The spot where each photon began its cross-cosmos journey is where it had smacked into the last electron that would ever stand in its way—the “point of last scatter.” As more and more photons escape unsmacked, they create an expanding “surface” of last scatter, some 120,000 years deep. That surface is where all the atoms in the universe were born: an electron joins an atomic nucleus, and a little pulse of energy in the form of a photon soars away into the wild red yonder.
