Question: what is the cosmic microwave background?

Matthew Malek answered on 10 Mar 2014:

The cosmic microwave background is the “footprint” of the Big Bang. It’s the earliest picture of the universe. That said, this “picture” dates from when the universe was 380,000 years old!

Now 380,000 years is still a pretty small fraction of the universe’s current age, 13.7 billion years. In fact, it’s less than a percent of a percent. Even so, 380,000 years is a pretty long time! Why is the earliest picture of the universe so late?

It’s because the early universe was not transparent to light. In the early days, the universe was really hot — too hot for atoms to form! So you had charged particles like protons and electrons moving about. Light interacts with charged particles, so the light got absorbed, scattered, re-emitted, and so on. Thus, the young hot universe was opaque.

At about 380,000 years, the universe had cooled to the point where the first neutral atoms could form. That’s when the universe became transparent to light, as the neutral matter allowed light to pass on by. So the cosmic microwave background is the earliest possible picture of the universe taken with light.

There are other ways to probe the universe at earlier times, some of which are hot areas of active research. If we could detect the cosmic neutrino background, or gravitational waves, both would give us direct information going further back than the light of the cosmic microwave background.

That said, I should point out that we have learned an awful lot from studying the cosmic microwave background, including the precise age of the universe, the amount of dark matter and dark energy, the amount of ordinary matter, the “shape” of the universe. Observing the CMB with satellite telescopes like CoBE, WMAP, and Planck has taught us so much! (In fact, the CoBE scientists got the Nobel Prize for their measurements of the cosmic microwave background!)

There’s quite a bit more that could be said about the CMB, but I’ll leave you with one fun fact: If you turn on your telly with an antenna (not cable) and tune in to a channel that isn’t broadcasting, the background noise (or snow) that you see is photons — particles of light — from the cosmic microwave background striking the antenna! Likewise, if you turn on your radio and find a spot on the dial with no station, the static noice you hear comes from CMB photons.