If the universe is expanding at 73 kilometers per second per megaparsec, what is the fastest expansion speed that we can observe from Earth? I'm assuming that's the edge of the observable universe since that's the furthest point from Earth. Couldn't find anything so my calculations say 3.47c, going with the (73 km/s)/mpc, but I wanted to verify.

Radius of 46.5 billion light years converted to megaparsecs = 14,257. Multiply that by 73 gets you 1,040,761 km/s. Divide by 300,000 km/s gets you 3.47c


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    $\begingroup$ Sounds about right; and in light of the definitions of conformal time and global velocity, its being superluminal is no problem. $\endgroup$
    – J.G.
    Commented Oct 16, 2021 at 10:33

1 Answer 1


Your calculation is correct. With the Planck 2018 values:

Cosmic time = 13,787 Myear

Hubble constant = 67.66 (km/s)/Mpc

Particle horizon = 46.19 Gly = 14,162 Mpc

Current recession speed of the particle horizon = 3.20 c

Best regards.


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