RE: Flat universe == Infinite Universe?

From: xgl (
Date: Mon Jun 05 2000 - 10:32:17 MDT

On Mon, 5 Jun 2000, White, Ryan wrote:

> I've been watching this thread for some time and my ignorance of these
> matters is driving me crazy - what is meant by 'flat' in this context? My
> immediate reaction to the adjective 'flat' is 'nearly two dimensional'.
> Certainly I have the wrong idea - given a >2 dimensional universe.

        the terms "open," "closed," and "flat" refer to curvature. thus
two parallel lines diverge in an universe with "open" or negative
curvature; the same two lines always meet in an universe with "closed" or
positive curvature; and they stay the same distance apart in an universe
with "flat" or zero curvature. since all three spatial dimensions curve,
it is useful to think of it as curvature on a fourth spatial dimension.

        other differences include -- the universe's expansion accelerates
if it's open, decelerates and reverses if it's closed, and decelerates
forever if it's flat.

> > And, I would love to hear more about,
> 'what eerie odds would be overcome if such were really the case.'

        hmm, reading the quote a second time, i'm afraid i didn't make
myself clear. what i meant to say was, what eerie odds would be overcome
if such were really the case _without_inflation_. which, i might add, is
just silly.

        this refers to the flatness problem back in the 70's. that is, for
the universe to be anywhere near flat today, it would have to have been so
close to perfectly flat when it started (99. 9 to some ungodly decimal
place %), that it would be almost too much to ask of chance.

        of course, then alan guth et. al. came up with inflation ... which
resolved a whole slew of cosmological problems and _predicted_ a flat (or
very nearly flat) universe. which (to come full circle) is why the latest
findings are such a forceful boost to inflationary theory (as if it needed

> Please enlighten me.

        again, i'm nooo expert.


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