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Just watched the 'How Big is the Universe?' TV program and it indicated that the universe is believed to be infinite in size. They used an approach involving measuring the internal angles of triangles. Or something like that anyway.

But hang on, the Universe has a known starting time and size followed by a finite time expanding at a finite speed. So surely, by definition, it cannot be infinite? I can accept it is honking big but by definition it cannot be infinite. Are they misleading me?

Qmechanic
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Phil Wright
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  • But what if the universe was already infinite at the 'known starting time' – QCD_IS_GOOD Feb 22 '15 at 07:39
  • Well I thought everyone said the Big Bang started as a singularity which implies it was pretty darn small. – Phil Wright Feb 22 '15 at 07:40
  • The Big Bang did not happen at a point. The singularity at the Big Bang was not a spacetime point. The spacetime geometry is undefined at the Big Bang, which is what we mean by the term singular. – John Rennie Feb 22 '15 at 07:50
  • Possible duplicates: http://physics.stackexchange.com/q/9419/2451 , http://physics.stackexchange.com/q/1915/2451 and links therein. – Qmechanic Feb 22 '15 at 09:38
  • That the universe had any "start" time is highly controversial: Two major cosmological models ("Conformal Cyclic Cosmology", by a winner of 2020's Nobel Prize in physics, Roger Penrose, and the torsion-based cosmological model by Nikodem Poplawski, described in numerous 2010-2021 papers whose preprints can be found by his name on the "Arxiv" website) are past- and future-eternal. – Edouard Jun 15 '21 at 17:15

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There is difference between what there is and what we observe. We do observe finite amount of matter and energy exactly because it all started somewhere and moves away, while speed of light is pretty constant. In that, most useful, sense universe is finite. We can speculate about size of what "there is", but it still will be baseless theories.

aaaaa says reinstate Monica
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  • If the universe is finite, then it is theoretically possible that at some time in the future this can be confirmed by observation. This future time would occur when the observable universe grows to include the entire finite universe. This observation will confirm that each pair of some distance points seen in opposite directions will be identifiable as the same object. If the universe is infinite, then such pairs can never be observed. – Buzz Jun 23 '21 at 16:28