Where do the CMB Photons Come From?


Back in time denser / hotter.

Eventually hot enough that atoms become ionized:
Plasma - The Fourth State of Matter

Three familiar states of matter:
Solid - atoms are bonded together rigidly
Liquid - atoms are bonded less rigidly
Gas - atoms move fast and freely

Fourth state:
Plasma - atoms slam together so hard electrons are knocked free.
Becomes a "soup" of nuclei, electrons and photons.