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”?

  • $\begingroup$ See if this video helps: youtube.com/watch?v=AwwIFcdUFrE $\endgroup$
    – Paul B
    Mar 23 '17 at 21:41
  • $\begingroup$ 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. $\endgroup$
    – JMLCarter
    Mar 23 '17 at 21:48

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).

  • $\begingroup$ 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? $\endgroup$
    – Thriveth
    Mar 24 '17 at 15:26
  • $\begingroup$ @Thriveth: Good question. I don't know if it makes sense to talk about the arrival of a single photon in the radio regime. $\endgroup$
    – pela
    Mar 24 '17 at 17:29
  • $\begingroup$ 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... $\endgroup$
    – Thriveth
    Mar 24 '17 at 21:42
  • $\begingroup$ @Thriveth: There's material for you next paper! $\endgroup$
    – pela
    Mar 25 '17 at 5:34
  • 1
    $\begingroup$ Me doing a theory paper - that would be the day... $\endgroup$
    – Thriveth
    Mar 25 '17 at 9:55

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