The night sky appears completely black, stretched between the stars. The heavens, however, are literally glowing, everywhere, with microwave light left over from their violent infancy. This is the cosmic microwave background radiation (CMB for short). The CMB is the foundation of the most important discoveries of modern cosmology: the history of the very early universe and its continual expansion since the very beginning.
A Map of the CMB across the sky, taken by the Planck space telescope. (Public Domain.)
The CMB didn't start out as microwaves. Its light was born as warm infrared and bright visible radiation. This spectrum of colors perfectly matches those given off by an object heated to about 3000 Kelvin (close to 5000 degrees F). The gleam given off by a hot object, called black body radiation, is familiar to everyone as the orange glow from a stove burner or the bright filament of an incandescent light bulb.
What gave off this black body glow? The entire universe at that time! The CMB provides clear evidence that the very early cosmos was entirely in a state called plasma: a soup of electrons, protons, neutrons and light waves careening through space with too much energy for any atoms to form. Any larger pieces of matter that formed were blown apart by fast-moving particles. The light waves were trapped in the thick soup, continuously being absorbed and re-emitted, giving the subatomic particles too much energy to coalesce.
Things cooled down enough eventually that the light waves no longer had enough energy to tear atoms apart before they could fully form. Atoms fused together; they could only absorb a few specific colors of light, allowing most of it to escape. These photons were all given off at one black body temperature. This means that from one moment to the next, the universe transformed from a glowing opaque fog into a transparent open space, filled with patches of gases and solids. Most of the photons burst out freely into space in a huge flash, creating the CMB.
What cooled the photons down? Physicists realized that it was due to another very important property of the universe: the expansion of space itself. As space enlarges, photons still travel at the same speed through it, but their waves are stretched longer, like the coils of a spring being pulled apart. This gradually drains their energy. In the 13 billion years since the creation of the CMB, these photons have gradually become weaker and weaker. They are now stretched out roughly 1000 times, from their original length of a micrometer to nearly a millimeter. This is clear evidence for space itself expanding through its entire history.
The greatest triumph of the CMB is reconstructing the origin of the universe. Its discovery was the first overwhelming evidence for the Big Bang. The Big Bang concept originated when astronomer Edwin Hubble observed that everything in the universe seems to be moving away from everything else, but this was considered unproven. Theories then created for a Big Bang suggested the existence of a CMB as the cosmic afterglow of the primordial explosion.
The CMB shows us that the universe used to be a tiny, compact, extremely hot place. This is a reference point looking back in time toward the instant of the Big Bang. Extrapolating to the moment the CMB was released, the universe was only 370,000 years old and billions of times smaller in volume than today. Additionally, small fluctuations in the CMB show that tiny variations in the density of the young universe seeded the complex structure of galaxies we see in the universe now. Areas with slightly more matter gradually pulled further together gravitationally and later formed clusters of galaxies like ours.
Before discovery of the CMB, a model in which the universe exists roughly as it is now -- forever the same, past and future -- was still plausible. Now we have strong evidence for a Big Bang, an early universe made of plasma, and expansion and cooling of everything over the 13.8 billion years since.
We are all familiar with these concepts, but, much like plate tectonics, this theory is only a few decades old, enabled by modern physics instruments and theories!