© Emilio Segre Visual Archives/American Institute of Physics/Science Photo Library
Holmdel horn antenna, Bell Laboratories, New Jersey, USA.
Holmdel horn antenna, Bell Laboratories, New Jersey, USA.
In 1963, two young researchers, Arno Penzias and Robert Wilson, joined forces to adapt a radio telescope, originally designed to establish the feasibility of global satellite communications, for astronomical work. The telescope, at Crawford Hill in New Jersey, was owned by the Bell telephone company, which had a policy of allowing its research teams time for purely scientific projects. Before this team could start making astronomical observations, they had to calibrate the telescope and remove all sources of interference – ‘noise’. It was during this process that they unexpectedly made a discovery that would earn them the Nobel Prize.
The business end of the telescope incorporated a very sensitive receiver to detect weak radio emissions from objects in space. Astronomers call these ‘signals’, but they are not artificial signals like TV transmissions, just the natural radio emissions produced by astronomical objects such as active galaxies. The strength of these signals is measured in terms of the temperature of equivalent blackbody radiation, and the receiver, a radiometer using maser amplifiers, was so sensitive that it could detect radiation as cold as a few degrees on the Kelvin scale (K), just above minus 273 degrees Celsius.
To calibrate the telescope, it was pointed at the sky (at nothing in particular) and the radiometer was switched between the signal coming from the antenna and the signal from a ‘cold load’ kept at a temperature of just over 4 K using liquid helium. This showed them how much hotter or colder the antenna signal was than the signal from the cold load. Penzias and Wilson expected the ‘temperature of the sky’ to be zero, so in this way they could make a calibration and subtract out all the known sources of noise, such as the temperature of the air above the antenna. What was left would be noise due to the antenna itself, which they intended to remove by appropriate techniques, such as polishing the metal to a smooth surface. But no matter how they tried, the team were never able to reduce the noise in the system to zero. They even tried laboriously cleaning out the droppings left in the antenna by nesting pigeons, and sticking shiny aluminium tape over all the riveted joints, but to no avail. They were left with what they called an ‘excess antenna temperature’ of between 2 K and 3 K, meaning that the radiation coming in to the radiometer from the antenna was at least 2 K hotter than they could explain. The signal was the same day and night, day after day and week after week. It corresponds to a very weak hiss of radio noise at microwave frequencies, like the ‘signal’ from a very cold microwave oven.
Throughout 1964, Penzias and Wilson remained baffled, and their whole radio astronomy project hung in the balance. But then news of their predicament reached a team of radio astronomers at nearby Princeton University. They were interested in the possibility that the Universe as we know it had emerged from a hot, dense state (the Big Bang), and they had calculated that this event should have filled the Universe with electromagnetic radiation that would by now have cooled to a temperature of a few K (unknown to them, the same prediction had been made in the 1940s, by Ralph Alpher and Robert Herman, but had been ignored). The Princeton team were in the process of building a radio telescope to look for this radiation when news of the work at Crawford Hill reached them.
When the two teams got together to discuss the discovery, it became clear that the radio noise from space found by Penzias and Wilson might be the echo of the Big Bang. Penzias and Wilson were not entirely convinced, but they were relieved to have any explanation, and felt confident enough to publish their results in 1965, in a paper with the cautious title ‘A Measurement of Excess Antenna Temperature at 4080 Mc/s’, without actually claiming that they were detecting radiation from the birth of the Universe. As Wilson put it in his Nobel lecture: ‘Arno and I were careful to exclude any discussion of the cosmological theory of the origin of background radiation from our letter because we had not been involved in any of that work. We thought, furthermore, that our measurement was independent of the theory and might outlive it.’48
But the announcement triggered a wave of further experiments which confirmed that the Universe is filled with a sea of microwave radiation at a temperature of 2.712 K, and this remains compelling evidence that there was a Big Bang. In the twenty-first century, observations of this microwave background radiation have refined our understanding of the Universe and revealed details of its properties (see here).
In 1978, Penzias and Wilson received a share of the Nobel Prize in Physics ‘for their discovery of cosmic microwave background radiation’.