The Big Bang produced a hot, dense, nearly homogeneous universe. As the universe expanded and cooled, particles, then nuclei, and finally atoms formed. At a redshift of about 1100, equal to about 400,000 years after the Big Bang, when the primordial plasma filling the universe cooled enough for protons and electrons to combine into neutral hydrogen atoms, the universe became optically thin by which photons from this early era no longer interacted with matter. We detect these photons today as the cosmic microwave background (CMB). The CMB shows that the universe was still smooth and uniform.