We can model the motions of planets in the solar system quite accurately using Newton’s laws of physics. But in the early 1970s, scientists noticed that: this didn’t work for disk galaxies — stars on their outer edges, far from the gravitational pull of all matter at their center — moved much faster than Newton’s theory predicted.
This led physicists to suspect that an invisible substance called “dark matter” added gravity, causing the stars to accelerate — a theory that has become hugely popular. However, in a recent reviewmy colleagues and I suggest that observations across many scales are much better explained in an alternative gravitational theory proposed in 1982 by Israeli physicist Mordehai Milgrom, called Milgromian dynamics of Monday – no invisible matter needed.
Mond’s main postulate is that when gravity becomes very weak, as happens at the edge of galaxies, it behaves differently from Newtonian physics. In this way it is possible to to explain why stars, planets and gas in the outskirts of more than 150 galaxies rotate faster than expected based on their visible mass alone. But Mond doesn’t do it alone to explain such rotational curves, in many cases, predicts them.
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Philosophers of Science have argued that this predictive power makes Mond superior to the standard cosmological model, which states that there is more dark matter in the universe than visible matter. This is because, according to this model, galaxies have a very uncertain amount of dark matter that depends on details about how the galaxy formed – which we don’t always know. This makes it impossible to predict how fast galaxies should rotate. But such predictions are routinely made with Mond, and so far they have been confirmed.
Suppose we know the distribution of visible mass in a galaxy, but not yet its rotational speed. In the standard cosmological model it could only be said with some certainty that the rotational speed at the edge will be between 100 km/s and 300 km/s. Mond makes a more definitive prediction that the rotational speed should be in the range of 180-190 km/s.
If observations later show a rotational speed of 188 km/s, then this is in line with both theories – but it is clear that Mond is preferred. This is a modern version of Occam’s razor – that the simplest solution is preferable to more complex ones, in this case that we have to explain observations with as few “free parameters” as possible. Free parameters are constants – certain 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 why they should have a certain value – so we have to 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 against various astronomical observations, such as the rotation of galaxies and the motions within galaxy clusters.
Each time we gave a theoretical flexibility score between –2 and +2. A score of -2 indicates that a model is making a clear, accurate prediction without looking at the data. Conversely, +2 means “anything goes” – theorists would have been able to fit almost any plausible observation result (because there are so many free parameters). We also assessed how well each model matches the observations, with +2 indicating excellent agreement and -2 reserved for observations that clearly show that the theory is wrong. We then subtract the theoretical flexibility score from that for the agreement with observations, since a good match of the data is good, but being able to fit everything is bad.
A good theory would make clear predictions that are confirmed later, ideally getting a combined score of +4 in many different tests (+2 -(-2) = +4). A bad theory would get a score between 0 and -4 (-2 -(+2)= -4). Accurate predictions would fail in this case – unlikely to work with the wrong physics.
We found an average score for the standard cosmological model of -0.25 over 32 tests, while Mond achieved an average of +1.69 over 29 tests. The scores for each theory in many different tests are shown in Figures 1 and 2 below for the standard cosmological model and Mond, respectively.
It is immediately apparent that no major problems have been identified for Mond, which is at least plausible with all the data (note that the bottom two rows denoting counterfeits are empty in Figure 2).
The Problems With Dark Matter
One of the most notable flaws of the standard cosmological model relates to “galaxies” — rod-shaped bright regions made of stars — which spiral galaxies often have in their central regions (see lead image). The rods rotate in time. If galaxies were embedded in massive dark matter halos, their bars would slow down. However, most, if not all, observed galaxies are fast. This one forged the standard cosmological model with great confidence.
Another problem is that the original models who suggested that galaxies have halos of dark matter made a big mistake — they assumed that the dark matter particles gave gravity to the matter around them, but were unaffected by the gravity of normal matter. This simplified the calculations, but it does not reflect reality. When this was taken into account in subsequent simulations it was clear that dark matter halos around galaxies do not reliably explain their properties.
There are many other shortcomings of the standard cosmological model we explored in our review, with Mond often being able to: explain of course the observations. The reason the standard cosmological model is nevertheless so popular may have to do with calculation errors or limited knowledge about its errors, some of which were discovered quite recently. It could also be due to people’s reluctance to adapt a theory of gravity that has been so successful in many other fields of physics.
Mond’s huge lead over the standard cosmological model in our study led us to conclude that Mond is highly favored from the available observations. While we’re not claiming Mond is perfect, we still think it gets the big picture right — galaxies really don’t have dark matter.
This article by Indrani BanikPostdoctoral Researcher Astrophysics, University of St Andrews has been reissued from The conversation under a Creative Commons license. Read the original article†