In “Circles in the Sky,” Cornish, Spergel and Starkman explained how cosmological data might reveal that our universe has topology like that of a 3D torus (one of many shapes they considered). They proposed looking for this evidence in the cosmic microwave background (CMB), a steady stream of photons from the early universe that reaches us from all directions. The CMB tells us how the universe looked just 380,000 years after the Big Bang, when light was first able to travel through the cosmos unimpeded. By observing these photons today, we can map a spherical surface called the last scattering surface (LSS) — a snapshot of the universe at that early time.
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Cosmologists Try a New Way to Measure the Shape of the Universe | Quanta Magazine
Is the universe flat and infinite, or something more complex? We can’t say for sure, but a new search strategy is mapping out the subtle signals that would reveal if the universe had a shape.
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