<p>The relative recent <a href="https://en.wikipedia.org/wiki/WMAP" rel="nofollow noreferrer">WMAP</a> result shows, the Universe seems infinite.</p>

<p>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.</p>

<p>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 <a href="https://physics.stackexchange.com/q/88786/32426">at least 300billion light years</a>.</p>

CC BY-SA 3.0
2018-01-24T11:07:51.843Z
Note that WMAP shows that the Universe is likely flat, not necessarily infinite. There are topologies that are both flat and finite.
Sir Cumference
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2018-01-24T11:32:06.723Z
@SirCumference Yes, I agree (I didn't want to detail it so deeply).
peterh
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2018-01-23T21:29:15.893Z
2018-01-23T21:29:15.893Z
peterh
<p>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?</p>

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2018-01-23T09:19:48.120Z
You should split this in 2 questions.
J. Chomel
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2018-01-23T16:42:25.223Z
"Does infinite universe entails infinite black holes or infinite stars?" If the cosmological principle holds, then yes.
Sir Cumference
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2018-01-24T10:26:01.397Z
@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)?
peterh
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2018-01-24T11:10:13.553Z
@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.
Sir Cumference
<black-hole><cosmology><infinite>
Measuring the size and the geometry of Universe
Gauti
# Measuring the size and the geometry of Universe

(Asked by: username_0 on 2018-01-23)

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?

**Tags:** black-hole, cosmology, infinite
> **Comments:**
> **username_1**: You should split this in 2 questions.  
> **username_2**: "Does infinite universe entails infinite black holes or infinite stars?" If the cosmological principle holds, then yes.  
> **username_3**: @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)?  
> **username_2**: @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.  

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## 1 answers

### Answer by username_3 on 2018-01-23. Score: 2

The relative recent [WMAP](https://en.wikipedia.org/wiki/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](https://physics.stackexchange.com/q/88786/32426).

> **Comments:**
> **username_2**: Note that WMAP shows that the Universe is likely flat, not necessarily infinite. There are topologies that are both flat and finite.  
> **username_3**: @SirCumference Yes, I agree (I didn't want to detail it so deeply).  

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