Montgomery, Colin Robert Lister (2014) From Michell to MACHO: changing chronological perspectives on the concept of 'dark matter' in astronomy and astrophysics. Masters (Research) thesis, James Cook University.

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Abstract

The underlying theme of this thesis is 'missing mass' which extends back to John Michell's (1767) paper on the parallax of fixed stars. He foresaw that our galaxy was part of a system in our own part of the Universe, independent of other systems which existed at great distances. This understanding of the independence of our galaxy was necessary before its mass could be calculated and the final recognition that there was 'missing mass'. In 1933 Fritz Zwicky published a paper in Helvetica Physica ACTA in which he introduced the concept of 'dark matter' to explain the 'missing mass' in the Andromeda Nebulae, specifically its Coma Nebulae. This launched a series of studies which culminated, during the 1990s, in a number of international teams (including two from Australia) using gravitational microlensing to search for evidence of 'dark matter' in our Galaxy.

This thesis examines these 'dark matter' studies and demonstrates that the concept of 'dark matter'—in one guise or another—extends back to John Michell and a seminal paper that he published in 1784. It also reveals that in the late nineteenth and early twentieth centuries, following the emergence of astrophysics, various astronomers foreshadowed Zwicky's pioneering studies. The history examines the ways in which the changing concept of 'dark matter' has contributed to our understanding of astronomy and astrophysics between 1784 and the present day.

John Michell was interested in the concepts of probability. After reviewing William Herschel's paper on double stars he suggested that if there were luminous double stars there was a high probability that one of a pair of stars could be non-luminous. He then proceeded to calculate the size of a body from which light could not escape because of its force of gravity. This body eventually became known as a 'black hole'. Michell also suggested that the proper motions of a luminous body could be disturbed by the nearby presence of a non-luminous body, known as perturbation.

Fifty years later Fredrich Bessel verified Michell's concept of perturbation by gravity, by applying the corrected clock times of 1834 to the proper motions of Procyan and Sirius, which had been previously recorded in 1755. J.F.W. Herschel confirmed this fact and concluded "The existence of numberless visible stars can prove nothing against the existence of numberless invisible ones" (Herschel, 1845:141).

Early in the 20ᵗʰ century Edward E. Barnard produced his catalogue of 'dark patches in the sky' and disagreed with other astronomers who suggested that some of the dark patches were formed by 'opaque mass of some sort'.

One of the biggest changes on our perspective of 'dark matter' was a result of the 'Great Debate' in 1920 between Harlow Shapley and Herber D. Curtis which led to the acceptance of Curtis' view of the size of our galaxy confirming Michell's (1767) foresight. The acceptance of an independent galaxy was necessary before the amount of 'missing mass', which was holding the Galaxy together, could be calculated.

Jacobus C. Kapteyn took the first step with his paper in 1922 entitled "First Attempt at a theory of the Arrangement and motion of the Sidereal System" in which he estimated the number of luminous stars and their average mass and suggested that this only made up half of the mass of the Galaxy and the balance must be 'dark matter'. This work was further confirmed by Kayteyn's student Jan H. Oort (1926) paper "Non -light-Emitting matter in the Stellar System".

This work was confirmed by Fitz Zwicky in important papers published between 1933 and 1942, mostly based on the rotational speed of the Coma Cluster. Zwicky arrived at a figure of 1000 km/sec, which was 400 hundred times that which was expected.

No further research was done in related to 'dark matter' until 1970 when Kenneth Freeman renewed the interest in the mass of spiral galaxies, which eventually led to the discovery that there was 'missing mass'. His studies on their circular-velocity field and mass-angular momentum densities and their proportion of the total light for each of the spheroidal and exponential components became known as Freeman Law.

Two short papers one by Einasto et al., (1974) and another by Ostriker et al., (1974) discussed the missing mass around galaxies and of the universe. They concluded that there existed non-observable mass within the galaxies and a large part of the mass was on the outer regions. These papers add a little more understanding of the proposals by Freeman (1970).

After a few similar conceptual papers over the next decade by Margon (1975), Shields (1978) and Bailey (1982), Rubin et al., (1983) attempted to answer the question "Is the distribution of luminosity in galaxies a reliable distribution of mass?" (Rubin. et al., 1983). They also applied the same question to the universe. This was a very detailed paper and did indicate the locations of mass in galaxies and concluded that there was an unknown fraction of mass in spiral galaxies and clusters hidden by a non-luminous constituent.

Peebles (1984) attacked the question of 'dark matter' from a different perspective moving from the observation of the effect of 'dark matter' objects on nearby luminous matter to a theoretical analysis. He based his model on a set of interesting assumptions which he hoped would lead to a realistic model. However this model does not seem to have evolved.

In 1987 the most cited paper on the subject of 'dark matter' was published by Albada and Sancisi (1987). They proposed that the amount of 'dark matter' can be calculated by comparing the observed rotation curves with the rotation curves based on the distribution of light. The approach was on the basis that galaxies had distinct components, e.g. a bulge, a disk and a dark halo, but this was idealistic because of the difficulty of separating the components.

It could be said at that stage, new conceptual papers entered into a pause and the interest changed to review papers on what had been written and researched. Noted papers were by Trimble (1987) and the presentations at the 1987 IAU Conference "Dark Matter in the Universe" most of which clarified results that had been previously published elsewhere.

In a different isolated approach Carignan and Freeman (1988) asked the question "DDO 154: A Dark Galaxy?" and they went onto show that 90% of its mass at 7.6 kpc is contributed by the dark component.

During the thirty year period from 1970 there were numerous research papers on the observational and theoretical aspects of the existence of 'dark matter' without a definitive answer to what it really comprised.

In 1989 the MACHO Collaboration was formed which eventually led to the identification of some non-luminous bodies in binary systems using gravitational microlensing. This collaboration continued with observations and assessment up to and past 2000. The work is currently being continued by the SuperMacho group in South Africa and Chile.

The above foregoing review demonstrates that over three hundred years or so the concept of 'dark matter' has undergone a major change moving from the probability that dark suns exist, through to the need for 'dark matter' to prevent the rotating Galaxy flying apart. The existence of dark bodies suggested by Michell (1784) was finally proven by the MACHO projects at the turn of the twentieth century.

Item ID:	41364
Item Type:	Thesis (Masters (Research))
Keywords:	18th century; 19th century; 20th century; astronomers; astronomy; astrophysics; black holes; chronology; cosmology; dark matter; extra galactic; galactic; galaxies; galaxy; general relativity; history; MACHO; massive compact halo objects; missing mass; nebulae; the great debate; universe
Date Deposited:	01 Dec 2015 05:01
FoR Codes:	02 PHYSICAL SCIENCES > 0201 Astronomical and Space Sciences > 020105 General Relativity and Gravitational Waves @ 40% 02 PHYSICAL SCIENCES > 0201 Astronomical and Space Sciences > 020104 Galactic Astronomy @ 30% 02 PHYSICAL SCIENCES > 0201 Astronomical and Space Sciences > 020103 Cosmology and Extragalactic Astronomy @ 30%
SEO Codes:	97 EXPANDING KNOWLEDGE > 970102 Expanding Knowledge in the Physical Sciences @ 100%
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