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Infinite Universe Theory?

15 points by naveen99 ↗ HN
Is there any well developed theory of an infinite universe even if theoretical? Would an infinite universe have cosmic background radiation ? Would an infinite universe have most observers see far away objects moving away from them ?

14 comments

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nabla9
Yes.Current standard theories fit both finite and infinite universes. We don't know if the universe is finite or infinite. Only that observable universe is finite and expanding.

>Would an infinite universe have cosmic background radiation ?

Yes.

>Would an infinite universe have most observers see far away objects moving away from them ?

Yes.

Infinite universe would still have big bang, infinite early universe can be extremely dense, and infinite universe can have metric expansion.

naveen99
What about the differences between finite and infinite universes ? Would there be any ?
garmaine
Due to the speed of light, there would be no observable differences.
naveen99
there must be theoretical differences between finite and infinite universes.
garmaine
Which aren't observable because the end of the finite universe is beyond our relativistic sight horizon.
naveen99
I would think the timeline of the far future would be different in an infinite universe vs. a finite universe. Namely there would be no heat death in an infinite or closed universe.
garmaine
It’s admittedly non intuitive, but no there’s no difference. The reason is that space isn’t expanding into something else, but rather space time itself is stretching. Which reduces the energy density, which cools things down. In exactly the same way whether the universe is finite or infinite.
naveen99
I mean a theoretical universe without this space time expansion. Could it not be compatible with distant objects moving further away without space time expansion ?

Besides what time is spacetime expanding over ?

garmaine
It’s not expanding over anything. Expansion is an intrinsic property.
MrGuts
Check out Olber's Paradox. Basically, if the Universe were infinite, any line of sight would eventually intersect a star. Since radiation decreases in intensity with the square of the distance, but the area of "shells" of the universal volume increase with square of the radius, the light level of each shell would be roughly the same throughout the volume. In other words, everywhere you looked would be the temperature of the surface of a star, evenly paved in your sky. The fact that it is dark at night speaks against an infinite Universe.
chrispeel
I think the original question was about infinite spatial extent. One can certainly have infinite space which has existed for only a finite time since the big bang. Your argument using Olber's Paradox only argues for a finite time (say since the Big Bang), and says nothing about infinite space.
MrGuts
If we had an infinite space constructed in a finite time, then surely there would be a history of the sky getting hotter as more radiating matter is included in our causal horizon (light cone). Instead we see a sky at a near homogeneous temperature, meaning that every part of the Universe must once have been very close to every other part for a time sufficient to reach equilibrium, and then cooled down. The temperature history of the Universe appears to speak against infinite space.
naveen99
but we have cosmic wave background radiation... and matter may intercept enough of the radiation to make it dark.
MrGuts
Olber's Paradox covers this case also. Obscuring matter would not make the sky dark. Any matter in your line of sight would also itself "observe" a hot sky, just as you do. Any such "observing matter" would reach equilibrium with incoming energy by re-radiating that energy at the same temperature. An infinite Universe looks like the surface of a star unrolled in your sky. All matter in the volume is baked, and re-radiates, at this intensity. I grant you only a moment to witness this spectacle, before you yourself reach equilibrium temperature with your sky.