<p>No. In fact the opposite is the case.</p>

<p>(See the last paragraph for an intuitive explanation.)</p>

<p>It is a common misbelief that galaxies receding faster than the speed of light are not visible to us. This is not the case; we easily see galaxies moving at superluminal velocities. This does not — as I think most people would think — contradict the theory of relativity, which says that nothing can travel <em>through</em> space faster than $c$. Galaxies do not travel <em>through</em> space (except with <a href="https://astronomy.stackexchange.com/a/22653/5264">small velocities of 100-1000 km/s</a>); rather, space itself is expanding, causing distances between the galaxies to increase.</p>

<h2>We see "super-luminal" galaxies</h2>

<p>The recession velocity $v_\mathrm{rec}$ of a galaxy is given by Hubble's Law:
$$
v_\mathrm{rec} = H_0 \, d,
$$
where $H_0 \simeq 67.8\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$ is the Hubble constant (<a href="https://arxiv.org/abs/1502.01589" rel="noreferrer">Planck Collaboration et al. 2016</a>). This law implies that galaxies farther away than
$$
r_\mathrm{HS} \equiv \frac{c}{H_0} \simeq 4400\,\mathrm{Mpc} \simeq 14.4 \, \mathrm{Gly}\,\,\mathrm{("\!\!Giga\mbox{-}lightyears\!\!")}
$$
recede faster than $c$. Here, the subscript "HS" is chosen because the ragion within which galaxies recede slower than $c$ is called the "Hubble sphere". Objects at a distance of $r_\mathrm{HS}$ have a redshift of $z\simeq1.6$.</p>

<p>Consider a photon emitted from a distant galaxy (say, <a href="https://en.wikipedia.org/wiki/GN-z11" rel="noreferrer">GN-z11</a> at redshift $z=11.1$) in the past, in the direction of the Milky Way (MW). What special relativity tells us is that <em>locally</em>, the photon always travels through space at $v=c$. Initially, the photon thus increases it distance from GN-z11 at velocity $c$. However, <em>even though the photon travels toward us, its distance to MW increases</em>, due to the expansion of the Universe. As the photon increases its distance to GN-z11, the same expansion causes it to recede from GN-z11 at an ever-increasing velocity. Moreover, as it travels toward MW, it will slowly "overcome" the expansion until it reaches the point where $v_\mathrm{rec} = c$. For an infinitesimally small period, it will stand will wrt. MW, after which it will begin to travel faster and faster as measured from MW. Eventually, its velocity — still in MW's reference frame — will reach $c$, at which point it will have reached MW.</p>

<p>Thus, even though GN-z11 and MW recede from each other at $v_\mathrm{rec} = 2.2c$, we are still able to see it. What is perhaps even more counterintuitive is that when GN-z11 emitted the light we see today, it receded <em>even</em> faster, at $v_\mathrm{rec} \sim 4c$.</p>

<h2>We see more and more distant galaxies</h2>

<p>There is, however, a limit to how fast a galaxy visible to us can recede, given by the distance $r_\mathrm{PH}$ that light has had the time to travel since the Universe was created. Light comes to us from all directions, so we're situated in the center of a sphere of radius $r_\mathrm{PH}$. This sphere is called "the observable Universe", and its surface (which is not a physical thing) is called the <em>particle horizon</em> (hence the subscript "PH"). Galaxies at the particle horizon are receding at $v_\mathrm{rec}\simeq3.3c$.</p>

<p>As time goes by, light from ever-more-distant galaxies$^\dagger$ will reach us; that is $r_\mathrm{PH}$ increases. In other words, <em>the observable Universe always increases in size, and no galaxy visible today will ever leave the observable Universe, no matter its speed</em>. </p>

<p>However, since future observable galaxies will be more and more redshifted, their light will eventually shift out of the visible range and into longer and longer radiowaves. Furthermore, the time between each detected photon will increase, so they will be dimmer and dimmer, and thus in practice, they <em>will</em> disappear.</p>

<h2>Intuitive explanation</h2>

<p>A good analogy for better understanding why light can reach us from a galaxy that recedes faster than light, is the "worm on a rubber band": Attach an (infinite stretchable) rubber band (of length, say, 10 cm) to a wall and walk away at any constant speed you choose, e.g. 1 m/s. Before you start, put your pet worm at the end near the wall. It wants to get back to you, and starts crawling at 1 cm/s, i.e. 100× slower than you. Will it ever reach you? If you look at it from the perspective of the wall, both you and the worm move away, but whereas you recede at a constant speed, the worm, although slower in the beginning, accelerates because it moves on the rubber band, but the part of the rubber band between the worm and the wall increases in size. The rest of the rubber band of course also increases in size, but that doesn't matter — as long as you have a constant speed, and the worm accelerates, it <em>will</em> reach you (although in this example, it will take the worm $10^{26}$ billion years, at which point it may have lost its patience. But if you walk at only 10 cm/s, it will take just 6 hours).</p>

<p>In this analogy, you're the MW, the wall is GN-z11, and the worm is a photon. Now if you don't walk at a constant speed, but <em>also</em> accelerate (this is an analogy of the effect of dark energy), the worm may or may not reach you, depending on your speeds. Just like there is a limit to how distant galaxies we will ever be able to see.</p>

<hr>

<p>$^\dagger$<sub>Note that since large distances also means looking back in time (since the light has spent a long time traveling), we actually don't see galaxies this far away, as they hadn't formed this early in history. We do however see the gas from which the galaxies were born, as far back as 380,000 years after Big Bang.</sub></p>

CC BY-SA 3.0
2016-03-29T20:20:02.163Z
Does this mean border of observable universe 'recedes' with the same speed which have galaxies placed on it? Does this mean escape velocity is constant on whole sphere defined by given radius?
Waldemar Gałęzinowski
CC BY-SA 3.0
2016-03-29T20:49:21.733Z
@WaldemarGałęzinowski: I'm not sure I understand this question: A galaxy currently located at the border recedes at v = 3.3c. The border itself moves away at additionally 1c, since as time goes, we see light from galaxies increasingly farther away (ignoring the fact that we don't actually see _any_ galaxies this far away, since they haven't yet formed). Concerning your last remark, there is no such thing as an "escape velocity", but if you mean is the _recession_ velocity independent of the direction from us, then yes, it depends only on the distance.
pela
CC BY-SA 3.0
2016-07-01T05:07:42.610Z
So you're saying the expansion of the universe is faster than the speed of light?
iMerchant
CC BY-SA 3.0
2016-07-04T12:23:38.403Z
@iMerchant: Yes indeed, but I'm not fond of the phrasing "expanding faster than [some velocity]". The whole of space expands. That means that stuff at a given distance _recedes_ at a given velocity, and stuff at twice that distance recedes at twice that velocity. For sufficiently large velocities, that velocity exceeds the speed of light. The expansion is currently H = 70 km/s per megaparsec (Mpc). Hence, the velocity exceeds c at a distance d = c/H = 4300 Mpc = 14 billion lightyears. This doesn't violate relativity, as nothing travels _trough_ space faster than light. Space just expands.
pela
CC BY-SA 3.0
2016-07-04T12:39:02.783Z
Okay, re-reading my answer I see that I basically just repeated myself. But the important point is the last part: Nothing travels through space faster than the speed of light. Galaxies and other stuff lie approximately still in space, but the expansion carries it away at arbitrarily large velocities.
pela
CC BY-SA 3.0
2016-07-10T21:53:28.113Z
I think one part this otherwise great answer misses is that galaxies *do* leave our cosmic event horizon all the time, which is kinda-sorta the effect the OP asks about. The effect is basically the same: Cross a certain horizon, and no more light emitted from that galaxy will ever reach us. Only difference is that instead of the light from that galaxy suddenly switching off, we see the galaxy's history increasingly redshifted and slowed down until it grinds to a complete halt at infinite redshift at that event. The event that is on the horizon.
Thriveth
CC BY-SA 3.0
2016-07-11T13:46:18.007Z
@Thriveth: But our event horizon increases in the future from its current value of 47 Gly to some 63 Gly in comoving coordinates (see Fig. 1 in Davis & Line weaver 2004), so the observable Universe must encompass more and more galaxies.
pela
CC BY-SA 3.0
2016-07-11T17:17:31.843Z
@pela I believe you are talking about the particle horizon. In same figure, the event horizon is shrinking, from around same 63 Glyr at t=0, to around 15 Glyr at the present time, to 0 at $t=\infty$.
Thriveth
CC BY-SA 3.0
2016-07-11T20:46:19.893Z
@pela In short, the particle horizon is our future light cone at $t=0$, while the event horizon is our past light cone at $t=\infty$. The region from which we can see what happens at $t=0$ is the one you are describing, and it is indeed ever increasing. The region in which we will one day be able to observe what happens right now is the one I am describing, and it is decreasing in our cosmology. Both can lay a somewhat righteous claim on the term "the observable Universe", but the last one is the one I think is closer to what the OP is asking about.
Thriveth
CC BY-SA 3.0
2016-07-12T09:35:55.080Z
@Thriveth: You're right, I used the wrong name. But I'm not sure… Today we're seeing galaxies as far away as 47 Gly (forgetting for a moment that galaxies hadn't formed at the time). In the future, we will see galaxies from larger distances, _in comoving coordinates_, eventually up to 63 Gly away. If this distance is monotonously increasing, then how can any galaxy escape the observable Universe? In the future, a galaxy that is today at $d = 47$ Gly, will at $t = \infty$ be well inside the observable Universe, right?
pela
CC BY-SA 3.0
2016-07-12T10:34:59.557Z
I just discussed this with Tamara Davis (the creator of [this](https://smp.uq.edu.au/people/TamaraDavis/scienceimages/Spacetime_diagrams.pdf) fantastic plot, who is visiting us at the moment). I see now how "leaving the observable Universe" can be interpreted differently, but I still think my answer is what the OP has in mind when asking "disappearing from the sky". No galaxy that is observable today ever disappears (but does become increasingly redshifted). But there _are_ galaxies that we can see now (how they looked in the past), but that we will never be able to see how they look _today_.
pela
CC BY-SA 3.0
2016-07-12T11:57:39.963Z
I think the key here is that "the observable universe" is an ambiguous term. 

In Tam's plot that you link to, you see that if you follow one of the world lines (dotted black), we only ever have access to the part of it that is inside the yellow region. Once the object leaves this region, it is no longer observable, so in that sense, it has definitely left the observable Universe (even though the last moments are stretched out indefinitely like the last piece of cake at a Swedish coffee break).
Thriveth
CC BY-SA 3.0
2016-07-12T12:11:54.533Z
Mmm… I miss fika… Anyway, in my opinion "the observable Universe" is unambiguous. Wikipedia defines it as "…galaxies and other matter that can, in principle, be observed from Earth at the present time because light […] has had time to reach Earth since the beginning of the cosmological expansion", which agrees with my interpretation. The key is "at present time […] because it has had the time to reach us". In the future, "present time" is that point in time, and light will have had more time to travel, so we will see more distant galaxies, and thus the observable Universe will have increased.
pela
CC BY-SA 3.0
2017-01-31T17:54:03.133Z
"In other words, the observable Universe always increases in size, and no galaxy visible today will ever leave the observable Universe, no matter its speed." Depends on the equation of state used. The event horizon in a universe dominated by phantom energy will shrink.
Sir Cumference
CC BY-SA 3.0
2017-01-31T18:56:24.377Z
@SirCumference: You're right that I only consider standard cosmologies.
pela
CC BY-SA 3.0
2017-09-28T17:51:44.530Z
Why do you make the distinction between not detectable due to red shift and moving beyond the observable universe? Suppose a galaxy is red shifted at the edge of detection today by all means and expansion is accelerating, tomorrow, it won't be detectable any more. There is a point where the photon wavelength is greater than the observable universe.
A. C. A. C.
CC BY-SA 3.0
2017-09-28T18:19:53.587Z
@A.C.A.C. The whole point is that nothing ever "moves beyond the obbservable Universe", no matter the acceleration. Once inside, you can never leave. The light from sources inside _does_ however redshift beyond practical observation. Photon wavelengths will never be greater than the obs. Uni., though — the factor by which $\lambda$ increases is proportional to the size of the obs. Uni, which means that if $\lambda$ is at some point smaller the the obs. Uni., it will stay smaller than the obs. Uni.
pela
CC BY-SA 3.0
2017-09-28T18:22:52.517Z
@A.C.A.C.: I think perhaps you think that "the observable Universe" is the region within which a galaxy may emit light _now_, which at some point we may detect. That is not the case. The definition is the region from which we may receive light that has been emitted at _any time_ — in particular, light that was emitted (arbitrarily close to) Big Bang. Could this be the case?
pela
CC BY-SA 3.0
2017-09-28T18:30:39.203Z
@pela It is true that is what I meant but to me, space and time is the same thing so saying we won't detect the photons that are emitted now is the same as the object will eventually be beyond the cosmic horizon. Also, if the acceleration makes the relative velocity >>>>>> c, wouldn't the wavelength eventually be so large that no matter what you consider the observable universe that the wavelength will be bigger?
A. C. A. C.
CC BY-SA 3.0
2017-09-28T19:37:27.820Z
@A.C.A.C. Well, according to your definition, you are right about both things. But "cosmic horizon" (or more commonly the "particle horizon") and "observable Universe" has specific meanings in cosmology. The horizon you refer to is called the _event horizon_, and is currently much closer (~16 Gly) than the particle horizon (47 Gly). That is, everything closer than 16 Gly may emit a photon _today_, and we will at some point be able to receive (although it may take billions and billions of years, and the photon is redshifted far, far into radiowaves).
pela
CC BY-SA 3.0
2017-09-28T19:40:08.737Z
Because of the expansion, the particle horizon always grows, and because of the _accelerated_ expansion, the event horizon shrinks (if the acceleration doesn't evolve in time).
pela
CC BY-SA 3.0
2017-10-01T13:55:19.460Z
I can’t think of any positive outcome for that worm.
iMerchant
CC BY-SA 4.0
2020-06-14T21:28:11.803Z
There are some helpful spacetime diagrams showing the particle horizon, event horizon, and past light cone today (the last of which is usually taken to define the size of the 'observable universe' today) on p. 3 of [this paper by Tamara Davis and Charles Lineweaver](https://arxiv.org/pdf/astro-ph/0310808.pdf), showing them in different choices of space and time coordinate, though all assume the definition of simultaneity commonly used in cosmology (relativity doesn't require you to use that definition though). Also see [this physicsforum post](https://physics.stackexchange.com/a/63780/59406).
Hypnosifl
CC BY-SA 4.0
2021-01-29T22:05:25.460Z
Professor Tamara Davis's [personal page](https://people.smp.uq.edu.au/TamaraDavis/) has changed and your link no longer works. (I get 403 Forbidden for `https://people.smp.uq.edu.au/TamaraDavis/papers/`, so *presumably* the directory still exists). But there are many papers linked on her [UQ staff page](https://smp.uq.edu.au/profile/186/tamara-davis).
PM 2Ring
CC BY-SA 3.0
2016-03-28T15:06:49.253Z
2017-09-29T12:10:10.090Z
2017-09-29T12:10:10.090Z
pela
<p>As time passes, there are galaxies that are currently not in the observable universe which will become observable But this is not a sudden winking on. Instead, over hundreds of millions of years we will see a proto galaxy evolve into a mature galaxy. </p>

<p>For example there is a  <a href="http://arxiv.org/abs/astro-ph/0512396">"blob" of hydrogen</a> that some interpret as being the accretion of hydrogen onto a dark matter halo.  If this interpretation is correct, then the galaxy that eventually forms from it is outside the observable universe. But it won't remain so. Over billions of years the hydrogen will have formed stars, and the galaxy will be in our observable universe. We don't see the sudden appearance of a new galaxy, rather we see the evolution over billions of years.</p>

<p>There is an effect of greater red-shifting. Ultimately galaxies will begin to retreat fast enough that they are red-shifted below the level of detectability. It is suggested that in about 2 trillion years only local galaxies will be visible. This again is not a rapid process(!) </p>

<p>Thus we do not observe galaxies disappearing over a cosmic horizon, and do not expect to do so.</p>

CC BY-SA 3.0
2016-03-28T16:20:10.453Z
2016-03-28T16:54:37.307Z
2016-03-28T16:54:37.307Z
James K
<p>If farthest galaxies run away from us with acceleration making them exceed speed of light, we should expect them to disappear from sky among time with increasing quantity. Did we observe this? Can we indicate next galaxies to eliminate and their time of decline?</p>

<p>My question concerns galaxies moving with all speed ranges, not only ones greater than speed of light.</p>

CC BY-SA 3.0
2016-04-03T18:32:43.350Z
This YouTube video shows why galaxies become visible even though they are very far away. https://www.youtube.com/watch?v=gzLM6ltw3l0 (Fast forward to 6 minutes and 50 seconds and watch until about 8 minutes and 50 seconds.) And if you keep watching past 9 minutes, it will say how far away a galaxy must be to never be seen by us because   the universe will expand faster than light.
RichS
CC BY-SA 3.0
2016-07-10T21:55:14.623Z
@pela It's a matter of definition of course, but I disagree here. As mentioned in my comment below, galaxies leave our event horizon all the time. In a sense, that *is* leaving the observable Universe.
Thriveth
CC BY-SA 3.0
2016-07-11T13:46:47.913Z
@Thriveth: See comment under your other comment.
pela
<galaxy><observational-astronomy><expansion><observable-universe><horizon>
Are there any galaxies which fell out of sight horizon due to cosmic expansion?
Waldemar Gałęzinowski
# Are there any galaxies which fell out of sight horizon due to cosmic expansion?

(Asked by: username_0 on 2016-03-28)

If farthest galaxies run away from us with acceleration making them exceed speed of light, we should expect them to disappear from sky among time with increasing quantity. Did we observe this? Can we indicate next galaxies to eliminate and their time of decline?

My question concerns galaxies moving with all speed ranges, not only ones greater than speed of light.

**Tags:** galaxy, observational-astronomy, expansion, observable-universe, horizon

> **Comments:**
> **username_1**: This YouTube video shows why galaxies become visible even though they are very far away. https://www.youtube.com/watch?v=gzLM6ltw3l0 (Fast forward to 6 minutes and 50 seconds and watch until about 8 minutes and 50 seconds.) And if you keep watching past 9 minutes, it will say how far away a galaxy must be to never be seen by us because   the universe will expand faster than light.  
> **username_2**: @username_3 It's a matter of definition of course, but I disagree here. As mentioned in my comment below, galaxies leave our event horizon all the time. In a sense, that *is* leaving the observable Universe.  
> **username_3**: @username_2: See comment under your other comment.  

---

## 2 answers

### Answer by username_3 on 2016-03-28. Score: 23

No. In fact the opposite is the case.

(See the last paragraph for an intuitive explanation.)

It is a common misbelief that galaxies receding faster than the speed of light are not visible to us. This is not the case; we easily see galaxies moving at superluminal velocities. This does not — as I think most people would think — contradict the theory of relativity, which says that nothing can travel *through* space faster than $c$. Galaxies do not travel *through* space (except with [small velocities of 100-1000 km/s](https://astronomy.stackexchange.com/a/22653/5264)); rather, space itself is expanding, causing distances between the galaxies to increase.

## We see "super-luminal" galaxies

The recession velocity $v\_\mathrm{rec}$ of a galaxy is given by Hubble's Law:
$$
v\_\mathrm{rec} = H\_0 \, d,
$$
where $H\_0 \simeq 67.8\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$ is the Hubble constant ([Planck Collaboration et al. 2016](https://arxiv.org/abs/1502.01589)). This law implies that galaxies farther away than
$$
r\_\mathrm{HS} \equiv \frac{c}{H\_0} \simeq 4400\,\mathrm{Mpc} \simeq 14.4 \, \mathrm{Gly}\,\,\mathrm{("\!\!Giga\mbox{-}lightyears\!\!")}
$$
recede faster than $c$. Here, the subscript "HS" is chosen because the ragion within which galaxies recede slower than $c$ is called the "Hubble sphere". Objects at a distance of $r\_\mathrm{HS}$ have a redshift of $z\simeq1.6$.

Consider a photon emitted from a distant galaxy (say, [GN-z11](https://en.wikipedia.org/wiki/GN-z11) at redshift $z=11.1$) in the past, in the direction of the Milky Way (MW). What special relativity tells us is that *locally*, the photon always travels through space at $v=c$. Initially, the photon thus increases it distance from GN-z11 at velocity $c$. However, *even though the photon travels toward us, its distance to MW increases*, due to the expansion of the Universe. As the photon increases its distance to GN-z11, the same expansion causes it to recede from GN-z11 at an ever-increasing velocity. Moreover, as it travels toward MW, it will slowly "overcome" the expansion until it reaches the point where $v\_\mathrm{rec} = c$. For an infinitesimally small period, it will stand will wrt. MW, after which it will begin to travel faster and faster as measured from MW. Eventually, its velocity — still in MW's reference frame — will reach $c$, at which point it will have reached MW.

Thus, even though GN-z11 and MW recede from each other at $v\_\mathrm{rec} = 2.2c$, we are still able to see it. What is perhaps even more counterintuitive is that when GN-z11 emitted the light we see today, it receded *even* faster, at $v\_\mathrm{rec} \sim 4c$.

## We see more and more distant galaxies

There is, however, a limit to how fast a galaxy visible to us can recede, given by the distance $r\_\mathrm{PH}$ that light has had the time to travel since the Universe was created. Light comes to us from all directions, so we're situated in the center of a sphere of radius $r\_\mathrm{PH}$. This sphere is called "the observable Universe", and its surface (which is not a physical thing) is called the *particle horizon* (hence the subscript "PH"). Galaxies at the particle horizon are receding at $v\_\mathrm{rec}\simeq3.3c$.

As time goes by, light from ever-more-distant galaxies$^\dagger$ will reach us; that is $r\_\mathrm{PH}$ increases. In other words, *the observable Universe always increases in size, and no galaxy visible today will ever leave the observable Universe, no matter its speed*.

However, since future observable galaxies will be more and more redshifted, their light will eventually shift out of the visible range and into longer and longer radiowaves. Furthermore, the time between each detected photon will increase, so they will be dimmer and dimmer, and thus in practice, they *will* disappear.

## Intuitive explanation

A good analogy for better understanding why light can reach us from a galaxy that recedes faster than light, is the "worm on a rubber band": Attach an (infinite stretchable) rubber band (of length, say, 10 cm) to a wall and walk away at any constant speed you choose, e.g. 1 m/s. Before you start, put your pet worm at the end near the wall. It wants to get back to you, and starts crawling at 1 cm/s, i.e. 100× slower than you. Will it ever reach you? If you look at it from the perspective of the wall, both you and the worm move away, but whereas you recede at a constant speed, the worm, although slower in the beginning, accelerates because it moves on the rubber band, but the part of the rubber band between the worm and the wall increases in size. The rest of the rubber band of course also increases in size, but that doesn't matter — as long as you have a constant speed, and the worm accelerates, it *will* reach you (although in this example, it will take the worm $10^{26}$ billion years, at which point it may have lost its patience. But if you walk at only 10 cm/s, it will take just 6 hours).

In this analogy, you're the MW, the wall is GN-z11, and the worm is a photon. Now if you don't walk at a constant speed, but *also* accelerate (this is an analogy of the effect of dark energy), the worm may or may not reach you, depending on your speeds. Just like there is a limit to how distant galaxies we will ever be able to see.

---

$^\dagger$Note that since large distances also means looking back in time (since the light has spent a long time traveling), we actually don't see galaxies this far away, as they hadn't formed this early in history. We do however see the gas from which the galaxies were born, as far back as 380,000 years after Big Bang.

> **Comments:**
> **username_0**: Does this mean border of observable universe 'recedes' with the same speed which have galaxies placed on it? Does this mean escape velocity is constant on whole sphere defined by given radius?  
> **username_3**: @username_0: I'm not sure I understand this question: A galaxy currently located at the border recedes at v = 3.3c. The border itself moves away at additionally 1c, since as time goes, we see light from galaxies increasingly farther away (ignoring the fact that we don't actually see _any_ galaxies this far away, since they haven't yet formed). Concerning your last remark, there is no such thing as an "escape velocity", but if you mean is the _recession_ velocity independent of the direction from us, then yes, it depends only on the distance.  
> **username_4**: So you're saying the expansion of the universe is faster than the speed of light?  
> **username_3**: @username_4: Yes indeed, but I'm not fond of the phrasing "expanding faster than [some velocity]". The whole of space expands. That means that stuff at a given distance _recedes_ at a given velocity, and stuff at twice that distance recedes at twice that velocity. For sufficiently large velocities, that velocity exceeds the speed of light. The expansion is currently H = 70 km/s per megaparsec (Mpc). Hence, the velocity exceeds c at a distance d = c/H = 4300 Mpc = 14 billion lightyears. This doesn't violate relativity, as nothing travels _trough_ space faster than light. Space just expands.  
> **username_3**: Okay, re-reading my answer I see that I basically just repeated myself. But the important point is the last part: Nothing travels through space faster than the speed of light. Galaxies and other stuff lie approximately still in space, but the expansion carries it away at arbitrarily large velocities.  
> **username_2**: I think one part this otherwise great answer misses is that galaxies *do* leave our cosmic event horizon all the time, which is kinda-sorta the effect the OP asks about. The effect is basically the same: Cross a certain horizon, and no more light emitted from that galaxy will ever reach us. Only difference is that instead of the light from that galaxy suddenly switching off, we see the galaxy's history increasingly redshifted and slowed down until it grinds to a complete halt at infinite redshift at that event. The event that is on the horizon.  
> **username_3**: @username_2: But our event horizon increases in the future from its current value of 47 Gly to some 63 Gly in comoving coordinates (see Fig. 1 in Davis & Line weaver 2004), so the observable Universe must encompass more and more galaxies.  
> **username_2**: @username_3 I believe you are talking about the particle horizon. In same figure, the event horizon is shrinking, from around same 63 Glyr at t=0, to around 15 Glyr at the present time, to 0 at $t=\infty$.  
> **username_2**: @username_3 In short, the particle horizon is our future light cone at $t=0$, while the event horizon is our past light cone at $t=\infty$. The region from which we can see what happens at $t=0$ is the one you are describing, and it is indeed ever increasing. The region in which we will one day be able to observe what happens right now is the one I am describing, and it is decreasing in our cosmology. Both can lay a somewhat righteous claim on the term "the observable Universe", but the last one is the one I think is closer to what the OP is asking about.  
> **username_3**: @username_2: You're right, I used the wrong name. But I'm not sure… Today we're seeing galaxies as far away as 47 Gly (forgetting for a moment that galaxies hadn't formed at the time). In the future, we will see galaxies from larger distances, _in comoving coordinates_, eventually up to 63 Gly away. If this distance is monotonously increasing, then how can any galaxy escape the observable Universe? In the future, a galaxy that is today at $d = 47$ Gly, will at $t = \infty$ be well inside the observable Universe, right?  
> **username_3**: I just discussed this with Tamara Davis (the creator of [this](https://smp.uq.edu.au/people/TamaraDavis/scienceimages/Spacetime_diagrams.pdf) fantastic plot, who is visiting us at the moment). I see now how "leaving the observable Universe" can be interpreted differently, but I still think my answer is what the OP has in mind when asking "disappearing from the sky". No galaxy that is observable today ever disappears (but does become increasingly redshifted). But there _are_ galaxies that we can see now (how they looked in the past), but that we will never be able to see how they look _today_.  
> **username_2**: I think the key here is that "the observable universe" is an ambiguous term.   In Tam's plot that you link to, you see that if you follow one of the world lines (dotted black), we only ever have access to the part of it that is inside the yellow region. Once the object leaves this region, it is no longer observable, so in that sense, it has definitely left the observable Universe (even though the last moments are stretched out indefinitely like the last piece of cake at a Swedish coffee break).  
> **username_3**: Mmm… I miss fika… Anyway, in my opinion "the observable Universe" is unambiguous. Wikipedia defines it as "…galaxies and other matter that can, in principle, be observed from Earth at the present time because light […] has had time to reach Earth since the beginning of the cosmological expansion", which agrees with my interpretation. The key is "at present time […] because it has had the time to reach us". In the future, "present time" is that point in time, and light will have had more time to travel, so we will see more distant galaxies, and thus the observable Universe will have increased.  
> **username_5**: "In other words, the observable Universe always increases in size, and no galaxy visible today will ever leave the observable Universe, no matter its speed." Depends on the equation of state used. The event horizon in a universe dominated by phantom energy will shrink.  
> **username_3**: @username_5: You're right that I only consider standard cosmologies.  
> **username_6**: Why do you make the distinction between not detectable due to red shift and moving beyond the observable universe? Suppose a galaxy is red shifted at the edge of detection today by all means and expansion is accelerating, tomorrow, it won't be detectable any more. There is a point where the photon wavelength is greater than the observable universe.  
> **username_3**: @A.C.A.C. The whole point is that nothing ever "moves beyond the obbservable Universe", no matter the acceleration. Once inside, you can never leave. The light from sources inside _does_ however redshift beyond practical observation. Photon wavelengths will never be greater than the obs. Uni., though — the factor by which $\lambda$ increases is proportional to the size of the obs. Uni, which means that if $\lambda$ is at some point smaller the the obs. Uni., it will stay smaller than the obs. Uni.  
> **username_3**: @A.C.A.C.: I think perhaps you think that "the observable Universe" is the region within which a galaxy may emit light _now_, which at some point we may detect. That is not the case. The definition is the region from which we may receive light that has been emitted at _any time_ — in particular, light that was emitted (arbitrarily close to) Big Bang. Could this be the case?  
> **username_6**: @username_3 It is true that is what I meant but to me, space and time is the same thing so saying we won't detect the photons that are emitted now is the same as the object will eventually be beyond the cosmic horizon. Also, if the acceleration makes the relative velocity >>>>>> c, wouldn't the wavelength eventually be so large that no matter what you consider the observable universe that the wavelength will be bigger?  
> **username_3**: @A.C.A.C. Well, according to your definition, you are right about both things. But "cosmic horizon" (or more commonly the "particle horizon") and "observable Universe" has specific meanings in cosmology. The horizon you refer to is called the _event horizon_, and is currently much closer (~16 Gly) than the particle horizon (47 Gly). That is, everything closer than 16 Gly may emit a photon _today_, and we will at some point be able to receive (although it may take billions and billions of years, and the photon is redshifted far, far into radiowaves).  
> **username_3**: Because of the expansion, the particle horizon always grows, and because of the _accelerated_ expansion, the event horizon shrinks (if the acceleration doesn't evolve in time).  
> **username_4**: I can’t think of any positive outcome for that worm.  
> **username_7**: There are some helpful spacetime diagrams showing the particle horizon, event horizon, and past light cone today (the last of which is usually taken to define the size of the 'observable universe' today) on p. 3 of [this paper by Tamara Davis and Charles Lineweaver](https://arxiv.org/pdf/astro-ph/0310808.pdf), showing them in different choices of space and time coordinate, though all assume the definition of simultaneity commonly used in cosmology (relativity doesn't require you to use that definition though). Also see [this physicsforum post](https://physics.stackexchange.com/a/63780/59406).  
> **username_8**: Professor Tamara Davis's [personal page](https://people.smp.uq.edu.au/TamaraDavis/) has changed and your link no longer works. (I get 403 Forbidden for `https://people.smp.uq.edu.au/TamaraDavis/papers/`, so *presumably* the directory still exists). But there are many papers linked on her [UQ staff page](https://smp.uq.edu.au/profile/186/tamara-davis).  

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### Answer by username_9 on 2016-03-28. Score: 8

As time passes, there are galaxies that are currently not in the observable universe which will become observable But this is not a sudden winking on. Instead, over hundreds of millions of years we will see a proto galaxy evolve into a mature galaxy.

For example there is a ["blob" of hydrogen](http://arxiv.org/abs/astro-ph/0512396) that some interpret as being the accretion of hydrogen onto a dark matter halo. If this interpretation is correct, then the galaxy that eventually forms from it is outside the observable universe. But it won't remain so. Over billions of years the hydrogen will have formed stars, and the galaxy will be in our observable universe. We don't see the sudden appearance of a new galaxy, rather we see the evolution over billions of years.

There is an effect of greater red-shifting. Ultimately galaxies will begin to retreat fast enough that they are red-shifted below the level of detectability. It is suggested that in about 2 trillion years only local galaxies will be visible. This again is not a rapid process(!)

Thus we do not observe galaxies disappearing over a cosmic horizon, and do not expect to do so.


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