Why were/are released photons red-shifted?
The cosmic microwave background radiation is thermal radiation that was originally emitted at much higher frequency corresponding to a relatively warm 380,000 year old universe (3000 K). Since then, the universe has expanded, stretching the photons’ wavelengths with it- imagine wavy lines drawn on the surface of the balloon stretching out as the balloon is blown up (this is cosmological redshift). By now, the universe has expanded by a factor of about 1,000, and the light has stretched to microwave wavelengths.
