The cosmologist Arno Penzias, who discovered the cosmic microwave background (CMB) with Robert Wilson, died on 22 January at the age of 90. He shared a half of the 1978 Nobel Prize for Physics with Wilson, with the other half awarded to Pyotr Kapitsa for his work in low-temperature physics.

Penzias was born in Munich, Germany, on 26 April 1933. At the age of six, Penzias and his family fled Nazi Germany, first to England before settling in New York in 1940. In 1954 Penzias graduated in physics from the City College of New York before serving as a radar officer in the US Army Signal Corps until 1956.

He then moved to Columbia University’s radiation laboratory working on microwave physics, earning a PhD in 1962 under the guidance of maser inventor Charles Townes.

Penzias then took a position at Bell Labs, New Jersey, developing microwave receivers for radio astronomy. There he worked with Wilson on a 6 m-diameter horn-reflector antenna with a 7 cm ultra-noise receiver. In 1964 the pair came across an excess source of radiation at 3 K that they could not eliminate.

Initially, they thought the hiss of radio waves at a wavelength of 7.35 cm had a terrestrial origin, given that it was approximately uniform in all directions on the sky. They even famously wondered if it was caused by pigeon excrement on the antenna.