The MOND hypothesis
A “fiery minority”[1] of physicists has long disputed the existence of dark matter, and recent surveys of the heavens – including one encompassing 400 stars within 13,000 light years from Earth - have failed to find any dark matter in places where the standard dark matter theory would tend to suggest it should be, including in our own Milky Way galaxy. The question therefore arises whether scientists are just working overly hard to keep the dark matter hypothesis alive, similar to the discredited theory of epicycles, whereby 16th century astronomers endeavoured to retain geocentrism by adding a constant series of tweaks to a fatally flawed theory [2].
An alternative model exploiting this forensic void at the centre of the ongoing quest to substantiate the existence of dark matter and known as modified Newtonian dynamics (MOND) is that gravity works differently at vast cosmological scales than on the scales we are used to, but until recently most physicists have been of the view that dark matter as a theory is too successful to abandon and MOND too immature to replace it [3].
Also scant attention was initially paid to MOND because it dealt only with the framework of Newtonian gravity whereas Einstein’s theory of general relativity demonstrated that gravity is not a force and arises from the curvature of space and time. [4] However, in its present framework, MOND consists of a tweaking of Einstein’s equations by changing the mathematical rules that govern how the force of gravity arises from mass. MOND can therefore be made compatible with general relativity, with each using different types of fields that behave slightly differently to describe how gravitational attraction arises from mass. It is this 10 or so more complete theories describing this process that we collectively describe as modified gravity [5].
As the ongoing quest for dark matter comes up empty, [*] and likewise the search for its imaginary particle, the axion, and the search for mirror particles for every known particle collectively grouped together under the rubric of supersymmetry, various pundits have stepped forward to assert that perhaps MOND should be taken more seriously. Though it is not without its problems [6], modified gravity can satisfactorily predict phenomena in a number of areas far more directly and succinctly than can its dark matter counterpart: instances such as why the total gravity present in a study of more than 150 galaxies is directly proportional to the amount of visible matter contained therein, and how small galaxies behave when trapped in the gravitational field of larger galaxies - in stark contrast to the complex dark matter simulations to explain the same phenomena [7].
The point of all this being that, in the absence of a verified corroboration of the existence of dark matter, and its corollaries the axion and supersymmetry, we should all at least keep an open mind on the viability of the MOND line of thinking that in fact the subject matter of this long and fruitless search doesn’t really exist after all. Furthermore, research in other areas suggests that the universe contains more matter than we can see is an illusion caused by the reaction of space to the presence of mass [8]. Alternatively, perhaps the truth may occupy a region in between: a kind of dark matter masquerading as modified gravity [9]. We await the dark energy Survey, the Large Synoptic Survey Telescope and the launch of the James Webb Space Telescope in 2020 for further developments.
[1] Lisa Grossman, "A curious case of missing darkness", New Scientist, 28 April 2012, 6.
[2] Bogdan A Dobrescu and Don Lincoln, “Mystery of the Hidden Cosmos”, Scientific American, July 2015, 27.
[3] Grossman, op cit, 6-7.
[4] Sabine Hossenfelder and Stacy S McGaugh, “Is dark matter real”, Scientific American, August 2018, 28-35 at 32.
[5] Ibid.
[*] See also Candidates for dark matter: 2 Primordial black holes and Candidates for dark matter: 3. Neutralinos and axions
[6] The subject of elaboration in ibid, 35.
[7] Ibid 33-35.
[8] Research by Erik Verlinde of the University of Amsterdam, noted in ibid, 35.
[9] Ibid.