(MOND) as an alternate explanation for the observations generally attributed to dark matter (e.g., Hartnett 2002; Worraker 2002). Proposed by a non-creationist (Milgrom, M. 1983), MOND hypothesizes that Newton’s simple inverse square of the distance law of gravity is a good description of gravity over relatively short distances (at least on the scale of the solar system), but for distances spanning thousands of light years, Newtonian gravity fails to adequately describe how gravity operates. Thus, Newtonian gravity must be modified in such a way that its long-scale behavior is masked over distances spanning less than a few thousand light years. This modification comes across as radical, but is it? Newton derived his law of gravity by comparing the measured acceleration of gravity on the earth’s surface to the centripetal acceleration required for the moon to orbit the earth, and he further tested his hypothesis by showing it accurately described the motions of the planets and the natural satellites of Jupiter and Saturn (Faulkner 2017b). MOND proposes to extend Newton’s original approach of fitting orbital motions of objects at distances seven or more orders of magnitude greater than those directly tested in the solar system. In this sense, MOND would just be a modification of our understanding of gravity, with Newtonian gravity as a special limiting case, much as general relativity modified our understanding of gravity a century ago. So, perhaps MOND is not so radical after all. However, since Newtonian gravity and general relativity are consistent with one another in the regimes under discussion, MOND would require modification of general relativity as well. To my knowledge, this has not yet been attempted. There are tests of MOND that we can perform. If MOND correctly describes how gravity operates, then MOND ought to apply to all galaxies of sufficient size. However, astronomers have found a few galaxies that have no need of dark matter, that is, spiral galaxies which have rotation curves that follow Keplerian behavior outside their nuclei. The first example of this was the galaxy NGC 1052-DF2 (Faulkner 2018). If MOND properly describes the observed departure from Keplerian motion outside the nuclei of most spiral galaxies, then why does it not apply to galaxies with Keplerian motion outside their nuclei? It may seem counterintuitive, but the existence of a few large galaxies that have no evidence of dark matter amounts to evidence of dark matter in other galaxies. Another test of MOND was presented by the discovery of the interacting galaxy cluster 1E0657-558, aka the Bullet Cluster (Clowe, Gonzalez, and Markevitch 2008). This object is two galaxy clusters that appear to have recently undergone a collision. Most of the emitting mass in clusters of galaxies is in the form of hot intergalactic gas. Stars are very small compared to the scales of galaxies and clusters of galaxies, so when clusters of galaxies collide, the stars and galaxies largely pass through one another with only modification of their trajectories. However, the diffuse intergalactic clouds directly collide and stall, leaving the gas originally in the two clusters between them. The high-temperature intergalactic gas is detected by the X-rays they emit, while the stars and galaxies are detected by optical light. When images of the two are superimposed as in Fig. 4, the intergalactic gas is located between the two galaxy clusters. Since the mass of the intergalactic gas dominates the visible mass, MOND would predict that most of the mass would be aligned with the gas and not the visible galaxies. However, dark matter does not appear to interact with normal matter, so the prediction of the mass distribution based upon the assumption of dark matter is that most of the mass would align with the galaxy clusters, not the gas. 1E0657-558 acts as a gravitational lens of more distant objects, allowing its distribution of mass to be mapped. The bulk of the mass is centered on the two clusters, not the interposing stalled gas, thus MOND is eliminated as a possibility. The same sort of observations and reasoning applied to the colliding cluster MACS J0025.4-1222 reach the same conclusion (Brada et al. 2008). It’s not as if dark, or yet unseen, matter is a new concept. Neptune was discovered in 1846 based upon calculations of a hypothetical planet responsible for perturbations of the orbit of Uranus. In 1980, the two Voyager spacecrafts discovered that Saturn’s F Ring appeared braided. The inferred explanation was that there were two small natural satellites, or moons, nearby that perturbed ring particles to produce the braiding. A search for these shepherd moons quickly led to the discovery of Prometheus and Pandora. More recent studies suggest that Prometheus plays the dominant role in this process. One can even argue that Wolfgang Pauli’s 1930’s proposal of the neutrino to salvage the conserveation of energy, linear momentum, and angular momentum in beta decay was a form of dark matter because neutrinos remained undetected until 1956. To be fair, the hypothetical planet Vulcan that was proposed in the 19th century to explain the anomaly in the perihelion advance of Mercury’s orbit is an example of a failed dark matter prediction. The solution to the problem with Mercury’s orbit came with the publication of Einstein’s theory of general relativity in 1915. Hence, there is precedent for new physics. However, the question is which of the two, dark matter or new physics, best explains what we see today. Dark matter is a much better explanation than new physics. There has been at least one refreshing approach for an alternative Figure 4. Composite image of 1E0657-558 (the Bullet Cluster). Superimposed over a visible light image of the galaxies is an x-ray image (pink), showing the emission of gas, and the inferred distribution of dark matter (blue) from gravitational lensing. The dark matter coincides with the galaxies, not the intergalactic gas, which has greater mass than the galaxies. MOND would predict the coincidence of the gas and the need for unseen matter. FAULKNER Dark matter and dark energy 2023 ICC 6
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