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What about the volume of the observable universe?

Can we find it?

And what is the result in cubic light years?

James K
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Panagiotis Makris
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  • 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! – simon at rcl Jul 24 '21 at 09:59
  • The radius is 46 billion years?How did they calculated that if i may ask? thanks – Panagiotis Makris Jul 24 '21 at 10:10
  • @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"... – James K Jul 24 '21 at 10:28
  • The 46 billion value is the co-moving distance. The whole notion of "distance" becomes complex Read https://en.wikipedia.org/wiki/Distance_measures_(cosmology). The value 46 billion is calculated based on observations and using a model of the expanding universe. – James K Jul 24 '21 at 10:31
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    @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. – simon at rcl Jul 24 '21 at 10:45
  • @simonatrcl so the radius of the universe is not defined very well right? I read on wikipedia that the volume of the observable universe is 10 ^32 cubic light years.Hows that sounds? – Panagiotis Makris Jul 24 '21 at 15:05
  • @simonatrcl so space is expanding so no absolute term about the radius. I see.The indication above is based on the figure of 46 b light year radius.Could it be smaller or not? – Panagiotis Makris Jul 24 '21 at 15:31
  • @JamesK In standard cosmology, comoving distance and proper distance are two closely related distance measures used by cosmologists to measure distances between objects; the comoving distance is the proper distance at the present time.----so 46 billion light years at present times.How accurate is that? – Panagiotis Makris Jul 24 '21 at 15:42
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    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. – pela Jul 25 '21 at 13:16
  • @pela thanks a lot for your answer!! – Panagiotis Makris Jul 26 '21 at 06:45
  • You're welcome :) I realized I basically answered this question before, so I'll vote to close your question, linking to the other one (because we generally try to have only one version of each question). – pela Jul 26 '21 at 08:25

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