cosmic microwave background
/ C-M-B /
Point a sensitive microwave antenna at any patch of empty sky, away from stars and galaxies, and you still catch a faint, steady hiss coming from every direction. That hiss is the cosmic microwave background, or CMB — the oldest light in the universe, a relic glow left over from when the cosmos was young, hot, and glowing. It is, quite literally, the afterglow of the Big Bang, filling all of space.
Here is its origin. For its first 380,000 years the universe was so hot that atoms could not hold together; it was an opaque fog of free electrons and bare nuclei that scattered light, like the inside of a dense cloud. As expansion cooled the universe to about 3,000 degrees, electrons finally combined with nuclei into neutral atoms (called recombination), the fog cleared, and light streamed free for the first time. That released light has been travelling ever since, stretched by the expanding universe by a factor of about 1,100, cooling from a hot orange glow to faint microwaves at just 2.7 degrees above absolute zero. The CMB has the perfect spectrum of a glowing object at that temperature — a near-flawless blackbody — and it is astonishingly uniform across the whole sky, identical in every direction to about one part in 100,000.
The CMB is arguably the most important single dataset in cosmology. Its near-perfect blackbody shape confirms the hot Big Bang; its tiny temperature ripples (one part in 100,000) are the seeds from which all galaxies grew, and the precise pattern of those ripples lets cosmologists measure the universe's age, geometry, and contents to remarkable precision. A common confusion: the CMB is not radiation coming from one distant object or one explosion site, but light that was emitted everywhere at once and is reaching us now from the edge of what we can see — the surface of last scattering.
Old analogue televisions tuned between channels showed snow and static; a small fraction of that flicker was the universe's CMB hitting the antenna. Detected directly in 1965, the same glow is now mapped sky-wide by satellites to fractions of a millionth of a degree.
The CMB is the relic light from when atoms first formed, now cooled to microwaves.
The CMB is not light from a single distant source. It was emitted everywhere at once when the universe became transparent, and reaches us from the surface of last scattering, the boundary of the observable universe in time.