Could This Particle 'Clean Up' A Cosmic Mystery?

Physics has a bit of a messy problem: There's matter missing in our universe. Something is there that we can't see but can detect! What could this mysterious substance be? A lot of astronomers are searching for the answer. And some, like theoretical particle physicist Chanda Prescod-Weinstein https://chanda.science/, think a hypothetical particle called the axion may make this problem a little ... tidier. That's right: hypothetical. Scientists have never seen one, and don't know if they exist. So today, we point our cosmic magnifying glasses towards the axion and ask how scientists could find one — and if it could be the neat solution physicists have been searching for. Help shape the future of Short Wave by taking our survey: npr.org/shortwavesurvey http://npr.org/shortwavesurvey Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave http://plus.npr.org/shortwave. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

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• 0:59Regina Barber?

  Physics has a bit of a messy problem. There's matter missing in our universe.

• 1:07Regina Barber?

  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.

• 1:32Regina Barber?

  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.

• 1:44Speaker 02

  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.

• 1:53Regina Barber?

  That's Chanda Prescod Weinstein. She's a theoretical particle physicist at the University of New Hampshire. And she says that this missing matter...

• 2:01Speaker 02

  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.

• 2:12Regina Barber?

  That's why it's often called dark matter. It makes up over a quarter of the entire universe.

• 2:21Regina Barber?

  Scientists don't know what it is, but they do know whatever it is has to have a few key components.

• 2:27Speaker 02

  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.

• 2:49Regina Barber?

  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.

• 3:02Speaker 02

  Frank Wilczek, who named the axion, named it after the laundry detergent.

• 3:07Regina Barber?

  An axion is smaller than an atom and hypothetical. Meaning scientists have never seen one and don't know if they exist.

• 3:21Regina Barber?

  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 Short Wave, the science podcast from NPR.

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• 5:05Regina Barber?

  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?

• 5:14Speaker 02

  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.

• 5:43Regina Barber?

  Our listeners are going to love that. They love the wave-particle duality.

• 5:45Speaker 02

  Yeah, I mean, don't we all? Because it really challenges us to rethink our intuition about what constitutes normal in the universe.

• 5:59Speaker 02

  Like, I always just 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.

• 6:20Speaker 02

  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.

• 6:51Regina Barber?

  I loved those when I was in undergrad. Why don't you tell everyone what a Bose-Einstein condensate is?

• 6:56Speaker 02

  Yeah, so 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.

• 7:30Regina Barber?

  It's the flash mob all the time.

• 7:32Speaker 02

  Yes, flash mob particles. That's like essentially what a Bose-Einstein condensate is, is it's like a flash mob of particles that have all committed to doing the same thing at the same time.

• 7:42Regina Barber?

  And they love it.

• 7:43Speaker 02

  They love it. One of the things I love about axions is they're like the flash mob dark matter candidate, right? 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?

• 8:22Regina Barber?

  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?

• 8:32Speaker 02

  No.

• 8:32Regina Barber?

  Okay.

• 8:34Speaker 02

  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 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 gonna, that's gonna make it happen.

• 9:18Regina Barber?

  It's not gonna work.

• 9:18Speaker 02

  You're not gonna make fetch happen. That's so you just like, you don't make axions happen that way.

• 9:24Regina Barber?

  She doesn't even go here.

• 9:25Speaker 02

  She doesn't even go here. That's exactly it. She doesn't even go here.

• 9:28Regina Barber?

  Okay, so like, how do we go into space and look at our telescopes? And, and like, how do we find confirmation that axions are really doing this work?

• 9:38Speaker 02

  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 tweak 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 the, those very small changes that we make.

• 10:23Regina Barber?

  Oh, that's so cool. Tell me a little bit more about that study that just came out and 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 gonna tell us a little bit more about dark matter?

• 10:40Speaker 02

  Yeah, so neutron stars, 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, is potentially leftover on the other end.

• 11:06Speaker 02

  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.

• 11:51Regina Barber?

  So to find axions, scientists could look for excess photons, these particles of light, and that might tell us like some interaction happened.

• 12:00Speaker 02

  You can have axions going through neutron stars 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.

• 12:22Regina Barber?

  For much smaller stars.

• 12:24Speaker 02

  Much smaller stars, yes.

• 12:25Regina Barber?

  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?

• 12:35Speaker 02

  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.

• 13:01Regina Barber?

  Yeah.

• 13:01Speaker 02

  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 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?

• 13:35Regina Barber?

  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?

• 13:47Speaker 02

  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 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.

• 14:18Regina Barber?

  Why?

• 14:19Speaker 02

  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. 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.

• 14:50Regina Barber?

  Yeah.

• 14:51Speaker 02

  That would be very exciting.

• 14:52Regina Barber?

  Shana, thank you so much for like enlightening me about axions. Pun intended, they could turn into photons.

• 15:01Speaker 02

  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.

• 15:12Regina Barber?

  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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