# Why does the Hubble flow does not carry photons with it?

I have this naive question. We know that the expansion of the universe "takes" or "carries" the galaxies with it, separating them, but this doesn't apply to photons. My guess is that it has to do with the fact that photons are massless (i.e. they always move with c), but I am not sure and would like a more physical explanation or insight.

• Why do you say "this doesn't apply to photons"? From astronomy.stackexchange.com/a/39379/16685 "The hitherto most distant observed galaxy, GN-z11, has a redshift of $z=11.09$. It was only $2.7\,\mathrm{Glyr}$ from us when it emitted the light we see today, but in the $13.4\,\mathrm{Gyr}$ it took the light to reach us (most of the age of the Universe), GN-z11 moved out to a current distance of $32.2\,\mathrm{Glyr}$!" – PM 2Ring Oct 18 '20 at 23:45
• Thank you, good way to see it! – xkudsraw Oct 22 '20 at 15:44

## 1 Answer

Expansion isn't some mysterious force that "carries" galaxies. There's a very common misconception that it is, but it's not true. The expansion is just the recessional motion of galaxies and other matter.

Light does participate in the expansion. Even though it moves at a constant speed (and therefore can't move with the galaxies as you correctly observed), it still thins out with time. This is counterintuitive, but it happens even in a special relativistic cosmological model (the Milne model, which is a special case of FLRW cosmology). If you think of the big bang as an explosion of light bulbs, the bulbs that you see from a particular era in the universe's history have higher and higher relative speeds as time goes on (since the light from nearer, slower bulbs has already reached and passed you), and therefore are more redshifted, and therefore emit photons at a lower rate (since redshift affects the perceived rate of every process). So the relic photons from a particular era of the universe's history get more redshifted and less dense as time goes on.

• You say “expansion is just the recessional motion of galaxies and other matter.” This is fundamentally wrong on two levels. Firstly, you’re conflating the observation (recession) with the underlying cause (metric expansion of space). Secondly, the expansion of space is NOT due to a Big Bang “explosion”, it’s a change in the scale of the metric of spacetime. – Chappo Hasn't Forgotten Monica Oct 19 '20 at 0:13
• @ChappoHasn'tForgottenMonica As I said, it's a very common misconception. I know Davis & Lineweaver missed it; they aren't infallible. As usual in GR it's a result of treating coordinates as though they have physical meaning. It's the same thing that leads people to think that it takes forever to cross an event horizon, etc. The Milne model is a good source of counterexamples to misconceptions about cosmology, like Rindler coordinates are with event horizons. In the Milne model $a(t)\propto t$, but it's just Minkowski space. You can show cosmological and SR redshifts are equal, etc. – benrg Oct 19 '20 at 0:55