Posts Tagged ‘McKellar

The cosmic microwave background and Andrew McKellar

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Andrew McKellar

It is generally supposed that the time order in which discoveries are made should have no eventual influence on our beliefs. But it is very evident that they do, and perhaps nowhere more importantly than with the cosmic microwave background.
The cosmic microwave background first showed itself observationally to astronomers in the late 1930s and early 1940s, when Adams and Dunham observed a number of interstellar absorption lines in the blue, identified by Swings and Rosenfeld (L 1937, Ap. J., 94, 381), by McKellar (1940, Pub. Astr. Soc. Pacific, 52, 307), and by Douglas and Herzberg (1941, Ap. J., 94, 381), as arising from the diatomic molecules CH, CH+ and CN. Of these, those lines from CN were the most immediately important. One was from J=1 level of the ground state. The first of these was to the J=0 rotational state of the B2Σ multiplet, and others were to the J=1 and J=2 rotational state of higher B2Σ multiplet. In 1941, McKellar (1941, Pub. Dom. Astrophys. Observatory, Victoria, B.C., 7, 251) interpreted the required population of the J=1 rotational level of the ground state as being caused by radiative excitation from the J=0 level. The excitating radiation was taken to be black body, and the temperature required for it, in order to explain the relative intensities of the observed lines, was 2.3 K.
This detection of the microwave background was so very explicit that its discovery could quite properly be dated in 1941, if it suited astronomers to do so. The sociological problem of course in 1941, was that Europe was deeply into a disastrous war. And so, within a few more months, would be the United States. There was simply no opportunity for McKellar’s work to be adequately discussed. It might have been discussed after the Second World War, but the world recovered only very slowly, and it was really not until the meeting of the IAU in Rome in 1952 that an opportunity became available. By McKellar’s paper had, unfortunately, lain too long in observatory files and had become forgotten.
A further problem is that McKellar published in an Observatory publication which almost no physicist heard of. The extent of which this still remains true is shown by the reference list of the otherwise excellent article by Boesgaard and Steigmann (1985, Ann. Rev. A. & A, 23, 319) with the title “Big Bang Nucleosynthesis: Theories and Observations”. This article contains of the order of 500 references on the cosmic microwave background. But it does not contain any reference to McKellar’s determination of its temperature. The attitude of the post-1965 astronomical world is apparently that the work of the 1940 period was all a peculiar aberration best now forgotten. The hope of the present authors, however, is that by 2041, when the centenary of McKellar’s discovery comes round, it will at last be remembered, and that by then it will be the strident cosmological pronouncements of the last third of twentieth century on this matter that have become forgotten…..
Fred Hoyle, Geoffrey Burbidge and Jayant V. Narlikar,
A Different Approach to Cosmology, from a Static Universe through the Big  Bang towards Reality”, Cambridge University Press, 2000

Written by physicsgg

June 19, 2011 at 12:36 pm