# How long would a photon take to reach us if it's emitted from a galaxy receeding at c?

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?

• I'm voting to close this question as off-topic because it feels like a physics question, not an astronomy question Nov 21 '18 at 14:44
• 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. Nov 21 '18 at 15:55
• @RoryAlsop astrophysics is on-topic on this site. This question should not be closed, but you should downvote it if it's not useful. Nov 21 '18 at 22:53
• 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. Nov 21 '18 at 23:13
• 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. Nov 22 '18 at 9:30