Arno Penzias and Robert Wilson, radio astronomers at Bell Telephone Laboratories in Holmdel (New Jersey), detected in 1964-65 an isotropic, constant microwave noise with no identifiable source while calibrating a horn antenna originally designed for satellite communications. After methodically ruling out every possible source of interference — including, famously, pigeon droppings roosting in the antenna — they concluded the signal was real and of cosmic origin. Simultaneously and unaware of the other group's work, Robert Dicke and his team at Princeton University were building a detector specifically to search for this radiation, theoretically predicted by George Gamow, Ralph Alpher, and Robert Herman in 1948 as a thermal remnant of the Big Bang. A phone call between the two groups in 1965 confirmed that Penzias and Wilson's signal was exactly the radiation Dicke was looking for. The discovery constitutes the first direct observational evidence of the Big Bang, transforming cosmology from a theoretical debate between the 'expanding universe' model and the 'steady state' model into a discipline with decisive empirical evidence favoring the former. Later missions (COBE 1989, WMAP 2001, Planck 2009) map the tiny temperature variations of this radiation with increasing precision, making it the central observational tool of modern cosmology.