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Dec11-07, 10:04 AM
P: 119
Quote Quote by Wallace View Post
I think we are getting somewhat off track in this discussion! No one is suggesting that we can conclusively prove the Universe is infinite but I'm curious to know why you think 'the burden of proof' rests on the 'case for infinite' side of the argument?
Because that's a positive claim: 'The Universe is infinite.' Okey, fine: Maybe you're right. Why is that so? What empirical evidence do you have in support?

Quote Quote by Wallace
It could equally be argued that if one was to suggest the Universe is finite, then evidence for this must be presented.
Indeed. And in a previous post I made precisely this point. If I were prepared to make a rigorous claim that the Universe were finite, the onus would be on me to defend it.

Quote Quote by Wallace
However, neither of the above arguments I just made are reasonable. Clearly we do not have conclusive evidence either way. What we do know is that we have not reason to doubt that the cosmological principle applies for the extent of our observable universe, for this we do have hard evidence such as the isotropy of the CMB (the anisotropies are of course of great interest, but are very small in agreement with the cosmological principle), and the general isotropy of galaxies and QSO's seen in large redshift surveys.
Thanks for this response. But why isn't the onus on you to justify your claim: 'The universe is isotropic from Earth. Therefore, the universe is isotropic from every point in the observable universe.' That's a leap that does not seem to be entailed by the empirical evidence. The Bedouin would be wrong to say, 'The world looks desert-y in all directions to me. Therefore, the world looks desert-y from all spots on the Earth.' How does going from local isotropy (observationally true) to the cosmological principle not make the same mistake?

Quote Quote by Wallace
Given this data then, we can see that if the Universe is finite, it is clearly at least much bigger than the observable Universe, and for all intents and purposes is infinite. Assuming the Universe is infinite makes the equations easier to deal with, since there are less parameters.
But here's a technical question I asked earlier that has not been answered: Is there any amount of matter that would halt the expansion of the Universe (that is, such that the Universe asymtotically approaches a maximum volume)? If so, the Universe is identical to the observable universe (right?) and if there's a boundary, we could eventually find it (our progeny, that is). Other than the cosmological principle and (your) intuitions about simplicity, is there anything that weighs against a finite universe?

Quote Quote by Wallace
Theories are constructed based upon what is observed, and are made to be as simple as possible given those observations. This is why we assume the Universe is infinite, in full knowledge that this is not absolutely neccessarily the case.
Okey. Our inuitions just differ on this point. Thanks again for your very clear and informative reponse.


Quote Quote by pervect
One thing that pushes my buttons is Ordo's idea that SpaceTiger should have unlimited amounts of time to answer his (Ordo's) questions. In an ideal world, this would be nice, but people have to be able to live with the fact that moderators and mentors may actually have other things in their life other than PF, hard as it may be to believe....
Are you talking about someone else here? I never claimed any such thing; in fact, I've asked SpaceTiger to stay out of the discussion. Part of being a good scientist--indeed, a good human being--is honesty in argument, pervect.