The universe might be like a room where every wall is a mirror. Light from a distant galaxy, having circled the cosmos, reaches us from both directions at once, painting identical patterns on the sky. Astronomers hunt for such reflections in the ancient light, cooled after the [tag:big_bang]Big Bang[/tag]. Using computer models, they predict what traces a closed universe would leave. So far, no matches: the nearest 'copy' of us, if it exists, is hidden beyond the visible horizon.

Future telescopes, like LiteBIRD, will be able to catch the subtlest hints in the direction of this light's oscillations and in its breakdown into colors, to measure the speed of cosmic expansion. Key clues will also come from light bending around massive objects and tremors in space-time.

The most astonishing implication: if the cosmos is a mirror room, somewhere there might be an exact copy of Earth, repeating our every move. This idea was first entertained by Einstein, and later expanded by Lemaître and Guth, linking the universe's geometry to invisible matter and explosive inflation in the first moments.