Measurements of the cosmic microwave background—the ancient light left over from the Big Bang—indicate that the universe is remarkably flat. That flatness has often been taken as a clue that the universe might be infinite, stretching out in all directions without curvature or boundary.
But physicists caution that the conclusion does not follow. The observable cosmos could be only a small patch of a much larger universe that is curved on scales we cannot see. In other words, the flatness we detect locally does not rule out a global geometry that is curved, or even finite.
Space could even be flat while still wrapping back on itself in some exotic topology, the source notes. The answer to whether the universe is infinite, it seems, gets weird fast—and depends on what lies beyond our cosmic horizon.