In the days before remote controls and cable TV, changing the channel often took you through unused channels filled with static. Most of that static came from the afterglow of the Big Bang — a background energy that fills the universe. It was first studied in detail by a spacecraft that was launched 25 years ago today. Big Bang theory says the early universe was incredibly hot and dense. As it expanded, it cooled. We can still see the energy produced by that early period, though, as a microwave glow that comes from every direction in the sky. Scientists discovered that glow in 1964. Early observations found absolutely no variations in it — it looked exactly the same in every direction. But that was a problem. It was hard to explain how such a smooth early universe could give rise to the “lumpy” modern universe, which is filled with galaxies and clusters of galaxies. COBE — the Cosmic Background Explorer — was designed to look for tiny variations in the microwave background. And it foun
