cosmic background radiation

“Electromagnetic echo” of the early universe; first predicted by the big bang models in the context of general relativity; later, from the 1960s on, observed with radio telescopes.

The cosmic microwave background contains important information about the properties and the earliest history of the universe. For instance, it can be used to deduce whether space is curved or Euclidean; more information about this can be found in the spotlight text Cosmic sound.