the cosmic microwave background

Turn a sensitive radio antenna to any patch of empty sky, well away from stars and galaxies, and you still catch a faint hiss of microwaves, coming from every direction with almost exactly the same intensity. This is the cosmic microwave background, the oldest light in the universe, released when the cosmos was a few hundred thousand years old. It is a photograph of the infant universe, and its discovery in 1965 turned the Big Bang from a speculation into the standard model of cosmology.

For its first 380,000 years the universe was an opaque plasma of free electrons and nuclei; light scattered off the electrons and could not travel far. As expansion cooled the plasma to about 3000 K, electrons and protons combined into neutral atoms, an event called recombination, and suddenly the fog cleared: photons streamed freely and have travelled ever since. Those photons are the CMB. Cosmic expansion has stretched their wavelengths by a factor of about 1100, so a plasma once at 3000 K now appears as a near-perfect blackbody at T = 2.725 K, peaking in the microwave band, following Planck's radiation law with astonishing fidelity.

The CMB is extraordinarily uniform, but the tiny departures from uniformity, temperature ripples of about one part in 100,000, are the treasure. They are the seeds of all later structure, and their statistical pattern encodes the geometry and contents of the universe: the spacing of the ripples reveals that space is flat, and their amplitudes pin down the amounts of ordinary matter, dark matter, and dark energy. A common confusion: the CMB does not come from a special place. It surrounds us because the plasma that emitted it filled all space; we see the shell of it whose light is only now reaching us.