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Radius of the observable universe is 47.5 billion light years,which means we are seeing things which are about 48 billion light years away. At a rough estimate,at what distance would our finest instruments and telescopes be unable to see or detect anything at all,& is there anything in the idea that if we were able to see far enough,we might see our own galaxy as it appears from an immense distance,its light having circled the universe? And as space is not a perfect vacuum,wouldn't the light be significantly retarded by the end of such a long journey?

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  • $\begingroup$ chat.stackexchange.com/transcript/message/50517334#50517334 $\endgroup$ – PM 2Ring Jun 12 at 11:45
  • $\begingroup$ PM,I remember we tried to use the chatline about a week ago,& I found it unsatisfactory. You agreed at the time it needed rationalisation. $\endgroup$ – Michael Walsby Jun 12 at 11:49
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    $\begingroup$ I said that chat is somewhat limited. But it is still quite usable. I put that link here because in that chat I mentioned that yes, in theory a photon could "circumnavigate" the universe, if the universe has positive curvature, and if the rate of expansion were small enough, but that current measurements indicate that neither of those things are likely to be true. $\endgroup$ – PM 2Ring Jun 12 at 11:59
  • $\begingroup$ You seem to be mixing two questions. First - can we reliably detect single photons. Second, what's the most distant object that was within our observable sphere at the time it emitted a photon? $\endgroup$ – Carl Witthoft Jun 12 at 18:03
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    $\begingroup$ @MichaelWalsby suspect that you are not only asking about detection limits but also angular resolution limits (i.e. resolving small and faraway objects into sensible pictures). Maybe you want to add that part in explicitly? $\endgroup$ – Ingolifs Jun 12 at 22:56
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The furthest we can "see" is the cosmic microwave background at a redshift of about 1100.

The proper distance of the CMB-emitting gas that we see today is about 46 billion light years.

If you are talking about galaxies, then the first are thought to have formed at redshifts of about 20 (current distance 36 billion light years) and beyond that are the universe's "dark ages" where no stars or galaxies should exist; the most distant galaxies observed at the moment are at redshifts of about 10 (current distance 31 billion light years).

  • All current distances depend on the adopted cosmological model. The numbers are for a flat $\Lambda$CDM universe with the Planck cosmological parameters.

There is no chance of seeing our own Galaxy, since it only formed about 12 billion years ago and the universe is a lot bigger than 12 billion light years.

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  • $\begingroup$ I was thinking more in terms of galaxies rather than gases,but in particular I wanted to know how far would it be until the best instruments & telescopes available in the foreseeable future could detect nothing at all. Already the farthest galaxies are becoming small & faint. $\endgroup$ – Michael Walsby Jun 12 at 20:21
  • $\begingroup$ This is a thought provoking answer! However I'm not sure if said provoked thoughts I'm having are correct. So I've just asked Is the CMB's "surface of last scattering" 46 billion light-years away? $\endgroup$ – uhoh Jun 13 at 1:25

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