American astrophysicist and Nobel laureate Arno Penzias, who has died aged 90, was responsible for one of the greatest cosmological revelations of the 20th century: the discovery of the cosmic microwave background, the leftover radiation from the Big Bang.
He first demonstrated its existence in 1965 with his collaborator Robert Wilson, and five years later-along with Wilson and another astrophysicist, Keith Jefferts-went on to detect the presence of interstellar carbon monoxide, expanding the field of molecular line astronomy.
When Penzias discovered the cosmic microwave background, cosmology was still in a state of uncertainty. Einstein's proposal that the universe was homogeneous, the same at every location, and isotropic, the same in every direction, was still valid, but had not yet been conclusively demonstrated.
Alexander Friedmann had shown what cosmological histories were possible within the context of general relativity, and most of these arose in a state of infinite density, which the British astronomer Fred Hoyle sarcastically called the Big Bang.
The properties of the early Big Bang universe had been investigated by Edwin Lemaître, George Gamov, and the Soviet cosmologist Yakov Zeldovich, and collaborators of both Gamov and Zeldovich had predicted that there would be remnant background radiation left over from the hot Big Bang phase.
The laconic one-page 1965 article by Penzias and Wilson announcing an isotropic excess background radiation of 3.5 K at a frequency of 4080 MHz had an explosive impact. To most cosmologists, this showed that the universe really did form from a hot big bang, and it marked the beginning of the end for Hoyle's steady state theory.
As Chicago cosmologist Michael Turner put it, "The discovery of the cosmic microwave background transformed cosmology almost overnight from the domain of a handful of astronomers into a 'respectable' branch of physics."
Penzias was born in Munich, Germany, the son of Justine (née Eisenreich) and Karl Penzias, who ran a training company. After narrowly avoiding deportation to Poland in 1938, Arno, aged six, and his brother Gunther, aged five, were evacuated to Britain in 1939 via the Kindertransport, the operation that saved tens of thousands of Jewish children.
His parents also managed to reach Britain, and in 1940 the family gained entry to the US, thanks to a Jewish American foreigner who acted as a sponsor. They settled in the Bronx, New York, where his parents secured housing by acting as supervisors of an apartment complex.
Penzias graduated from Brooklyn Technical High School in 1951 and enrolled to study chemistry at the City College of New York, switching to physics before graduating in 1954, the year he married Anne Barras, a student at Hunter College.
After serving two years as a radar officer in the US Army Signal Corps, he started as a research assistant at Columbia University's radiation laboratory, which at the time was heavily involved in microwave physics. He worked under Charles Townes, who later received the Nobel Prize for the invention of the maser, a device that produces and amplifies microwave radiation at a very precise frequency used in atomic clocks.
Penzias enrolled as a graduate student at Columbia University in 1956 and completed a doctorate in physics in 1962 before moving into the commercial sector with the Bell Labs radiophysics group in Holmdel, New Jersey.
There he especially hoped to make use of the large microwave horn antenna built for the Echo experiment, in which radio signals were transmitted from a similar antenna in France, reflected by the moon, and then received in New Jersey.
Together with Wilson, he embarked on two experiments, one to measure the radio spectrum of the Milky Way, and the second to make an absolute flux measurement of Cassiopeia A, a supernova that emits strong radio waves.
Penzias and Wilson soon encountered a problem well known to Bell Labs engineers: that the horn antenna was surprisingly noisy. One problem was that pigeons were nesting in the horn and as a result the antenna was covered in what Penzias subtly described as "a white dielectric substance".
Both the substance and the pigeons were removed, then the antenna was taken apart and reassembled to ensure the joints had good electrical contact, while the horn's response was calibrated by flying a helicopter with a transmitter above the antenna. But despite their best efforts, they were left with a persistent background noise, which was the same no matter which direction the antenna was pointed.
At this point they had a stroke of luck. Penzias mentioned the sound in a telephone conversation with Bernie Burke, a radio astronomer at nearby Princeton. Burke told him that a group of scientists at Princeton University led by Robert Dicke were building equipment to look for microwave background radiation. In a Eureka moment, Penzias then realized that the hiss he couldn't get rid of was actually the cosmic microwave background the Princeton scientists were looking for.
Penzias and Wilson then wrote their one-page article, which was published alongside an article by Dicke, Jim Peebles, Peter Roll, and David Wilkinson explaining that the microwave background radiation was the expected remnant of the hot Big Bang phase of the early universe.
Penzias and Wilson then began bringing the sensitive Bell Labs microwave receiver to the Kitt Peak Millimeter Wave Telescope in Arizona, and in 1970, together with Jefferts, they announced the detection of the interstellar carbon monoxide molecule in Orion's star-forming cloud. Within a year, Penzias, Wilson and other collaborators had discovered six more interstellar molecules, and by 1976 more than thirty had been found; today the number stands at 233.
In the dense clouds of dust and gas where new stars form, the gas is in the form of molecular hydrogen, while a large number of other molecules are present. Penzias' discovery of interstellar molecules enabled the development of molecular line astronomy, in which the physical conditions of the gas are investigated and the complex chemistry of the clouds is modeled.
Between 1967 and 1985, Penzias had a part-time collaboration with Princeton and supervised PhD students there. At Bell Labs, he served as director of the radiophysical research laboratory from 1976 to 1979 and as vice president of research at AT&T Bell Laboratories from 1981 to 1995. In 1978 he shared the Nobel Prize for physics with Wilson.
Penzias' marriage to Anne ended in divorce in 1995, and that year he moved to California to become chief scientist of Bell Labs spinoff company Lucent Technologies. When he retired in 1998, he joined a venture capital firm in Silicon Valley, where he advised small start-up companies.
In 1996, he married Sherry Levit, a Silicon Valley executive.
She survives him, along with three children from his first marriage, Sherry's two children from a previous relationship, 12 grandchildren and Gunther.
* Arno Allan Penzias, astrophysicist, born April 26, 1933; died January 22, 2024
