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, 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/shortwavesurveyListen 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.Learn more about sponsor message choices: podcastchoices.com/adchoicesNPR Privacy Policy
Chapter 1: What is the mystery of missing matter in the universe?
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Just go to npr.org slash shortwave survey. We'll also put a link in our show notes. Thank you. Okay, on to our show. 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.
Chapter 2: Who is Chanda Prescod-Weinstein?
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.
Chapter 3: What is dark matter and why is it important?
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.
Chapter 4: What are axions and how do they relate to dark matter?
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.
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. 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?
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Chapter 5: What is a Bose-Einstein condensate?
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. Oh, that's so cool.
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, just 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?
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Chapter 6: Can we find axions using particle accelerators?
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?
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 trying to look for an axion using the exact same mechanism that 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. 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.
Chanda, thank you so much for like enlightening me about axions. Pun intended, they could 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.
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