cosmic microwave background
/ see-em-bee /
Point a radio antenna at any patch of empty sky, in any direction, and you will pick up a faint hiss that does not come from any star or galaxy. That hiss is the oldest light in existence — a glow left over from the universe when it was a hot, glowing fog, only about 380,000 years after the Big Bang. It is called the cosmic microwave background, or CMB, and it fills all of space. In the 1960s two engineers stumbled on it as unexplained static in their antenna, mistook it at first for pigeon droppings, and ended up with a Nobel Prize for the discovery.
Here is what it is. In the very early universe everything was so hot that atoms could not hold together; matter was a glowing plasma of bare nuclei and free electrons, and light could not travel far before scattering off a charged particle, like trying to see through fog. As the universe expanded and cooled, there came a moment when electrons and nuclei finally joined into neutral atoms, the fog cleared, and light streamed free for the first time. That light has been travelling ever since, stretched by the expanding universe from a fierce glow into faint microwaves — radiation at a temperature of just 2.7 degrees above absolute zero, almost perfectly uniform across the whole sky.
The CMB is a particle-physics relic and one of our most precious sources of information. Its faint patterns — tiny variations in temperature, about one part in a hundred thousand from place to place — are a snapshot of the seeds from which all galaxies grew, and reading them tells us with remarkable precision how much ordinary matter, how much dark matter, and how much dark energy the universe contains. The CMB is in fact one of the strongest pieces of evidence for dark matter, and its near-perfect uniformity is a key clue pointing toward cosmic inflation in the universe's first instant.
If you tune an old analogue television between channels, a small fraction of the static snow on the screen is actually the cosmic microwave background — ancient light from the infant universe being picked up by your set's antenna.
The oldest light in the universe still fills the sky.
The CMB is not light from the Big Bang explosion itself, and not light from any object. It is the moment the whole universe became transparent, 380,000 years later — a glow coming uniformly from every direction at once, not from a point in the sky.