Could This Particle 'Clean Up' A Cosmic Mystery?
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You're listening to Shortwave from NPR. Physics has a bit of a messy problem. There's matter missing in our universe. Something's there that we can't see, but we can detect it. This mysterious substance behaves a lot like the matter we know. You know, the matter that makes up you, me, the sun, the planets, and the stars. At least in the way that matter attracts other matter. Stars can orbit other stars. Galaxies, collections of billions of stars, can orbit other galaxies.

And looking at those orbits or the way things move around other things in space can tell us how massive the object in the center is. But sometimes we can't see what is really causing that movement. When we look at how stars move in galaxies, they move as if there is a lot of matter there that we can't see. That's Chanda Prescott-Weinstein. She's a theoretical particle physicist at the University of New Hampshire. And she's

And she says that this missing matter... It's actually most of the matter in the universe. And it is not visible. And when we say it's not visible, we mean it doesn't interact with light in any way that we've so far detected. That's why it's often called dark matter. It makes up over a quarter of the entire universe. ♪

Scientists don't know what it is, but they do know whatever it is has to have a few key components. We want it to be something that doesn't interact very strongly with light, if at all. So we want it to be effectively transparent, effectively invisible, and we also want it to be relatively slow moving. So if it's fast moving, then it won't clump together under gravity. It will escape gravity, and then you won't form galaxies.

So what could this mysterious substance be? A lot of astronomers are searching for the answer, and some, like Chanda, think a particle called the axion may help make the dark matter problem a little tidier. Frank Wilczak, who named the axion, named it after the laundry detergent. An axion is smaller than an atom and hypothetical, meaning scientists have never seen one and don't know if they exist.

Today on the show, what does it mean if axions exist? Could they be the solution to the mysterious dark matter problem? And how can scientists find one? I'm Regina Barber, and you're listening to Shortwave, the science podcast from NPR.

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Okay, Chanda, tell me more about axions and your research. Like, what are they? What are you looking into? And what would you like to find out? So axions are essentially a class of models that all look kind of similar. So they tend to be lighter in mass.

And they also have these very interesting properties that they behave more like a wave than like a particle, depending on the situation you are looking at, the physical scenario you're looking at. Our listeners are going to love that. They love the wave-particle duality. Yeah. I mean, don't we all? Because it really challenges us to rethink our intuition about what constitutes normal in the universe. Yeah.

Like I always think about in that context, the axion is actually one of these dark matter candidates that challenges us to rethink, oh, it's just a different type of particle. Because in our head, when we think about particles, we tend to think of them as like maybe little billiard balls bouncing off of each other or something like that.

And the axion really requires us to think it's not that because it does behave like a wave in key physical scenarios that are of interest to us for the purposes of dark matter. And this makes it distinct from other dark matter candidates. And this is actually one of the reasons that I got into the axion as a dark matter candidate is I was like, oh, I like this wave stuff. And in particular, I liked that it potentially formed...

A state of matter known as a Bose-Einstein condensate. I loved those when I was an undergrad. Why don't you tell everyone what a Bose-Einstein condensate is? Yeah. So a Bose-Einstein condensate is a state of matter that can only be formed by bosons. So all types of matter are either bosons or fermions. So what's the difference?

So I like to think of fermions as stacking particles. They don't all like to be in the same place, in the same status, the same energetic state at the same time.

Whereas bosons are like the pep squad. They will go and do everything together, a large number of people all doing the exact same thing at the exact same time. It's the flash mob all the time. Yes, flash mob particles. That's essentially what a boson sting condensate is. It's like a flash mob of particles that have all committed to doing the same thing at the same time. And they love it. They love it. One of the things I love about axions is they're like the flash mob dark matter candidate, right? Yeah.

It's an awesome characteristic. And so the thing that I have been most concerned about in my research, particularly over the last decade, is what are the implications of this particular aspect of axion properties that they do this Bose-Einstein condensate flash mob thing.

And what are the implications for the nature of galaxies and how galaxies evolve if dark matter does in fact behave like this on very large scales?

Okay, and so are they a kind of particle we can find by just like smashing things together? Like, can we use particle accelerators like CERN to find them? No. Okay. So the short answer is, this is not a particle that you go looking for by smashing things together. And there's a little bit of potential there, but because of how this particle... So let me think about how we would say this. So the way that we would say this is,

in physicist speak, is that the particle doesn't couple very strongly to standard model particles. And so when you are colliding particles together and looking for other particles, that assumes that there is such a strong relationship between those particles and whatever you're trying to create that it will pop out when you smash them together. But if that relationship doesn't really exist in the first place,

There's no amount of smashing together that's going to make it happen. It's not going to work. You're not going to make fetch happen. So you just like you don't make axions happen that way. She doesn't even go here. She doesn't even go here. That's exactly it. She doesn't even go here. Okay. So like how do we go into space and look at our telescopes and like how do we find confirmation that axions are really doing this work? So this is where computation can be really useful.

And so you can imagine a scenario where there are two galaxies that are maybe colliding with each other and basically collided them to see what would happen. And then we tweaked the properties of the axion-like particle to see if the collision happened differently depending on how we tweaked the properties.

And so this is an example of why you would call it, for example, particle cosmology, because this is one where we're making changes to the characteristics of a very small object. But then we're looking at large scale astrophysical implications for those very small changes that we make.

Tell me a little bit more about that study that just came out talking about like these axion clouds, not just around, you know, big galaxies, but around these like dense dead stars, these neutron stars. Is that going to tell us a little bit more about dark matter? Yeah. So neutron stars are

You know, to back up a little bit, neutron stars are stellar remnants. So they are objects that are formed when a massive star reaches the end of its life, goes through a supernova experience, and neutron star is potentially left over on the other end.

So this is not well understood, but neutron stars often have a magnetic field associated with them. And when I say it's not well understood, we don't really understand where the magnetic field comes from. There are good models for it, but this is actually still an active area of research. So I made this claim that dark matter doesn't really interact with light, but axions do actually have a very mild, tiny, tiny, tiny interaction with light.

So you can have a situation where an axion is traveling over long distances through a galactic magnetic field and converts into a photon, so a little particle of light. So to find axions, scientists could look for excess photons, these particles of light, and that might tell us some interaction happened.

You can have axions going through a neutron star's magnetic field and turn into a photon, and then potentially we can see that photon. And so this is an active area of research. People also look for these kinds of interactions around white dwarfs, which are another possible outcome for a star at the end of its life. For much smaller stars. Much smaller stars, yes.

So this is kind of understood that there might be these axions around neutron stars? Have they been found? Or is it just like we're still just looking around these neutron stars? So at this point, I think...

the way we've been thinking about both white dwarfs and neutron stars in this kind of scenario is that it allows us to rule out axions with certain properties because we go looking for evidence that there was this phenomenon happening and we don't see it. And so then we can say, okay, this axion with this kind of characteristic is not out there. So one of the data sets that I've been interested in is

from the Gaia Space Telescope. This was a European Space Agency mission. And what they did is they characterized the motions of stars and they characterized the motions of a lot of stars and

And so this allows us to get into these questions of if there is this flash mob core thing happening and it's affecting how the stars move in a way that's unique to the Axion scenario, can we look for evidence of that in the stars? So what would strengthen the idea that

Axions are the best possible solution for like solving the dark matter problem. In your mind, what would happen? What would have to happen? I mean, obviously we should find one. That would be good. So one thing I didn't talk about is that people do have these ground-based experiments. And this is actually a lot of the global investment is actually in transits.

trying to look for an axion using the exact same mechanism that we might use to look for axions around neutron stars. So they basically take a microwave cavity, they turn on a giant magnetic field inside of it, and then hope an axion will fly through and become a photon. And yeah, so this is the biggest type of experiment like this in the United States is the axion dark matter experiment, which is housed at the University of Washington. And

But there are experiments that are similar to this around the world. And so there is some possibility that we will actually what we would call directly detect one. So instead of looking for how it impacts how structures form, that we would actually see evidence that one went through our laboratory. So that would be awesome. Yeah. That would be very exciting. Yeah.

Chanda, thank you so much for enlightening me about axions. Pun intended, they turn into photons. Thank you for having me. And maybe next time we can talk about axion laundry detergent and other weird axion paraphernalia that I know about. If you liked this episode, check out our episodes on black hole jets and neutrinos. Also, make sure you never miss a new episode by following us on whichever podcasting platform you're listening from.

This episode was produced by Rachel Carlson and edited by showrunner Rebecca Ramirez. Tyler Jones checked the facts. Robert Rodriguez was the audio engineer. Beth Donovan is our senior director, and Colin Campbell is our senior vice president of podcasting strategy. I'm Regina Barber. Thank you for listening to ShoreWave, the science podcast from NPR. ♪

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