From: Anders Sandberg (asa@nada.kth.se)
Date: Mon Feb 10 2003 - 05:14:28 MST
On Sun, Feb 09, 2003 at 10:39:50PM -0800, Ramez Naam wrote:
> From: Anders Sandberg [mailto:asa@nada.kth.se]
> > I think most cosmologists think of spatially
> > unbounded universes when they consider the open
> > universe scenario (although it is not topologically
> > required). These would have an infinite number of
> > galaxies, although I don't think this is widely
> > remarked on.
>
> Really? I'm no cosmologist, but this doesn't jive with my
> understanding. An open universe is one that will expand infinitely
> and is therefore *eventually* infinite in volume.
No, look at the metric - the spatial metric of the vanilla open universe
has an infinite volume.
> However, an open universe has finite mass, which clearly limits the
> number of galaxies.
Not if there are counteracting terms like the cosmological constant
around. That way the total mass energy can be zero or even negative.
> Indeed, if an open universe had an infinite
> number of galaxies it would have infinite mass, which would result in
> a mass density that would guarantee collapse, thus implying a closed
> universe.
Already Newton saw that if you distributed an infinite number of stars
in a pattern that would today be called fractal the gravitational
contributions from remote stars can be made to decrease, produce only a
finite force which in turn would not be able to cause a global collapse.
-- ----------------------------------------------------------------------- Anders Sandberg Towards Ascension! asa@nada.kth.se http://www.nada.kth.se/~asa/ GCS/M/S/O d++ -p+ c++++ !l u+ e++ m++ s+/+ n--- h+/* f+ g+ w++ t+ r+ !y
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