<p>The spacetime geometry of the universe is determined by the <em>total</em> energy density, which includes contributions from matter (including dark matter), dark energy, radiation and curvature (if non-zero).</p>
<p>In the present-day universe the sum of these, dominated by matter and dark energy, adds up to something that is consistent with the critical density and the universe must be very nearly flat.</p>
<p>I think the particular observations that are highly constraining are detailed modelling of the cosmic microwave background angular fluctuations, combined with baryon acoustic oscillation data. These suggest that any curvature energy density is limited to 0.2% of the critical density (1 sigma) - <a href="https://arxiv.org/abs/1807.06209" rel="nofollow noreferrer">Planck collaboration 2018</a>.</p>

CC BY-SA 4.0
2021-12-26T21:28:30.123Z
2021-12-27T21:52:22.723Z
2021-12-27T21:52:22.723Z
ProfRob
<p>What about astronomical observations makes scientists believe our universe is flat, at least as far as they can tell?  Despite the critical Friedmann density being less than one, plus the existence of dark energy, etc.?</p>

CC BY-SA 4.0
2021-11-26T12:00:07.427Z
Kurt, before you ask more questions, would you mind going through your previously asked questions and mark the responses that best answered them as "Accepted"? This is not only to give the people who spend time explaining some credit for their effort, but also to show future readers what is the best answer in your opinion. Best, Peter.
pela
CC BY-SA 4.0
2021-11-26T14:37:47.573Z
The critical *energy density* includes the contribution from dark energy, which is about 70% of the total. (Thinking of it in terms of just "mass" is a bit misleading.)
Peter Erwin
CC BY-SA 4.0
2021-11-26T18:33:49.987Z
does this answer your question? https://www.forbes.com/sites/startswithabang/2021/03/05/ask-ethan-why-is-the-universe-flat/?sh=1675e319365e
Daddy Kropotkin
<observational-astronomy><cosmology><observable-universe><density><friedmann-equations>
If the ratio of actual total mass to critical mass (Ω) in the universe is only about 0.3, why do cosmologists believe it is flat, as far as we see?
Kurt Hikes
# If the ratio of actual total mass to critical mass (Ω) in the universe is only about 0.3, why do cosmologists believe it is flat, as far as we see?

(Asked by: username_0 on 2021-11-26)

What about astronomical observations makes scientists believe our universe is flat, at least as far as they can tell? Despite the critical Friedmann density being less than one, plus the existence of dark energy, etc.?

**Tags:** observational-astronomy, cosmology, observable-universe, density, friedmann-equations

> **Comments:**
> **username_1**: Kurt, before you ask more questions, would you mind going through your previously asked questions and mark the responses that best answered them as "Accepted"? This is not only to give the people who spend time explaining some credit for their effort, but also to show future readers what is the best answer in your opinion. Best, Peter.  
> **username_2**: The critical *energy density* includes the contribution from dark energy, which is about 70% of the total. (Thinking of it in terms of just "mass" is a bit misleading.)  
> **username_3**: does this answer your question? https://www.forbes.com/sites/startswithabang/2021/03/05/ask-ethan-why-is-the-universe-flat/?sh=1675e319365e  

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## 1 answers

### Answer by username_4 on 2021-12-26. Score: 2

The spacetime geometry of the universe is determined by the *total* energy density, which includes contributions from matter (including dark matter), dark energy, radiation and curvature (if non-zero).

In the present-day universe the sum of these, dominated by matter and dark energy, adds up to something that is consistent with the critical density and the universe must be very nearly flat.

I think the particular observations that are highly constraining are detailed modelling of the cosmic microwave background angular fluctuations, combined with baryon acoustic oscillation data. These suggest that any curvature energy density is limited to 0.2% of the critical density (1 sigma) - [Planck collaboration 2018](https://arxiv.org/abs/1807.06209).


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