The American astrophysicist and Nobel laureate Arno Penzias, who has died aged 90, was responsible for one of the biggest cosmological revelations of the 20th century – the discovery of cosmic microwave background, the leftover radiation from the big bang.
He first demonstrated its existence in 1965 with his collaborator Robert Wilson, and then, five years later – with Wilson and another astrophysicist, Keith Jefferts – went on to detect the presence of interstellar carbon monoxide, launching the field of molecular line astronomy.
When Penzias discovered 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, still held sway, but had not been conclusively demonstrated.
Alexander Friedmann had shown which cosmological histories were possible within the context of general relativity, and most of these originated 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 explored by Edwin Lemaître, George Gamov and the Soviet cosmologist Yakov Zeldovich, and collaborators of both Gamov and Zeldovich had predicted that there should be relic background radiation left over from the hot big bang phase.
The laconic, one-page, 1965 paper by Penzias and Wilson announcing a 3.5K isotropic excess background radiation at a frequency of 4080 MHz had an explosive impact. For most cosmologists this demonstrated that the universe really did originate in a hot big bang, and it marked the beginning of the end for Hoyle’s steady state theory.
As the Chicago cosmologist Michael Turner put it: “The discovery of the cosmic microwave background transformed cosmology from being the realm of a handful of astronomers to a ‘respectable’ branch of physics almost overnight.”
Penzias was born in Munich, Germany, the son of Justine (nee Eisenreich) and Karl Penzias, who ran a leather business. Having narrowly avoided 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 the UK, and in 1940 the family were granted entry to the US, thanks to a Jewish-American stranger who acted as a sponsor. They settled in the Bronx, New York, where his parents secured housing by acting as superintendents of an apartment building.
Penzias graduated from Brooklyn technical high school in 1951 and enrolled to study chemistry at the City College of New York, later switching to physics before graduating in 1954, the year he married Anne Barras, a student at Hunter College.
After serving for two years as a radar officer in the US Army Signal Corps he started out as a research assistant at Columbia University’s radiation laboratory, which was then heavily involved in microwave physics. He worked under Charles Townes, who was later awarded the Nobel prize for the invention of the maser, a device that produces and amplifies microwave radiation of a very precise frequency used in atomic clocks.
Penzias enrolled as a graduate student at Columbia University in 1956, completing a PhD in physics in 1962 before moving into the commercial sector with the Bell Labs radio-physics group at Holmdel, New Jersey.
There he particularly hoped to make use of the large microwave horn antenna that had been built for the Echo experiment, in which radio signals were transmitted from a similar antenna in France, bounced off the moon, and then received in New Jersey.
Teaming up 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 quickly ran up against a problem well known to the Bell Labs engineers: that the horn antenna was surprisingly noisy. One problem was that pigeons were nesting in the horn and that the antenna was consequently coated with what Penzias delicately 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 response of the horn was calibrated by flying a helicopter with a transmitter above the antenna. But despite all their efforts they were still left with a persistent background noise, which was the same in whichever direction the antenna was pointing.
At this point they had a stroke of luck. Penzias mentioned the noise in a phone conversation to Bernie Burke, a radio astronomer at nearby Princeton. Burke told him that a group of scientists at Princeton University led by Robert Dicke was building equipment to look for microwave background radiation. In a Eureka moment, Penzias then realised that the hiss that he could not get rid of was actually the cosmic microwave background that the Princeton scientists had been searching for.
Penzias and Wilson then wrote their one-page paper, which was published alongside a paper from Dicke, Jim Peebles, Peter Roll and David Wilkinson explaining that the microwave background radiation was the expected relic of the hot big bang phase of the early universe.
Penzias and Wilson then started to take the sensitive Bell Labs microwave receiver to the Kitt Peak Millimetre Wave telescope in Arizona, and in 1970 they announced the detection, with Jefferts, of the interstellar carbon monoxide molecule in the Orion star-forming cloud. Within a year Penzias, Wilson and other collaborators had discovered a further six interstellar molecules and by 1976 more than 30 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, with a host of other molecules present. Penzias’s discovery of interstellar molecules allowed the development of molecular line astronomy, in which the physical conditions of the gas are probed and the complex chemistry of the clouds is modelled.
Between 1967 and 1985 Penzias had a part-time association with Princeton and supervised PhD students there. At Bell Labs he was director of the radio physics research laboratory from 1976 to 1979 and vice-president of research at AT&T Bell Laboratories from 1981 to 1995. He shared the Nobel prize for physics with Wilson in 1978.
Penzias’s marriage to Anne ended in divorce in 1995, and that year he moved to California to become chief scientist of the Bell Labs spin-off company Lucent Technologies. On retiring in 1998 he joined a Silicon Valley venture capital firm, advising 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 26 April 1933; died 22 January 2024
