1 upvotes, 2 direct replies (showing 2)
View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science
How fast is the universe expanding at its furthest point? I know the rate is different based on how far away something is from us and that it is some speed greater than light. I've heard c^3, is that accurate?
Comment by dubcek_moo at 14/09/2022 at 22:42 UTC
4 upvotes, 1 direct replies
c^(3) is not a speed. It would have units of m^(3) / s^(3) and a speed only m / s.
Ok, so the Hubble Law says the expansion goes faster with distance, so this says expansion speeds can be faster than light.
Let me drop some subtleties. Speed of light as a speed limit is basic to SPECIAL relativity. When we talk about expansion speed, we're not talking about motion of something moving past you ("locally") which has that speed limit but the expansion of space itself. In A SENSE, faraway galaxies can move away faster than c, and we may be even able to see some of them--the light moves towards us, towards slower expanding regions that don't carry it away so fast.
But the real correct answer is that in General Relativity, speeds ONLY have local meaning. It's meaningless to compare a speed here and far away. Look up parallel transport[1] in curved space (or spacetime). Velocity is a vector, and moving a vector through a curved space can give you a different result depending even on the path with which you move it through.
1: https://en.wikipedia.org/wiki/Parallel_transport
Comment by Wooden_Ad_3096 at 14/09/2022 at 22:17 UTC
0 upvotes, 1 direct replies
https://www.sciencedaily.com/releases/2021/03/210308165239.htm
“73.3 kilometers per second per megaparsec, give or take 2.5 km/sec/Mpc -- that lies in the middle of three other good estimates, including the gold standard estimate from Type Ia supernovae. This means that for every megaparsec -- 3.3 million light years, or 3 billion trillion kilometers -- from Earth, the universe is expanding an extra 73.3 ±2.5 kilometers per second. The average from the three other techniques is 73.5 ±1.4 km/sec/Mpc.”