How do astronomers find the oldest light in the universe?

The cosmic microwave background, or CMB, is the faded afterglow of the hot early universe. It was released about 380,000 years after the Big Bang, when electrons and protons combined to make neutral atoms and light could travel freely. Today it fills all of space and is observed with microwave telescopes as a nearly uniform background at 2.725 K. Tiny variations in its temperature encode the seeds of galaxies and the composition of the cosmos.