gravity forced atoms together, they collided more often and jiggled more frenetically. That drove up temperatures in the clumpier regions, as more heat was concentrated in smaller volumes of space. (The same principle explains why a tire gets warmer when you pump it up.) While most of the universe kept cooling, the clumpy bits began to heat up again. Eventually, some clumps got so hot that protons could no longer hold on to their electrons. Atoms broke apart, re-creating inside each clump the charged plasma, crackling with
