Daniel and Jorge Explain the UniverseDaniel and Jorge break down a new study that looks for dark matter using both gravity and light.
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Hey Dannie, I'm curious how does a physicist make mental images of some of the tricky things you do research on?
You know, I'm not sure you really want to see inside the brain a physicists. It's a bit of a mess.
Well, let's find out what's your mental image of dark matter?
Okay, that's a really tough one because it's invisible. I guess I sort of imagine it like water, which is, you know, mostly transparent, but you can definitely tell it's there.
So we're all swimming in a bath of dark matter. I hope there's some dark matter rubber duckies out there, some dark duckies. I wonder if they have a cute song for that on Sesame Street.
That would make dark matter bath time, lots of fun.
It's a special episode on the Letter D for dark Matter.
Hi.
I am Horam mccartoonist and the author of Oliver's Great Big Universe.
Hi, I'm Daniel. I'm a particle physicist and a professor you see Irvine, And this episode is brought to you by the taxpayers of California.
Is it They're not paying me?
This professor is brought to you by the taxpayers of California.
I should say, I thought our heart worth paying you. Are you double dipping here, Daniel? Sounds like it.
I am multifaceted.
Your multi paid is all you're saying by multiple people. Sounds like a good setup there.
I got no complaints, but.
Anyways, Welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio.
In which we try to bring multiple streams of understanding together into your brain. There's so many ways to understand the universe, so many questions to ask about it, so many things to understand, and the challenge of physics is to weave all those together into one coherent, comprehensive story that explains the universe to us and to you and to our kids. And that is the goal of this podcast.
That's right because it is a vast universe full of all kinds of things that start with all kinds of letters, the letter A, the letter B, the letter C, all of the letters in the universe, even the ones we haven't discovered.
Yet, even letters in other alphabets. In particle physics, we often reach to the Greek alphabet to name our particles.
Is that why it's all Greek? To me?
That's exactly right. And sometimes when we run out of Greek letters, we dip into the Hebrew alphabet. M.
I thought you were going to say, we're gonna you're gonna dip into alien alphabets or one day we might have variables that are only defined in an alien language.
That sounds wonderful. I look forward to figuring out how to write alien letters in word.
You're gonna need like another app like trio Linguo or something.
I'm sure this's gonna be a Unicode symbol for alien languages.
Oh so, I mean they'll be alien emojis.
What if the first message we get from aliens is just in emojis that we need our kids to.
Help us interpret it.
Yeah, let's hope it's not like food icons like hey, hey dinner, Hey hot dog, hamburger, pizza.
M I think the food emojis have other meanings sometimes, you know, the eggplant for example.
I don't know what you're talking about, Daniel, what kinds of websites have you been surfing?
Ask your kids about it later.
I doubt it.
But anyways, it is a big universe full of amazing things to study and observe if we can actually see them.
One of the challenges of understanding the universe is first figuring out what's out there. We begin by using the natural senses. We're all familiar with our eyes, our ears, our noses, et cetera. But there's so much more out there that we can detect with more subtle methods.
Yeah, because there's a lot of stuff out there that we can see, but also a lot of stuff we can't see. The most amazing discoveries in recent decades has been the idea that most of the universe is out there, but we can't see it or touch it or I feel.
But as we progress scientifically and technologically, we build new kinds of eyeballs, new kinds of sensors, new kinds of ears that let us detect things happening in the universe that were otherwise invisible and impossible for us to notice. Sometimes that means seeing entirely new signals like photons of very high energy or low energy. Sometimes that means noticing patterns in other signals, like the rotations of galaxies and gravitational hints in the motions of other objects that tell us there's so much more going on out there than we could immediately see.
Yeah, and one of the biggest pieces of stuff out there that we can see is called dark matter.
It accounts for about twenty seven percent of the universe, right, Daniel.
That's right, In any given chunk of the universe, just over a corridor of the energy is devoted to dark matter, and only five percent of it is made of the kind of stuff that you and I are made out of, baryons, quarks, leptons, this kind of stuff, which means that there's a lot more of the invisible stuff out there than the visible stuff.
Yeah, which can of make you wonder if maybe we're the ones that are invisible.
Maybe we needs learn those dark matter emojis pretty quickly, but could.
We see them? Maybe you are receiving dark matter emojis, you just don't know it.
I've kind of an outdated operating system, so a lot of the emojis that get are question marks anyway, Maybe those are the aliens trying to talk to me.
Mmm, maybe they're actually sending you question marks. They're like, Daniel, why are you getting paid so many ways for the same thing?
You know, I do get lots of questions from listeners, so maybe some of those are actually coming from the aliens.
Interesting, are you calling our listeners aliens?
I'm saying we're inclusive, right, everybody is welcome. We try to reach everybody, not just humans.
Well, there you go.
If you want to stump a physicist, just send them a question in the form of emojis. You know how they summarize movies sometimes with just emojis. I wonder if you could do that with physics theory.
What they summarize movies with just emojis?
Yeah?
Or books or stories or news items.
Oh, I feel really out of touch.
Speaking of out of touch, that's what dark matter is. It's stuff that's out there that you can't see and you can't even touch, right because it doesn't feel the electromagnetic force, so you can feel it with your fingers.
That's right. It doesn't emit light, give off light, reflect light, or interact with light at all, which makes it pretty dark. And if a huge chunk of the universe is so dark but so important, then scientists really have to figure out how to study, how to understand what it is. So we're doing our best to be creative to find new ways to look.
For it so that we can study it and kind of figure out what it's made out of. So there are maybe new ideas out there about how to do this. So today on the podcast, we'll be tackling the question could dark matter be making flashes of light? I always figured dark matter was pretty flashy.
Well, I don't know the name dark matter is more mysterious than flashy, isn't it.
Well, that's just what we'll call it. Maybe it's something we can see, but really inside it's fancy and flashes.
Maybe when we finally meet the beings made of dark matter, they'll be like annoyed or offended or disappointed that we call them dark matter.
Hopefully they won't flash us.
You mean like zap us with a laser being from orbit or open the trench code.
I mean like Senda's fruit emojis. So as usual, we.
Were wondering how many people out there had thought about the idea of seeing dark matter through flashes of light.
Thank you very much to our group of volunteers who answers these questions. We'd love hearing your thoughts on the topic of the day. If you would like to contribute, please don't be shy. Everybody is welcome. Whether you've been listening for years or weeks or days, or this is your first episode. Just write to meet too questions at Danielandhorge dot com.
So think about it for a second.
Do you think dark matter could be making flashes of light that we could see.
Here's what people had to say.
I don't know but that'd be cool.
The concept of dark matter making flashes of light is quite interesting, since dark matter seems to make up a lot arn't part of the universe. I don't see why it could be giving off light pulses given the right circumstances.
I think that only directly by making stuff made of regular matter to behave in a certain way due to gravitational effects, since.
It doesn't interact with the electra week force, I think.
Not all right.
I think that pretty much summarizes the episode here. I don't know, but that would be cool. I feel like that's almost every episode.
That's the emoji version of the episode. Yeah, if you have to summarize it, so that would be what shrug question mark check mark? Is how you summarize an emojis.
Yeah, or like a black square for dark matter? Question mark trump? And then and then the emoji with the sunglasses for cool yeah?
Or sparkles. Isn't there a Sparkles emoji?
Well, depends how flashy you want to get, Daniel.
Let's go all out. We've got multiple funding sources here.
There you go.
Let's go out with a flash of jail time. But anyways, let's get down to it. Daniel, let's recap for listeners, what is dark man in the first place.
It's important that we explain what we mean when we say dark matter, because I noticed there's lots of different ideas out there about what dark matter is. Online you see a lot of people saying dark matter is just a placeholder, is just a way to say we don't know. Other people talk about dark matter as if it was a very specific theory of a very specific particle. There's a whole bunch of people in the middle to talk about dark matter as a sort of general catalog of ideas. But all of these things are there to explain something that we don't understand, which is that there's a lot of gravity happening in the universe that we cannot explain, Like when you look at how galaxies spin, and when you look at how the universe formed and all the gravity necessary to pull the stars together into galaxies. We just cannot explain all of that galaxy using the stuff that lights up, using the stuff made of quarks, it either glows or reflects light or gives off light. We just cannot tell the story of the universe and how it makes sense without something else out there. Providing a bunch of gravity. So much gravity is missing that you need five times as much of this mysterious stuff we call dark matter as there is normal matter. So very briefly, dark matter is just an idea to explain all this unexplained gravity in the universe.
Yeah, and there are different ways that scientists have sort of found or think that dark matter is there and there. As you said, they all relate to gravity. But I think basically the main idea is that what we see of the universe tells us that there's more matter out there than the stuff that glows or that we can see and feel right exactly.
And the story started with the galaxy rotation curves. We looked at galaxies and measured how fast they spin, and we notice that they spin really really fast, and that if you add up all of the stars and the gas and the dust in those galaxies, they don't provide enough gravity to hold that galaxy together as it spins. So that was evidence number one, and for decades we knew about that, but it was sort of hard to accept the idea that there could be so much more missing matter out there. It was just one piece of evidence, but slowly, over the decades, we've pieced together lots of totally independent measurements that tell us that there is missing stuff out there, that there's matter out there providing gravity that we cannot see.
Like initially, for example, it could have just been that galaxies out there had a lot of like dark rocks, right or gas that you couldn't see through the telescope exactly.
Or it could have been that gravity work differently over really really long distances, like we've measured gravity in the Solar System and on Earth, but maybe over hundreds of thousands of light years gravity operates differently than Newton and even Einstein suggested that could have been the explanation when you were just looking at one example, just at galaxy rotation curves. But now we have lots of other ways to probe this, you know, we look, for example, at the structure of the universe. How did it come together? How did you go from blobs of gas mostly dispersed through the universe clumping together into stars and galaxies, And that requires gravity. And if you run the universe without any dark matter, just with the kind of matter we can see, you don't get stars and galaxies. After fourteen billion years, there isn't enough gravity to do it. You've got to add in the dark matter and boom, then you get a universe that looks just like ours. So it's another very convincing piece of evidence that there is matter out there. It's not just like gravity operates differently at those distance scales. There really is missing matter.
But could you ask the same question about these large scale structure theories. Could it be just that gravity works differently though we thought at different scales, and that might explain why galaxies form the way they are.
It is possible, and people who work on these theories they're called like Mond modified Newtonian dynamics, have tried to tweak them. I've not seen one that can successfully explain both the galaxy rotation curves and the large scale structure of the universe. Mostly these theories are tuned to explain the galaxy rotations, and they don't even try to explain the other evidence for dark matter.
What's this other evidence?
Maybe one of the most compelling and precise piece of evidence for dark matter is seeing its effect on the very very early universe plasma before we start were formed, and we really had any structure in the universe at all, it was just huge blobs of hot gas everywhere in the universe, and we see the glow from that gas in the cosmic microwave background radiation. When the gas cooled enough to become transparent, when it formed neutral atoms, so photons mostly could fly through it. Those photons are still around and we see them, so they're like the last glow of this plasma from the very early universe. And we see ripples in that plasma, ripples that show us how the normal matter and the dark matter and the photons were all sloshing around. And it's a very very precise measurement, and it tells us how much normal matter there is in the universe, how much radiation there was in the universe, how much dark matter there was in the universe back then. It's very very precise because we've taken these very very detailed maps of this early universe plasma, and that tells us that there is a component of that plasma that doesn't interact with photons. It's not buryonic, it's not our kind of matter at all. So it's another completely independent measurement that tells us there's a dark but gravitationally active component to the universe.
Now, is that also due to gravity? Like, does that evidence also depend on our current model of gravity?
It does, but those are much shorter distances. Those oscillations ended up seeding the larger scale structure of the universe, but it was before a lot of the expansion, So we're talking about things that happened over short distance scales.
There were a short distance but there are large distance now.
That's right, But the CMB comes from when it was short distance. Earlier you were asking if this could all just be due to like long distance gravitational effects, and the CMB proves shorter distance scales gravity because the picture we have of it comes from back when things were much more cozy and compact.
All right, So then the prevailing picture is that there's a lot of stuff out there, stuff that creates gravity and feels gravity, but we just can't see it.
So let's get into what it could be, how.
We might see it, and what might be new ways to figure out where it is and what it's doing.
So we'll dig into that, but first let's take a quick break.
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All right, we're talking about dark matter now, Daniel. How many episodes now have we talked about dark matter?
Oh? Man, so many episodes. But it's still a thing people are most confused and most interested about. In the emails we get from.
People, people are still in the dark about it.
Yeah, and it's a really active area of research, obviously because it's one of the biggest open questions in modern physics. But are constantly trying to come up with new theories for what dark matter might be. Is it this particle, is it that particle, Is it not a particle at all? Is it something else entirely? And new ways to spot dark matter. If it's this kind of particle, how could we see it could we build a detector to observe it. So it's a huge area of active research.
Yeah, I guess we can't just wait for the Sesame Street episode on it.
We could just sit back and wait for the aliens to show up and tell us the answers to the universe. But that'd be kind of embarrassing if they showed up and we had nothing to contribute.
Yeah, it'd be more ambassing if they have to do it over dinner while they eat us. I wonder if they would feel less bad about eating us if they knew we weren't. You know, dark matter intelligence, All right, Well, we just kind of define what dark matter is. It's the stuff we think that's out there to explain a lot of the gravitational effects we see in how the universe form and also in the microwave background radiation. Now there are candidates where what we think this stuff might be, right Daniel.
There are lots of ideas for what dark matter might be, and people initially thought, well, maybe dark matter isn't some new, weird kind of stuff. It's just more of the kind of stuff we already know about. But you know, it's just hiding or something. People thought maybe dark matter is like neutrinos, because neutrinos are tiny little particles that don't interact with photons, they don't give off light, they don't reflect light, they pass right through a lot of stuff. So they seemed initially like a really good candidate maybe for dark matter, like maybe the universe is just filled with uncountable numbers of neutrinos. That would have been amazing. But neutrinos don't have a lot of mass. They're very very low mass particles, which means they almost always move nearly the speed of light. And one thing we do know about dark matter is that it's kind of slow moving. It's not a fast particle.
Well, I guess that's Those are two questions. First of all, why can't thetrinos go slow even if they're light? Couldn't you still stop them from moving?
You can stop a neutrino because they have very low mass, but their mass is so low it's like thousands of times lower than even an electron that essentially, when the energy in trino has means it's moving at almost the speed of light. So it's almost impossible to have a natural process that produces a huge number of neutrinos and have them be slow moving.
And you also said dark matter is cold. How do we know it is cold if we can't feel it.
Yeah, we know that dark matter can be moving very fast because of the wave its influenced the structure of the universe. Like, dark matter is mostly responsible for what we see out there. The reason you have a galaxy here or a galaxy there is because there's a huge blob of dark matter that created gravitational attraction that pulled all that gas in to form the stars and the galaxy. Now, if dark matter was moving really really fast, if it was hot, then it wouldn't clump the same way. It would spread itself out much more. So we know that dark matter has to be below a certain speed basically, or it wouldn't have clumped into these blobs which then form the structure of the universe. Like you run simulations where dark matter is a fast moving particle, you don't get the same kind of structure that we see in our universe. That means dark matter has to be slow moving or cold, as physicists say, and that means that it can't be neutrinos.
Couldn't it be like super massive but hot too well.
Hot essentially refers to its velocity, regardless of its mass. The issue is the velocity, and the problem is neutrinos basically can't have low velocity. We know that dark matter can't be moving very fast, otherwise it would spread itself out and wash out the structure of the universe.
All right, well, then how is it that we can see it? How can we hope to see it and study it?
Then? Yeah, it's tricky, you know. The one thing that we do know is that dark matter feels gravity, so gravitational studies are surefire way to detect dark matter. The problem is that gravity is super duper weak. It's like the weakest of the fundamental forces. If it even is a fundamental force, it's weaker than the weak force by like ten to the thirty, which means that it's basically impossible to use gravity to detect tiny bits of dark matter like we can see like Solar System sized chunks of dark matter maybe, but anything smaller than that, the gravity from it is too weak for us to even detect it. And that means that you couldn't really use gravity to detect the particle nature of dark matter, like one of the deepest questions is what is dark matter made out of? Is that this kind of particle is that kind of particle. But gravity is really too weak to tell us anything about the particle nature of dark matter.
And we don't even know if it is a particle, right, it could be that dark matter is something that doesn't form into particles.
Is that possible.
It's totally possible. It's not a mainstream idea, Like most of modern physics right now is focused on the idea of particle dark matter, because the kind of matter that we know is all made of particles, and so we extrapolate, We say, well, probably this other kind of matter is made out of particles. And you might think that sounds reasonable. It's a pretty basic assumption, but it's also extrapolating from five percent of the universe to like twenty five percent of the universe. It's entirely possible that the physics of this other huge section of the universe is very different from anything we've imagined. We've talked in the podcast before about unparticles or other weird kind of theories that describe it as not made of particles. So like as you zoom in, it doesn't ever change it's not like there's a basic of it. You can just keep zooming in forever and it always looks the same. That would be really weird but super awesome. But the mainstream theories are mostly particle dark matter because that's what we know how to think.
About, because that's kind of the only way how to think about things, right.
Well, you know, there are people trying to think outside the box. That requires a lot more creativity and flashes of insight. But there definitely are people out there working on sort of crazier theories of what dark matter is. But I think most people are working on particle dark matter because yeah, that's what we're good at thinking about. I mean, you talk to a particle physicist, you're going to get a particle explanation for.
Everything, I guess.
I mean, like, we haven't seen anything that isn't explained by particle theory, right, and we wouldn't even know what that math would look like.
Yes, and no, we've never seen anything that we've been able to explain with a non particle based theory. But remember there's ninety five percent of the universe dark matter and dark energy that we still can't really explain. So particle based theories are the only ones that have ever been successful, but they've only been successful in five percent of the universe.
So yes and no, I guess there's a lot out there that we can't still explain. So maybe our current theory only covers five percent of the universe.
Yeah, that's exactly right. So we should definitely keep an open mind to other crazy theories of what dark matter might be. But currently we're mostly working on is dark matter a particle? Is it a new kind of particle? What does it do? How could we possibly spot it? If it is a particle?
All right, so if it is a particle, how do we see it? How do we study it? We know that it feels gravity, what else does it feel or not feel?
So the short answer is we have no idea. Like, it could be that dark matter feels some new kind of force with itself. It could be the dark matter feel to no forces other than gravity, and like that could totally be our universe, in which case it's almost impossible to ever discover the particle nature of dark matter. It could just be a mystery forever because gravity is just so weak. There's another possibility that dark matter is kind of misnamed that dark matter does actually interact with our kind of matter through some mechanism we haven't discovered yet, that it's not really truly one hundred percent dark.
But I guess you mean using electromagnetic forces or other kinds of forces like it dark in terms of light or dark in terms of all the forces in nature we're imagining.
Maybe there's a new force out there also, So we're suggesting maybe dark matter is some new kind of particle, right, And in addition, maybe there's a new force out there, a force that helps dark matter particles interact with our kind of particles. So call this a new dark force or a portal to the dark sector or whatever. If there is this new kind of force, then maybe it helps dark matter particles bump into our kind of particles, or turn into our kind of particles, or somehow interact with our kind of particles, which would make them effectively visible.
But would you need to come up with a new kind of force. Couldn't interact with us through the weak force or the strong force somehow?
It's totally possible that dark matter could have interacted with us via one of the forces we know already, But we basically ruled that out with.
Our experiments, have we really is that official.
If dark matter interacted with a strong force, it would be a very powerful interaction because the strong force is super duper strong, and that would actually be pretty easy to find. We look for dark matter giving off photons. We've never seen that, so we don't think that dark matter actually directly interacts with photons. We've also looked for dark matter interacting via the weak force, and this is probably the biggest area of experimental dark matter particle physics right now. With these huge tanks underground very quiet liquid like xenon, and we wait for dark matter to pass through the Earth and interact with one of these xenon molecules and give it a little kick. So these huge tanks of underground liquid xenon are just waiting for a dark matter particle to bump into it via the weak force, and if it does interact via the weak force, we can calculate how often that should happen. And we've been running these experiments for years and years and years, and we've never seen a blip that looks like dark matter kicking one of these xenon molecules via the weak force. So now we can pretty definitely rule it out.
I see, we've been putting stuff out there to hopefully interact with dark matter through the weak force, but so far nothing has been bumped that way. So now you're saying the dark matter probably doesn't interact with the weak force.
As time goes on and we don't see any of those interactions, we more confidently say that they never happen. If you only listen for a day, it might be that they only happen once a year and you just haven't seen one yet. But after you've listened for five years, ten years, twenty years, either you're getting very very unlucky or it just doesn't happen. And so now we've had big enough detectors running for enough years that were pretty confident ruling that out. But it doesn't mean that there isn't a new kind of force, like an even weaker force, that would allow dark matter to interact with our detectors. So that's what they're looking for now, Like they call it the feeble force.
It's weaker than weak, that's the idea, exactly weaker than week.
We only we have seen that already with our detectors, with the tanks of Xenon there sitting there waiting.
It depends on how weak it is. If it's super duper extra week, if it's the feeblest force, you can imagine it might take a very very long time. It might be so unlikely they have to run for ten years or one hundred years in order to see it. And that's why we're also at the same time using other methods to try to look for these kinds of interactions, Like we're hoping to use this new feeble force to create dark matter at particle colliders.
What do you mean, like when you collide protons or quarks like that, it might make dark matter.
Yeah, if this feeble force exists that allows dark matter to bump into protons and neutrons inside a xenon atom, then in principle you can reverse that. You can say, well, what if we smash protons together, maybe sometimes they can use the feeble force to create dark matter. Because that's what we do with the particle collider. We annihilate protons together and create new kinds of stuff. And the cool thing about a collider is that anything that's out there, you can make it as long as your protons can interact with it. So anything that protons interact with we produce at the collider eventually. Things that interact with protoons, it's a lot. We produce them all the time. Things that don't interact with protons very often, we produce them more rarely. So Higgs bosons are pretty rare, for example. But we comb through all of those collisions looking for evidence of the production of dark matter.
Wouldn't we have seen evidence of that already?
I mean, you've been running the LHC for a long time and particle colliders for decades. You know, if there was some sort of unaccounted for force, wouldn't we have seen it by now?
We haven't seen anything, You're right, And again it's a question of the strength of that force. If that force was as strong as the weak force, yeah, we probably would have seen it. And the longer we run our colliders and don't see it, the weaker that force has to be to still be consistent with our data, to still be hiding from all of these experiments. But we don't know how weak that force is. Maybe it's really ridiculously weak, so weak that we haven't seen it underground and we haven't seen it in our colliders, So we actually turn to another mechanism to try to see this super duper weak force in action, which is to look at the center of the galaxy to see if dark matter is smashing into itself.
I see use the center of the galaxy as a particle collider.
Exactly because one problem with the particle collider is that it's not very dense. Right. We have protons smashing into protons, but it's like very few protons, and we try to run it as often as we can, and it adds up to zillions of collisions, but it just might not be enough. But we think that the center of the galaxy is very very dense with dark matter. We can map out where the dark matter is in the galaxy by looking at how things rotate and how fast things are moving, and we suspect that the center of the galaxy is very dense with dark matter. And so if dark matter has this feeble force, this super weak force, then occasionally two dark matter particles should bump into each other and produce normal matter particles like the opposite of what we think might happen in the collider, or two protons smash together to make dark matter run that backwards in time, we hope that's the operation happening in the center of the galaxy. So we turn these special telescopes to the words the center of the galaxy and look for these characteristic flashes of life that might come from those collisions.
Interesting, that sounds a little cheaper than building a thirty billion dollar collider here.
Well, it was only ten billion dollars, right, so we can save you twenty billion off the top right there. And it wasn't that cheap because you're building a particle detector and launching it into space, which is never simple.
All right, Well, let's get into the details of how we're using the center of the galaxy as a collider to look for dark matter. Let's dig into that, but first, let's take another quick break.
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All right, we're looking for dark matter. I had some right here, Daniel. You don't know where I put it.
I can give you emoji based directions to it, Derry, I'll send you some arrows.
That might not be helpful there. But yeah, there's a lot of matter missing in the universe. All twenty seven percent of the universe is out there, but it's invisible. We can't see it or touch it called dark matter, and our only hope for ever studying its particle nature is that there's some sort of new force that we haven't discovered yet, some super weak force that you're calling the feeble force. Although Daniel, I'm kind of disappointing you didn't call it the dark force.
Because then forever it would dominate my destiny.
Exactly right, Then you get the cool red lightsabers or the cool laser pointers when you give your lectures.
Yeah, that's true, but I didn't want to have to grow those horns.
Not all sith have horns.
That's relief.
You need to brush up on your Star Wars there.
But yeah, our only hope forever studying the particle nature of dark matter is that it feels a new kind of force we haven't seen before, and we can't generate that force apparently here in our colliders. But there's hope that maybe you can use the center of the galaxy as a collider to maybe study this part of this aspect of dark matter we think we're hoping is there.
Exactly, And the idea is dark matter smashes into itself and then via this new feeble force, turns into standard model particles, maybe tau leptons or be quarks or something, and those particles can then give you flashes of light because those particles do interact with light. So what we do is we turn this telescope to the center of the galaxy and we look for these flashes of light that we can't otherwise explain. If you see flashes of light that look like they come from dark matter turning into particles, we know, then you can say, oh, there's evidence for dark matter there.
Wait, the idea is that dark matter smashes into itself. Why does it have to smash into itself.
It doesn't have to. It's possible for dark matter also interact with normal matter particles in the center of the galaxy the way it might interact, for example, with our tanks of Xenon underground. But the signature we're looking for would come from two dark matter particles smashing into itself, producing like a pair of bottom quarks or a pair of tau leptons, which then give off some photons which travel to our telescope and we observe them.
Wait, so two dark matter particles can interact with each other and generate regular kind of matter?
Yeah, exactly?
Does that work the other way too? Like, can regular matter interact and come up with dark matter?
That's what we're trying to do with the large hair dark collider is smash regular matter together two protons and turn it into dark matter. We haven't seen that yet, so we're trying to do the opposite, smash dark matter together and turn it into regular matter. It's like the inverse collider.
What's the inverse of a collider? An expander as a separator.
It just runs it the other direction, you know, Instead of colliding normal matter to make dark matter, we collide dark matter to make normal matter.
So the picture is that in the center of the galaxy, you're saying, there's a high density of dark matter, and some dark matter particle just happens to smash head on with another dark matter particle, hopefully they're going fast enough to actually sort of interact and create a lot of energy, and then out of that comes a regular matter particle from the center of the galaxy that then we see here on Earth and go, yep, that came from two dark matter particles.
Yeah, And if you want the microparticle physics explanation, two dark matter particles smashed together form some new kind of particle that mediates this feeble force, call it a dark photon or something. And then that dark photon turns into a pair of normal particles like two bottom corks or two tau leptons or two muons or something, and then those produce photons, which then we see here.
But then how can we hope to like discern that from all the way here the center of the galaxy is, you know, fifty thousand light years away.
It's very tricky, and that's exactly the difficulty of this method. Number one is far away. We think that these photons would be very high energy, so we think that gamma rays would survive travel from the center of the galaxy. But the real problem is that we don't understand the center of the galaxy. It's a lot of stuff going on there making flashes of light anyway that we can't really understand. And so actually we have seen flashes of light from the center of the galaxy that we cannot explain that No astrophysical explanation has been provided to explain these flashes of light, and so some people are actually pretty excited about that. They say, whoa, maybe this is dark matter. But you know, the problem is, we don't really know what's in there in the center of the galaxy. It's a very weird, dense region. We just did a whole episode about how weird it is and how little we understand about what's there. So it could just be new, weird stuff made of normal matter in the center of the galaxy flashing lights and ways we don't understand. Or it could be dark matter creating these flashes of light.
Wait, wait, wait, what do these flashes look like? Like?
You're just looking at the galaxy center of the galaxy and suddenly there's like a spike or a pulse or a train of pulses. What do these flashes look like?
So these flashes come as individual photons. So this telescope, a Fermi telescope, can see gamma ray photons, which are just photons in a certain energy range, in this case from fifty million electron volts up to a trillion electron volts. And they see a bunch of these gamma ray photons from the center of the galaxy, and we cannot explain.
Them otherwise, like one at a time, Well, the.
Detector can only see one photon at a time.
Yeah, I guess.
I mean, like you see one and then you don't see one for another year, or is it do you see like a whole bunch of them in a cluster or something.
So the higher energies are definitely more rare than they are at the lower energies, but there's a lot of them. It's not just like one or two or seven photons. You know, We're talking about thousands of photons here, accumulated over many, many years, and so people are trying to understand where are these photons coming from. Is there something going on with normal matter at the center of the galaxy? Or is this actually dark matter?
But I guess are they unexplainable or why are they so mysterious? Isn't it just like, Oh, here's a photon with a lot of energy.
Well, we have an idea for what's in the center of the galaxy. We think that there's a black hole there, and we think there's a swirl of stuff around the black hole. We think there's a certain density of stars. We think there's gas and dust and all sorts of stuff, and we use that model to predict what we should see from the center of the galaxy, and we see all that stuff, plus we see more. We see another spectrum of photons that have sort of like a different shape, like they emit light in different patterns. They're distributed around the center of the galaxy, and their brightness and their wavelengths can't be explained by any of the components that we do know about in the center of the galaxy. So it's definitely something new happening in the center of the galaxy emitting photons in a way that we can't otherwise explain, and it's consistent with some theories of dark matter.
All right, well, let's dig into that part of it. Then what's the connection between gravity and or what might be the connection between gravity and these flashes of light.
Yes, we've been seeing these flashes for a while, and there's been theories of dark matter for decades from the center of the galaxy, but people have been unsure about what it means because we don't understand what's going on at the center of the galaxy very well. So recently people came up with a really cool clever idea to combine gravitational information with these flashes of light. They say, look, if there is dark matter creating these flashes of light, we should be able to zero in on where these flashes come from and see if there's dark matter there. By seeing if there's a gravitational effect from that dark matter, Like if there's a blob of dark matter there that's extra dense and making these flashes, we should be able to see gravitational lensing from that dark matter blob to tell us, oh, that really is dark matter and not just some star going crazy.
Couldn't it just be a star going crazy, or maybe like a quasar or something like that.
Doesn't that seem more likely? But like, how do you know which is more likely?
Exactly we don't really understand what's there, so it is tricky. But the first thing they did is they tried to map all the gravitational lensing near the center of the galaxy and measure that by seeing, like how that distorts light from behind the galaxy, Like galaxies in the background far away, how is their light being distorted as it passes through the center of the galaxy. So that's the gravitational lensing measurement. They use that as a way to just see, like, are there pockets of extra gravity in the center of the galaxy. So they make a map of all this gravitational lensing and then they line that up with the map of these extra flashes from the center of the galaxy, and boom, boom, boom, they do line up. They line up very nicely. So there seems to be this correlation between where there's extra gravitational lensing and where there's more gamma ray flashes.
That's pretty interesting, although it could just be something else. I wonder, like, you know, where maybe where.
There's a lot of dark matter, there are also a lot of black holes exactly.
And everybody's very skeptical at first, right, you don't want to just like jump up and down and claim you discover dark matter. So then they try to explain this with other sources, you know, and one idea they have in this paper is that maybe it's blazars. Blazars are these super awesome galaxies that are generating a really powerful beam of light from the center of the galaxy. If you have a super massive black hole the core of a galaxy, then its magnetic field can funnel radiation up and down the north and south pole of that galaxy and produce a very powerful beam of light. And if that beam of light is pointed right at the Earth, the relativistic effects effectively they make the Earth a little shorter and pile up the light beams a little bit to make it extra bright. So these things are called blazars, and blazars could also explain these effects that we're seeing. And so in the paper they analyze like, well, is this just more blazars than we anticipated, or is it dark matter? And they can explain part of this effect with blazars, but there's a part of it that blazars cannot explain, and that is very nicely explained by dark matter. And so this is the challenge with studying dark matter. It's like, is what we're seeing dark matter or is it something else we don't understand because dark matter is still kind of a fuzzy idea.
M yeah, I mean we don't really know or have a clear idea what it's nature is. I feel like we're sort of like putting assumptions into of assumptions and top of assumptions and then checking that.
That is what we're doing. We're making hypotheses and we're checking them, and we're coming up with alternatives, like is this dark matter? Is this something else? What we know is that there is something new happening at the center of the galaxy. Something is generating these flashes of light that we do not understand. Maybe it's dark matter, Maybe it's some other new astrophysical source. Maybe it's some new process that's distorting light from behind the galaxy. We don't know. There's something new to be discovered there. We don't know if that lines up with these other mysteries we're seeing, like galactic rotation curves and the structure of the galaxy. So maybe this reveals something about dark matter, but maybe it reveals something about something else. Right, we're so clueless about the nature of the universe that we're trying to put these puzzle pieces together, but maybe they don't click. Maybe they're parts of different puzzles. It's just because we're at the beginning of understanding the universe that we're so clueless about how to put this all together.
Yeah, I guess it could be anything. Could be alien sending us emojis.
Maybe those high speed photons are their version of emojis exactly.
Maybe there are express.
Emojis, hopefully they're safer work.
This is a very difficult kind of science because the data is sparse. You just have like these flashes of life from the center of the galaxy, and we're very far away from the center, as you said. So I've been to like whole conferences dedicated just to this signal and understanding it, and people show completely different interpretations. They say, oh, I can explain all of it using some astrophysical source, and other people say, no, that's impossible, it can't describe it. And other people say, well, you forgot to account for all these uncertainties in that, and if you let these things float and consider other possible ways that these things get interact, then maybe it explains it. So we really are at the beginning days of understanding what it is we're seeing. But it's cool to see people try to line these things up, you know, gravity and flashes of light and other ways to try to get ideas for what dark matter might be.
All right, well, it sounds like it's still kind of an open question and a big mystery, but it is a pretty exciting idea to kind of like use one mystery to try to explain another mystery. It's like you're piling on the mysteries.
We are piling on the mysteries, and we're trying to line things up, you know, we're trying to come up with a coherent explanation for everything we see out there in the universe. It's not fair to have one story you tell in one case and a completely different story you tell in another case. Right, You need a single theory that explains everything we see, and that's really a challenge, especially because the data is not as good as we'd like it to be. We'd like to have these sensors at the center of the galaxy probing these things in detail with very fine spatial resolution, but instead we're limited to these like tiny cameras orbiting very far from the center of the galaxy trying to get a glimpse. Like, imagine you were trying to understand what was happening in Manhattan and all you had was like a camera in Indiana. It'd be pretty tricky.
Yeah, especially with all those buildings, be hard to kind of see what's going on between there. So it sounds like maybe we just need to send a pro to the center of the galaxy right, figure out what's there, you know, and just wait fifty thousand years or so.
Yeah, some poor graduate student is going to wait fifty thousand years to defend their thesis.
I'll get paid for that. I could get paid for that while I'm doing this podcast.
Sounds good.
I mean, as long as we're double dipping.
Are your check going to come from the sense of the galaxy because you might be waiting a while?
Yeah, non, I know they're going to come in a dollar bill emojis they magically transform into cash, into my Venmo or the new.
X app quantum cash entanglement.
There you go. I'll be rich and not rich at the same time.
I hope your finance system collapse.
All right, Well, another reminder that there are still humongous mysteries out there in the universe, and that there are scientists out there actively searching for the answers and searching for it, trying to find ways to study it and reveal to the rest of us what its true nature is.
So remember that dark matter is really a whole suite of ideas. It's not just like a blank check placeholder for things we don't understand. It's an empty chalkboard for us to fill in with details, and scientists are constantly coming up with ideas for what dark matter might be and then being creative about how we might discover it because we hope one day to fill that chalkboard in with something we really do understand.
Does that mean you have to call it dark matters or dark matter?
These are the types of questions that I get paid to think about from multiple people.
That sounds good. Sounds like you are qualified to answer.
That question, as are we all all right, Well, we hope you enjoyed that. Thanks for joining us, See you next time.
Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app Apple podcasts or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases. Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
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Go safely.
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