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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

by MrGuts 5y ago

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 5y ago
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 5y ago
  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 5y ago
but we have cosmic wave background radiation... and matter may intercept enough of the radiation to make it dark.
- MrGuts 5y ago
  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.

by MrGuts 5y ago

chrispeel 5y ago

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 5y ago
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 5y ago

but we have cosmic wave background radiation... and matter may intercept enough of the radiation to make it dark.

MrGuts 5y ago
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.