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62–65 billion light years away is the “future visibility limit”, according to Wikipedia.

Does this also mean that objects past this 62–65 gyr range have red-shifted already? And that’s why we’ll never see them or detect their light/presence ever, which is why they’re in the “unobservable universe”?

Have there been any galaxies that we have seen or observed/detected in the past (that is, within 62–65 gyr, and within 46 billion light years), but have already red-shifted so much that they're beyond detection? I don’t think so, because this would mean that the objects 46 billion light years away would have already red-shifted beyond detection because they’re the furthest away. Am I correct?

But even if they do red-shift beyond detection, they’re still there even though we can’t see them. So does that mean that they move farther away, in terms of “proper distance”?

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Tommy D
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  • See if this video helps: https://www.youtube.com/watch?v=AwwIFcdUFrE – Paul B Mar 23 '17 at 21:41
  • Your question seems to suggest that the change in frequency of their emissions due to red shift makes them undetectable. However cosmic microwave background has a huge red-shift and can still be detected. The boundary of the observable universe is not caused by red-shift. – JMLCarter Mar 23 '17 at 21:48

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The term "beyond detection" could mean two things: As objects redshift, they may eventually redshift so much that we don't have the technology do see them. A more interesting interpretation of the term is "is the object detectable in principle?", i.e. with any conceivable instrument that doesn't violate the laws of physics.

Objects that today are farther away than roughly 63 Gly ("billion lightyears") will never be detectable, simply because the expansion of the Universe carries then away too fast for the light to reach us. Objects somewhat nearer than this may also be undetectable, because the light both is redshifted far into the radio regime, and because the rate at which photons arrive becomes so low that they appear too dim for us to see.

But let's assume that the receiving of a single radio wave is enough for us to call it "a detection".

Objects within the 63 Gly have redshifted already, just as all other objects lying at cosmological distance (i.e. far enough that they're not gravitationally bound to the Milky Way).

But the observable Universe always increases in size, because the light from increasingly distant galaxies, as time goes, has had the time to reach us. That means that once a galaxy is within the observable Universe, it never leaves. It may redshift so much that puny humans can't see them, but radio creatures with eyes the size of a galaxy can still see them.

"Comoving coordinates" are defined such that they coincide with the real, physical coordinates today. In 15 Gyr ("billion years") or so the Universe has expanded to twice the size of today (linearly speaking), so the physical distance to the "border" outside which we will never be able to know about will have grown to 126 Gly, and in 25 Gyr it will have grown to 252 Gly. In other words, galaxies always move farther away in physical coordinates, but remain stationary in comoving coordinates (except for small so-called "peculiar" velocities of a few 100 km/s).

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pela
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  • In classical theory, an object never becomes undetectable in finite time. But I wonder if there is an energy limit below which it does indeed become undetectable, even in theory, when taking quantum fluctuations into account? – Thriveth Mar 24 '17 at 15:26
  • @Thriveth: Good question. I don't know if it makes sense to talk about the arrival of a single photon in the radio regime. – pela Mar 24 '17 at 17:29
  • I don't think there's any intrinsic difference, but on the other hand I guess standing waves could bump into the same kind of limit...? Just speculating. My gut feeling suggests it would be so but I don't have the theory it takes to show it... – Thriveth Mar 24 '17 at 21:42
  • @Thriveth: There's material for you next paper! – pela Mar 25 '17 at 05:34
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    Me doing a theory paper - that would be the day... – Thriveth Mar 25 '17 at 09:55
  • So, an object that is 13.3 billion light years away (the MACS0647 galaxy https://en.wikipedia.org/wiki/MACS0647-JD) -- the light that we're seeing from this object is 13.3 billion years old (which means that we're seeing it as it were 13.3 billion years ago). It hasn't completely redshifted yet, with a redshift value of about z = 10.7-11.

    At what point in time should we expect this galaxy to redshift beyond detection?

     – Tommy D Mar 26 '17 at 02:24
  • @TommyDang: If by "completely redshifted" you mean "redshifted beyond possible detection", then you're right. But it has redshifted, by a factor of ~11. But as I say in my answer, since it is inside our observable Universe, it will always be observable, at least in principle. The photons we see from it will just slowly be redder and redder, and arrive at a slower and slower rate. When it will unobservable in practice is another question, which is impossible to answer since it depends on our technology billions of years from now. – pela Mar 26 '17 at 02:34
  • Ok, that makes more sense. So my question now is this. MACS0647 is 13.3 bly away, which means we're seeing this galaxy as it was 13.3 billion years ago. In the (far) future, will it EVER be possible to see it as it is/was in the year 2017? It's not as simple as waiting 13.3 billion yrs, since it's getting further away from Earth every passing moment, meaning that it will take even longer than that. Also, since this galaxy's LIGHT is 13.3 bly away RIGHT NOW, where do we estimate the ACTUAL galaxy to be RIGHT NOW? Is it around 40 some bly away, since the furthest observable ones are 46 bly away? – Tommy D Mar 27 '17 at 04:40
  • @TommyDang: No, the most "recent version" of MACS0647 we will ever see, is the version that is currently roughly 10 Gyr old. Light that is emitted at a later point will never reach us. And the most distant galaxy that we will ever see the 2017 version of is currently ~17 Gly away. Such a galaxy is currently redshifted by a factor of roughly 1.9, and the Universe was roughly 3.5 Gyr old when the light we see today was emitted. Calculating these numbers is not trivial, but not extremely hard either. It involves integrating the Friedmann equation, for which various tools exist. I use Python. – pela Mar 27 '17 at 20:42
  • I have another question. Is this "currently ~17 Gly away" the same thing as the "cosmic event horizon"? According to Wikipedia, the CEH is roughly 16 billion light years away, so since they are close in terms of number (16 and 17 billion light years) are they the same thing and it's just a rounding thing, or am I getting my terms mixed up here?

    https://en.wikipedia.org/wiki/Observable_universe

     – Tommy D Mar 30 '17 at 22:48
  • Plus, what Python program do you use to calculate these numbers? Is it an online tool? Also, where do we estimate the ACTUAL galaxy of MACS0647 to be RIGHT NOW? – Tommy D Mar 30 '17 at 23:09
  • @TommyDang: Yes, the CEH in that article is exactly that. The "17" was my quick estimate. And yes, there are online tools, but I don't know of any that calculates the CEH, actually. I use Python, but much can be estimated from a spacetime diagram such as the one in Davis & Lineweaver 2004, or Pulsar's version thereof. It shows time/size/redshift as a function of distance from us in comoving coordinates, i.e. the current distance if we freeze the Universe and lay out measuring rods. – pela Apr 01 '17 at 07:37
  • The current distance to MACS0647 is equal to the comoving distance given by its redshift (z = 10.7), and is equal to 10 Gpc, or roughly 32.5 Gly. Btw, if you want to use Python, you'll want to use the astropy or CosmoloPy modules. Have fun! – pela Apr 01 '17 at 07:44
  • I have some additional questions. I'm having trouble understanding the relationship between an object's distance and its redshift. Is it true that the further away an object is, the more it has redshifted? So like, is the reason why distant objects like galaxies "disappear" from sight (but ARE STILL POTENTIALLY DETECTABLE with any conceivable instrument) is because it has red shifted so far and that their photons just don't have time to reach us? And if objects within this 65 gly HAVE ALREADY red shifted beyond our abilities to see it, what makes some objects more red shifted than others? – Tommy D Apr 08 '17 at 18:27
  • If the inability to "see" an object (that again, is still OBSERVABLE with any conceivable instrument) is NOT CAUSED by its red shift, what is it caused by then? Is it its distance? If so, have there been any objects that HAVEN'T red shifted too much, but that we can no longer "see" (but can still be observed in principle)? – Tommy D Apr 08 '17 at 18:31
  • @TommyDang: The other effect that makes objects unobservable in practice is the rate at which photon arrive at our detector. For sufficiently distant objects, photons arrive so rarely that they do not really form an image, but rather arrive one by one. And yes, the primary reason we don't see most distant galaxies is not their redshift (which is only by a factor of the order 10), but the fact that their photons arrive so rarely that their surface brightnesses are too dim too be detected. – pela Apr 08 '17 at 22:54
  • I have a followup question and I apologize for my ignorance of these topics. Have there been any objects that we HAVE detected in the past, but have by now moved so far away that, although they are observable in practice (as objects can never leave the OU), we cannot detect their light anymore (but theoretical radio creatures can)? For example, an object that was possible to be detected 10,000 years ago (that is, its light was able to reach Earth 10,000 years ago)--is it possible that we can no longer detect this object's light in the year 2017, because its photons could be arriving so rarely? – Tommy D Apr 09 '17 at 04:08
  • @TommyDang: Don't apologize :), But no, because although galaxies and other cosmological object recede immensely fast (by human standards), 10,000 yr or even 1,000,000 yr nothing, cosmologically speaking. At least today. But if you consider a hypothetical observer much closer in time to the Big Bang, when the Universe expanded much faster. For instance, when the CMB was emitted 380,000 yr after BB, the expansion rate was 20,000 times faster than today. – pela Apr 09 '17 at 07:06
  • Ok. That clears things up a bit. So to summarize: EVERYTHING that we HAVE DETECTED in the past--we CAN STILL DETECT ("detect" meaning "detect their light") TODAY, correct? (I know that they are observable in principle; they are detectable with any conceivable instrument) In this case, do we have an estimate for when the first object/galaxy will move so far away that their photons arrive too slowly or redshift too far for our instruments to detect? – Tommy D Apr 10 '17 at 00:37
  • @TommyDang: Yes, you're right. But I wouldn't dare to estimate that, since it requires extrapolating current technology billions of years into the future. But if you look at the figure I linked to, you can see that an object like MACS0647 which today has a redshift of ~10, will have a redshift <50 even after the Universe is 40 Gyr old, and a redshift <1000 for many, many billions of years. It's probably not impossible that, in billions of years, we could observe objects at z ~ 50 or even z ~ 1000 (provided our civilization still exists). So my best answer is: "Many billions of years". – pela Apr 10 '17 at 08:29
  • Awesome! Thanks for answering all my questions​! I really appreciate it! I do have an additional question that's not really important but I still want to know the answer to. What is the Hubble Sphere, and how is it related to all this (expansion of the universe, etc.)? Also, if you don't mind me asking, I was just wondering how you know so much about all these topics. Are you like an astronomy major or something? – Tommy D Apr 11 '17 at 02:16
  • @TommyDang: You're very welcome. The Hubble sphere is simply the spherical volume centered on us, inside which the Universe expands slower than the speed of light. It doesn't really bear any physical significance, as we can still see galaxies outside this sphere. In the linked figure, it's the innermost green region. And yes, I'm an astronomer (postdoc) at the University of Oslo. :) – pela Apr 11 '17 at 06:42