Radio telescopes are important in astronomy because optical telescopes can only observe objects that emit or reflect light. As light comprises only a narrow range of the electromagnetic spectrum, radio telescopes have played a vital role in obtaining knowledge of the universe.
Probably the single most important discovery made through the use of radio telescopes occurred in 1963. Astronomers Robert Woodrow Wilson and Arno Penzias were experimenting with the Holmdel Horn Antenna, which kept picking up interference. They could find no mechanical or natural reason for the persistent background noise, but they found a theoretical explanation: the amount of radiation they were detecting matched the cosmic microwave background radiation predicted by the Big Bang theory. Penzias and Wilson won a Nobel Prize for their accidental discovery.