The dark matter hypothesis isn’t perfect, but the alternatives are worse
You might not be a fan of dark matter, the hypothetical particle that makes up most of the mass of the universe. And it’s true that the dark matter hypothesis has its flaws – and, of course, we haven’t found dark matter particles yet. But the truth is that the alternatives are much worse.
The universe is full of unexplained mysteries (that’s what keeps astronomers and astrophysicists happy), and many of those mysteries surround gravity. As we watch stars orbit around the centers of their galaxies, we find that they are moving far too quickly considering the amount of visible matter that can hold them in those orbits with its gravity.
Galaxies the buzz around galaxy clusters also moves far too quickly considering the amount of mass visible in the clusters. Those same clusters bend the background light way too much. Even large structures have appeared far too quickly in our universe 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 disparate observations is that there is a new type of particle, known as a black matter, who inhabits the cosmos. This particle would be almost entirely invisible (hence its name), rarely (if ever) interacting with normal matter. This idea isn’t as far-fetched as it sounds; neutrinos are particles with exactly these 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 galaxies dominated by dark matter should have incredibly high densities at their centers. Observations of real galaxies show higher densities in their nuclei, but not enough as predicted by these simulations. Moreover, simulations of dark matter evolving in the universe predict that each galaxy should have hundreds of smaller satellites, while observations are consistently lacking.
The case of MOND
Given that the dark matter hypothesis is not perfect – and we have no direct evidence for the existence of candidate particles – it is worth exploring other options.
One such option was introduced in the 1970s alongside the original idea of dark matter, when the astronomer Vera Rubin discovered the problem of stars moving too quickly inside galaxies. But instead of adding a new ingredient to the universe, the alternative changes the recipe by altering how gravity works on a galactic scale. The original idea is called MOND, for “modified Newtonian dynamics”, but the name also applies to the general family of theories arising from this original concept.
With MOND, you pretty much get what’s on the label. On the scale of the planet or the solar system, NewtonGravity works just fine (except, of course, where you need the more detailed gravity calculations provided by general relativity). But once you get big, the usual F = my that we know doesn’t quite apply, 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 slight adjustment to the gravitational force. And because the gravity setting under MOND is explicitly designed to explain the motions of stars in galaxies, it naturally does this very well. The theory also does not suffer from the overproduction of satellites and extremely high galactic nuclei of dark matter.
The Imperfect Champion
But MOND is far from perfect. Modifications to gravity to explain stellar motions struggle to explain motions of galaxies within clusters and the lensing of background light. And MOND is not a fully relativistic theory (all modern theories of physics must be compatible with Relativity). An equivalent MOND update, called TeVeS, can compete with general relativity – and is far from good. Models based on modified gravity have great difficulty in explaining the growth of structure in the universe, the characteristics of cosmic microwave background and more – all the places where dark matter works quite well.
There is no MOND-like theory that can account for every observation when it comes to dark matter; all fail at least one test. Although MOND can still be accurate when it comes to galaxy rotation curves, there are enough observations to tell us that we would be 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, scientists can’t just follow their gut or pick one that sounds cooler or sounds simpler. We must follow the evidence wherever it leads. In nearly 50 years, no one has come up with a MOND-like theory that can explain the wealth of data we have about the universe. It doesn’t make WORLD Wrongbut that makes it a much weaker alternative to dark matter.
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