What is an axion? How could axions make a star, and what does that have to do with dark matter? What would these stars look like? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO GENERATED)

Sometimes on a quiet, dark night, I love to look up at the stars and wonder what I'm not seeing. What's beyond the stars and what's between them? The galaxies too far for the naked eye. The red dwarfs and rogue planets and black holes lurking in the depths. And I wonder if there's something more.

Something even more hidden. More mysterious. I wonder that if when I look at a blank patch of night sky and I look at a point right between the stars, I wonder if I'm looking directly at something without even realizing it. And I need to start out this episode with the usual dark matter disclaimer. Yeah.

We have no idea what's going on. I mean, we have some idea what's going on. I've done episodes on dark matter before, so I'm not going to belabor the point, But the gist is that something funny is definitely going on with mass in the universe. We strongly, strongly, all caps, strongly believe that there is a huge component to the universe that is some kind of particle currently unknown to physics and that this particle is both, a, abundant, and, b, nearly, if not completely invisible. That's about it.

There are a lot of finer details that aren't super important right now, but that that's it. We strongly believe that dark matter exists, and it is some form of particle. But the real sticking point is that we have no direct evidence for what this dark matter particle could be. We don't have a signal in a detector or a trapped one in a bottle, and don't ask me how we would trap it. It's just it's just a figure of speech.

We have all this circumstantial evidence, and we know something weird is definitely happening. We can say that for sure. And we've ruled out a lot of possibilities, and all that's left is basically a new kind of particle, but but we don't have the identity of that particle. And what's just a tiny, tiny, tiny little bit frustrating is that for a couple decades, we thought we were on the right track in terms of identifying the particle behind the dark matter. For a long time, our best candidate was called WIMP, a weakly interacting massive particle.

And the reason this was a strong candidate is that there were other noncosmology reasons to believe that this kind of particle existed. There were some very small extensions to the standard model coming out of something called supersymmetry that predicted the existence of this class of particles that were massive, like, in the mass range of some of the heavier known particles, but also hardly if ever interacted with normal matter or with light and also could be produced in abundance in the early universe. And so since this idea of a WIMP was motivated by not cosmology, we had strong reasons to suspect that wimps existed and they existed with a certain set of properties, a certain mass range, interaction strength, and so on. And so, we built dozens of experiments around the world, many of them that are still running, to go and hunt for wimps. Now, okay.

You know, if they have some sort of interaction and some sort of mass range, we should be able to catch it with a detector like this over such and such time scale, but but slowly and steadily, WIMPs are getting ruled out by experiment. You know, it's not never a definitive yes or no answer. You know, science, you know, life is rarely binary like that, And that's because WIMPs are just a particle. They're a class of particles, a range of possible pro particle properties that are predicted by these various supersymmetry theories. And so when we run our detectors, we look for a range of masses of what the wind could be.

We look for a range of interaction strengths and so on, and then we don't find anything. And we haven't found anything for a couple decades now. And so we are chipping away, and we don't know what the dark matter particle is if it's a WIMP, but we can definitely say what it isn't that is the benefit of running these experiments. We we learn what things aren't, which is still knowledge, and there are still room for wimps. Of course, there are ranges of masses and interaction strengths that we haven't yet touched with our detectors, but it's looking less and less likely with every passing year.

Yeah. We there's could be a surprise around the corner, but it's getting to the point that that we should start looking for other possibilities, which is fine. Nature is under no obligation to go along with our first guesses and science in general is a process of guessing and seeing if our guesses turn out right. So with WIMPS, we took a big swing, and it looks like we're getting a big miss, but the game isn't over yet. And here's the curious thing, something else that motivates this kind of discussion.

Even though WIMPS are slash were our best guess, they're not perfect. You know, wimps are naturally emerge out of supersymmetry theories. They have all the right properties for dark matter. They can fill up the universe. They can do all the dark matter y things, but then when we go to take this wind theory and say, okay.

We've got this particle with with this kind of mass and this kind of interaction strength, this kind of abundance in the early universe, then we then we try to understand how that particle would behave in the universe, to see if anything interesting crops up in addition to just existing as the dark matter. Is there anything else that would happen? And for the most part, wimps agree as a model with all of our observations, but there are some difficulties in the the cores of galaxies. When we run simulations of the evolution of the universe with WIMPs, cores of galaxies end up being way, way, way denser than they actually are. And this has been a long standing problem with the WIMP dark matter model, this this little corner where, like, okay.

We get 9 out of 10, observational check marks. But this last one, the galactic cores, it's it's a little bit sticky. We can't quite explain what's going on. And so that's always been a little bit of a thorn in the WIMP side. But you know what?

If we end up finding a WIMP, okay, then we know the dark matter is the WIMP, and then we have to figure out what's going on in galactic cores. And so we're like, it's fine. We'll we'll solve wimps later. We'll solve this problem later, after we find the wimp. Now we're not finding the wimp.

So these kinds of questions start to, take on a little bit more weight. And and I know you're asking, if if wimps aren't perfect, why keep searching for them? I mean, you gotta love the one we you're with. Alright. You just move on.

So, anyway, so the fact that wimps haven't been directly observed and the fact that they have some weaknesses opens up the physics equivalent of a playground. We just keep cooking up stuff, toying around with ideas, taking wild stabs in the dark. It's actually quite thrilling if you're into it, which I understand not everybody is, but different strokes for different folks and all that. And with that spirit of relentless guessing until we get it finally right, there is a cornucopia of dark matter candidates. These candidates are all over the map.

Some have huge masses. Some have tiny masses. Some require only small additions to the standard model. Some require a bit more. And some are just purely hypothetical with little to no connection to established knowledge at all.

But no matter what, all dark matter candidates share 2 things in common, which is that the dark matter particle candidate must be, a, abundant, and, b, nearly if not completely invisible. And I'd like to use this episode to highlight the strange properties of one of these candidates, a particle generally regarded as our second best guess to the wimps. Like, it was always on deck, always in the background, the vice president of dark matter, the runner-up to the missed dark matter u universe contest. Should WIMPS fail to perform their duties as a dark matter particle, then this one shall assume full Let me introduce you to the axion. Now the axion is the only hypothetical particle that I am aware of that's named after a brand of laundry detergent, but I'll get to that in a second.

The reason that cosmologists are really into the axion as a dark matter candidate is that just like the WIMP, there are other already existing decidedly noncosmological related reasons why we think the axion might exist. That's right. Particle physicists, all on their own without ever calling cosmologists or even texting or even, looking up from their phones when we pass by them in the hallway, it gets super awkward. They decided on their own that they need axions and the motivation for the existence of the axion comes in a little problem with the strong nuclear force. See, the strong nuclear force obeys 2 important symmetries in nature.

Symmetries are everywhere, and various forces obey certain numbers of them. In this case, the strong nuclear force obeys the symmetry of charge and the symmetry of parity. What this means is that if you take an interaction involving the strong nuclear force, you chart the paths of all the particles and how they interact with the strong nuclear force and then go on their separate ways or transform, you know, do whatever they need to do with the strong nuclear force. If you take that interaction you imagine taking a picture of it, and then you flip all the charges for their opposite charge. So every plus becomes a minus.

Every minus becomes a plus. Every neutral, stays neutral. And you take that interaction and you flip it around so you're looking at it in the mirror, you end up with the exact same reaction, which is pretty cool. But while we see these symmetries appear in observations, there's nothing in the theory itself to make it obey those symmetries. There's nothing in the math that says, oh, yeah.

Yeah. Yeah. When you flip the charges and run it in a mirror, you get the same result. There's nothing there in the math equations themselves. It's something we only see.

So we have to ask, how does it happen? What makes the strong force obey these symmetries? Well, we found one way to do it. If we hack the math just a little bit, we can get the strong nuclear force to obey these symmetries of charge and parity, and the way we do it is to introduce a new variable, a new parameter, into the equations and then set the value of that parameter to 0, which enforces, through various hand wavy stuff that I'm skipping over, this charge and parity symmetry. That's lame.

We don't like that. We like it when our theories are natural and elegant and explain nature on their own without us needing to go in with duct tape and wrenches to keep it all working. We don't like just, yeah, insert this parameter and then send it 0. That's that's so oh, gosh. But we did find a way to make things a little more natural.

If we promote that variable, that extra parameter, if we give it a new job, Instead of just being a number in the equations, if we promote it into being a brand new quantum particle, then through various interactions, the value represented by that particle naturally goes to 0 and the theory is safe, hence the name. The Axteon brand of laundry detergent cleans up messes, and physicists thought this would be funny. And if you think this all sounds a little weird just making up particles willy nilly to solve random problems that appear in theories, there is a long and storied tradition in physics. I mean, this is how we got the neutrino in the Higgs boson, just to name two examples, so it's not completely off base. So axions might exist.

They they are predicted to exist in this solution to what's called the strong CP problem, the the problem of charge and parity symmetry appearing in the strong nuclear force. Once we realize that axions might exist, we can start playing with their properties and say, okay. If we have a universe with axions in it, where do they go? What do they do? Who do they hang out with?

You know, just what are they like? And as we fleshed them out, as we ran through the calculations of what axions might act like and behave like and hang out with in the universe, we realized that they were, a, abundant, and nearly or completely invisible. Ding ding ding. That's why they are a candidate for dark matter because if you decide that axions exist, they can very naturally exist in abundance and barely, if ever, interact with normal matter, which is exactly what we need dark matter to do. Okay.

So remember that game with WIMPs? Say, okay. We're gonna make dark matter the WIMPs. That means we know how many WIMPs there are because we can measure how much dark matter there is in the universe. And we're going to run some, simulations and some calculations with this amount of dark matter wimps, you know, doing wimpy things.

And then we're gonna see just how they manifest. How do they, spread themselves out in a galaxy? How compact do they get? How long do they travel? Say you have a clump of wimpy dark matter.

How long does it take to travel from one place to another? And this is how we discovered that they're not so great at explaining the cores of galaxies because wimpy dark matter really, really likes to clump up really, really tight to extremely high densities and extremely high masses in the centers of galaxies, which is far more than what we observe. So let's play the same game with axions. Like, let's flood the universe with axions and and see see what happens, And, the answer is they form stars. But, stars completely unlike anything we have ever encountered before in astronomy because they're not like normal stars.

They're not like red giants. They're not like red dwarfs. They're not like neutron stars or pulsars. They're not even like black holes. They are invisible stars which is wild.

So let me give you a recipe of how to make invisible stars. The first thing we need is axions. Check. What's so special about axions? Well, one, they have incredibly tiny mass.

We're talking oh, and just like the WIMPs, there are range of axion masses. There are range of models. There are different approaches to solving this, you know, etcetera, etcetera, etcetera. So when you hear WIMP, I don't want you to think of one single kind of particle like a proton or a neutrino or a top quark. I want you to think of a category of possible particles that have a certain range of properties, same thing with the axion.

But axions in general have incredibly light masses. We are talking a trillionth of a trillionth of the mass of the electron. No joke. Orders of magnitude lighter than even the neutrinos, the lightest known particles. And this incredibly light mass means that they get to play some games that heavier particles don't, especially because their quantum wave nature manifests on very large scales.

Every particle is also a wave. We know this from quantum mechanics that that everything, every particle, every object of existence, you know, has a wave nature associated with it. In most cases, especially on macroscopic scales, the wave nature, the quantum wave nature, the wave nature that introduces all the weird quantum juiciness, goes away. Like, you and me technically have waves associated with us, probability waves, quantum waves, call them whatever you want, but they're so super tiny that you wouldn't know. In fact, we didn't know for most of the history of humanity.

Things like electrons, if we set up the experiment just right, their their wave nature comes out a little, and we can detect it and it's all cool and quantum y. These axions, because they're so light, their wave nature manifests on galactic scales. Like, you can we can measure the the wavelength of an of an electron, like, moving through a slit, and it's all it's all subatomic. It's all minuscule. The wavelength of a axion can be, like, 10,000 light years across.

So when you think of 1 axion, it's impossible to think of it as a single little particle, planted right there. Like, you point to it and say, yep. There is that axion right over there. No. Because it's so light, its way its wave nature can be spread out over 10,000 light years.

So if you want to point to an axion, you can only vaguely wave your hand and say, well, the axion is kind of spread out. That we're talking about 1 particle here, and it can be spread out over light years. Its quantum wave nature manifests not just on macroscopic scales, but on galactic scales. That's going to create the possibility for some interesting games in the second property of axions that allows them to do some interesting things like make invisible stars is the fact that they are bosons. I know.

I know. Anytime we venture into particle physics land, it just becomes one something on after another on. I'm sorry. But there is very one very important one that I need to mention. That's Patreon.

That's patreon.com/pmsutter, and it's how you can contribute to this show and keep it going. I truly, truly appreciate all of your contributions. That's patreon.com/pmsutter. But when it comes to bosons, look, I'm not in charge of naming things. We're gonna get through it together.

And trust me, I'm sparing you from some of the gory jargon like the fact that the axion is classified as a pseudo Nambu Goldstone boson. We're just gonna move right on past that. Feel free to ask if you want. I'll do an episode on it someday, but not today. Okay.

So what the heck is a boson, and why is it important? We might as well just jump all the way in. There are 2 kinds of particles in the universe, 2 broad categories, fermions and bosons. Fermions are named after Enrico Fermi, nicknamed the pope of physics, by the way. And bosons are named after Satyendra Nath Bose.

The basic idea is that, say, you've got a box and you're putting some particles in the box. With fermions, there are only so many fermions that you can fit in the box. Even if you were to lower the temperature of the fermions to absolute 0, I know you can't do that, but just pretend you can, and they were all the fermions, all the particles were in their lowest energy state. There are only so many fermions you can fit in the box until the box overflows with fermions. Fermions are like electrons.

They're protons. They're quarks, neutrinos. They're the building blocks of matter. Bosons, however, can keep getting shoved in a box. You can put as many bosons in a box as you feel like it.

They can all share the same quantum state, so they can just keep stuffing and piling onto themselves. Like, an example of a boson is a photon light. You can put as much light in a box as it can handle. It might get really, really, really hot, but you can keep pumping that light in. It's not gonna get full of light, and it's not like light is gonna ooze over the edges and spill out and and get all over your shoes.

You can just keep putting light in the box. That's because the bosons can keep piling on top of each other. I like to think of bosons as, tortilla chips. You know, you can just keep eating tortilla chips and somehow you never get full. You can just keep piling those tortilla chips.

Yeah. Give me a refill. Go for it. And you you they just go down and where did they go? I don't know.

They all share the same quantum state. Like, you can't get full on tortilla chips. Those are the bosons. The fermions are like cheesecake. You know, 3 bites and you're like, I'm done.

The cheesecake is literally going to spill out of my pores. I'm so full right now. So the WIMPs are our old friend WIMPs that, you know, aren't looking so hot nowadays. Those are fermions. You can only pack so many WIMPs in a box.

But the axions are bosons which means they can reach much much much much much much much higher densities. But, because the individual axions have so little mass, the insane densities of axions don't overwhelm an entire galaxy. You can just keep piling axions on in a galaxy and it doesn't cause things to go haywire and then you combine that with their wave nature and you have something really special that happens. It means that axions can all share the same quantum state. They can all pile on top of each other.

They don't mind. And, axions, individual axions, can be spread out in space. Their wavelength can be spread out in space. And you put this together and this is how you get invisible stars because you take a whole bunch of axions. They don't have a lot of mass but they have some mass.

That means they attract each other and themselves with gravity. They feel the gravitational pull of each other. But this gravitational pull is spread out over a large volume of space because of that quantum wave nature. And so you've got, like, one big spread out axion, and it feels gravitationally attracted to another big spread out axion. And so they the 2 axions meet, they clump together.

And so in that same large chunk of space, now you have 2 axions with their 2 wavelengths, you know, merge together and then you get a 3rd and a 4th and a 5th. And so what ends up happening is you get these bound structures of axions due to the fact that you can pile axions on as much as you want and in a very real sense, individual axions are spread out in space unlike, you know, any other known particle is capable of. You know, even the wave nature of the neutrino is still at microscopic scales. But now we're talking about galactic scales or stellar scales. You know, you know, it depends on the mass of the axion.

So you can build large clumps of axions very very easily. Axions naturally start to clump together from their own gravity. And in fact, in some models, they may have even emerged out of the early universe already in this state. And they can compress far tighter than WIMPs because they're bosons. You can add a lot more axions into a clump than you can a WIMP dark matter particle.

In fact, it's possible for axions to reach incredibly high densities and in the process form a completely new state of matter, a Bose Einstein condensate. Now, I should do a full proper episode on Bose Einstein condensate, so feel free to ask, but the short and sweet version is that this is a state of matter when bosons all occupy the same lowest quantum state. In other words, they begin acting like a single quantum particle, and all sorts of weird quantum effects become macroscopic and, in the case of axions, galactic. So far, the only place we've produced Bose Einstein condensates is in the laboratory but axions might do it on their own in the universe And what you end up with are these gigantic objects held together by their own gravity all acting as a single quantum thing, all sharing the same wave function spread out through space, all moving to the beat of the same drum. All of their individual quantum waves merge together and they share the same quantum wave function.

Imagine a single quantum particle that is bigger than a planet composed of trillions is that doesn't even do the the term justice. Like, the like innumerable amounts of axions clump together into a finite volume of space and all sharing a single quantum wave. A single particle bigger than a planet, a single gigantic wibbly wobbly object that's more wave than it is particle. Like, you ever hear about those rogue waves? You know, they're normal ocean waves that all sorts of heights and amplitudes and wavelengths, and then every once in a while, the waves can just combine together in the exact right way or exact wrong way depending on your perspective and generate a giant wave that can be tens of meters tall that appears out of nowhere on its own in the middle of the ocean.

We used to think this was just, you know, sailors telling us fun stories, but now there are direct observations of these rogue waves. I like to think of these axion stars as rogue waves in the universe where you have individual axions with their individual waves piling together with other axions and because they're bosons they can just pile on together and all their waves combine together, and they all start to act as a single entity. These objects, these entities have various names, dark stars, axion stars, ex stars, boson stars, exotic stars. I prefer the name dark stars because we've got sort of a dark theme going on when it comes to cosmological mysteries, but but feel free to pick your favorite. How big are these objects?

These dark stars. On the big side, the dark stars might be huge like, thousands of light years across for the very lightest axion masses. This can actually help alleviate the galaxy core problem. And this is because even though the axions are there are so many Axions that have reached reached so much density, or such a high density in the galactic core with all their wave functions piling on top of each other, but Axions are so light that it's still not a lot of stuff and that wave function prevents them from collapsing down into even tighter tighter tighter density. So, you actually spread out the mass of the axion dark matter within the core of the galaxy, and so that might help this problem because normal dark matter clumps up too tightly.

Axions do clump up tightly, but they do maintain this, like, spread out sense. So that might be nice, might solve that little problem. On the small end, you know, dark stars might be well, you know, have a masses around a solar mass, a planetary mass. These clumps of axions with enormous numbers clumped together all sharing a common ground state, all sharing a common wave function spread out in space, not acting when when I talk about a dark star, I don't want you to think of particles buzzing around like you might think of hydrogen and helium buzzing around the sun. No.

These are quantum exotic quantum objects where it's just this, like, like a rogue wave of dark matter in 3 dimensions. It's like spherical rogue wave that appears out of the background and and travels around the galaxy. There might be 1,000,000,000,000,000,000,000,000,000,000,000, trillions of these dark stars, these rogue waves sloshing through the galaxy. And then, on the other end of the spectrum, there might be dominated by a single rogue wave existing in the center of the Milky Way and every other galaxy. Invisible stars like an evil mirror version of the galaxy existing in parallel to it.

For every star you see on the sky, there might be 1, 10, a 1000 dark companions in the space around it. So what does this mean? I mean, it's fun to think about and all, but between our flights of fancy, we have to remember to do physics, which is to explore the physical consequences of this idea. What would it mean for dark matter to be made of these axion dark stars floating around the universe? On one hand it makes it very challenging for direct detection because unless one of these dark stars just happens to be passing through the solar system, you're not gonna, like, pick up an axion in one of your direct detection experiments.

If your experiment is tuned to find, rogue waves and you're just floating around, you know, rogue waves are kinda rare unless one washes over here you're not gonna see it. So that's a bummer, but on the other hand it does open up a lot of possibilities for identifying Axion as a dark matter because of what it might do. Axions might live inside of giant stars and modify, fusion reactions that because that's because there's a channel, there's a a way, a mechanism for axions to turn themselves into photons in the presence of strong magnetic fields. And so they might mess up nuclear fusion rates, and we might be able to detect that. Giant clumps of axions might actually go supercritical and turn into what's called a Bose Nova where very slight, very tiny interactions between the axions get them to cascade and release energy in a giant flash, and so we've got things to hunt in in the universe, which is always fun.

Like I said at the beginning, we don't know what we're doing. We're just guessing and we're seeing what sticks. Axions are very intriguing as a dark matter particle since we already have reasons to suspect that they exist and we are currently working through the implications of this idea to see how we could possibly detect them. That's exactly how science works. And in the process, we end up with these predictions where dark matter may not be, like, smooth, and continuous all throughout a galaxy, but it might be clumped up into the form of these dark stars.

These these rogue waves where these quantum wave functions all pile on top of each other, which is such a fun intriguing thought to entertain. So who knows? Maybe the axion is the dark matter. At this stage, we just don't know. We're working on it.

And in the meantime, as we look up into the night sky, we can keep imagining what we're not seeing, what's hiding in plain sight. Thank you to XinJian R for the question that led to today's episode. And thank you to everyone. Please keep those questions coming. That's askaspaceman.comoraskaspaceman@gmail.com.

Please keep leaving reviews on your favorite podcast platform that really helps the show visibility, and of course, thank you for all of your generous, generous contributions to Patreon. That's patreon.com/pmsutter. And, in fact, I'd like to thank just the top just the top Patreon contributors this month. They are Justin g, Chris l, Alberto m, Duncan m, Corey, d Robert b, Michael p, Nyla, Sam r, John s, Joshua, Scott m, Louis m, John w, Alexis, Gilbert m, Rob w, Jessica complete knowledge of time and space.

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