Malcolm S Longair Cambridge cosmology in the s

Malcolm Longair carried out his postgraduate studies at the Mullard Radio Astronomy Observatory of the Cavendish Laboratory, University of Cambridge from 1963 to 1967. He spent the academic year 1968-1969 as a

Royal Society Exchange Fellow to Moscow. Subsequently, he was Astronomer-Royal for Scotland and Director of the Royal Observatory Edinburgh from 1980 to 1991. He returned to Cambridge in 1991 as Jacksonian Professor of Natural Philosophy and from 1997 to 2005 was Head of the Cavendish Laboratory.

Cosmology in Cambridge in the 1950s and early 1960s was dominated by the controversy between Martin Ryle and Fred Hoyle over the interpretation of the number counts of extragalactic radio sources. I was only involved in this rather acrimonious debate toward the end of the most controversial period, which most people would concede ended with the establishment of the thermal nature of the CMBR and its remarkable isotropy.

I have given many more details and full references to the most significant events in the development of steady state cosmology and the controversy over the number counts of radio sources in my book, The Cosmic Century: A History of Astrophysics and Cosmology (Longair 2006).14 An interesting feature of the development of steady state cosmology was that most of the proponents were UK cosmologists and this at least partly accounts for the fact that steady state cosmology was taken much more seriously in the UK than in the USA.

The facts of the controversy and its resolution are now well known and will not be repeated here. What has been less appreciated are the very different scientific agendas which Ryle and Hoyle pursued. Both were larger than life personalities who had honed their research skills under periods of extremely high pressure during World War II. Hoyle was a brilliant astrophysicist whose extensive imagination led to some of his most remarkable achievements; for example, the prediction of the triple-a resonance, as well as to the controversies which dogged his career. Hoyle's biography is the story of a disadvantaged Yorkshire schoolboy with a very strong independent streak from the very beginning, who, by sheer ability and hard work, attained the premier position in British academic astronomy, the Plumian Professorship of Astronomy at the University of Cambridge.

Although trained as a physicist, Ryle was primarily an electrical engineer with a genius for making complex radio-receiving systems operate. In particular, he understood how aperture synthesis techniques could be used to provide both angular resolution and high sensitivity for radio astronomical observations. The principles of aperture synthesis were understood by 1954, but putting them into practice was a virtuoso technical achievement which

14 Helge Kragh's (1966) Cosmology and Controversy: The Historical Development of Two

Theories of the Universe can also be thoroughly recommended.

involved a considerable team of researchers and support staff under Ryle's charismatic leadership.

Ryle had remarkable physical intuition but little time for complex mathematical arguments or the intricacies of theory. Early in the history of the development of radio astronomy, profound personal animosities developed between a number of the participants in the debates. The excess of faint radio sources was certainly exaggerated in the 2C survey of 1954 and this experience made Ryle reluctant to release his results until he was sure they were fully understood and as secure as they could be. I was often present when he lamented the fact that what had taken him years to get right through the dedicated efforts of his team could be dismissed in a stroke by a theorist.

These events had positive and negative impacts upon the work of the Cambridge radio astronomy group. The positive side was that new astro-physical and cosmological opportunities had been opened up. In 1956, the radio observatory moved to a disused wartime Air Ministry bomb store at Lord's Bridge and, in acknowledgment of a grant of £100,000 from the electronics company Mullard Ltd, the Mullard Radio Astronomy Observatory was opened in 1957. The technical successes of the interferometry programs led to construction of the first fully steerable aperture synthesis radio telescope system, the One-Mile Telescope completed in 1965, and then to the 5-km Telescope completed in 1972. These were undoubtedly the most powerful radio telescope systems in terms of angular resolution and sensitivity for a number of years.

The negative side was that the group became very much more defensive in its interaction with outside groups. Great care was taken to ensure the reliability and completeness of subsequent catalogs and radio maps before they were made available outside the group. The accusation of secrecy recurred with Jocelyn Bell and Antony Hewish's discovery of the radio pulsars in 1967. This was such a surprising and important discovery that every effort had to be made to ensure that the data were absolutely secure before the data were made public. Even those of us in the next door offices did not know what was going on until Hewish gave a colloquium in the week of publication of the discovery paper in Nature.

The contrasting approaches of the experimental and theoretical astrophysicists were exacerbated by the split which occurred in the early 1960s between the Cavendish Laboratory and the newly formed Department of Applied Mathematics and Theoretical Physics (DAMTP). Hoyle was a member of DAMTP, as was Dennis Sciama, who was also a strong proponent of steady state cosmology. The differing points of view were institutionalized and observational and theoretical cosmologists pursued their research programs independently.

The resolution of the controversy over the radio source counts only came with the construction of the next generation of radio telescopes which had higher angular resolution and hence were less sensitive to the effects of source confusion. The radio source counts derived from the 4C catalogs showed a clear excess over the expectations of Euclidean world models. The optical identification programs led to the discovery of quasars in the early 1960s. By the mid-1960s, the evidence was compelling that there was indeed an excess of extragalactic radio sources at large redshifts and this was at variance with the expectations of steady state cosmology.

It was a great sadness that relations between Hoyle and Ryle were so soured by the controversy over the radio source counts. In early 1967, Peter Scheuer and I attempted a reconciliation between them. We believed the new data were very compelling that the excess of faint sources was real and that there was no need to prolong hostilities. The four of us got together in Hoyle's newly founded Institute of Theoretical Astronomy in Cambridge to try to effect a reconciliation. We talked for about 45 minutes but, sadly, there was no longer any common ground. Hoyle and Ryle simply repeated their entrenched views. It was one of the saddest events of my career.

In fact, during the 1960s, the differences in interpretation of the source count data were confined to a relatively small, but prominent, number of individuals. The great strength of Cambridge astrophysics expanded enormously through the 1960s. The foundation of Hoyle's Institute of Theoretical Astronomy in the mid-1960s provided positions for a very powerful group of post-doctoral fellows including Peter Strittmatter, John Faulkner, and Peter Eggleton as well as regular summer visitors including Geoff and Margaret Burbidge, William Fowler, and the next generation of their research associates including Don Clayton, Bob Wagoner, Wal Sargent, and their colleagues. A good appreciation of success of this venture is included in The Scientific Legacy of Fred Hoyle (Gough 2005) in which Hoyle's wide range of scientific interests are reviewed mostly by his collaborators. At the same time, in DAMTP, Dennis Sciama took on a succession of brilliant research students who worked on quite different problems in general relativity, relativistic astrophysics, and cosmology. Among these, George Ellis completed his PhD in 1964, Stephen Hawking in 1966, Brandon Carter in 1967, Martin Rees in 1967 and Malcolm McCallum in 1971. The intellectual vitality of Cambridge astrophysics and cosmology was remarkable.

Despite the differences between a few of the most senior astronomers, the staff members, postdocs, and graduate students of the various departments maintained good working relationships and, to encourage interactions, the Astronomers' Lunch Club met every week. This was open to all astronomers working in Cambridge and helped to maintain intellectual contacts between the groups. These were lively occasions and for many years I was secretary and treasurer of the Club which met regularly at Clare Hall.

In 1964, while I was completing my first year of research at Cambridge, I attended a course of lectures given by Hoyle on the problems of extragalac-tic research. It was a rather remarkable audience and included many who would become future leaders of astrophysical research - Martin Rees, Roger Tayler, Peter Strittmatter, John Faulkner, Russell Cannon, Bob Stobie, and many others. Hoyle would arrive with, at best, a scrap of paper with some notes and expound an area of current research. One week, the topic was the problem of the cosmic helium abundance.

At that time, helium was one of the more difficult elements to observe astronomically because its high excitation potential meant that it could only be observed in very hot stars. Already by 1961, Donald Osterbrock and John Rogerson had shown that the abundance of helium seemed to be remarkably uniform wherever it could be observed and corresponded to about 25% by mass. In Fred's lecture, he discussed the recent preprint by O'Dell, Peimbert and Kinman (1964) concerning the helium abundance in a planetary nebula in the old globular cluster M15. Despite the fact that the heavy elements were deficient relative to their cosmic abundances, the helium abundance was still about 25%.

Hoyle reviewed the evidence on the cosmic helium abundance and then described the work of Gamow, Alpher, Herman, and Follin concerning the problems of synthesizing the heavy elements in the early phases of the big bang. Although helium is synthesized in the central regions of stars during their long phases of evolution on the main sequence, it is most unlikely that this process could have created as much helium as 25% by mass of the baryonic matter in the universe.

By 1964, it was possible to carry out primordial nucleosynthesis calculations more accurately. At that time, Roger Tayler had just returned to Cambridge and was present in the audience. Hoyle and Tayler realized that they could undertake much more precise calculations and, in the next week, they and Hoyle's research student John Faulkner worked out the details of the formation of helium in the early phases of the big bang. The audience had the privilege of being present as a key piece of modern astrophysics was created in real time in a graduate lecture course.

John Faulkner told me about the remarkable events which took place over that weekend when they were analyzing the results of his computations.

Hoyle became very excited indeed about the results of the computations because he thought the models were predicting a very high helium abundance, which exceeded what was observed - he thought he had found a reason to reject the big bang scenario. In fact, however, the results settled down to about 25%, in remarkable agreement with observation and essentially independent of the overall baryonic matter density in the universe. Hoyle and Tayler's paper was published in Nature (Hoyle and Tayler 1964).

One consequence of the big bang model which Hoyle and Tayler did not mention explicitly in their paper was that the cooled remnant of the thermal radiation present during the very hot early phases should be detectable at centimeter and millimeter wavelengths. According to Roger Tayler, he had included this result in his draft of their paper, but it did not appear in the published version. Alpher and Herman's prediction had been more or less forgotten when Gamow's theory of primordial nucleosynthesis had failed to account for the creation of the chemical elements.

The very next year, the CMBR was discovered by Arnold Penzias and Robert Wilson (Penzias and Wilson 1965a), more or less by accident. There is no need to take this story further since many of the key players give a blow-by-blow account of this crucial discovery, the number of earlier "near-misses" and the remarkable story of the wealth of cosmological information which the CMBR contains.

Wagoner, Fowler and Hoyle (1967) repeated the analysis carried out by Hoyle and Tayler, but now using all the available cross sections for many more nuclear interactions between light nuclei and with the knowledge that the CMBR has radiation temperature 2.7K. When Hoyle first presented these results in Cambridge, many of us were surprised that it seemed as though he had been converted to the big bang picture. As in the paper itself, however, equal weight was given to the idea that these computations could also be applied at very much higher baryonic densities to very massive stars, which collapsed and "bounced". The nucleosynthesis of the expansion phase was exactly the same as a universe of very high baryonic mass density. The densities were so high that heavy elements could be synthesized in these stars. The subsequent paper by Wagoner (1973) concentrated upon the primordial synthesis of the light elements.

There is a delightful sequel to the story of Hoyle's approach to steady state cosmology. He always affirmed that the idea of the continuous creation of matter was the most important aspect of the theory. On the occasion of his 80th birthday in 1995, I invited him to lecture to the Cavendish Physical Society. He was delighted to accept this invitation because he had given his first lecture on steady state cosmology to the Cavendish Physical Society in 1948. Hoyle remarked wryly that, in his development of the theory of continuous creation, his only mistake had been to call his creation field C rather than The exponential expansion of the early universe according to the inflationary picture involves a scalar field ^ which performs exactly the same function as Hoyle's C field, or Lemaitre's cosmological constant A. Then, Hoyle would have been the originator of the inflationary picture of the early universe.

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