Dark matter — the unseen 80 percent of the universe’s mass — doesn’t emit, absorb or reflect light. Astronomers know it exists only because it interacts with our slice of the ordinary universe through gravity. Hence the hunt for this missing mass has focused on so-called WIMPs — Weakly Interacting Massive Particles — which interact with each other as infrequently as they interact with normal matter.
Physicists have reasons to look for alternatives to WIMPs. For two decades, astronomers have found less dark matter at the centers of galaxies than what WIMP models suggest they should. The discrepancy is even worse at the cores of the universe’s tiny dwarf galaxies, which have few ordinary stars but lots of dark matter.
About four years ago, James Bullock, a professor of physics and astronomy at the University of California, Irvine, began to wonder whether the standard view of dark matter was failing important empirical tests. “This was the point where I really started thinking hard about alternatives,” he said.
Bullock thinks that dark matter might instead be complex, something that interacts with itself strongly in the way that ordinary matter interacts with itself to form intricate structures like atoms and atomic elements. Such a self-interacting dark matter, Bullock suspects, could exist in a “dark sector,” somewhat parallel to our own light sector, but detectable only through the way it affects gravity.
He and his colleagues have created numerical simulations that predict what the universe would look like if dark matter feels strong interactions. They expected to see the model fail. Instead, they found that it was consistent with what astronomers observe.
Quanta Magazine spoke with Bullock about complex dark matter, how this mysterious mass might behave, and the best places in the universe to find it. An edited and condensed version of the interview follows.
QUANTA MAGAZINE: What do we know about dark matter?
JAMES BULLOCK: We are confident that it’s there, that it has mass, and that it tugs on itself and on other things via gravity. That’s about it. While dark matter has a gravitational tug, it doesn’t interact with normal matter — the stuff that makes up you and me — in a very intense way. It doesn’t shine. It’s invisible. It’s transparent. It doesn’t glow when it gets hot. Unfortunately, those are the ways astronomers usually study the universe; we usually follow the light.
So we don’t know what it’s made of?
We’ve come to understand that we can describe the world that we experience by the Standard Model of particle physics. We think of the particles that make up you and me as being broken down into constituent things, like quarks, and those quarks combine into neutrons and protons. There is a complicated dance that allows these particles to interact in certain ways. It gives rise to the periodic table of elements and all of the vast complexity we see around us. Just 20 percent of the mass of the universe is all of this complexity.
On the other hand, dark matter makes up something like 80 percent of the mass. First-guess models for what it is suggests that it is one particle that doesn’t really interact with much of anything — WIMPs. These are collisionless, meaning when two dark matter particles come at each other they basically go through each other.
Another possibility is this 80 percent of the universe is also complex. Maybe there’s something interesting going on in what’s called the dark sector. We know that whatever ties us to the dark matter is pretty weak or else we would have already seen it. This observation has led to the belief that all the interactions that could be going on with dark matter are weak. But there’s another possibility: When dark matter particles see themselves, there are complex and potentially very strong interactions. There even could be dark atoms and dark photons.
Those two worlds — this dark sector and our own sector — only communicate by gravity and perhaps other weak processes, which haven’t yet been seen.
How can you probe this dark sector if you can’t interact with it?
Now what we’re talking about doing is not just looking at the gross properties of the dark matter but the very makeup of the dark matter, too. The most obvious place to see those effects is where dark matter is bunched up. We believe the centers of galaxies and galaxy clusters are densest. And so by studying the behavior of dark matter by indirect methods — basically by the dynamics of stars and gas and galaxies in galaxy clusters — we can start to understand how dark matter is distributed in space. To start to discriminate between models, we can compare differences in dark matter’s spatial clumpiness in simulations, for example, and then look for those differences in data.
What does the data say?
In models using cold, collisionless dark matter — WIMPs — the dark matter is very dense at the middle of galaxies. It appears that those predicted densities are much higher than what’s observed.
What might be going on is that something a little more complex is happening in the dark sector, and that complexity is causing these slight disagreements between theory and observation at places where the dark matter is really clumped or starts congregating, like in the centers of galaxies or the centers of galaxy clusters.
I’m interested in running cosmological simulations of how the universe should evolve from the very beginning until now. I look at what happens, when I run those simulations forward, if I allow cold dark matter to occasionally collide and exchange energy. The simulations start with a small, almost-smooth primordial universe and end with beautiful agreement with large-scale structure — galaxies stretched out across the universe in the way we observe them. But the hearts of galaxies are less dense in dark matter in my simulations than they are in simulations where the dark matter is cold and collisionless.
How long have researchers known about these disagreements between the models and the data?
We’ve known that there’s a bit of a problem at the centers of galaxies for about 20 years. At first it was thought maybe we’re interpreting the data wrong. And now the question comes down to: Does galaxy formation eject dark matter somehow, or do we need to modify our understanding of dark matter?
Why did you start looking into self-interacting dark matter?
The first paper exploring ideas that the dark matter might be more complex was inPhysical Review Letters, April 2000, by David Spergel and Paul Steinhardt. I actually started working on this several years later when I began seeing papers from the particle physics community exploring these ideas. My initial reaction was, that couldn’t be true, because I had this prejudice that things work so well with collisionless dark matter.
In the first set of simulations we ran, we gave dark matter a cross-section with itself. The bigger the cross-section is, the higher the probability that these particles are going to run into one another in any given amount of time. We set the value of the cross-section to something we were convinced would be ruled out [by the data], but when we ran our simulation we found that we couldn’t see any difference between that model and the classic one. And so we thought maybe we don’t know quite as much as we thought we knew.
Then, we dialed it up and looked at a strong interaction similar to if you threw two neutrons together. We saw something that looks really close to observations on large scales but does produce differences in the hearts of galaxies. Rather than the dark matter getting denser and denser as you approach the center of the galaxy, it reached a threshold density.
Could it be that these little discrepancies we’ve been seeing in the observational data are actually a clue that there’s something interesting and fun going on in the dark sector that we weren’t thinking about before?
August 20, 2015