Adapting to an open universe

John K Clark (
Sun, 18 Jan 1998 23:11:38 -0800 (PST)


Nick Bostrom wrote:

>if d is big enough, then by the time the probes have traveled 10% of
>the way, the remaining 90% will have grown larger than the original
>100%, due to the expansion of space inbetween. So if that expansion
>rate settles down to a constant, then there must be a sphere centered
>around Earth such that nothing that is outside of that sphere can
>*ever* affect Earth, even if it travels with the speed of light.

Damien Broderick
>I believe you have just reinvented the canonical explanation for
>Olbers' Paradox.

That's true, but it causes another paradox. If we look at a galaxy 15 billion
light years away and then look at another galaxy in the opposite direction
that is also 15 billion years away, and if the universe is only 15 billion
years old and has always expanded at the present rate, then the two galaxies
could NEVER have had anything to due with each other. The trouble is that we
can see that the two look very similar to each other, and the laws of physics
seem the same in both places, that is, electrons produce photons in exactly
the same way. How do electrons in such distant places know to act identically
if they never ever had any causal relationship with each other?

The inflationary model of the Big Bang was invented to explain this paradox,
it postulated that for a extremely short time in the very early universe the
expansion was accelerating at an exponential rate, enormously faster than
light. If this is true then the matter in our two galaxies was once close
enough for light to travel between them, and that explains why the galaxies
are so similar even today.

Inflation also predicts that the universe should be right on the knife edge
between eternal expansion and collapse. According to inflation the universe
will not collapse, it will slow down but never completely stop expanding,
it will not grow arbitrarily large. The very recent experimental evidence
that the expansion is unbounded is NOT consistent with inflation theory.
If the evidence holds up the theory is in deep trouble and it's back to the
drawing board.

John K Clark

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