we can model the motions of the planets in the Solar System using quite precisely Newton’s laws of physics. But in the early 1970s, scientists noticed that this didn’t work for disk galaxies — stars at their outer edges, away from the gravitational pull of all the matter at their center — were moving much faster than Newton’s theory predicted.
This led physicists to propose that an invisible substance called “dark matter” provided an additional gravitational pull, causing stars to speed up, a theory that has become very popular. However, in a recent reviewMy colleagues and I suggest that observations on a wide range of scales are much better explained in an alternative theory of gravity proposed by the Israeli physicist Mordehai Milgrom in 1982, called Milgromian dynamics or Monday — which does not require invisible matter.
Mond’s main postulate is that when gravity becomes very weak, as it does at the edge of galaxies, it begins to behave differently from Newtonian physics. In this way, it is possible explain why stars, planets, and gas in the outskirts of more than 150 galaxies are spinning faster than expected based on their visible mass alone. But Mond is not limited to explain such rotation curves; in many cases, it predicts to them.
philosophers of science they have discussed that this predictive power makes Mond superior to the standard cosmological model, which proposes 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 of 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, have been confirmed.
Imagine that we know the distribution of visible mass in a galaxy but we don’t yet know its rate of rotation. In the standard cosmological model, it would only be possible to say with any confidence that the rotation speed will be between 100 and 300 km/s in the outskirts. Mond makes a more definite prediction that the rate of rotation should be in the range of 180 to 190 km/s.
If subsequent observations reveal a rotational 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 ones; in this case, we should explain the observations with as few “free parameters” as possible.
Free parameters are constants, certain numbers that we need to plug into the equations for them to work. But they are not given by the theory itself, there is no reason for them to have any particular value, so we have to measure them by observation. 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 use this concept when testing the Mond and Standard Cosmological Model 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 score of -2 indicates that a model makes a clear and accurate prediction without looking at the data. By contrast, +2 implies “anything goes”: theorists would have been able to fit almost any plausible observational result (because there are so many free parameters). We also rate how well each model matches 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 the agreement with the observations, since matching the data well is good, but being able to fit anything is bad.
A good theory would make clear predictions that would then be confirmed, ideally getting a combined score of +4 on 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; they are unlikely to work with the wrong physics.
We find 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. The scores for each theory on many different tests are shown in Figures 1 and 2 below for the Mond and Standard Cosmological Model, respectively.
It is immediately apparent that no major issues were identified for Mond, which at least plausibly agrees with all the data (note that the bottom two rows denoting fakes are blank in Figure 2).
The problems with dark matter
One of the most striking flaws in the standard cosmological model relates to the “galaxy bars” (bright rod-shaped regions made of stars) that spiral galaxies often have in their central regions (see main image). The bars rotate over time. If galaxies were embedded in massive halos of dark matter, their bars would slow down. However, most, if not all, of the observed galaxy bars are fast. East falsifies the standard cosmological model with very high confidence.
Another problem is that the original models who suggested that galaxies have dark matter halos made a big mistake: they assumed that the 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 post simulationsit became clear that dark matter halos around galaxies do not reliably explain their properties.
There are many other flaws in the standard cosmological model that we investigate in our review, with Mond often able to explain naturally the observations. However, the reason the standard cosmological model is so popular could be due to miscalculations or limited knowledge about its flaws, some of which were only recently discovered. 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.
Mond’s large advantage over the standard cosmological model in our study led us to conclude that the available observations strongly favor Mond. While we’re not claiming that Mond is perfect, we still think he has the big picture right: galaxies really do lack dark matter.
This article was originally published on The conversation by Indranil Banik in the University of San Andres. Read the original article here.