Cosmic Microwave Background: Evidence for the Big Bang (Penzias and Wilson, 1964)

In 1964, Bell Labs radio astronomers Arno Penzias and Wilson accidentally detected a uniform microwave hiss coming from every direction in the sky — and could not eliminate it, even after ruling out pigeon droppings in their antenna. Physicist Robert Dicke at Princeton immediately identified it as the cosmic microwave background (CMB): the afterglow of light from 380,000 years after the Big Bang, stretched by 13.8 billion years of cosmic expansion into the microwave band. The CMB is the most direct observational evidence that the universe began in an extremely hot, dense state. Penzias and Wilson won the 1978 Nobel Prize. Subsequent CMB maps by COBE (1992), WMAP (2003), and Planck (2013) have pinpointed the universe's age, composition, and geometry to extraordinary precision.