What about the volume of the observable universe?
Can we find it?
And what is the result in cubic light years?
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$\begingroup$ Th volume of a sphere is (4/3) · π · r3 (cubed, not times 3), so set r to be the radius of the visible universe and off you go! $\endgroup$– simon at rclJul 24, 2021 at 9:59
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$\begingroup$ The radius is 46 billion years?How did they calculated that if i may ask? thanks $\endgroup$– Panagiotis MakrisJul 24, 2021 at 10:10
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$\begingroup$ @simon, not sure it is that simple, or if the question has a single answer since space has expanded. Take two galaxies at a light travel distance of 11 billion years, and an angular separation of 60 degrees. What is the distance between the galaxies? Do we mean "light travel distance or "co-moving distance" and do we mean 11 billion years ago or "now"... $\endgroup$– James KJul 24, 2021 at 10:28
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1$\begingroup$ @JamesK - indeed. I know enough to know the question of what r for the universe is is at best ambiguous and at worst not defined or unanswerable. I could have put some emphasis on the difficulty assigning a value to r, but didn't know what level of complexity is appropriate, so just left it. Possibly not my most helpful comment. $\endgroup$– simon at rclJul 24, 2021 at 10:45
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1$\begingroup$ The comoving radius — which by definition coincides with the physical radius today, i.e. the radius that you're interested in — is calculated by integrating the Friedmann equation, using observed values of the Hubble constant $H_0$ and the density parameters $\Omega_{\mathrm{m},\Lambda,\mathrm{k},\mathrm{r}}$. The result is some 46 Glyr. The accuracy of that number depends on which dataset you trust the most. E.g. Planck 2015 parameters give you 46.3, while 2018 data give you almost 47 Glyr. But including BAO and lensing data again modifies the result. $\endgroup$– pelaJul 25, 2021 at 13:16
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