A Matter of Universal Interest

thesis A matter of universal interest

A research psychologist friend of mine the luminous stars and gas. By this time, recently asked me about dark matter. Is Dark matter seems to be a Bertone and Hooper’s history concludes, the that conjecture really on solid ground? He fairly sensible hypothesis idea of dark matter was becoming broadly expressed worry that physicists may, in accepted — and for pretty normal reasons. this case, have invoked the invisible and yet the idea that it may Physicists since then have mostly turned undetectable a little too readily. When faced actually be a grand illusion their attention to finding plausible candidates with a puzzle, isn’t it rather unscientific to for it, and trying to detect them. Today, the explain it away by assuming the existence still persists. front-runner is a class of non-baryonic weakly of some strange and invisible substance of interacting particles of various possible kinds; just the right kind, distributed through the Zwicky sought to check the consistency of a wide variety of other exotic possibilities — Universe in just the right way? Couldn’t it be, the observation by using the virial theorem of including primordial black holes — having he went on, that the apparent need for dark classical mechanics, which relates the average been more or less ruled out. matter just reflects a gap in our understanding potential energy among some gravitating So dark matter does seem to be a fairly of something fundamental about cosmology, bodies to their average kinetic energy, sensible hypothesis, suggested directly by including the workings of gravity? which is linked to the velocity dispersion. the available evidence. And yet the idea that Not at all expert on the topic, I didn’t His calculation led to an expected velocity dark matter may actually be a grand illusion know how to reply. So I started reading dispersion of 80 km s−1, some ten times still persists. and fortunately came across a recent, smaller than observed. “If this would be In the early 1980s, astrophysicist beautifully written history of the idea of confirmed”, he concluded, “we would get the Mordehai Milgrom suggested that it might dark matter that explores how and why surprising result that dark matter is present in be possible to explain the observed motions most physicists have come to believe in much greater amounts than luminous matter” of stars and gases within galaxies without it (G. Bertone and D. Hooper, preprint at (Helv. Phys. Acta 6, 110; 1933). invoking any hidden matter, but by instead http://arxiv.org/abs/1605.04909; 2016). This was anything but a decisive result. looking to variations in the behaviour of The story is reassuring — the idea of dark As Bertone and Hooper note, astronomers gravity. In particular, all would seem natural matter doesn’t seem any more dubious than and astrophysicists mostly kept open minds if for very small accelerations the force of earlier hypotheses about as-yet-unobserved over the next few decades, neither strongly gravity on a mass followed not Newton’s things such as atoms or black holes or believing or disbelieving in dark matter. second law, F = ma, but a slightly different microorganisms. Even so, it turns out that They awaited further evidence, which arrived law. Milgrom’s idea — now known as my friend’s intuition is also sound — there is in the 1970s with technological advances modified Newtonian dynamics — didn’t still room for doubts over the reality of dark that enabled more accurate measurements of conserve energy or momentum, nor was matter. A fundamental lack of understanding the velocities of stars within different parts it consistent with general relativity, but the of gravity is still legitimately in play. of galaxies. concept has been taken further since then. As astrophysicists Gianfranco Bertone and This made it possible to test with According to Bertone and Hooper, the Dan Hooper relate, the philosophically simple increasing precision expectations of how the most advanced project to date goes under idea that some things may be hidden from rotational speed of stars should change as the name TeVeS theory — the acronym us has been present in science for centuries. one moves outwards from the galactic centre. stands for tensor–vector–scalar gravity. In modern times, the mathematician According to well-accepted calculations of Developed by physicist Jacob Bekenstein, it is Friedrich Bessel argued in 1844 that the stellar motions under gravity, if the luminous consistent with general relativity, and offers a presence of unseen companion stars would stars in a spiral galaxy contain most of the generalization of it with two additional fields naturally account for oddities in the observed mass, then this galactic rotation curve should and some free parameters. It is also apparently motions of the stars Sirius and Procyon; he increase with radius, reach a peak and then consistent with a variety of cosmological data was right. Many scientists have convinced fall off outside the zone that contains most as well as the observed rotation curves of themselves through logical argument of the mass. In the early 1970s a study that hundreds of spiral galaxies. Pretty impressive, that most of the astronomical Universe compared the observed and expected radii although it does still have some problems is unobservable at present. Lord Kelvin of the rotation curve peaks for two galaxies, fitting the data for galaxy clusters. pioneered the use of the kinetic theory of M33 and NGC 300, found that the observed This story, presumably, will end as gases in the analysis of galaxies, and on its peaks were substantially larger. The author previous searches for hypothetical entities basis came to the conclusion that “many of Ken Freeman concluded that “there must have: either with an eventual definitive our stars, perhaps a great majority of them, be in these galaxies additional matter which detection of the stuff that makes up may be dark bodies”. is undetected. Its mass must be at least as dark matter, and a pleasing resolution There’s nothing at all strange in this and large as the mass of the detected galaxy” of the various puzzles surrounding the modern idea of dark matter isn’t a lot (Astrophys. J. 160, 811; 1970). the behaviour of galaxies, or with the different. In the 1930s, Fritz Zwicky identified Other astronomers soon found similar ultimate abandonment of the hypothesis a notable scatter in the apparent velocities results for a host of other galaxies. By 1980, if an exhaustive search over decades finds of eight galaxies within the Coma Cluster. observations on dozens of galaxies confirmed nothing. For now, it looks like an eminently The velocity dispersion ought to reflect both that all had increasing rotation curves out reasonable scientific speculation. ❐ the gravitational forces acting between the to the largest observed radius, indicating bodies and the overall mass of the cluster. that galactic mass kept growing out beyond MARK BUCHANAN