What is the faint glow left over from the Big Bang?

The faint glow left over from the Big Bang is the cosmic microwave background, a sea of microwaves that fills the whole universe. To an ordinary telescope, the space between stars looks black. But a sensitive radio telescope picks up a dim, almost perfectly even glow coming from every direction, belonging to no star or galaxy. It is light released when the universe was about 380,000 years old.

Before then, the young universe was a hot, glowing fog of charged particles that scattered light like a cloud. Once it cooled to about 3,000 degrees, electrons and protons joined into atoms, the fog cleared and the light flew free. It has been travelling ever since, stretched by the expansion of space into microwaves. Today its temperature is just 2.7 degrees above absolute zero.

It was found by accident. In 1964 Arno Penzias and Robert Wilson, working on a sensitive antenna at Bell Labs in New Jersey, couldn't get rid of a faint hiss that came from the whole sky, day and night. They checked their equipment and even evicted some pigeons and cleaned out their droppings. The hiss stayed. Meanwhile, physicists at Princeton, 60 kilometres away, were preparing to search for exactly this radiation. When the call came, Robert Dicke told his team: 'Boys, we've been scooped.'