Dark matter was proposed to explain why stars at the far end of a galaxy were able to move much faster than Newton predicted. An alternative theory of gravity could be a better explanation. Using Newton’s laws of physics, we can model the motions of the planets in the Solar System with great accuracy. However, in the early 1970s, scientists discovered that this did not work for disk galaxies – stars at their outer edges, away from the gravitational pull of all the matter at their center – moving much faster than predicted by Newton’s theory. As a result, physicists proposed that an invisible substance called “dark matter” provided extra gravitational pull, causing the stars to accelerate – a theory that has been widely accepted. However, in a recent review my colleagues and I suggest that observations over a vast range of scales are much better explained by an alternative theory of gravity called Milgromian dynamics or Mond – which does not require invisible matter. It was first proposed by Israeli physicist Mordehai Milgrom in 1982. Mond’s main postulate is that when gravity becomes very weak, as it does near the edge of galaxies, it starts to behave differently from Newtonian physics. In this way, it is possible to explain why the stars, planets and gas on the outskirts of more than 150 galaxies rotate faster than expected based on their visible mass alone. However, Mond does not simply explain such rotation curves, in many cases he predicts them. Philosophers of science have argued that this predictive power makes Mond superior to the standard cosmological model, which suggests that there is more dark matter in the universe than visible matter. This is because, according to this model, galaxies have a highly uncertain amount of dark matter that depends on the details of how the galaxy formed – something we don’t always know. This makes it impossible to predict how fast galaxies should rotate. But such predictions are usually made with Mond, and so far they have been confirmed. Imagine that we know the visible mass distribution in a galaxy but do not yet know its rotation speed. In the standard cosmological model, it would only be possible to say with some confidence that the rotation speed would be between 100km/s and 300km/s in the outskirts. Mond makes a more definite prediction that the rotation speed should be in the range of 180-190 km/s. If observations later reveal a rotation speed of 188 km/s, then this is consistent with both theories – but clearly, Mond is preferred. This is a modern version of Occam’s razor – that the simplest solution is preferable to the most complex, in which case we should explain the observations with as few “free parameters” as possible. Free parameters are constants – some numbers that we have to plug into equations to make them work. But they are not given by the theory itself – there is no reason for them to have any particular value – so we must measure them observationally. An example is the gravitational constant, G, in Newton’s theory of gravity, or the amount of dark matter in galaxies within the standard cosmological model. We introduced a concept known as “theoretical flexibility” to capture the underlying idea of Occam’s razor that a theory with more free parameters is consistent with a wider range of data – making it more complex. In our review, we used this concept when testing the Standard Cosmological Model and Mond on various astronomical observations, such as galaxy rotation and motions within galaxy clusters. Each time, we gave a theoretical flexibility score between –2 and +2. A score of -2 indicates that a model makes a clear, accurate prediction without looking at the data. Conversely, +2 indicates “anything goes” – theorists could match almost any plausible observational result (because there are so many free parameters). We also scored how well each model fit the observations, with +2 indicating excellent agreement and -2 reserved for observations that clearly show the theory is wrong. We then subtract the theoretical flexibility score from that for agreement with observations, since a good fit to the data is good – but being able to fit anything is bad. A good theory would make clear predictions that would later be confirmed, ideally getting a combined score of +4 on several different tests (+2 -(-2) = +4). A bad theory would get a score between 0 and -4 (-2 -(+2)= -4). Exact predictions will fail in this case – they are unlikely to work with the wrong physics. We found an average score for the standard cosmological model of -0.25 in 32 tests, while Mond achieved an average of +1.69 in 29 tests. Scores for each theory on several different tests are shown in Figures 1 and 2 below for the Standard Cosmological Model and Mond, respectively. Figure 1. Comparison of the established cosmological model to observations based on how well the data fit the theory (bottom-up improvement) and how much flexibility it had in fitting (left-to-right rise). The hollow circle is not counted in our evaluation, as this data was used to define free parameters. Reproduced from table 3 of our review. Credit: Arxiv Figure 2. Similar to figure 1, but for Mond with hypothetical particles interacting only through gravity called sterile neutrinos. Notice the lack of obvious tampering. Reproduced from Table 4 of our review. Credit: Arxiv It is immediately apparent that no significant problems were identified for Mond, which is at least in reasonable agreement with all data (note that the bottom two rows indicating falsifications are blank in figure 2).
The problems with dark matter
One of the most striking failures of the standard cosmological model relates to “bar galaxies”—bright, rod-shaped regions made of stars—that spiral galaxies often have in their central regions (see lead image). The bars rotate over time. If galaxies were embedded in massive haloes of dark matter, their bars would slow down. However, most, if not all, observed galaxy bars are fast. This falsifies the standard cosmological model with very high confidence. Another problem is that the original models that proposed that galaxies have dark matter halos made a big mistake – they assumed that dark matter particles provided gravity to the matter around them, but were unaffected by the gravitational pull of normal matter. This simplified the calculations, but does not reflect reality. When this was taken into account in subsequent simulations, it was clear that dark matter halos around galaxies did not reliably explain their properties. There are many other failures of the standard cosmological model that we explored in our review, with Mond often being able to explain the observations physically. The reason the standard cosmological model is still so popular could be due to computational errors or limited knowledge about its failures, some of which have only been discovered very recently. It could also be due to people’s reluctance to modify a theory of gravity that has been so successful in many other areas of physics. The huge lead of Mond over the standard cosmological model in our study led us to conclude that Mond is strongly favored by the available observations. While we’re not claiming Mond is perfect, we still think it’s got the big picture right – galaxies do lack dark matter. Written by Indranil Banik, Postdoctoral Research Fellow in Astrophysics, University of St Andrews. This article was first published on The Conversation. Citation: “From Galactic Bars to Hubble Tension: Weighing the Astrophysical Evidence for Milgromian Gravity by Indranil Banik and Hongsheng Zhao, June 27, 2022, Symmetry.DOI: 10.3390/sym14071331
