The American astrophysicist and Nobel laureate Arno Penziaswho died aged 90, was responsible for one of the greatest cosmological revelations of the 20th century – the discovery of cosmic microwave background, the leftover radiation from the big bang.
He demonstrated its existence for the first time in 1965 with his collaborator Robert Wilsonand then, five years later – along 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 suggestion that the universe is homogeneous, the same in every place, and isotropic, the same in every direction, still held, but was not conclusively demonstrated.
Alexander Friedmann showed what cosmological histories are 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 were investigated by Edwin Lemaître, George Gamov and the Soviet cosmologist Yakov Zeldovich, and collaborators of both Gamov and Zeldovich predicted that there would be residual 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 showed that the universe really did originate in a hot big bang, and it was 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 realm of a handful of astronomers to a ‘respectable’ branch of physics.”
Penzias was born in Munich, Germany, the son of Justine (nee Eisenreich) and Karl Penzias, who ran a leather business. 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 the UK, and in 1940 the family was granted entry to the US, thanks to a Jewish-American alien who acted as a sponsor. They settled in the Bronx, New Yorkwhere 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. , got married.
After serving as a radar officer in the US Army Signal Corps for two years, he began 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 at a very precise frequency used in atomic clocks.
Penzias entered Columbia University as a graduate student in 1956 and completed a PhD in physics in 1962 before moving into the commercial sector at the Bell Labs radio physics 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 sent from a similar antenna in France, reflected off the moon, and then received in New Jersey.
He teamed up with Wilson and undertook 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 encountered a problem familiar to the Bell Labs engineers: that the horn antenna was surprisingly noisy. One problem was that pigeons were nesting in the horn and the antenna was consequently covered in what Penzias finely described as “a white dielectric substance”.
Both the dust and the pigeons were removed, then the antenna was disassembled and reassembled to ensure that 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 whatever direction the antenna pointed.
At this point they had a stroke of luck. Penzias mentioned the noise in a phone 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 was 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 paper, published alongside a paper 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 taking the sensitive Bell Labs microwave receiver to the Kitt Peak Millimeter 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 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’ 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 modelled.
Between 1967 and 1985, Penzias had a part-time connection with Princeton, supervising PhD students there. At Bell Labs, he was director of the radiophysics research laboratory from 1976 to 1979 and vice president of research at AT&T Bell Laboratories from 1981 to 1995. He shared the 1978 Nobel Prize in physics with Wilson.
Penzias’ 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. When he retired 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.
