Hello everybody. We haven’t talked about dark matter in a while. So today I want to tell you how my opinion about dark matter has changed over the past twenty years or so. In particular, I would like to discuss whether dark matter is made up of particles or if not what else it could be. Let’s begin.
First things first, dark matter is the hypothetical material that astrophysicists believe makes up eighty percent of the matter in the universe, or 24 percent of the combined matter-energy. Dark matter should not be confused with dark energy. These are two completely different things. Dark energy is what makes the universe expand faster, dark matter is what makes galaxies rotate faster, although that’s not the only thing dark matter does, as we’ll see in a moment.
But what is dark matter? 20 years ago I thought that dark matter was most likely made up of some kind of particle that we have not yet measured. Because I am a trained particle physicist. And if a particle can explain an observation, why look any further? At this time there were also some suggestions for new particles that might fit the data, like some supersymmetric particles or axions. The idea that dark matter is made up of particles seemed plausible to me and likes the obvious explanation.
So, just among us, I always thought that dark matter wasn’t a particularly interesting problem. Sooner or later they’ll find the particle, give it a name, someone’s going to get a Nobel Prize, and that’s it.
But that didn’t happen. Physicists have been trying to measure dark matter particles since the mid-1980s. But no one has ever seen one. There were some anomalies in the data, but these all disappeared upon closer inspection. Instead, it has become increasingly difficult to explain some astrophysical observations with the particle hypothesis. Before I get to the observations that don’t explain particle dark matter, I’ll first briefly summarize what it explains. These are the reasons astrophysicists believed they existed in the first place.
Historically, the first evidence of dark matter came from galaxy clusters. Galaxy clusters consist of a few hundred to about a thousand galaxies that are held together by their gravitational pull. They move around each other and how fast they move depends on the total mass of the cluster. The more mass, the faster the galaxies move. It turns out that galaxies in clusters of galaxies move far too fast to explain this in terms of the mass we can ascribe to visible matter. In the 1930s, Fritz Zwicky suspected that there must be more matter in galaxy clusters, only that we cannot see it. He called it “dark matter” dark matter.
It is similar with galaxies. The speed of a star orbiting the center of a galaxy depends on the total mass within that orbit. But the stars in the outer parts of the galaxies are simply orbiting the center too quickly. Their speed should decrease with distance from the center of the galaxy, but it doesn’t. Instead, the speed of the stars at a great distance from the galactic center becomes almost constant. This creates the so-called “flat rotation curves”. Again, you can explain this by saying that there is dark matter in the galaxies.
Then there are gravitational lenses. These are galaxies, or clusters of galaxies, that bend light coming from an object behind them. That object behind you will then appear distorted, and from the amount of distortion you can deduce the mass of the lens. Here, too, the visible matter is insufficient to explain the observations.
Then there are the temperature fluctuations in the cosmic microwave background. You can see these fluctuations in this Skymap. All of these points here are deviations from the average temperature, which is around 2.7 Kelvin. The red spots are a little warmer, the bruises a little colder than this average. Astrophysicists analyze the microwave background based on its power spectrum, with the vertical axis roughly corresponding to the number of points and the horizontal axis its size, with the larger sizes on the left and the smaller and smaller points on the right. To explain this range of services, you need dark matter again.
Finally, there is the large-scale distribution of galaxies and galaxy clusters and interstellar gas and so on, as you can see in the picture of this computer simulation. Normal matter alone simply does not create enough structure on short scales to match what was observed, and again, adding dark matter removes the problem.
You see, dark matter was a simple idea that went with a lot of observations, which is why it was such a good scientific explanation. But that was the status 20 years ago. And what has happened since then is that observations have accumulated that dark matter cannot explain.
For example, the dark matter of the particles predicts a density in the cores of small galaxies that will peak, while observations say that the distribution should be flat. Dark matter also predicts too many small satellite galaxies. These are small galaxies that fly around a larger host. For example, the Milky Way should have many hundreds, but actually only has a few dozen. These small satellite galaxies are also often aligned in planes. Dark matter doesn’t explain why.
We also know from observations that the mass of a galaxy correlates with the fourth power of the rotation speed of the outermost stars. This is known as the Tully-Fisher baryonic relationship and is just an observational fact. Dark matter doesn’t explain it. It is similar with Renzo’s rule: if you look at the rotation curve of a galaxy, for every feature in the curve for visible emission, such as: B. a wobble or a bulge, also a feature in the rotation curve. Again, this is an observational fact, but it makes absolutely no sense if you think that most of the matter in galaxies is dark matter. The dark matter should eliminate any correlation between the luminosity and the rotation curves.
Then there are high-speed collisions of galaxy clusters, such as the globular cluster or the El Gordon cluster. These are difficult to explain with the dark matter of the particles, as dark matter creates friction and this makes such high relative speeds incredibly unlikely. Yes, you heard right that the bullet cluster is a dark matter PROBLEM, not proof of it.
And yes, you can fiddle around with the dark matter computer simulations and keep adding more and more parameters to try to get everything right. But that is no longer a simple explanation or a prediction.
So if it’s not dark matter, what else could it be? The alternative explanation to the dark matter of the particles is the modified gravity. The idea of modified gravity is that we are not missing a source for gravity, but that we have the law of gravity wrong.
Modified gravity solves all of the puzzles I just told you about. There is no friction, so high relative speeds are not a problem. It predicted the Tully-Fisher relationship, explained Renzo’s rule and satellite alignment, removed the problem of density peaks in galactic cores, and solved the problem of missing satellites.
However, modified gravity does not fit well with the microwave cosmic background and early universe, and there are some problems with galaxy clusters.
So this looks like a battle between competing hypotheses, and that’s certainly how it has been portrayed, and how most physicists think of it.
But here’s the thing. From a data point of view, the simplest explanation is that the particles’ dark matter works better in some cases and modifies gravity better in other cases. Many astrophysicists answer that. If you have dark matter anyway, why did you change gravity too? Answer: Because dark matter has difficulty explaining many observations. On its own, it is no longer the simplest parametric explanation.
But wait, you might want to say you can’t just use dark matter for observations a, b, c and modified gravity for observations x, y, z! Well, actually, you totally can do that. Nothing in the scientific method that forbids it.
But more importantly, if you look at the math, the particles’ modified gravity and dark matter are actually very similar. Dark matter adds new particles and modified gravity adds new fields. But due to quantum mechanics, fields are particles and particles are fields, so it’s really the same thing. The difference is the behavior of these fields or particles. It is behavior that changes from the scales of galaxies to clusters to filaments and the early universe. So what we need is some kind of phase transition that explains why and under what circumstances the behavior of these additional fields or particles changes, so that we need two different systems of equations.
And when you look at it that way, it’s obvious why we haven’t made any progress in so long on what is dark matter. It’s just the wrong people working on it. It’s not a problem that you can solve using particle physics and general relativity. It is a problem for condensed matter physics. That’s the physics of gases, liquids and solids and so on.
The conclusion I have come to is that the distinction between dark matter and modified gravity is a false dichotomy. The answer is not either – or it is both. The only question is how to combine them.