When we would be able to tell with surety that universe is finite or infinite? How can we exactly determine the density of the universe so, that we compare it with critical density, in order to determine its geometry? Does the infinite Universe entails infinite black holes or infinite stars?

  • $\begingroup$ You should split this in 2 questions. $\endgroup$ – J. Chomel Jan 23 '18 at 9:19
  • $\begingroup$ I also helps helps if you search and read a little (there are plenty of sites discussing the infinity of the universe), then ask what you don't understand. $\endgroup$ – user1569 Jan 23 '18 at 12:12
  • $\begingroup$ "Does infinite universe entails infinite black holes or infinite stars?" If the cosmological principle holds, then yes. $\endgroup$ – Sir Cumference Jan 23 '18 at 16:42
  • $\begingroup$ @SirCumference 1) What if it has some cyclic topology? 2) What if the universal constants are 100billion light years away a little bit different and there are no stars (for example, because the diproton is stable)? $\endgroup$ – peterh Jan 24 '18 at 10:26
  • $\begingroup$ @peterh 1) To my knowledge, there is no such thing as a "cyclic topology", unless you mean one that collapses (positively curved). If that's what you mean, then the Universe would be finite. 2) That's why I specifically said "if the cosmological principle holds". If stars stop existing some distance away from us, then that would be an inhomogeneity. $\endgroup$ – Sir Cumference Jan 24 '18 at 11:10

The relative recent WMAP result shows, the Universe seems infinite.

However, the "is the Universe infinite" question has a hidden problem: it doesn't matter, how far away can we see, we can't be sure, if anything out of it is finite or not. :-) Thus, it is about a definitely unknowable thing.

The WMAP measured essentially the curvature of the Universe and it found a zero curvature (i.e. it seems to be planar). If there is a minimal curvature, below a measurement precision, and we have a cyclic, spherical geometry, then its radius must be at least 300billion light years.

  • $\begingroup$ Note that WMAP shows that the Universe is likely flat, not necessarily infinite. There are topologies that are both flat and finite. $\endgroup$ – Sir Cumference Jan 24 '18 at 11:07
  • $\begingroup$ @SirCumference Yes, I agree (I didn't want to detail it so deeply). $\endgroup$ – peterh Jan 24 '18 at 11:32

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