Characteristic Color of the Big Bang


Anyone who has used a toaster or electric stove, lit a candle, or watched a campfire knows that extreme heat produces different colors.  The coolest visible color is red, emitted by an object about 3,000°C, only a little cooler than the universe on its 300,000th birthday. Hot objects radiate at all wavelengths, but not equally.  Heating elements, like the one in your toaster, though visible as "red", emit most of their light in the infra-red, which is invisible to us. Objects that are 3000C emit most of their light at red wavelengths. 
However, just as the light of stars and galaxies that are moving away from us at high speeds is Doppler shifted towards longer wavelengths, the red light that was released when atoms formed has since been stretched about 1,000 times, so that today it would appear to be light characteristic of an object just 3° above absolute zero.  The “color” that we observe would be in the microwave range, with a wavelength of about 1 millimeter.
If it seems odd to talk of microwaves as “light,” then the missing piece of information is that visible light, infrared and ultraviolet, microwaves, radio waves, X-rays, and gamma rays are really all the same thing—energy traveling through space with different wavelengths. The longest wavelengths are the radio waves, the shortest gamma rays, and visible light is somewhere in the middle.  Microwaves—the same sort of energy generated by a microwave oven—falls between infrared and radio waves.  These are all called electromagnetic waves since they are composed of oscillating electric and magnetic fields moving through space.