Well the main problem is the sheer severity of the Doomsday effect.
Suppose space-colonizing civilizations have an average population a billion times that of "doomed" civilizations, then the mediocrity argument implies that fewer than 1 in a billion civilizations become space-colonizing. If the population ratio is a trillion, then fewer than 1 in a trillion become space-colonizing.
But there are something like 10^22 stars in the observable universe, and a space-colonizing civilization could reach a very large portion of them; further, it would tend to do so, if there is no real competition from other colonizing civilizations (the competition would instead be arising at the edge of the expansion wave, causing travel speeds to increase and approach the speed of light). So the most likely population increase factor is something like a billion trillion or more, implying a chance of civilization survival of 1 in a billion trillion or less. That does seem unreasonably pessimistic.
Fair enough, and on reflection I agree that those kind of survival odds are unreasonably pessimistic given the information we currently have.
Hi gang,
for the last several months, I've intermittently been wondering about a curious fact I learned.
You see, I was under the impression that the universe (as opposed to just the currently observable universe or our Hubble volume) must be finite in its spa[t|c]ial dimensions. I figured that starting with a finite area of space, expanding with a finite (even if accelerating) expansion rate, could only yield a finitely sized volume of space (from any reference frame), a fraction of which constitutes our little Hubble bubble.
Turns out - honi soit qui mal y pense - my layman's understanding was wrong: "This [WMAP data] suggests that the Universe is infinite in extent (...)"
Now, most (non-computer-scientist) people I've bothered with that answered along the lines of "well, it's really big alright? (geez)".
However, going from any finite amount of matter/energy to an actual infinite amount (even when looking at just e.g. baryonic matter from the infinite amount of galaxies) still seems like a game-changer for all sorts of contemplations:
For example, any event with any non-zero probability of happening, no matter how large the negative exponent, would be assured of actually happening an infinite amount of times somewhere in the our very own universe (follows straightforwardly from induction over the law of large numbers).
Such as a planet turning into a giant petunia for a moment, before turning back.
The universe being infinite doesn't make that event any more likely in our observable universe, of course, but would the knowledge that given our laws of physics, there is an infinite amount of Hubble spaces governed by any sorts of "weird" occurrences - e.g. ruled by your evil twin brother - trouble you? Do we need to qualify "there probably is no Christian-type/FSM god" with "... in our Hubble volume. Elsewhere, yes."?
The difference, if you allow me a final rephrase, would be in going from a MWI-style "there may be another version - if the MWI interpretation is correct - that I cannot causally interact with" to a "in our own universe, just separated by space, there is an infinite amount of actual planets turning into actual petuniae (albeit all of which I also cannot interact with)".