The hot and cold spots from the hemispheres of the sky, as they appear in the CMB. This encodes a tremendous amount of information about the early Universe. Image credit: E. Siegel / Damien George / http://thecmb.org/ / Planck Collaboration.

Where is the Cosmic Microwave Background?

We claim it’s the leftover glow from the Big Bang, but where is this light actually coming from?

“We are told to let our light shine, and if it does, we won’t need to tell anybody it does. Lighthouses don’t fire cannons to call attention to their shining- they just shine.” -Dwight L. Moody

When you look out at the distant Universe, you’re also looking back in time, thanks to the fact that the speed of light — although huge — is finite. So if you look back at the farthest thing you can see, at the very first light visible to our equipment, you’re bound to reach something. In our Universe’s case, to the best of our knowledge, that’s the leftover glow from the Big Bang: the cosmic microwave background (CMB). But it’s possible that the Universe is infinite; there’s no reason to believe that the CMB we see is the edge or boundary in any way. So where, exactly, is the CMB?

The timeline of our observable Universe’s history. Image credit: NASA / WMAP science team.

Let’s start with the Big Bang itself, so we can put the CMB in perspective, and go from there. When the hot Big Bang first began — after a period of cosmic inflation that lasted for an indeterminate amount of time — the Universe…