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Dark matter has been proposed to explain why stars at the farthest edge of a galaxy move faster than Newton predicted. An alternative theory of gravity may be a better explanation.
Using Newton’s laws of physics, we can model the movements of the planets in the solar system quite accurately. But in the early 1970s, scientists discovered it this didn’t work disk galaxies – stars on their outer edges, far from the gravitational pull of all the matter at their center – were moving much faster than Newton’s theory predicted.
As a result, physicists “invisible matter calleddark matter“It provided the extra gravitational force that accelerated the stars – this theory was widely accepted. However, a final review my colleagues and I suggest that the large-scale observations are better explained in Milgrom dynamics or alternative gravity theory. Mond – does not require any invisible substances. It was first proposed in 1982 by the Israeli physicist Mordechai Milgrom.
Mond’s main postulate is that when gravity becomes very weak, as it does at the edges of galaxies, it begins to behave differently from Newtonian physics. It can be done this way explain why the stars, planets, and gas at the edges of more than 150 galaxies rotate faster than expected based on their apparent mass alone. However, Mond simply isn’t explain such rotation curves, in many cases, o predicts they are.
Philosophers of science they argued This predictive power makes Mondo superior to the standard cosmological model, which suggests that there is more dark matter than visible matter in the universe. That’s because, according to this model, galaxies have a very uncertain amount of dark matter that depends on the details of how the galaxy formed—which we don’t always know. This makes it impossible to predict how fast galaxies rotate. But such predictions are regularly made with Mond, and so far they have been confirmed.
Imagine that we know the apparent mass distribution of the galaxy, but we don’t yet know its rotation rate. In the Standard Cosmological Model, one can only say with some certainty that the rotation speed outside would be between 100km/s and 300km/s. Mond predicts more precisely that the rotation speed should be in the range of 180-190km/h.
If observations later reveal a rotation speed of 188km/s, then this is consistent with both theories – but clearly, Mon is favored. This is the modern version Occam’s razor – that the simplest solution is superior to the more complex ones, in which case we need to explain the observations with as few “free parameters” as possible. Free parameters are constants—certain numbers that we have to plug into the equations to make them work. But they are not given by the theory itself—there is no reason why they should have any particular value—so we must measure them observationally. An example is the gravitational constant G or quantity in Newton’s theory of gravity dark matter in galaxies within the standard cosmological model.
We introduced a concept known as “theoretical flexibility” to capture the basic idea of Occam’s razor, that a theory with more free parameters fits a wider range of data – making it more complex. In our review, we used this concept to test the standard cosmological model and Mond against various astronomical observations, such as the rotation of galaxies and motions within galaxy clusters.
Each time we gave a theoretical flexibility score between -2 and +2. A value of -2 indicates that the model makes a clear, accurate prediction without looking at the data. Conversely, +2 means “anything goes” – theorists could fit almost any plausible observational result (since there are so many free parameters). We also rated how well each model fit the observations, with +2 indicating excellent agreement and -2 being reserved for observations that clearly show that the theory is wrong. We then derive a theoretical flexibility score for agreement with observations, because a good fit to the data is good – but a good fit is bad.
A good theory will make clear predictions that are later confirmed, ideally scoring a combined +4 in many different tests (+2 – (- 2) = +4). A bad theory will receive a score between 0 and -4 (-2 – (+ 2) = -4). In this case, accurate predictions will fail – they are unlikely to work with incorrect physics.
We found an average score of -0.25 for the standard cosmological model in 32 tests, while Mond averaged +1.69 in 29 tests. Scores for each theory in many different tests are shown in Figures 1 and 2 below for the standard cosmological model and Mond, respectively.
Figure 1. Comparison of the standard cosmological model with observations shows how well the data fit the theory (improves from bottom to top) and how flexible the fit is (increases from left to right). The void circle is not considered in our estimation because this data was used to determine the free parameters. It is given from table 3 of our review. Credit: Archive
Figure 2. Similar to Fig. 1 , but for Mond with hypothetical particles that only interact gravitationally, called sterile neutrinos. Pay attention to the absence of obvious forgeries. It is given from table 4 of our review. Credit: Archive
It is immediately clear that no major problem has been identified for Mond, which at least agrees with all the data (note that the bottom two rows reporting forgeries are empty in figure 2).
Problems with dark matter
One of the most striking failures of the standard cosmological model concerns the “galactic bars”—bright rod-shaped regions of stars—often found in the central regions of spiral galaxies (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, of the observed galactic bars are fast. This falsifies standard cosmological model with very high confidence.
Another problem is that original models Those who claimed that galaxies are dark matter halos made a big mistake – they assumed that dark matter particles provide the attraction of the matter around them, but are not affected by the gravity of normal matter. This simplifies calculations, but does not reflect reality. When this is considered subsequent simulations it was clear that dark matter halos around galaxies do not reliably explain their properties.
There are many other failings of the standard cosmological model that we examine in our review, Mond often naturally explains observations. However, the reason the standard cosmological model is so popular may be due to limited knowledge of its computational errors or failures, some of which have only recently been discovered. It may also be due to people’s reluctance to change the theory of gravity, which has been very successful in many other areas of physics.
The large superiority of the Mond over the standard cosmological model in our study leads us to conclude that the Mond is strongly favored by the available observations. While we don’t claim Mond is perfect, we still think he gets the big picture right—there really is no dark matter in galaxies.
Written by Indranil Banik, postdoctoral researcher in astrophysics at the University of St Andrews.
This article was first published Conversation.
Reference: “From Galactic Bars to the Hubble Stress: Measuring the Astrophysical Evidence for Milgrom Gravity
By Indranil Banik and Hongsheng Zhao, June 27, 2022, Symmetry.
DOI: 10.3390/sym14071331
