The Early Universe

After the Big Bang but before matter had thinned and cooled enough for radiation to move around, the early universe was one gigantic black body. The constant absorption and remission of high-energy photons spread energy evenly throughout the universe, and when the light finally leapt free of matter, it bore the mark of that thermal equilibrium. The cosmic microwave back-ground had the telltale characteristics of black-body radiation. At the time of decoupling, the radiation was a black body shining at several thousand degress with a mix of wavelengths. Now, billions of years later, it has cooled somewhat: it has become a black body at just about 2.7 degrees above absolute zero. Absolute zero is the coldest temperature possible. What we call temperature is really a measure of the average energy of motion of the particles of matter. Even in a piece rock the atoms, fixed in their places, are vibrating with energy at ordinary temperatures. Absolute zero is the point, -273C or -459F, where even that motion stops.