Re: poly: eschatology

From: Hal Finney <>
Date: Sun Jan 18 1998 - 14:59:09 PST

Peter C. McCluskey, <>, writes:
> (Robin Hanson) writes:
> >The theoretical attraction is for a very nearly critical mass
> >universe. The key observation is that the mass density relative
> >to critical density diverges with time. If this parameter is near
> >one, then it must have been very very near one when the universe
> >was orders of magnitude younger than it is now. And if it is now
> >almost exactly one now, then it will be far from one when the
> >universe is a few orders of magnitude older than now, and thus
> >we are living at the unusual time when this parameter goes from
> >being near one to being far from one. Theorists don't have any
> >ideas I know of for this coincidence, so they'd rather it wasn't
> >there.
> Barrow and Tipler mention on pages 494 and 500 of The Anthropic
> Cosmological Principle a theory that all possible maximum sizes for
> a closed universe are equally probable, implying that we should expect
> omega to be infinitessimally close to one. (I don't know how this
> theory handles the possibility of an open universe).

Sounds like a good topic for Wei Dai's "all universes exist" discussion
list. It's interesting BTW how these arguments take on a different spin
when we look at them from the point of view believing that omega being
equal to 1 has been ruled out. It makes you want to go back and take
a closer look and see if there was some flaw in the reasoning. I think
we'll be seeing the same thing about Tipler's criticisms of Dyson, as
Nick Bostrom mentioned recently.

Presumably there is an anthropic argument for omega to be in the
general vicinity of 1. If omega were much less than 1 then matter
would have been too dispersed for stars and galaxies to form, and if it
were much greater than 1 then the universe would not last long enough.
So within broad limits of several orders of magnitude, we could expect
omega to be near 1 because otherwise our form of life could not exist.
As Robin describes, this constrains omega to have been very close to
1 in the early universe, but it doesn't force it to be exactly 1.

Of course, causative explanations like inflation are much more powerful
than anthropic arguments.

Received on Mon Jan 19 00:02:06 1998

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