back in time and can only see back to the time
        of "freeze-out".
2. Is the radius of the universe finite on a spacelike surface?  That's
a different question because we can't make spacelike observations.  If
everything came out of a "big-bang" that subsequently expanded with the
speed of light, then yes.  But the rate of expansion need not have
been limited (inflation doesn't violate relativity), so the question
cannot be answered by direct observation.

        "Open" and "closed" can be used in different contexts.  In
the context discussing the rate of expansion, "open" means expanding
forever (without coming asymptotically to rest), "closed" means that
the expansion will one day reverse itself.  The in-between case is
sometimes called "critical".
                                        Regards,
                                                Jack
> > on cosmology written for the intelligent lay person.  It's available
> > in paperback from Amazon.com.
>
> I dunno.  Timothy Ferris????  Comments??
>
>
> Jim Green
> mailto:JMGreen@sisna.com
> http://users.sisna.com/jmgreen