### Which star / galaxy is moving away from us the fastest?

• I know that we have measured the rate a lot of stars and galaxies move away from us using the doppler shift, and I know that the further a star / galaxy is the faster they accelerate away from us due to space inflation. I'm wondering, did anyone categorise which star / galaxy is moving towards us / away from us the fastest? How fast are they going in relation to our solar system?

Another way to ask this question might be "What object has the highest observed cosmological redshift *z*?"

I would assume the stars at the very edge of the observable universe are moving away the fastest, if indeed the farther they are from *us* they are moving away faster

The closer a galaxy is to satisfying your criterion, the more difficult it is to detect. Given any object that is currently the one with the greatest cosmological redshift, we are essentially guaranteed that there are others with a greater cosmological redshift, which is why we can't detect them. *the faster they accelerate away from us due to space inflation.* The quantity to talk about in Hubble's law is velocity, not acceleration, and none of this has anything to do with inflation, which is a different phenomenon. We actually don't know for sure whether inflation happened.

2 years ago

When a galaxy recedes from us, the light we see from it is redshifted. For galaxies at cosmological distances, this redshift is fundamentally different from a Doppler shift; whereas the latter is due to a velocity difference between the emitter and the receiver, a cosmological redshift is due to photons traveling through an expanding space$$^\dagger$$.

Hence, as @uhoh comments, your question is equivalent to asking "Which galaxy has the highest measured redshift?". Redshift is arguably the single most important concept in astronomy, and indeed we do catalogue the redshift of all galaxies, if possible. For our adopted cosmological model, the cosmological redshift can be translated to both a recession velocity, a distance, and an age of the Universe when the light was emitted.

The answer is GN-z11 (Oesch et al. 2016) which has a redshift of $$z=11.09$$. This corresponds to a distance of $$d = 32.2\,\mathrm{Glyr}$$ (i.e. billion light-years), and hence, by Hubble's law, to a recession velocity of
$$v = H_0\,d = 670\,000\,\mathrm{km}\,\mathrm{s}^{-1},$$
or more than twice the speed of light. Furthermore, the light we see today was emitted when the Universe was only 410 Myr (i.e. million years), or 3% of its current age.

You may think that "twice the speed of light" violates the theory of relativity, but this velocity is not a velocity through space. Both our galaxy (the Milky Way) and GN-z11 move through space at modest velocities of a few 100 km/s. The recession is merely due to space expanding, and space is allowed to expand at whatever rate is wishes.

$$^\dagger$$"Fundamentally different" might be a too strong statement, since the cosmological redshift can be interpreted as infinitely many infinitesimally small Doppler shifts. However, a hypothetical scenario that emphasizes the difference between the two types of redshifts is the following:

If an emitter and an observer are stationary wrt. each other when the emitter emits a photon, then start moving away from each other while the photon is traveling, then stop again before the observer receives the photon, the observer would measure zero redshift.

On the other hand, if space is static when the emitter emits the photon, then while the photon is traveling suddenly expands by a factor of, say, four, and then is static again when the observer receives the photon, the observer would measure a redshift of $$z+1 = 4$$.

Wouldn't there be some unknown number of galaxies beyond the cosmological horizon that are receding even faster? All one could answer of course is the galaxy receding fastest among all known galaxies.

@nasch Oh yes, definitely! Unless our understanding of the Universe is severely flawed, it looks (on average) the same everywhere and in all directions. I'm only describing the most distant one _observed_, and when we observe the distant Universe we're looking to the past, so we see GN-z11 as it looked more than 13 Gyr ago. More galaxies are "almost surely" beyond GN-z11, and even beyond the edge of the observable Universe. The Universe may even be infinite, in which case their is no limit to how fast a galaxy recedes.

*Both our galaxy (the Milky Way) and GN-z11 move through space at modest velocities of a few 100 km/s.* This is not quite right. GR reduces to SR at small scales, and SR says there is no way to define how fast something is moving "through space." I think what you probably mean is that 100 km/s is the speed relative to the Hubble flow, which is basically just the average state of motion of nearby matter.

@BenCrowell You're right that it's relative to the Hubble flow. It's also relative to most other sensible inertial frames you can think of, unless you choose the inertial frame of some individual, fast-moving particle. I think it makes sense to say that galaxies, stars, and bicycles move through space. You're of course free to choose some reference frame where a galaxy, or a bike, _doesn't_ move, but then you just make other galaxies, or bikes, move at a somewhat different velocity.

So if the recession velocity from a galaxy is greater than the speed of light, we will be unable to ever see the photons emitted from that galaxy right now?

@NotTelling Good question! This is indeed what many people think — even Neil deGrasse Tyson gets this wrong in his new book. All galaxies more distant than $d=c/H_0\simeq$ 14.4 Glyr recede faster than light. But our _event horizon_ — the maximum distance from which we may receive a signal sent right now — is at a distance of 16.5 Glyr. So all galaxies lying in the shell between 14.4 and 16.5 Glyr may emit a photon today which we in the future may observe (in principle; in practice it will take many billions of years, and the photon will be heavily redshifted).

@pela I cannot understand how it is possible to reach the light of an galaxy that is moving away faster than light? What determines our event horizon? Age of the universe? Would you explain it with more details please.. Space expands faster then light, but light manages to reach us? I'm very confused..