Cosmic microwave background radiation reaches the Earth from all directions and is like an echo of the Big Bang. The background pattern we see today emerged when the cosmos was only 400,000 years old. By that time its temperature had dropped to less than 2700c, and what had been hot, opaque plasma became transparent gas through which electromagnetic radiation could travel freely. Since then, the universe has expanded and cooled so that the temperature of the cosmic background radiation is now only 2.7c above absolute zero (-273.15c). Thanks to tiny variations in the cosmic background radiation we can hope to learn what happened at the time of the Big Bang.