“Protons, electrons and photons were scattered equally throughout, so that the emission and absorption of energy balanced each other in perfect equilibrium: a state of affairs known as 'black-body radiation'. At something over 379,000 years after the big bang, the universe had cooled down sufficiently to form neutrally charged atoms, which could not absorb all the thermal energy as perfectly as before. Instead, high-energy photons began to travel on their own through space, rendering the universe transparent instead of opaque. As the universe expanded over the subsequent 15 billion years these photons lost energy and red-shifted.” Artistic LicenseWriters Can T MathWriters Cannot Do Math Book:Science: Antiquity and Its Legacy Source: Science: Antiquity and Its Legacy