Gold , Thomas

(1920–) Austrian–American astronomer

Born in Vienna, Gold became a refugee from the Austrian Anschluss and gained his BA in 1942 from Cambridge University, England. He lectured there in physics from 1948 to 1952 before he joined the Royal Greenwich Observatory as chief assistant to the Astronomer Royal. He moved to America in 1956, becoming director of the Center for Radiophysics and Space Research at Cornell from 1959 to 1981, and professor of astronomy from 1971 to 1986.

Gold is best known for his contribution to cosmology, the study of the origin, evolution, and large-scale structure of the universe. In the 1940s the prevailing cosmological model was the big-bang theory originally proposed by Georges Lemaître. Since this theory postulated a ‘beginning of time’ when the incredibly compact universe exploded into being, it was regarded with suspicion and alarm by many astronomers. In 1948 Gold published with Hermann Bondi The Steady-State Theory of the Expanding Universe. At the heart of this paper was the adoption of what became known as the ‘perfect cosmological principle’. This was an extension of the cosmological principle, which states that the universe looks basically the same from whichever point one observes it; Gold and Bondi added to this that the time of observation was as irrelevant as the place. Thus the universe, on a large scale, is unchanging in time and space. It had no beginning, will never end, and a constant density of matter throughout space will always be maintained.

This theory needed to be reconciled with the work of Edwin Hubble, which Gold and Bondi accepted and which showed that the galaxies are receding and the universe is expanding. To maintain the steady state of their universe, Gold and Bondi had to introduce an original and startling proposition, namely, that there must be continuous creation of new matter from nothing. They calculated the amount needed as about one hydrogen atom per cubic kilometer of space every ten years, an amount too small to be detected. Although this proposition conflicted with such deep physical assumptions as the conservation of matter and the laws of thermodynamics they found that it was compatible with all astronomical data.

Consequently the steady-state theory proved attractive to a number of cosmologists and crucial evidence only emerged against it in the 1960s. Then Arno Penzias and Robert Wilson discovered the background microwave radiation in 1965 and Maarten Schmidt produced a survey of the distribution of quasars that seemed to support the evolving universe of the big-bang theory.

In 1968 news of a new type of star, a pulsar, was published by Jocelyn Bell and Antony Hewish. The distinguishing features of the pulsar were its high-frequency radio signals that had a periodicity of the order of a second or less. Gold quickly proposed a structure capable of producing such an effect: rapidly rotating neutron stars. The same theory was proposed independently by Franco Pacini. Neutron stars are extraordinarily dense stars that have undergone such extreme gravitational collapse following exhaustion of their nuclear fuel that their constituent protons and electrons have combined to form neutrons. These stars would be small and dense enough to rotate with a period equivalent to that of the radio pulses. It had also been shown that they would radiate energy in a narrow beam. If the Earth happened to be in the direction of the beam it would be picked up as a source of pulses, much as the beam of a lighthouse is observed as a series of flashes. The theory of Gold and Pacini was eventually accepted once pulsars rotating even faster than the original one were detected in the Crab and Vela nebulae.

Gold was able to make a prediction that has since been confirmed. He argued that pulsars should be slowing down by a small but measurable amount, because of the loss of energy. Following careful observation of the pulsar in the Crab nebula it was found to be slowing down and its period increasing by 3.46 × 10^–10 seconds per day.