“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
The farther away in space we look, the farther back in time we’re seeing. Light arriving from a star ten light years away is ten years old; light that took a billion-year journey from a distant galaxy is a billion years old. If we look out today at the most distant light we can see, we discover that it originates from the Big Bang itself: the Cosmic Microwave Background, or CMB.
But this doesn’t mean the light has never interacted with anything since the birth of the observable Universe. In fact, many arose from matter/antimatter annihilations, all of them have scattered off of charged particles, and the CMB photons we detect today were all released when the Universe was a few hundred thousand years old. Because of the way the Big Bang works, the particles are literally everywhere, all at once, including right here.
The cosmic microwave background appears very different to observers at different redshifts, because they’re seeing it as it was earlier in time. Image credit: Earth: NASA/BlueEarth; Milky Way: ESO/S. Brunier; CMB: NASA/WMAP.