When we look at the night sky, we can see the moon, stars, planets and sometimes even faraway clouds of gas and dust. All that visible matter — the stuff we can see — has a gravitational force, the same way the moon pulls on our oceans and creates the tides. But for decades, physicists have noticed something weird: There’s more gravity in the universe than we should expect. Why?
Physicists think the answer lies with dark matter, an invisible form of matter that accounts for that extra gravity they’re observing. University of Washington physicist Alvaro Chavarria helped build a dark matter detector deep below the French Alps. Chavarria joins us to help demystify dark matter, how the detector works and its potential applications.
Note: The following transcript was transcribed digitally and validated for accuracy, readability and formatting by an OPB volunteer.
Dave Miller: This is Think Out Loud on OPB. I’m Dave Miller. We start today with one of the enduring mysteries of our universe. When we look up at the night sky, we can see our moon, the stars and planets, sometimes even faraway clouds of gas and dust. All of that visible matter has a gravitational force, the same way that the moon pulls on our oceans and creates the tides. But for decades, physicists have noticed something weird. There is way more gravity in the universe than can be accounted for by that visible matter. Scientists think the answer lies with dark matter, an invisible form of stuff that explains the extra gravity they’re observing, but they’ve had a really hard time actually finding it. They now have a new tool that they hope will help. Alvaro Chavarria is a physicist at the University of Washington. He helped create the dark matter detector deep below the French Alps. He joins us now to talk about dark matter, this new detector, and the future of the field. It’s great to have you on Think Out Loud.
Alvaro Chavarria: Hi, Dave. Thanks for having me.
Miller: I want to start with the potential scale of dark matter in the universe. How much of the gravity that we can observe can be attributed to visible matter?
Chavarria: Studies in cosmology, which basically look at the dynamics of the universe as a whole and the distribution of galaxies, estimate that about 3/4 of the source of gravity or the matter in the universe is so-called dark matter.
Miller: Three-quarters. And how much do we, at this point, truly understand about something that makes up three-quarters of the gravitational force of our universe?
Chavarria: Maybe it would be useful to think a bit about what physicists mean when we talk about matter, because matter has, really, two definitions. The first one, something that is matter, it has a mass and it attracts other objects, like gravitational force. But matter also is something that is made of building blocks – atoms and things even smaller than atoms, subatomic particles.
So when we talk about dark matter and whether it exists or not, it really exists and we know it exists from the first definition, whether it’s a source of gravity. But the mystery is, if it’s a source of gravity like everything else that we know that is a source of gravity, then what is it made of? What are the building blocks of the dark matter? This is the question that we’re trying to answer.
Miller: Do we know that it has mass, or is it possible that it could exert gravity without having mass, the way visible matter does?
Chavarria: As a source of gravity, it must have mass. In some ways, the definition of something having mass is tied to gravity.
Miller: OK, but you said that the big question still out there is, how does it operate? What’s it made of? What are the various theories that have been put forward to explain that, to explain what dark matter is or how it operates?
Chavarria: The first thing that you would try to do is whether dark matter is made of something that you already know about. We know that ordinary matter is made of atoms, which are made of neutrons and protons and even more fundamental particles. Then you think about the easiest explanation, say it could be something like very dim, dark stars like black holes or something historically called the massive contact halo objects. We realized that none of any type of body that you could make with ordinary matter of what we know of could explain both the properties of dark matter in terms of its distribution in the universe. And so the mainstream view is that, then, the dark matter must be made of some type of particle that we have yet to discover.
Miller: That’s the mainstream view. Is there another competing view?
Chavarria: Yes, the field is very broad, and you could find other theories that try to explain the dynamics of the universe without the need of dark matter. Some of them required the modification of the theory of gravity, which is known as Einstein’s General Theory of Relativity, but those theories are not able to explain most of the observables in the universe as well as the dark matter hypothesis can do it. So most physicists and cosmologists really prefer the dark matter hypothesis than, say, modifications of gravity.
Miller: It also requires less of a fundamental rethinking of the way the universe works, if I understand you correctly.
Chavarria: Yes, but it’s mostly that it explains better what we observe in the universe, when we do surveys of the universe at large scales.
Miller: Why has it been such a challenge to get more definitive answers about dark matter?
Chavarria: We’re constantly learning about it from the gravitational sense, like, where is the dark matter? Right now, we can make very clear maps of where in the universe the dark matter is, and how it’s distributed. But it’s been a huge challenge to figure out what it’s made of, because when you’re dealing with a hypothesis that says it’s made from a new particle, the information in the new particle is very little. When you just suppose that it’s something that doesn’t have much interaction, for it to remain dark, you don’t know how it could talk, if it does at all, in some way, to ordinary matter and how you could somehow discover it.
People and theoretical physicists have proposed a large number of theories that lead to particles that could be the dark matter, and all these theories are tied, of course, to how they’re produced in the early universe. Because, as most people probably have heard, there’s a common origin of everything in the universe, of the Big Bang. So usually, you start with that and then there’s a large number of possibilities of particles that you could make to hypothesize it could be the dark matter.
Miller: Let’s turn to the new work that you’ve been a part of, and this detector that you were the lead on, as part of a consortium of researchers from all across the world. Can you describe the basic idea behind this new set of sensors?
Chavarria: The idea is related to the fact that the dark matter, we know its distribution, so we know there’s dark matter in the Milky Way, which is our galaxy. And the dark matter, it would be made of particles that are basically zipping through the earth, because we can’t see them and detect them. That means that their interactions with ordinary matter have to be very feeble, which means that they basically just go through the earth.
As we move, or as the solar system moves around the Milky Way, there is this wind of dark matter particles that would be blowing through the earth, and even though most of them will just zip through, it might be possible that one of them has an interaction or a collision, say, with an atom of ordinary matter and excites it. So the entire field that we call direct detection is to try to build highly sensitive instruments on earth that can sense these very rare atomic excitations.
Miller: Why did you have to build this a mile below an alpine mountain? This is in the French Alps.
Chavarria: In our case, the devices that we used, they’re called charge-coupled devices or CCDs. If someone here is into imaging or electronics in the audience, they probably have heard of CCDs as components of modern cameras.
These detectors are extremely sensitive for these excitations, but if you run them on the surface, say, in your lab, on UW campus, then you would be flooded by radiation from the environment, and that includes particles that come from space, cosmic rays, which are the ones, for example, that give you the auroras. To get away from those cosmic rays, then we put the detector deep underground, so the earth, the mountain, acts as a filter for the cosmic particles because you’re trying to get in the end in a very dark environment deep underground where there’s very little radiation.
Miller: So, you have a mile of granite and other things, other rock, between the sensor and the surface of the earth, and then all of the stuff hitting the surface of the earth. Is the idea that this particular sensor has to be hit by a particular particle, or that it will somehow detect that rare interaction somewhere in the vicinity between that dark matter particle and some other atom? Does it actually have to hit this one sensor by kind of the needle flowing through the haystack?
Chavarria: Yes, the only way that you can achieve insensitivity to it, if the interaction happens inside the sensor. So what you’re actually doing is that you’re monitoring the silicon that constitutes these charge-coupled devices because these are microfabricated semiconductors, they’re like microchips. And what we have deep underground is about one billion pixels, and we’re constantly monitoring all the silicon atoms within each one of them, trying to look for one of these very rare excitations.
Miller: How long has this sensor now been in operation?
Chavarria: We’ve had a staged approach. We run our detector modules, and we started running, since February 2023, two of these modules, and we’re slowly scaling up to make it bigger. By the middle of next year, we’ll be running 26 of them, which is the one billion scale pixels that I just referred to now.
Miller: So, scaling up over time, but they have been in operation for a couple of years now. Have there been any hits?
Chavarria: Unfortunately, no. Probably people would have heard it much earlier than now, if we had seen a signal. Right now, what we do is we see nothing and that also puts a constraint, say, on how weakly the dark matter interacts. The unfortunate situation is that there is no requirement on whether we can see them or not, since they’re only known through their gravity or gravitational interactions, there is no guarantee that they would be able to excite an atom. That’s only hypothesized.
Miller: Right. So, the fact that you haven’t gotten one yet doesn’t necessarily mean that they haven’t even hit the sensor, you can’t know that for sure. Does the fact that you haven’t gotten any hits yet, does it give you any truly definitive information? Have you already learned something from the experiment so far?
Chavarria: Yes, the theories that we explore, and as I mentioned, there are many of them, and some of them are simpler than others. Some theories that would be preferred, if people have heard about Occam’s razor, that usually the simplest explanation is the correct one. The simplest ones have been ruled out slowly. And we very recently, with our latest results, ruled out some of those. We basically know what the dark matter, even though we don’t know what it is yet, we for sure are learning what it’s not.
Miller: What would it be like for you to get some message, say in the middle of the night, that this sensor that you were the leader for, for this device, that it got a hit?
Chavarria: Disbelief, first of all. You would think that, you know, there’s probably a reason why it’s not.
Miller: That’d be the first thing as a scientist that goes through your mind now, something must be wrong.
Chavarria: Immediately you will try not to get too excited, and try to rule out every other possibility, because the stakes of discovering dark matter are very high. It would be a huge achievement for science to know what it’s made of. We’re talking, really, about discovering what three-quarters of the matter in the universe is really made of.
You would first be skeptical about it, and in my experience, these things, excitement builds up as you start ruling things out and coming to the conclusion that it really might be the dark matter. I don’t think it would be a moment where, whether we know it exists or not, or whether we know what it’s made of or not, it would be more of a process.
Miller: Alvaro Chavarria, thanks very much.
Chavarria: You’re welcome.
Miller: Alvaro Chavarria is an associate professor of physics at the University of Washington’s Center for Experimental Nuclear Physics and Astrophysics. He was the leader of the piece of this project focused on the dark matter detector. It is a broad project, with scientists from all over the world, that has put this new dark matter detector deep below the French Alps.
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