Circles-in-the-sky searches and observable cosmic topology in a flat Universe

Understand

[Abridged] In a Universe with a detectable nontrivial spatial topology the last scattering surface contains pairs of matching circles with the same distribution of temperature fluctuations - the so-called circles-in-the-sky.

• Searches for nearly antipodal circles in maps of cosmic microwave background have so far been unsuccessful.
• This negative outcome along with recent theoretical results concerning the detectability of nearly flat compact topologies is sufficient to exclude a detectable nontrivial topology for most observers in very nearly flat positively and negatively curved Universes ($0<|\Omega_{tot}-1| \lesssim 10^{-5}$).
• Here we investigate the consequences of these searches for observable nontrivial topologies if the Universe turns out to be exactly flat ($\Omega_{tot}=1$).