This physicist prefers a new theory of gravity

by Indranil Banik, Postdoctoral Researcher in Astrophysics, University of St Andrews
10 July 2022

Dark matter has been proposed to explain why stars at the far end of galaxies can move faster than Newton thought. An alternative theory of gravity may be a better explanation.

Using Newton’s laws of physics, we can model the motion of the planets in the solar system with complete accuracy. However, in the early 1970s, scientists discovered that it didn’t work for him. Disc galaxies The stars on their outer edges, away from the gravitational force of all matter at their center, are moving much faster than Newton’s theory predicted.

As a result, physicists suggest that the invisible substance called “dark matter” exerts an additional gravitational pull, causing the star to accelerate — a widely accepted theory. However, in a recent review my colleagues and I suggested that observations at multiple scales are much better explained in an alternative theory of gravity called Milgromian or Mond dynamics – it does not require invisible matter. It was first proposed by Israeli physicist Mordechai Milgrom in 1982.

Mond’s basic assumption is that when gravity becomes too weak, as happens near the edges of galaxies, it begins to behave differently from Newtonian physics. In this way, it might explain why the stars, planets, and gas in the periphery of more than 150 galaxies are spinning faster than expected based on their apparent mass alone. However, Mond not only explain Like the rotation curve, in most cases, Expect they.

philosophers of science argue that this predictive power makes Mond superior to standard cosmological models, which suggest that there is more dark matter in the universe than visible matter. This is because, according to this model, galaxies contain very uncertain amounts of dark matter that depend on the details of how galaxies form – which we don’t always know. This makes it impossible to predict how fast the galaxy is rotating. But such predictions are routinely made with Mond, and it has been confirmed so far.

Imagine that we know the distribution of visible mass in a galaxy but don’t yet know its rotational speed. In standard cosmological models, it is only possible to say with certainty that the rotational speed will be between 100 km/s and 300 km/s in the suburbs. Mond gives a more specific prediction that the rotational speed should be in the range of 180-190 km/s.

If subsequent observations reveal a rotational speed of 188 km/s, this fits both theories – but Mond is definitely the favourite. This is the latest version of Occam’s razor – that the simplest solution is better than the more complex solution, in which case we must define the record with the least number of “free parameters”. Independent parameters are constants – certain numbers that we have to plug into the equation for it to work. But the theory itself doesn’t give them – there is no reason for the existence of any particular value – so we have to measure it by observation. Examples are the gravitational constant, G, in Newton’s theory of gravity or the magnitude of dark matter in galaxies in the Standard Cosmological Model.

We introduce a concept known as “theoretical elasticity” to capture the idea behind Occam’s code that the theory with the most independent parameters is consistent with a wider range of data – making it more complex. In our review, we used this concept when testing the Standard and Mond cosmological models against various astronomical observations, such as the rotation of galaxies and motion within galaxy clusters.

Each time, we assign a theoretical elasticity score between -2 and +2. A score of -2 indicates that the model makes clear and accurate predictions without looking at the data. On the other hand, +2 stands for “anything” – theorists can have almost any reasonable observation result (because there are so many independent parameters). We also assess how well each model fits the observations, where +2 indicates an excellent fit and -2 is reserved for observations that clearly indicate that the theory is wrong. We then reduce the degree of theoretical flexibility from dealing with observations, because fitting data well is good – but being able to fit anything is bad.

A good theory will make clear predictions which are later confirmed, and a combined score of +4 on many different tests will be better (+2 – (- 2) = +4). A bad theory will score between 0 and -4 (-2 – (+ 2) = -4). Accurate predictions may fail in this case – and may not work with faulty physics.

We found the mean score for the Standard Cosmological Model -0.25 across 32 tests, while Mond achieved a mean score of +1.69 across 29 tests. Scores for each theory on the many different tests are shown in Figures 1 and 2 below for the Standard and Mond cosmological models, respectively.

Image 1. Compares standard cosmological models with observations based on how well the data fit the theory (bottom-up optimization) and how flexible the adjustments are (height left-to-right). Hollow circles were not taken into account in our evaluation, because these data were used to set the independent parameters. Reproduced from Table 3 of our review. credit: Arxiv

Figure 2. Similar to Figure 1, but for Mond with virtual particles that interact only by gravity, they are called sterile neutrinos. Note that there are no obvious fakes. Reproduced from Table 4 of our review. credit: Arxiv

It is immediately clear that no significant problems have been identified for Mond, which at least fairly fits all the data (note that the bottom two rows showing falsification are blank in Figure 2).

dark matter problem

One of the most striking failures of the Standard Cosmic Model has to do with “bar galaxies” – bright, bar-shaped regions made of stars – where spiral galaxies are often found in their central regions (see main image). The bar rotates from time to time. If galaxies were embedded in a large circle of dark matter, their rods would slow down. However, most, if not all, of the observed galactic bands are fast. this is a false Standard cosmological model with a high degree of confidence.

Another problem is that the original models that proposed galaxies have dark matter halos made a huge mistake – they assumed that dark matter particles exert gravity on the matter around them, but are not affected by the gravitational pull of ordinary matter. This simplifies calculations, but does not reflect reality. When this was taken into account in subsequent simulations it became clear that the halos of dark matter around galaxies could not reliably explain their properties.

There are many other failures of the Standard Cosmological Model that we saw in our reviews, and Mond can often explain the Notes naturally. However, the reason why the Standard Cosmological Model is so popular can be due to computational errors or limited knowledge of its failures, some of which have been discovered recently. It may also be due to people’s reluctance to modify the theory of gravity which has been so successful in many other areas of physics.

Mond’s great advantage over the Standard Cosmological Model in our study led us to conclude that the available observations strongly support Mond. While we don’t claim that Mond is perfect, we still think it corrects the big picture – galaxies really lack dark matter.

Written by Indranil Banik, Postdoctoral Researcher in Astrophysics, University of St Andrews.

This article was first published in Conversation.

Reference: “From the Galactic Trunk to the Hubble Tension: Considering the astrophysical evidence for Melgromian gravity
By Indranil Banik and Hongsheng Zhao, 27 Jun 2022 Available here symmetry.
DOI: 10.3390 / sym14071331

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