Using Newton’s laws of physics, we can accurately model the planets’ movements in the solar system. But in the early 1970s, scientists discovered that this didn’t work for disc galaxies—stars at their outer edges, far from the gravitational pull of all the matter at their center—moving much faster than Newton’s theory predicted.
As a result, physicists have proposed that an invisible substance called “dark matter” provides the extra gravitational pull, causing the stars to accelerate—a theory that has become widely accepted. However, in a recent review, my colleagues and I suggest that observations across a wide range of scales are much better explained by an alternative theory of gravity called Milgrom dynamics, or Mond – requiring no invisible matter. Israeli physicist Mordehai Milgrom first proposed it in 1982.
Mond’s primary postulate is that when gravity becomes very weak, as it does near the edge of galaxies, it begins to behave differently from Newtonian physics. This way, it is possible to explain why the stars, planets, and gas at the edge of more than 150 galaxies rotate faster than expected based on their visible mass alone. However, Mond does not just explain such rotation curves; in many cases, it predicts them.
Philosophers of science have argued that this predictive ability makes Mondo superior to the standard cosmological model, which suggests that there is more dark matter than visible matter in the universe. According to this model, galaxies have a highly 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 should rotate. But such predictions are routinely made with Mondo, and they have been confirmed so far.
Imagine that we know the distribution of visible matter in a galaxy but don’t yet know its rotation rate. In the standard cosmological model, it would be safe to say that the rotation speed at the periphery would be between 100 km/s and 300 km/s, and Mond makes a more accurate prediction than the rotation speed must be in the range of 180-190 km/s.