by You might not be a fan of dark matter, the hypothetical particle that makes up the bulk of the mass in the universe. And it’s true that the dark matter hypothesis has its flaws – and of course we haven’t found any dark matter particles yet. But the truth is that the alternatives are much worse.
The universe is full of unexplained mysteries (which is what keeps astronomers and astrophysicists happy to work), and many of these mysteries surround gravity. While we watch stars orbiting the centers of their galaxies, we find that they are moving far too fast given the amount of visible matter that can hold them in these orbits with gravity.
Galaxies buzzing around galaxy clusters is also moving far too fast given the amount of visible mass in the clusters. The same clusters bend the background light far too much. Even the large structures arose in our universe far too quickly without an additional source of mass.
Related: Should we be so sure that dark matter exists?
The best hypothesis scientists have to explain all these different observations is that there is a new type of particle, known as dark matter, who live in the cosmos. This particle would be almost completely invisible (hence the name), and would rarely (if ever) interact with normal matter. This idea is not as far-fetched as it seems; neutrinos are particles with exactly these properties. They do not have enough mass to explain dark matter, but they show that such particles can exist.
But the dark matter hypothesis is not perfect. Computer simulations of the growth of galaxies suggest that dark matter-dominated galaxies should have incredibly high densities at their centers. Observations of real galaxies show higher densities in their cores, but not nearly as much as these simulations predicted. Also, simulations of dark matter evolving in the universe predict that each galaxy should have hundreds of smaller satellites, while observations consistently fall short.
The case for MOND
Given that the dark matter hypothesis is not perfect—and that we have no direct evidence for the existence of any candidate particles—it is worth exploring other options.
Such an option was introduced back in the 1970s along with the original idea of dark matter, when the astronomer Vera Rubin first discovered the problem of stars moving too fast inside galaxies. But instead of adding a new ingredient to the universe, the alternative changes the recipe by changing how gravity works on galactic scales. The original idea is called MOND, for “modified Newtonian dynamics”, but the name also applies to the general family of theories that derive from the original concept.
With MOND, you mostly get what is written on the label. On planetary or solar system scales, Newtonits gravity works just fine (except, of course, where you need the more detailed calculations of gravity provided by general relativity). But when you first grow up, the usual F = ma we are familiar with does not apply entirely, and the relationship between force and acceleration follows a different rule.
Under MOND, there is no need for an extra particle to explain the observations – just a small adjustment of the gravitational force. And because the tweaking of gravity under MOND is explicitly designed to explain the motions of stars in galaxies, it naturally does it very well. The theory also does not suffer from the overproduction of satellites and the extremely high galactic nuclei of dark matter.
The flawed master
But MOND is far from perfect. The changes made to gravity to explain the motions of stars have trouble explaining the motions of galaxies in clusters and the lensing of background light. And MOND is not a fully relativistic theory (all modern theories of physics must be compatible with special relativity). A corresponding update to MOND, called TeVeS, can compete head-to-head with general relativity – and falls far short. Models based on modified gravity have significant problems in explaining the growth of structure in the universe, functions of cosmic microwave background and more – all places where dark matter works quite well.
There is no MOND-like theory that can explain every single observation when it comes to dark matter; everyone fails at least one test. Although MOND may still be accurate in terms of galaxy rotation curves, there are enough observations to tell us that we still need dark matter to exist in the universe.
No, the dark matter hypothesis is not perfect. But again, no scientific hypothesis is. When evaluating competing hypotheses, researchers can’t just go with their gut, or choose one that sounds cooler or seems simpler. We must follow the evidence, wherever it leads. In almost 50 years, no one has come up with a MOND-like theory that can explain the amount of data we have about the universe. MOND does not errorbut that makes it a far weaker alternative to dark matter.
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