Written by Kat Jivkova
There is a tendency for historians of science to reduce the development of scientific theories to a few key works, chronologically ordered and produced by a select number of individuals – histories of dark matter are no exception to this. In the past, accounts of dark matter’s “discovery” have focused predominantly on Swiss astronomer Fritz Zwicky’s work on the velocities of the Coma Cluster, or Vera Rubin’s research into galactic rotations. But the absurdness of dark matter could not have possibly arisen solely from these experiments. It goes beyond the visibility of normal matter. It does not interact with the electromagnetic force at all, and subsequently does not interact with light. In other words, it does not exist to humans in any other way than the imagination. The development of dark matter theory was a product of multidisciplinary research within the context of a twentieth-century rise in theoretical cosmology. The mystery of dark matter was pursued by a worldwide community of scientists all working to justify its existence and role in modern cosmology. This was a collective effort as opposed to an isolated timeline of independently operating scientists.
If then, dark matter could not be detected by scientists, how did the idea arise? Theoretical science is very different from experimental science in the sense that it does not require observation of physical phenomena. It applies a mathematical framework onto the physical world, and explains anomalies through abstractions – in this case, through dark matter. Scientists of the twentieth century noticed that gravity generated by observable matter, also known as baryonic matter, could not have possibly held galaxies together with the immense speeds at which they were rotating. They argued that there had to be some other kind of matter which gave galaxies extra mass to keep them in one piece. This was named dark matter, on the account that it could not be seen. Traditional narratives would name Zwicky as the “father” of dark matter and, to some extent, this is true. In 1933, he studied the radial velocities of the Coma Cluster, a dense collection of 1,000 galaxies, and could not explain how they even formed a cluster at all, given their individual speeds. Following the traditions of theoretical physics, he used a mathematical model, the virial theorem, to find a solution to the problem. The theorem is as shown, where M is the mass of the galaxy, R is its radius, V is the mean velocity of its stars and G is Newton’s gravitational constant:
Upon applying this model to the Coma Cluster, Zwicky calculated an immense mass for each galaxy that could not be accounted for by visible matter, which would place dark matter more abundant than “radiating matter.” It is widely accepted that Zwicky was the first to use the virial theorem to determine the mass of galaxies, and was also the first to coin the phrase “dark matter” in his work published by the Swiss journal, Acta Helvetica Physica. However, dark matter was just as much developed by Zwicky than it was by his predecessors, Lord Kelvin, Ernst Ӧpik and Jacobus Kapteyn. The rash historical tradition of naming an individual the “father” or “founder” of a scientific discovery jettisons the legacies of previous scientists who built the foundations for that same discovery, as in this case. At the beginning of the twentieth century, it was Lord Kelvin who argued that many stars in the Milky Way “may be dark bodies.” Notably, French mathematician Henri Poincare commented on the topic in 1911, arguing that “matière obscure” (dark matter) probably existed in smaller amounts than visible matter. Four years later, Estonian astronomer Ernst Ӧpik summarised the motion of stars and also determined that there must exist a presence of invisible matter. These scientific enquiries were therefore happening in dialogue with one another, in response to a growing scientific interest in modern cosmology.
The Dutch astronomer, Kapteyn, was responsible for the creation of a quantitative model of the Milky Way galaxy. Similarly to Ӧpik, he examined the motion of stars and quickly realised the possibility of dark matter with identical reasonings. Kapteyn’s pupil, Jan Oort, also made contributions to the dark matter question by providing a more accurate account of the “kinematics of stars” (their relative velocities). All of these works created the foundation for Zwicky’s findings and its consequences. Unsurprisingly, the search for dark matter was exacerbated in the latter half of the twentieth century. Princeton-based mathematicians Franz Kahn and Lodewijk Woltjer, were surprised to detect that the Milky Way and its neighbour, M31, were travelling toward one another with a velocity of 300 km/s – this was the complete opposite to what they should have been doing, had they been following Hubble’s law for the expansion of the universe. The only reason for this reversal could have been gravity, created by a mass between the two galaxies far greater than what was observable. Meanwhile, other astronomers and mathematicians including Arrigo Finzi and Rudolph Kurth studied the mass of galaxies and their relationships to the velocities of star clusters. All of these findings pointed to the same thing – the existence of dark matter – however, a confusing situation began to trouble the scientists of the field
It was Finzi who, upon summarising the findings that dark matter had produced, outlined the problem. Summarising the research made by Kurth, he stated that the mass enclosed from the distance of the centre of a galaxy increased with that distance, showing that the mass discrepancy of an astronomical system increases with scale. In 1970, Ken Freeman asserted this with the following statement:
“If [the data] is correct, then there must be in these galaxies additional matter which is undetected …and its distribution must be quite different from the exponential distribution which holds for the optical galaxy.”
Furthermore, it was assumed that most of a galaxy’s mass was located at its centre, thus it was expected that stars further from the centre would move at lower velocities to those that were closer. Developments in radio astronomy in the 1950s enabled scientists to conduct radio observations on galaxies, which they did on both the Milky Way and the Andromeda. Both studies showed that stars moved at the same speed, despite their distance from the centre of their respective galaxies. American astronomer Morton Roberts affirmed this in 1973, stating that these results implied “a significant mass density at these large distances.” These results enabled scientists including Vera Rubin and Kent Ford to produce works suggesting that galaxies may have dark matter haloes – a region of dark matter which surrounds individual galaxies. This would account for the rotation curves of galaxies observed, and for the distribution of matter within them. The idea of “haloes” of dark matter surrounding galaxies is now a part of the standard model of cosmology, however it can never be confirmed due to the obvious fact that dark matter cannot be detected. This is, of course, the largest problem that dark matter scientists tackle with in itself: dark matter’s invisibility.
There have existed, and still exist, a large number of physicists and astronomers who devote themselves to the detection of dark matter, despite its very existence being theoretical. The Soudan mine at Lake Vermillion, for example, became the experimental base for the direct detection of dark matter particles for a project facilitated by 12 universities and institutions. A more recent experiment conducted at the University of Chicago Fermi lab tried to achieve direct detection, using bubble chambers at high liquid temperatures to trace the paths of dark matter particles. The Large Hadron Collider at CERN in Geneva has also made efforts to find dark matter through high-energy collisions that could produce supersymmetric particles, which many scientists believe dark matter contains. These dark matter searches exist in a plethora of others that have dominated twenty-first-century research into the field, and have involved thousands of physicists conducting multiple experiments. The amount of time, energy and funding invested in detection experiments certainly shows the high standing that dark matter has been regarded with. It is, after all, the search to find what makes up around 30 per cent of the universe in contrast to visible matter’s 0.5 per cent (the rest is made up of dark energy, which warrants an article of its own).
The development of dark matter as a theory came about through the collective efforts of scientists in its field, and ultimately shows science to be a social activity. The scientific community involved in the search for dark matter were driven by their peers, basing their own research on their predecessors, and forming sub-communities involved in the strengthening of a paradigm. From the early developments of dark matter, it is clear that there were various scientists all working to solve the same problem, the behaviour of galaxies, which progressed into a worldwide movement by the late-twentieth century. Dark matter is not only a theory which explains the composition of particles, but also the gateway to a passionate scientific community interwoven by a desire to “see” the invisible.
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