The Cosmic Past

• recent past
• distant past
• the big bang (pardon my French)

The Cosmic Microwave Background: Discovery and Average Properties

• discovery; Nobel-prize winning 1.5-page paper
• temperature
• spectrum: purely thermal within current limits

The Atomic Age

• microphysics: recombination
  • ionized (plasma) to neutral (gas)
  • opaque to transparent
• surface of last scattering