The dark matter hypothesis isn’t perfect, but the alternatives are even worse

You may not be a fan of dark matter, the hypothetical particle that makes up the bulk of the universe. And it’s true that the dark matter hypothesis has its flaws – and of course, we haven’t found a single dark matter particle yet. But the truth is that the alternatives are much worse.

The universe is full of unexplained mysteries (which is what makes astronomers and astrophysicists happy to work), and many of these mysteries surround gravity. When we watch stars revolve around the centers of their galaxies, we find that they are moving too fast given the amount of visible matter that can keep them in those orbits due to their gravity.

Galaxies orbiting clusters of galaxies also move too fast given the amount of visible mass in the clusters. The same clusters distort the background light too much. Even large structures arose too quickly in our universe without an additional source of mass.

On the subject: Should we be so sure of the existence of dark matter?

The best hypothesis that scientists can explain to all these scattered observations is that there is a new kind of particle known as dark matter inhabiting the cosmos. This particle would be almost completely invisible (hence the name), rarely (if ever) interacting with ordinary matter. This idea is not as far-fetched as it seems; Neutrinos are particles with just such properties. They don’t 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 galaxy growth suggest that dark matter-dominated galaxies must have an incredibly high density at their centers. Observations of real galaxies do show a higher density in their cores, but far from sufficient, as these simulations predicted. In addition, simulations of the evolution of dark matter in the universe predict that each galaxy must have hundreds of smaller satellites, while observations consistently fail.

Case for MOND

Given that the dark matter hypothesis isn’t perfect – and that we don’t have direct evidence for the existence of any candidate particles – it’s worth exploring other options.

One such possibility was introduced back in the 1970s, along with the original idea of ​​dark matter, when astronomer Vera Rubin first discovered the problem of stars moving too fast within galaxies. But instead of adding a new ingredient to the universe, the alternative changes the recipe, changing how gravity works on a galactic scale. The original idea is called MOND, which means “modified Newtonian dynamics”, but the name also applies to the general family of theories that stem from this original concept.

With MOND, you pretty much get what the label says. On planetary or solar system scales, Newtonian gravity works just fine (except, of course, when you need the more detailed gravity calculations provided by general relativity). But once you get big, the usual F = ma that we’re familiar with doesn’t quite apply, and the relationship between force and acceleration obeys a different rule.

In MOND, there is no need for extra particles to explain the observations—just a slight tweak to the gravitational force is enough. And since the MOND gravity setting is specifically designed to explain the movement of stars within galaxies, it naturally does so very well. The theory also does not suffer from the overproduction of satellites and extremely high galactic dark matter nuclei.

Imperfect Champion

But MOND is far from perfect. Changes made to gravity to explain the movement of stars make it difficult to explain the movement of galaxies within clusters and the lensing of background light. And MOND is not a completely relativistic theory (all modern physical theories must be compatible with special relativity). An equivalent update to MOND, called TeVeS, can compete head-to-head with general relativity—and falls far behind. Models based on modified gravity have serious problems explaining the growth of structure in the universe, the features of the cosmic microwave background, and more – in all places where dark matter works quite well.

There is no theory like MOND that can explain every single observation when it comes to dark matter; they all fail at least one test. While MOND can still be accurate when it comes to 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, there is no scientific hypothesis. When evaluating competing hypotheses, scientists cannot simply rely on their gut or choose the one that sounds cooler or seems easier. We must follow the clues wherever they lead. In nearly 50 years no one has come up with a theory like MOND that could explain the wealth of data we have about the universe. This doesn’t make MOND wrong, but it does make it a much weaker alternative to dark matter.

Learn more by listening to the Ask an Astronaut podcast, available on iTunes. (will open in a new tab) and (will open in a new tab). Ask your question on Twitter using the hashtag #AskASpaceman or by following Paul @PaulMattSutter. (will open in a new tab) and (will open in a new tab).

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