# Do we observe galaxies disappearing at the edge of the observable universe? **URL:** \ **Category:** Factual Questions\ **Created:** [October 12, 2012, 8:13pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558 "2012-10-12T20:13:25Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![sweeteviljesus](https://avatars.discourse-cdn.com/v4/letter/s/898d66/32.png) [@sweeteviljesus](https://boards.straightdope.com/u/sweeteviljesus)\ **Post date:** [October 12, 2012, 8:13pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/1 "2012-10-12T20:13:25Z")
Since the universe is expanding at greater than the speed of light, do we observe galaxies disappearing off the edge as it were? I am pretty sure we don’t but I am having trouble trying to figure out what we do observe. Lots of red shift? Thanks, Rob ---
**Author:** ![robert\_columbia](https://avatars.discourse-cdn.com/v4/letter/r/e79b87/32.png) [@robert\_columbia](https://boards.straightdope.com/u/robert_columbia)\ **Post date:** [October 12, 2012, 8:51pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/2 "2012-10-12T20:51:58Z")
I thought the observable universe was expanding with the passage of time. For it to “disappear”, it would seem that the _relative_ velocity between us and that other galaxy must be greater than c. I thought that wasn’t possible. ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [October 12, 2012, 9:01pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/3 "2012-10-12T21:01:15Z")
It’s my understanding that what we see is the hot plasma that filled the universe before it became transparent. Recall that the farther we see, the farther back in time the objects we see are; go back far enough and the universe wasn’t transparent. Edit: Also; go back far enough and there weren’t any galaxies for us to observe, either. ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [October 12, 2012, 9:12pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/4 "2012-10-12T21:12:53Z")
> [@robert\_columbia](#): > > I thought the observable universe was expanding with the passage of time. For it to “disappear”, it would seem that the _relative_ velocity between us and that other galaxy must be greater than c. I thought that wasn’t possible. The explanation in General Relativity, as modified by people like Hawking, Penrose, and others, is that the space itself is expanding faster than c, even though nothing is ever actually moving through space faster than c. Extrapolations of cosmic expansion – there was a Scientific American article on this, not too long ago – say that, yes, as time goes by, the most distant galaxies will wink out. They will drop beyond the horizon, just like a ship sailing away from an observer on the coastline. In the long run, we might have our own galaxy to observe…and nothing else. [Here is a short article on the subject](http://digitaljournal.com/article/325370); for the answer to the OP, just jump to the last sentence in the article. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [October 12, 2012, 9:23pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/5 "2012-10-12T21:23:46Z")
> [@sweeteviljesus](#): > > Since the universe is expanding at greater than the speed of light, do we observe galaxies disappearing off the edge as it were? I am pretty sure we don’t but I am having trouble trying to figure out what we do observe. Lots of red shift? > > Thanks, > Rob We probably haven’t looked at any sufficiently distant galaxies for long enough to see one disappear this way. Galaxies are big things, many light years across, so even at around light speed they are going to take some time to pass over the threshold, as it were. They are not simply going to suddenly blink out. ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [October 12, 2012, 9:34pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/6 "2012-10-12T21:34:27Z")
The Hubble extreme deep field pictures were recently published. All the dots in the image are distant galaxies. I would guess that the galaxies get too faint from being so far away before they get to far away for light to reach us. The linked article says that this image was created with a 50 day total exposure (spread over a few years). [http://hubblesite.org/newscenter/archive/releases/2012/37/image/a/format/large\_web/](http://hubblesite.org/newscenter/archive/releases/2012/37/image/a/format/large_web/) [http://hubblesite.org/newscenter/archive/releases/2012/37/image/a/](http://hubblesite.org/newscenter/archive/releases/2012/37/image/a/) ---
**Author:** ![RedSwinglineOne](https://avatars.discourse-cdn.com/v4/letter/r/a9adbd/32.png) [@RedSwinglineOne](https://boards.straightdope.com/u/RedSwinglineOne)\ **Post date:** [October 12, 2012, 10:13pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/7 "2012-10-12T22:13:47Z")
> [@Trinopus](#): > > as time goes by, the most distant galaxies will wink out. They will drop beyond the horizon, just like a ship sailing away from an observer on the coastline. I’m not sure “wink out” is a very good term. They won’t just disappear. Wouldn’t it just gradually redshift beyond the visible spectrum? ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [October 12, 2012, 10:17pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/8 "2012-10-12T22:17:35Z")
> [@RedSwinglineOne](#): > > I’m not sure “wink out” is a very good term. They won’t just disappear. Wouldn’t it just gradually redshift beyond the visible spectrum? The Hubble deep field pictures are false color because those galaxies are already redshifted out of the visible spectrum. To take the pictures they use infrared cameras. ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [October 12, 2012, 11:43pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/9 "2012-10-12T23:43:44Z")
> [@RedSwinglineOne](#): > > I’m not sure “wink out” is a very good term. They won’t just disappear. Wouldn’t it just gradually redshift beyond the visible spectrum? A little of both? I think they’d also stretch, like the Enterprise in ST:TNG (or the Coyote and Roadrunner…) Not sure, though… ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [October 12, 2012, 11:49pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/10 "2012-10-12T23:49:48Z")
As has been said, the farther away you look, the farther back in time you look. As I understand it, the most distant (and therefore the oldest) signals we’ve detected come from some time not long after the Big Bang, when the universe cooled off enough to become transparent to electromagnetism. Or something like that. The point is, we can see back way waaaaay earlier than the formation of the first galaxy. IANACosmologist ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 13, 2012, 5:40am UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/11 "2012-10-13T05:40:32Z")
> [@Smeghead](#): > > As has been said, the farther away you look, the farther back in time you look. As I understand it, the most distant (and therefore the oldest) signals we’ve detected come from some time not long after the Big Bang, when the universe cooled off enough to become transparent to electromagnetism. Or something like that. The point is, we can see back way waaaaay earlier than the formation of the first galaxy. This is true. The oldest/furthest thing we see is the cosmic microwave background radiation (CMBR), which was emitted by the universe when it was only 380,000 years old. And it’s always going to be the oldest/furthest thing we see. Because the universal expansion is accelerating, distant galaxies (and the CMBR) will be red shifted further and further and faster and faster. So we’ll see them when they are younger and younger. Eventually we won’t be able to see them because they are too dim and small to see. That’s basically what’s meant by them winking out. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [October 13, 2012, 7:16am UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/12 "2012-10-13T07:16:39Z")
Before any visible galaxies ‘wink out’ of visibility, the much more distant Cosmic Microwave Background will ‘wink out’; that hasn’t happened, so no galaxies are going to be winking out soon. If expansion continues to accelerate, we will eventually lose sight of (first) the CMBR and next the most distant galaxies, as no photons would ever get to us from those locations any more. When that happens depends entirely on the rate of acceleration of expansion. ---
**Author:** ![Esox\_Lucius](https://avatars.discourse-cdn.com/v4/letter/e/848f3c/32.png) [@Esox\_Lucius](https://boards.straightdope.com/u/Esox_Lucius)\ **Post date:** [October 13, 2012, 2:25pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/13 "2012-10-13T14:25:42Z")
> [@Smeghead](#): > > As has been said, the farther away you look, the farther back in time you look. As I understand it, the most distant (and therefore the oldest) signals we’ve detected come from some time not long after the Big Bang, when the universe cooled off enough to become transparent to electromagnetism. Or something like that. The point is, we can see back way waaaaay earlier than the formation of the first galaxy. > > IANACosmologist I didn’t think we had to look very far for the CMB because it permeates the whole universe. It’s the oldest radiation we can detect, but it’s right here, right now, and always has been, while the light from a distant galaxy takes billions of years to get here. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [October 13, 2012, 2:37pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/14 "2012-10-13T14:37:24Z")
Yeah, probably. I don’t know. I’m a geneticist. For me, distances come in two categories: “within the cell” and “big”. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [October 13, 2012, 5:25pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/15 "2012-10-13T17:25:30Z")
> [@Esox\_Lucius](#): > > I didn’t think we had to look very far for the CMB because it permeates the whole universe. It’s the oldest radiation we can detect, but it’s right here, right now, and always has been, while the light from a distant galaxy takes billions of years to get here. Nope; that’s not right. The CMBR comes from the edge of the visible universe. See this thread for more details. > **[Cosmic microwave background radiation](https://boards.straightdope.com/sdmb/showthread.php?t=665131)** > > The CMBR is, in essence, the echo of The Big Bang…or more accurately, the flash of light emitted when the very young universe stopped being opaque and instead transparent to photons. Since, due to farther objects moving faster away from us, the... ---
**Author:** ![colonial](https://avatars.discourse-cdn.com/v4/letter/c/34f0e0/32.png) [@colonial](https://boards.straightdope.com/u/colonial)\ **Post date:** [October 13, 2012, 7:14pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/16 "2012-10-13T19:14:00Z")
> [@eburacum45](#): > > Before any visible galaxies ‘wink out’ of visibility, the much more distant Cosmic Microwave Background will ‘wink out’; that hasn’t happened, so no galaxies are going to be winking out soon. > > If expansion continues to accelerate, we will eventually lose sight of (first) the CMBR and next the most distant galaxies, as no photons would ever get to us from those locations any more. When that happens depends entirely on the rate of acceleration of expansion. I do not believe this is correct. CMB is nearly evenly distributed throughout the universe, and its continuing existence does not depend on a source on the outskirts of the visible universe. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [October 13, 2012, 10:22pm UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/17 "2012-10-13T22:22:45Z")
Did’t Hubble’s law say that the speed an object was moving away from us what directly proportional to the distance from us ? That is v=K.D where K is the Hubble’s constant and D is the distance. If that is the case - for a large enough D, v=c , the speed of light. So that would mean the Universe had a boundary - am I getting this wrong ? ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [October 14, 2012, 1:27am UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/18 "2012-10-14T01:27:29Z")
> [@colonial](#): > > I do not believe this is correct. > > CMB is nearly evenly distributed throughout the universe, and its continuing existence does not depend on a source on the outskirts of the visible universe. It is correct. It’s true that the source of the CMB is from the cloud of hot plasma that filled the entire universe until a few hundred thousand years after the big bang. However, the CMB that we on Earth can see comes from a particular part of that cloud – specifically, a thin spherical shell with Earth at the center. That’s the part of the primordial gas cloud that just happened to be the right distance away for its light to be arriving at Earth right now. As time goes by, the spherical shell emitting the CMB that is arriving at Earth gets larger and larger. It recedes from us faster and faster. Eventually it will redshift out of existence. ---
**Author:** ![levdrakon](https://avatars.discourse-cdn.com/v4/letter/l/49beb7/32.png) [@levdrakon](https://boards.straightdope.com/u/levdrakon)\ **Post date:** [October 14, 2012, 2:59am UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/19 "2012-10-14T02:59:05Z")
> [@am77494](#): > > Did’t Hubble’s law say that the speed an object was moving away from us what directly proportional to the distance from us ? That is v=K.D where K is the Hubble’s constant and D is the distance. If that is the case - for a large enough D, v=c , the speed of light. So that would mean the Universe had a boundary - am I getting this wrong ? This gives the _observable_ universe a boundary, which presently is thought to be 16 billion light years away. The universe could be infinite. I have no idea how fast an object infinitely distant from us could be receding from us. If we’re part of a multiverse, physical laws may not even be the same that far away. To the OP, I think the edge of the observable universe would look similar to how the night sky looks anyway. That is, more distant objects get smaller and dimmer until you can’t discern them anymore. IANAA. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [October 14, 2012, 5:45am UTC](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558/20 "2012-10-14T05:45:49Z")
> [@levdrakon](#): > > This gives the _observable_ universe a boundary, which presently is thought to be 16 billion light years away. Sorry to be a naysayer, but this isn’t right either. There are lots of different ways of measuring the distance of the edge of the visible universe, but none of them give the distance as 16 billion light years. See this wikipedia page; > **[Observable universe](https://en.wikipedia.org/wiki/Observable_universe)** > > The observable universe is a spherical region of the universe consisting of all matter that can be observed from Earth; the electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion. Assuming the universe is isotropic, the distance to the edge of the observable universe is the same in every direction. That is, the observable universe is a spherical region centered on the observer. Every location in the universe... your estimate seems to be the second one, > [@](#): > > 15.8 billion light-years > This is obtained in the same way as the 13.7 billion light year figure, but starting from an incorrect age of the universe that the popular press reported in mid-2006. The most realistic value for the size of the universe in my opinion is the **co-moving distance** , which gives a radius of 46 billion light years. The co-moving distance takes into account the expansion of space between our location and the most distant visible phenomenon, which is the cosmic microwave background; when the light that we now see left the CMB, 13.7 billion years ago, it was only 42 _million_ light years away. Space is weird. [Next page](https://boards.straightdope.com/t/do-we-observe-galaxies-disappearing-at-the-edge-of-the-observable-universe/637558.md?page=2)