With Tracy Slatyer
So you’ve probably heard a lot of hullabaloo about dark matter, how it’s all around us and there’s way more of it than of regular stuff (like atoms that make up everything we are generally familiar with.) It’s actually pretty disturbing how much more abundant it is than regular matter and yet we don’t concretely know a single thing about what it is. And it has a huge impact on our very lives: without dark matter, galaxies wouldn’t necessarily form and we wouldn’t be alive talking about it. So understandably, physicists would really like to know what the heck this stuff is and it’s a major pursuit right now in physics, including in our paper.
One way in which people have tried to answer the question of “what is dark matter?” has been by trying to see if we can detect it in some direct way. Lots of clever folks have designed really elaborate experiments to see whether they can maybe see ~1 dark matter particle per year. It’s the sort of challenge which makes looking for a needle in a haystack sound easy. Another approach is to look for dark matter at the Large Hadron Collider, because it’s possible that we could actually make dark matter in a lab by smashing together regular bits of matter. Another increasingly active area has been to see whether we can use astrophysics to answer this question. It’s actually kind of hipster when you think about it; the original way that we know dark matter exists is through looking at galaxies, clusters of galaxies, and the cosmic microwave background, among other things. So really by using astro to answer this question, we are going back to the roots of how the question was even originated.
In particular, you could say to yourself “I think I know how gravity works, and any deviations from that could be due to some additional dynamics coming from dark matter itself.” In fact, that is what many people are saying to themselves because of some observational questions that have come up in the past decade or so. Basically, if we look at satellite galaxies that are in the same neighborhood as the Milky Way, it looks like there are too few of them and they are not as dense as we would have predicted. The formation of these “dwarf” galaxies is very messy, but one particularly neat thing about them is that they are 99.9% dark matter, so they are quite pristine laboratories for testing dark matter models. Wouldn’t it be so cool if these anomalies in their abundance and density were coming from dark matter particle physics? That was a question that I looked into.
We studied a pretty simple model of dark matter where it is made up of two particles with a small mass difference between them. This small mass difference can arise via different mechanisms that happen at large energies, so if we detect this kind of dark matter we would be indirectly be probing its high energy behavior as well. But anyway, we considered a situation where the two kinds of particles interact with each other in the most simple and general way. These interactions affect the structure of these dwarf galaxies because if the dark matter is constantly bumping into itself then it could disrupt the gravity which binds the dwarf galaxy together. But we aren’t the first to do this-- what we brought to the table was the ability for the dark matter to scatter *inelastically* which comes from the fact that there are two dark matter particles with different masses. This is one of the cool things about quantum mechanics and relativity: you can convert mass into energy when you scatter particles together. In this case, you can have two of the more massive dark matter particles scatter to their lighter form and the extra mass gets converted into energy. This energy effectively gives the resulting dark matter particles a big kick. If you thought that pure dark matter scattering could solve dwarf galaxy problems, then this additional kick can *really* solve those problems. If you basically kick dark matter particles out of the centers of these dwarf galaxies, then you’d make them way less dense and would make it harder for them to form.
Another cool thing about this kind of dark matter: it can explain a mysterious x-ray signal coming from some galaxies and galaxy clusters. There are some constraints on the interpretation of this signal coming from dark matter vs. some other astrophysical process, but our mechanism is not subject to those constraints since in our scenario we are sensitive to density squared rather than just density (since there are two particles scattering rather than one particle decaying) and also because our cross sections are naturally velocity-dependent.
Update: Mark and Jesus have successfully implemented this dark matter model in this paper and indeed, it turns out that this has a lot of distinct implications for the formation of galaxies. Above, you can see what happens when you run a simulation with ordinary cold dark matter (left) vs. with our model (right.) While the difference may look small, with inelastic dark matter you get a distinct "fuzziness" in terms of how things are clustering. This fuzz is something we can measure and which has implications for the ability of low-mass galaxies to form.