# What Is The Maximum Distance Our Finest Instruments Could See When They're Perfected?

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?

• 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
• 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
• @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

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