The Cosmic Microwave Background: The Oldest Light We Can See

0:00Radio telescopes pick up a faint glow of microwaves from every direction in the sky.

0:07Radio astronomers found it by accident in 1965, as a hiss their antenna could not get rid of.

0:14It is called the cosmic microwave background, or CMB: microwaves from space filling the whole sky.

0:23It is not starlight: it is just as bright between the stars, and older than any star.

0:30After the big bang, the universe was too hot for atoms: a glowing fog of nuclei and free electrons, called plasma.

0:40Light kept bouncing off the free electrons, so it could not travel far.

0:45About 380,000 years after the big bang, it had cooled enough for electrons to join nuclei as the first atoms.

0:56Now its visible and infrared light could travel freely across space.

1:01As space expanded, it stretched the wavelength, the distance from crest to crest, over 1,000 times, into microwaves.

1:12Today, 13.8 billion years later, the glow's temperature is about 2.7 kelvin, just above absolute zero: about minus 270 degrees Celsius.

1:25Satellites have mapped this glow across the whole sky, shown here as one oval map.

1:31The colours show warmer and colder spots, differing by only about 1 part in 100,000.

1:39The spots show where matter was slightly denser. Gravity pulled in more matter there, and galaxies grew.

1:47The cosmic microwave background is the oldest light we can see, set free when the first atoms formed.