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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?

uhoh
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Michael Walsby
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  • https://chat.stackexchange.com/transcript/message/50517334#50517334 – PM 2Ring Jun 12 '19 at 11:45
  • 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. – Michael Walsby Jun 12 '19 at 11:49
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    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. – PM 2Ring Jun 12 '19 at 11:59
  • 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? – Carl Witthoft Jun 12 '19 at 18:03
  • No,I asked how far could we see before the most advanced instruments of the foreseeable future could detect nothing at all. Already the most distant galaxies we can see are only barely detectable. – Michael Walsby Jun 12 '19 at 20:25
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    @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? – Ingolifs Jun 12 '19 at 22:56
  • We can already see the most distant light: it’s impossible to detect any EMR older (ie more distant) than the CMB. Your question would be much more interesting if you moved beyond “seeing” (ie light) and explicitly included detecting things like gravitational waves and cold neutrinos, which exist from the first minutes after the BB. – Chappo Hasn't Forgotten Jun 13 '19 at 01:59
  • @chappo Then the answer becomes 47.5 billion light years. – ProfRob Jun 13 '19 at 06:21
  • @RobJeffries yes, if we could develop the technologies to detect them. Do you think either is a realistic possibility? For example, do we need to solve quantum gravity before we can model a BB GW signature? I’d have thought low-energy neutrinos would be just about impossible to detect, but then Einstein himself thought we’d never be able to detect GWs. – Chappo Hasn't Forgotten Jun 13 '19 at 08:16
  • There seems to be an assumption that gravitons exist & gravity can be quantised,but maybe there are no gravitons (or anti-gravitons),& gravity can't be quantised. If a graviton set out from Earth & hit a proton 2.5m light years away in the Andromeda galaxy,I'd say that was bloody good shooting,wouldn't you? Almost unbelievably good. – Michael Walsby Jun 13 '19 at 17:06
  • related and with potentially helpful answers Is there anything currently 46 billion light years away from Earth that we can see? – uhoh Jul 24 '22 at 13:43

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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.

ProfRob
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