A galaxy at the boundary of the Hubble sphere is receeding from us at the speed of light, right? If it emits a photon now, how long will it take to reach us?

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    $\begingroup$ I'm voting to close this question as off-topic because it feels like a physics question, not an astronomy question $\endgroup$
    – Rory Alsop
    Nov 21, 2018 at 14:44
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    $\begingroup$ It isn't. Expansion of the universe is not the same as speed of objects. Further, you seem to misunderstand special relativity: a photon moves at $c$ , period. $\endgroup$ Nov 21, 2018 at 15:55
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    $\begingroup$ @RoryAlsop astrophysics is on-topic on this site. This question should not be closed, but you should downvote it if it's not useful. $\endgroup$ Nov 21, 2018 at 22:53
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    $\begingroup$ It depends on the cosmological model, but in a universe with constant expansion the light emitted from the Hubble limit can by definition never reach us. As the evidence indicates an accelerating rate of expansion, the Hubble sphere is actually contracting, which means galaxies we can currently observe will eventually disappear over the horizon. Also, light at these distances becomes highly redshifted, so we would lose the ability to detect the photons well before they cease to reach us. $\endgroup$ Nov 21, 2018 at 23:13
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    $\begingroup$ It's also odd to downgrade this very interesting astronomy question. Just look at all the wrong answers already, it's clearly a subtle point that is actually quite interesting. Everything in astronomy is some other science too, calling this a physics question is not a valid criticism either. $\endgroup$
    – Ken G
    Nov 22, 2018 at 9:30

1 Answer 1


Actually, this is a perfectly good question if interpreted in a reasonable way, i.e., interpreted as asking how long will light take to get to us if emitted at comoving age 13.8 billion years from a galaxy whose comoving distance is currently increasing from us at rate c (and the Hubble law with H=70 tells us that comoving distance is 4.3 Gpc or 14 billion LY). That is not an infinite time, as that is not the definition of the Hubble sphere.

The answer to how long it would take, less than infinity, depends on the cosmology model, and is a little hard to calculate because cosmology calculators (such as http://www.astro.ucla.edu/~wright/DlttCalc.html) tend to focus on light reaching us now-- as that's what we see. For example, that calculator gives that a galaxy that is receding from us at rate c now would have had to emit light 9.25 billion years ago for us to be seeing it now, but that's not what was asked.

As mentioned, it is not easy to calculate the time it would take light emitted at the Hubble sphere now to get to us, but it is not infinite. The Hubble sphere would only be the edge of what we could ever see if the Hubble constant were constant with time (as in an accelerating universe completely dominated by a cosmological constant), but even though the expansion does seem to be accelerating, it is not yet completely dominated by a cosmological constant (assuming that is what is causing the acceleration). Hence, we would indeed eventually see the light emitted at the Hubble sphere now, but it might take a very long time, perhaps 50-100 billion years but that's just a guess without doing a real calculation with a full cosmological model.


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