<p>From what I know I don't think they have categorized that this specific galaxy, for example, is the fastest one moving away from the Milky Way Galaxy. There are two reasons for this:</p>
<ul>
<li>First of all, this isn't really that important. Measuring which galaxy is moving that fastest away from us can have some implications but not a whole lot. Scientists are however interested in the fastest speed any galaxy can move away from us. That does not, of course, violate the Theory of Relativity. And that speed is faster than the speed of light. Here is some extra info:</li>
</ul>
<blockquote>
<p>The Hubble constant is the measure of how fast the Universe is expanding today and its value has been measured to be 70 km/s per Megaparsec (a parsec is just a unit of distance equal to about 3.26 light-years, and a Megaparsec is a million parsecs). This means that on average, for every Megaparsec two galaxies are separated by, they are moving away from each other by 70 km/s. Therefore, to be moving away from each other at the speed of light, two galaxies would need to be separated by a distance of about 4,300 million parsecs. This is smaller than the radius of the observable Universe, therefore not only are there galaxies in the Universe that are moving away from us faster than light, but we can still see them!</p>
<p>Source: <a href="http://curious.astro.cornell.edu/legal-information/104-the-universe/cosmology-and-the-big-bang/expansion-of-the-universe/1066-can-two-galaxies-move-away-from-each-other-faster-than-light-intermediate" rel="nofollow noreferrer">http://curious.astro.cornell.edu/legal-information/104-the-universe/cosmology-and-the-big-bang/expansion-of-the-universe/1066-can-two-galaxies-move-away-from-each-other-faster-than-light-intermediate</a></p>
</blockquote>
<ul>
<li>Second, we can't really be sure even if we research really hard, because every day we detect new objects circulating in our solar system, and for galaxies, we are talking about mega-big distances.</li>
</ul>
<p><strong>Conclusion:</strong> So it is possible, but it is really time-consuming and abstract in a way, so it isn't something scientists are looking into today. But if you want some galaxies which are moving really fast away from us you can search the internet or read some books and you will find a lot. Of course, there is this certain galaxy which at the moment is categorized as the fastest or most-redshifted (if you understand what I'm saying) that we have detected (remark: detected) and it is <a href="https://en.wikipedia.org/wiki/GN-z11" rel="nofollow noreferrer">GN-Z11</a>. It is the most distant found galaxy in the whole observable universe, so the fact that it is the fastest moving away from us makes sense.</p>

CC BY-SA 4.0
2019-07-09T07:40:53.040Z
Something more, @pela, also puts the speed of GN-Z11 which is nice. Good work, @pela!
Gaurav Mall
CC BY-SA 4.0
2019-07-09T07:45:26.780Z
Hi Guarav. Thanks! You're right that _velocity_ is not really an important concept for cosmological distances, but it's not true that we don't know, nor that it's abstract. It might be time-consuming, but redshift is such an important concept that it's worthwhile. An once we know its redshift, we know exactly the recession velocity of a galaxy (although of course there is some uncertainty in the value of the Hubble constant used to calculate the velocity).
pela
CC BY-SA 4.0
2019-07-09T07:47:23.773Z
Thanks, @pela for the reply. Yes, I should have used different words there, like not in interest I think. Anyways I agree with your point about the redshift :)
Gaurav Mall
CC BY-SA 4.0
2019-07-09T07:47:41.997Z
Also, for low-redshift objects, it's actually quite normal to quote velocity rather than redshift (because peculiar velocities may dominate cosmological).
pela
CC BY-SA 4.0
2019-07-09T08:21:47.060Z
We use both, because they're completely interchangeable, but it's custom to quote velocities rather than redshift for $z\lesssim0.01$–$0.1$. This is probably to emphasize that the recession velocity is not completely cosmological; galaxies move _through_ space with velocities of 100-1000 km/s, so this so-called _peculiar_ velocity may dominate cosmological velocities or at least be significant for low redshifts.
pela
CC BY-SA 4.0
2019-07-09T08:22:54.523Z
Oh, so now I understand it better. Thanks for the answer!
Gaurav Mall
CC BY-SA 4.0
2019-07-09T09:44:08.163Z
You're welcome! Actually, right now I'm drinking coffee with the guy who found GN-z11 :)
pela
CC BY-SA 4.0
2019-07-09T09:44:52.917Z
Haha, amazing :-)
Gaurav Mall
CC BY-SA 4.0
2019-07-09T07:37:29.763Z
2019-08-06T09:35:38.803Z
2020-06-17T09:47:40.653Z
Gaurav Mall
<p>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<span class="math-container">$^\dagger$</span>.</p>
<p>Hence, as @uhoh comments, your question is equivalent to asking &quot;Which galaxy has the highest measured redshift?&quot;. 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.</p>
<p>The answer is <a href="https://en.wikipedia.org/wiki/GN-z11" rel="nofollow noreferrer">GN-z11</a> (<a href="https://arxiv.org/abs/1603.00461" rel="nofollow noreferrer">Oesch et al. 2016</a>) which has a redshift of <span class="math-container">$z=11.09$</span>. This corresponds to a distance of <span class="math-container">$d = 32.2\,\mathrm{Glyr}$</span> (i.e. billion light-years), and hence, by Hubble's law, to a recession velocity of
<span class="math-container">$$
v = H_0\,d = 670\,000\,\mathrm{km}\,\mathrm{s}^{-1},
$$</span>
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.</p>
<p>You may think that &quot;twice the speed of light&quot; violates the theory of relativity, but this velocity is not a velocity <em>through</em> space. Both our galaxy (the Milky Way) and GN-z11 move <em>through</em> 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.</p>
<hr />
<p><span class="math-container">$^\dagger$</span><sub>&quot;Fundamentally different&quot; 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:<br>
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.<br>
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 <span class="math-container">$z+1 = 4$</span>.</sub></p>

CC BY-SA 4.0
2019-07-09T16:28:28.007Z
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
CC BY-SA 4.0
2019-07-09T20:40:27.880Z
@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.
pela
CC BY-SA 4.0
2019-07-10T07:32:46.577Z
@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.
pela
CC BY-SA 4.0
2019-07-10T11:02:50.140Z
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
CC BY-SA 4.0
2019-07-10T11:36:52.053Z
@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
CC BY-SA 4.0
2019-07-10T15:08:27.270Z
@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..
Koray
CC BY-SA 4.0
2019-07-09T07:33:13.520Z
2021-03-31T10:52:11.633Z
2021-03-31T10:52:11.633Z
pela
<p>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?</p>

CC BY-SA 4.0
2019-07-09T07:04:59.823Z
Another way to ask this question might be "What object has the highest observed [cosmological redshift *z*](http://astronomy.swin.edu.au/cosmos/c/cosmological+redshift)?"
uhoh
CC BY-SA 4.0
2019-07-10T00:20:28.547Z
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
user17915
<star><galaxy><redshift><speed><movement>
Which star / galaxy is moving away from us the fastest?
ThePiachu
# Which star / galaxy is moving away from us the fastest?

(Asked by: username_0 on 2019-07-09)

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?

**Tags:** star, galaxy, redshift, speed, movement

> **Comments:**
> **username_1**: Another way to ask this question might be "What object has the highest observed [cosmological redshift *z*](http://astronomy.swin.edu.au/cosmos/c/cosmological+redshift)?"  
> **username_2**: 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  

---

## 2 answers

### Answer by username_4 on 2019-07-09. Score: 20 | Accepted answer

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 @username_1 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](https://en.wikipedia.org/wiki/GN-z11) ([Oesch et al. 2016](https://arxiv.org/abs/1603.00461)) 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 $z+1 = 4$.

> **Comments:**
> **username_5**: 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.  
> **username_4**: @username_5 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.  
> **username_4**: @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.  
> **username_6**: 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?  
> **username_4**: @username_6 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).  
> **username_7**: @username_4 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..  

---

### Answer by username_3 on 2019-07-09. Score: 4

From what I know I don't think they have categorized that this specific galaxy, for example, is the fastest one moving away from the Milky Way Galaxy. There are two reasons for this:

* First of all, this isn't really that important. Measuring which galaxy is moving that fastest away from us can have some implications but not a whole lot. Scientists are however interested in the fastest speed any galaxy can move away from us. That does not, of course, violate the Theory of Relativity. And that speed is faster than the speed of light. Here is some extra info:

> The Hubble constant is the measure of how fast the Universe is expanding today and its value has been measured to be 70 km/s per Megaparsec (a parsec is just a unit of distance equal to about 3.26 light-years, and a Megaparsec is a million parsecs). This means that on average, for every Megaparsec two galaxies are separated by, they are moving away from each other by 70 km/s. Therefore, to be moving away from each other at the speed of light, two galaxies would need to be separated by a distance of about 4,300 million parsecs. This is smaller than the radius of the observable Universe, therefore not only are there galaxies in the Universe that are moving away from us faster than light, but we can still see them!
>
> Source: <http://curious.astro.cornell.edu/legal-information/104-the-universe/cosmology-and-the-big-bang/expansion-of-the-universe/1066-can-two-galaxies-move-away-from-each-other-faster-than-light-intermediate>

* Second, we can't really be sure even if we research really hard, because every day we detect new objects circulating in our solar system, and for galaxies, we are talking about mega-big distances.

**Conclusion:** So it is possible, but it is really time-consuming and abstract in a way, so it isn't something scientists are looking into today. But if you want some galaxies which are moving really fast away from us you can search the internet or read some books and you will find a lot. Of course, there is this certain galaxy which at the moment is categorized as the fastest or most-redshifted (if you understand what I'm saying) that we have detected (remark: detected) and it is [GN-Z11](https://en.wikipedia.org/wiki/GN-z11). It is the most distant found galaxy in the whole observable universe, so the fact that it is the fastest moving away from us makes sense.

> **Comments:**
> **username_3**: Something more, @username_4, also puts the speed of GN-Z11 which is nice. Good work, @username_4!  
> **username_4**: Hi Guarav. Thanks! You're right that _velocity_ is not really an important concept for cosmological distances, but it's not true that we don't know, nor that it's abstract. It might be time-consuming, but redshift is such an important concept that it's worthwhile. An once we know its redshift, we know exactly the recession velocity of a galaxy (although of course there is some uncertainty in the value of the Hubble constant used to calculate the velocity).  
> **username_3**: Thanks, @username_4 for the reply. Yes, I should have used different words there, like not in interest I think. Anyways I agree with your point about the redshift :)  
> **username_4**: Also, for low-redshift objects, it's actually quite normal to quote velocity rather than redshift (because peculiar velocities may dominate cosmological).  
> **username_4**: We use both, because they're completely interchangeable, but it's custom to quote velocities rather than redshift for $z\lesssim0.01$–$0.1$. This is probably to emphasize that the recession velocity is not completely cosmological; galaxies move _through_ space with velocities of 100-1000 km/s, so this so-called _peculiar_ velocity may dominate cosmological velocities or at least be significant for low redshifts.  
> **username_3**: Oh, so now I understand it better. Thanks for the answer!  
> **username_4**: You're welcome! Actually, right now I'm drinking coffee with the guy who found GN-z11 :)  
> **username_3**: Haha, amazing :-)  

---

