Does dark matter exist? A provocative theory proposes a new rule of gravity
we can fashionl the movements of the planets in the solar system quite accurately using Newton’s laws of physics. But in the early 1970s, scientists noticed that it didn’t work for disc galaxies – the 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” provides additional gravitational pull, causing stars to accelerate – a theory that has become hugely 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 Israeli physicist Mordehai Milgrom in 1982, called Milgromian dynamics or Monday — requiring no invisible material.
Mond’s main postulate is that when gravity becomes very weak, as it happens at the edge of galaxies, it begins to behave differently from Newtonian physics. In this way it is possible to Explain why the stars, planets and gas on the outskirts of over 150 galaxies are spinning faster than expected based on their visible mass alone. But Mond is not content with Explain such rotation curves; in many cases it predicted their.
philosophers of science argued 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. Indeed, according to this model, galaxies have a very uncertain amount of dark matter that depends on the details of galaxy formation – something we don’t always know. This makes it impossible to predict how fast galaxies should spin. But such predictions are regularly made with Mond, and so far they have been confirmed.
Imagine that we know the distribution of visible mass in a galaxy but do not yet know its rotational speed. In the standard cosmological model, it would only be possible to say with some confidence that the rotation speed will come out between 100km/s and 300km/s at the periphery. Mond makes a more accurate prediction that the rotational speed should be between 180 and 190 km/s.
If observations later reveal a rotational speed of 188 km/s, 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 this case, the observations must be explained with as few “free parameters” as possible.
Free parameters are constants – certain numbers that we need to put 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 by observation. An example is the gravitational constant, G, in Newton’s theory of gravity or the amount of dark matter in galaxies in 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 to test the Standard and Mond Cosmological Model against various astronomical observations, such as the rotation of galaxies and movements within galaxy clusters.
Each time, we assigned a theoretical flexibility score between -2 and +2. A score of -2 indicates that a model makes a clear and accurate prediction without peeking at the data. Conversely, +2 implies “everything is fine” – theorists could have fitted almost any plausible observational 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 the theory to be wrong. We then subtract the theoretical flexibility score from the agreement with observations score, because matching the data well is good – but being able to adjust anything is bad.
A good theory would make clear predictions that would later be confirmed, ideally achieving 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 – they are unlikely to work with the wrong physics.
We found an average score for the Standard Cosmological Model of -0.25 across 32 tests, while Mond averaged +1.69 across 29 tests. The scores of each theory in many different tests are shown in Figures 1 and 2 below for the Standard and Mond Cosmological Model, respectively.
It is immediately apparent that no major issues have been identified for Mond, which is at least plausibly consistent with all data (note that the bottom two rows indicating falsifications are blank in Figure 2).
dark matter problems
One of the most striking failures of the Standard Cosmological Model are the “galaxy bars” – bright rod-shaped regions made up of stars – that spiral galaxies often have in their central regions (see main image). The bars rotate in time. If galaxies were embedded in massive halos of dark matter, their bars would slow down. However, most, if not all, of the galaxy bars observed are fast-moving. This falsifies the standard cosmological model with very high confidence.
Another problem is that the original models who suggested that galaxies had 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 the normal matter. This simplified the calculations, but it does not reflect reality. When taken into account in later simulationsit was clear that dark matter halos around galaxies do not reliably explain their properties.
There are many other failures of the standard cosmological model that we investigated in our review, with Mond often able to explain naturally observations. The reason the Standard Cosmological Model is nevertheless so popular could be due to miscalculations or limited knowledge of its failures, some of which have been discovered quite 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.
Mond’s huge lead over the standard cosmological model in our study led us to conclude that 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 – galaxies are seriously lacking in dark matter.