Daniel and Jorge Explain the UniverseDaniel and Jorge talk about where dark matter is in the Universe.
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Hey, Orgy, I think I may have lost something?
Well, oh, did you lose your mind again?
Of that happened a long time ago. This is something new?
Is it important?
It's actually kind of a big deal.
Is it like really small and easy to lose, like your keys or your wedding bed.
No, it's much more embarrassing because it's really enormous, like cosmically large, like most of the stuff in the universe.
And you can't find it.
I've been looking everywhere for it.
Well, I guess you know what to do.
Oh yeah, what's that?
You know? Give it a cool sounding name, and then ask the federal government to help you find it.
Do you think anybody would actually fall for that?
I think they already have. Hi am Jorham, a cartoonist and the creator of PhD comics.
Hi. I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I am actually paid to hunt for missing things.
Hmm. Interesting, You're like the Lost and Found of the Universe department.
That sounds very unglamorous. I like to think of myself more of the Sherlock Holmes of the universe.
I see you're not in like in the basement office with a little window that people go there and claim lost things.
That's right. I'm not sitting here surrounded by people's lost purses or socks.
Where do all the socks and the dryers go? That's what I want to know. We should have a whole episode about that.
They're orbiting the Earth in the ho zone layer.
Well that socks may they're all connected by wormholes? You think dryers, all that spinning and that heat, could that create a wormhole?
Keep running your dryer let us know.
But welcome to our podcast Daniel and Jorge Explain the Universe, a production of iHeartRadio.
Which we operate as a sort of lost and found of ideas about the universe. Everything we have found about the universe and all the ideas we have lost. Everything we do know about what's going on out there, the size of the universe, the number of dimensions of space, whether or not there are alien craters on other planets looking through their telescopes at us. Every deep and enormous question about the universe you might consider we talk about here on the podcast.
Yeah, because it is a huge universe full of amazing and wonderful things that seem to be screaming at us for us to find them. We get light from distant stars all the time at all times, radiation, cosmic rays, and it's all coming to us sort of one thing for us to find out and what's out there and to discover and learn about how it all works.
You make the universe sounds sort of cooperative, like it wants to play a role in our science and be helpful.
To need the universe right, I mean, it's constantly shouting at us, isn't it?
It is? But sometimes these clues are very, very subtle and frustratingly difficult to grasp. I feel like the universe is sort of playing cat and mouse with us, sometimes hiding all the best bits.
It's like a koi. Universe means like it wants us to find it, but it's not telling us everything that it can about itself.
Yeah, it doesn't just come right out and tell us it's deep nature. It sometimes seems to make sense, and then you dig deeper and discover oh my gosh, it's bonkers underneath. But there is a lot of fascinating information in those little hints that do arrive here on Earth.
Yeah, and it's pretty amazing that, you know, if you think about it, that we're sitting on this little rock, floating through space in a corner of the galaxy, in a little corner of the universe, and somehow, from this light that we're getting from distant stars, we can somehow piece together the whole structure almost of the universe.
I like how you just say somehow, you know, it's like the information comes and dot dot dot, we know these things. You just like, somehow my entire career right there.
Yeah, somehow it goes to the lost and Found department in the basement and we get a little memo about what happened.
That's right. Somehow, because the government decides to fund the basic science research, we have uncovered some truths about the universe.
I guess what I mean by somehow thought dot dot is you know, mostly engineering and then some science involved.
Some happy collaboration between scientists and engineers.
But yeah, we are learning more and more about the universe. And there's a big question, especially a big question about the universe, about what it's made out of and how it's what structure it has out there in the galaxies and also in between galaxies. What does it all look like how is it all put together?
It's really one of the biggest questions in the universe, and one of the first questions you might ask, which is what is in the universe? What's the universe made out of? Where is all the stuff out there? And what kind of stuff is it? Really it's sort of shocking and amazing that we don't know the answer to that very basic question about the nature of our own reality.
Yeah, because the universe sort of pulled the fast one on us, right like it did this big reveal twist somewhere a few decades ago where we thought we knew what the universe was made out of. It was stars and matter and atoms and things like that, But suddenly we learned that that was only like five percent of it. You know. It's kind of like when you're watching a show and Sony you realize that you still have like ninety five episodes ago.
It's sort of like the universe was wearing a mask and then somebody pulled it off and we discovered, oh my gosh, there's a lot more to it. So much for the universe being helpful and revealing, right.
Well, I think it just wants you to keep watching. I guess it's trying to parse out the information, trying to keep it interesting.
Maybe the showrunners of the Universe are just trying to dole out the reveals a little bit at a time.
Show runners. Huh, so you're a multi theist.
I think it's got to be a committee. I mean, whoo, there's so much to do.
Oh that's not a flattering, I guess description of you. It looks like it was made by a committee of the universe. Looks like it had too many writers.
Well, it does sometimes seem inconsistent, you know.
Well, it is sort of a mysterious question. What is the universe made out of? And as we've learned in the last few decades, it's not made out of just stars and atoms and elements. It's mostly made out of other things, namely dark matter and dark energy.
The kind of stuff that makes up you and me and hamsters and ice cream and baloney sandwiches is actually quite unusual in the universe. Most of the stuff that's out there is not stars and gas and dust and all the visible stuff. It's something else, something dark, something we've only recently discovered exists out there.
Yeah, and so dark energy is this sort of the phenomenon where the universe is expanding faster and faster every second. But dark matter is especially sort of weird and concerning because it's matter, it's stuff. It's like exerting gravity. You can feel it, but we can't see it, which means we don't know where it is exactly.
We know that dark matter is some kind of stuff. There's something out there exerting gravity on the rest of the universe, and we can sort of tell that it exists, but we don't know what it is. And we've done a lot of podcasts digging into what it might be. Is it axions, is it black holes? Is it whimps? Is it something else? Something weird? And they are even more crazy ideas we haven't yet covered.
It's sort of like we know there's an elephant in the room or in the galaxy or the universe, but we don't know what this elephant is doing, or you know, what is it like striking a pose? Is it jogging? Is it sleeping. It's kind of a big mystery, like we know it's there, but we sort of don't know what it's doing or what structure it has.
Yeah, you can ask so many fascinating questions. So far, we've mostly focused on what is dark matter? Is it this kind of particle? Is it that kind of particle? But equally interesting without even knowing what it's made out of, is just wondering like where is the dark matter? Is it here with us in this room? Is it out there in deep space? Does it form planets and other kinds of structure? Is it smoothly spread out throughout the universe? Where in the universe is all of this stuff?
So today on the podcast, we'll be asking the question, how do we know where dark matter is? It sounds like you lost you lost it again. Then I feel like we said of knew where it was, but now we don't know where it is.
It is a really important question because knowing where the dark matter is can help us get clues about what it might be, because certain kinds of dark matter might clump up in different ways, and other kinds of dark matter might not.
Oh yeah, is it chunky or smooth? I guess it's a big question about the peanut butter dark matter of the universe.
Well, I hope the showrunners don't disagree about that, because then you might get chunky parts of the universe and creamy parts of the universe.
You're not a creamy peanut butter fan.
It's all about the chunks, man, It's all about the chunks.
Well, to each their own may there's a different flavor of dark matter.
For every tape Nutella there you go. Nutella is the best flavor of dark flavor.
And everyone gets a heart attack from being surrounded by all this saturated fat. But yeah, it's a big question. Where is dark matter and sort of like what structure it has in the universe. Is it sort of smooth out there like a big cloud or is it a chunky? Is it in like strands? Is it in clumps? What's it doing out there?
And this is something that scientists are eager to figure out because they just want to know where all of this stuff is. They're trying to develop a picture of the universe both visible and invisible, and of course the invisible stuff much harder to see, but since there's much more of it than there is the visible stuff, it's a very important question.
So, as usual, we were wondering how many people out there had thought about this question of the location and structure of dark matter. So Daniel went out there to ask people on the internet. How do we know where dark matter is?
So, if you'd like to put your brain to the test and answer questions that leading physicists don't know the answer to, please write to me two questions at Danielandjorge dot com.
Here's what people had to say.
Well, dark matter reacts with gravity, so by looking out into the universe we can sort of detect where the dark matter is because of its effect on gravity.
I know that it's biogravity.
Is the gravitational lensing that makes it possible to have like some kind of ideas where it could be dark matter in the universe. The distortion of light suggests that there are more matter out there that we can actually see whenever it like bands.
Well, once again, I assume this has to do with the part with the examples and instances you're talking of these massive tubs of argon. If I'm not misticn that are deep underground.
We cannot see what dark matter itself is. Light travels straight through it, but we can see other planets and we can see light being distorted by the gravitational effects of what dark matter is itself.
I'd say we know where the dark matter is because we can see the gravitational force it exerts on the visible matter around it. Next to that, it also bends light from distant galaxies when it comes towards us. So I'd say gravity is the usual suspect for us knowing where dark matter is.
Who is DM?
I think I've heard that dark matter is everywhere, that it just kind of permeates the universe, so all over the place.
So my guess is for what we know dark matter is, or how we know it is, if it gives off gravitational waves because it interacts with stuff through gravity, then we would use something that detects gravitational waves to see either how far away it is or where it is in respect to something else.
I think we know where dark matter is by looking for localized gravitational effects like lensing or relative velocities that aren't completely explained by matter we can observe in other ways.
All right, pretty interesting answer is a lot of people went with gravity, which is sort of true, right, Like that's how we initially discovered dark matter is through gravity.
Yeah, that's basically the only way we can sense dark matter, and so gravity is basically the answer. Gravity is the reason we know dark matter exists, and dark matter is basically an explanation for otherwise unexplainable gravity. So yeah, gravity is basically our portal into the dark universe.
And I like this person who said who is dark matter? It's like who is new phone?
I think that's because in the question I wrote DM instead of dark matter, assuming that everybody would know what DM meant.
Right, who doesn't know what DM is?
Of sliding into some of these dms with that one, I guess.
You are being a physicist using acronyms on people who had no idea what those acronyms are. But yeah, a lot of people seem to sort of have a basic idea that it's through gravity. But the picture is a little bit more complicated than that, right, I mean, we sort of know that it's there because of gravity, but instead of finding out exactly where it is or whether it clumps or or smooth, that's much harder because we can't see it right exactly.
We can't see it in the way that we can see the other kinds of matter. It doesn't give off light, it doesn't reflect light. Remember that dark matter is a bit of a confusing name because dark matter is not actually dark. It's transparent. It's invisible. It's not like a cloud of dark matter between you and another star would block your view of that star. If that were true, would be much easier to see dark matter than it is today. Instead, light passes right through dark matter.
So give us, maybe start us off with a refresher of dark matter, or DM as the physicists call it. You know, what do we know about it?
Well, we know that dark matter is about twenty five percent of the energy budget of the universe. That means if you take like a cubic light year of space, or any volume of space, then twenty five percent of the energy in that space is devoted to the mass of dark matter, whereas five percent of the energy budget of any chunk of space on average, you know, averaging over big distances, is devoted to making things like stars and galaxies and dust and giraftes and all that kind of normal matter made out of atoms. And that means that there's a huge amount of dark matter that a galaxy, for example, is mostly dark matter. That the universe, the stuff in the universe, the matter, the physical form of the universe, is mostly dark matter. So we've been studying the universe for thousands of years looking up at the sky wondering how things work, and only recently have we discovered that we've been missing most of it. So that's pretty exciting. And we know that dark matter is not made out of atoms, not made out of the kind of stuff that you and I are made out of. If it were, then it would probably interact with light. And we can also do some careful accounting from the very beginning of the universe, where we know something about how much material there was to make atoms, we can kind of account for where all of that went. So we're pretty sure. We're almost certain that dark matter is some kind of matter that doesn't give off light or reflect light. It must be made out of something else. And we know that it doesn't move very fast. We call it cold dark matter because if it did, it would spread out much more throughout the universe.
Yeah, and it's kind of interesting because I think I almost feel like, in a way, physicists called this thing or named it a little bit too early, you know what I mean, Like we gave it a name like dark matter, maybe a little too early, Like maybe you should have kept going and say that you know, twenty five percent of the universe is just something that we don't understand, or something that is not like the rest of the stuff in the universe.
Well, that doesn't work as well in grand proposals as a nice ZINGI phrase like dark matter.
But I yeah, but I know, But I guess the point is that really we don't know that much about it. I mean, we sort of know its presence, or at least we know it's the effect on the rest of the universe, but we don't even know if it's matter.
Right, Well, we know that it generates gravity, which suggests that its curves space according to general relativity, and so either our understanding of how space curves is wrong, or it's some new kind of energy in matter that does curve space. And it's possible that we don't understand gravity. I mean, it's certain that we don't have complete understanding of gravity. There are alternative ideas to explain dark matter using variations on gravitational theory, but none of them can really explain everything that we see. And so you're right that we're not one hundred percent certain that it's matter. But it's the simplest explanation that fits all of the data. A new kind of particle that only interacts gravitationally explains basically everything that we see out there in the universe, from the ripples in the earliest light to the structure of the universe today to the rotations of galaxies. So we're not certain, but it's the best candidate.
Maybe should have called it dark probably matter or dark most likely matter DMML DMLM. But we, as you said, so far, we only know about it because of its gravitational effects, right, But we sort of know quite a few things about sort of generally where it is.
We do have a good idea of where it might be because of its gravitational effects. Right, it is invisible, It doesn't give off light or interact with any other kind of force. But gravity is local. Right, if you are close to something, it tugs on you more strongly than if you're far from something. So if you're measuring the gravity of an object, if you can only tell that something that's there because of its gravity, you can get an idea for where it is based on what it tugs on. For example, we can see the black holes are there because of the way the stars move around them without actually directly seeing black holes, and so gravity definitely can give you a picture as to where things are in the universe, and we have a rough idea for where dark matter is. We think that dark matter is mostly lined up with the normal matter. That where you see a galaxy is where there's a huge clump of dark matter. So every galaxy, we think, for example, is embedded in a huge cloud. We call it a dark matter halo for every galaxy.
Yeah, it's like where you see regular stars and planets, you see dark matter. Or it's almost like the opposite, Right, it's like where you see dark matter is where all the stars and planets formed in.
A way, Yes, stars are more like the tracers for the rest of stuff. Right, dark matter leads the way. There's more dark matter than everything else, and so it's actually like where the dark matter started clumping is where the normal matter fell into it because of its gravity and then formed galaxies and stars and all kinds of stuff that we can see. So you know how when the military is fighting at night, they shoot bullets and then occasionally, like one out of every thousand bullets is a tracer. It's like glows, so they can see where they're shooting. Stars are sort of like that. They follow the dark matter, and they give us a clue as to where that dark matter is. And that's why we think that most galaxies are embedded in this cloud, this halo of sort of spherical, sort of a little bit elliptical dark matter that goes well beyond actually where the stars are.
Yeah. I've heard this sort of analogy that regular matter like planets and stars is sort of like the sprinkling on the icing of a cupcake. Like not just like in terms of our relative importance to that and the size of the universe, but also kind of like you know, sprinkles stick to the icing in a cupcake. You know, you can't sort of have sprinkles anywhere else. They're like, you know, the sprinkles sort of tell you where the icing is.
Yeah, the sprinkles are sort of the stars, and the icing is sort of dark matter, and the cupcake itself is dark energy. That gives you sort of a sense of the relative fractions of the energy budget of the universe.
Yeah, so I guess we are just the hanger ons of the universe. We're just hanging onto dark matter. And so let's get into a little bit more detail about what we know about this halo around galaxies and also how we know where it is. But first, let's take out quick break.
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All right, we're talking about dark matter and where exactly it is because I guess we know it's there, but if physicists can't find it.
It's pretty tricky to nail down an individual piece of dark matter, for example, because it only interacts gravitationally. Imagine trying to find like an invisible piece of sand in your room. How would you detect it? It's gravity is essentially nothing because gravity is a really really weak force. All the other forces of electromagnetism, even the weak force is much stronger than gravity, So in order to detect something through gravity, you need to have a huge force. You need to have like a planet sized force, Solar system sized force. Because gravity is so weak, so when we use gravity to look for dark matter, we can only sort of tell the large scale structure. It's very difficult to get a fine grained picture of where things are.
But even though it's very weak gravitation, and we do have kind of a pretty good idea of what shape it has in the galaxy, right like this halo, it's not just like a blob. It has some sort of shape to it.
That's right. It tends to be denser at the core and thinner further out, much like the visible matter in the galaxy. And we can tell where it is because it has an effect on how the stars spin. Right, Like the old familiar story is that we know that dark matter is there because we see the speed of stars is way too high. If dark matter wasn't there, then if the galaxy was spinning this quickly, it should be throwing its stars out into interstellar space. It needs more gravity something out there to hold those stars in place. For the galaxy to spin this fast. That's the old story that just tells us that dark matter is there. But we can get much more fine grained information. We can tell where in the galaxy that dark matter is by measuring the velocity of stars at different points as you move closer in or further out from the center of the galaxy.
Right, Because I guess what you're saying is that if the dark matter was all clumped together in the very very center of the galaxy, the stars will move differently than if it was more spread out throughout the whole galaxy. Right.
Yeah, If you are a star moving through the galaxy, then the thing that determines to her speed is how much stuff there is closer to the center of the galaxy than you are. You're not sensitive to anything that's further out than you. It's sort of like if you dig into the Earth, then everything that's further out from you, that's above you doesn't affect your gravity at all because it all cancels out. It's only stuff that's closer to the center of the Earth than you are. So it's the same for a star. The star's speed basically tells you how much stuff there is between it and the center of the galaxy. So as you look at the velocity of the star as you move further out from the center, it gives you a picture for where that dark matter has to be to explain that velocity. If dark matter was all clumped together at the very center of the galaxy, then stars, you know, halfway out from the disc would be moving faster because there'd be a stronger force from all that gravity. Instead, if it's spread out really really far, then some of that stuff is outside those stars and it doesn't affect them. It doesn't pull them towards the center, it doesn't speed them up as much.
Yeah, I guess it's sort of like, if you're in the middle of a cloud of dark matter, you're not going to feel its gravitation effects so much because it's pulling you in all directions, Whereas if you're really really far away from it, the whole blob, then you are going to feel sort of its entire gravity. Yeah, And so that's how we know that it's more dense in the middle. And that's kind of important, right, It.
Is important, and it's not something that we really fully understand, Like if you do simulations and you say we think we understand how galaxies might have formed, and how this halo formed and all the dark matter swirls together to make this well that forms the galaxy, And then we predict a certain shape for that dark matter density. We predict it to be sort of like peaky near the center, that like most of the dark matter should be right at the center, and then it should fall off kind of quickly. But what we observe when we go out there and we look at the dark matter see where it actually is based on these rotation curves, is that it's not as peaky near the center. It's more like a broad flat core, like a big blob of dark matter. It's not as like pointed at the center. So that's a current mystery we don't really understand. Dark matter doesn't seem to be as clumped towards the center as we thought.
Now, I guess part of it is that, you know, a lot of people sort of wonder like, if there is that much dark matter out there, why doesn't it just collapse into a dark matter black hole. But we've talked about before how dark matter basically is not sticky with itself, like it doesn't feel besides gravity, it doesn't feel any other force. That would make it stick together, like our items have the electromagnetic force to make them stick, but dark matter it doesn't appear to have something like that.
And that force is important if you're going to fall into the black hole, because you have to have some way to lose your angular momentum. Dark matter, like everything else, is spinning and swirling. And the reason that things don't fall into a black hole is because they are swirling around it, the way the Earth is orbiting the Sun and not falling into it. For the Earth to fall into the Sun, it would have to somehow lose its velocity, it would have to bump into something, it'd have to get slowed down. That only happens if there's some sort of like sticky force that can do that. So for dark matter, that's really hard because it passes right through itself, it passes right through normal matter. It's very hard for it to lose its speed or its angular momentum. So that's why this halo of dark matter is bigger than the visible galaxy because dark matter actually finds it harder to collapse, harder to fall into black holes.
Right, Yeah, And so it's been this big diffuse and it has this interesting shape. So then how else can we sort of know about its structure besides its sort of general blobby shape.
Well, one of the listeners got it right, thinking about how dark matter distorts the path of light. We can tell when there's a big blob of dark matter between us and something else because it acts like a lens in the sky. Remember that dark matter, even though it's invisible and light can pass through it, it does change the shape of space. And that means a space can act like a lens, and so light will pass through it, but it will get bent on the way. And so if there's a big blob of dark matter between us and a distant galaxy, for example, it will change the shape of that galaxy distorted, just as if there was a lens there. So we can use that to try to get ideas for where dark matter might be interesting.
So dark matter does seem to clump within our galaxy, Is that what you're saying? Like, maybe within our galaxy there are spots where dark matter seems to be denser than others.
That's hard to tell because this kind of gravitational effect is kind of rare. Like you need a clear background galaxy, and then you need a blob of dark matter right in front of it has to be like perfectly lined up, so it's tough to use this. They call this strong gravitational lens to get a clear picture for where the dark matter is, because we don't have really enough examples, so it's not a great way to tell where the dark matter is. A better way to tell if dark matter clumps up is to look for its effect on stars. So not just like the velocity of stars as they weave around the center of the galaxy, but their motion in other directions, like if there is a big clump of dark matter, and especially dense blob of dark matter, it will affect how stars are moving around it. It will change the motion of those stars. It'll attract them, it will deflect them.
I see. So if you look sort of look at the overall motion of all the stars in the galaxy, if you see that there are you know, sort of wiggles here and there, or little you know, eddies or little clumps of stars forming, then you know that there's something else there and that it's not the dark matter is not perfectly smooth.
Yes, And we recently launched a satellite called Gaya, which is mapping the galaxy in four dimensions. It measures the position, the location of all these stars and their velocity. I think this incredible map. It has like a billion stars with their position and their velocity map, and we can use this to look for deviations where like, well, if dark matter was perfectly smooth, what would we expect all these stars to be doing. And are any stars doing anything weird? And if they are, we can sort of back that out and figure out where dark matter has to be to explain any weird patterns of the star motion.
Right, Because I guess if dark matter was perfectly smooth peanut butter, like this kind of smooth a cloud, then you would expect all the stars to be basically moving along as if it was in a lazy river, right, Like everyone sort of moving of the same. You know, nobody would be going much faster than or slower than any of the other stars.
Yeah, at the same radius.
Right.
We expect as you go out, as you change your radius relative to the center of the galaxy, these things will change, just like they do in our solar system. Pluto is not moving around the Sun as fast as Jupiter, which is not moving as fast as Mercury, because as you go further out, the gravitational force is weaker, but you'd expect at the same radius to basically be having the same motion. And so if you see deviations, then you know dark matter is there, and we do sort of expect there to be clumps. We expect that the galaxy, for example, has lots of other galaxies inside of it that it has absorbed. We think that the history of our galaxy includes lots of collisions to form the Milky Way, and you would suspect that those galaxies might still have like their dark matter halos embedded within.
Ours, because I guess you would expect darmatter to be clumpy because regular matter is clumpy, so in a way like would in our regular matter also sort of catalyze or trigger dark matter to clump.
Our matter is clumpy, but that's because it's sticky, right, It can form these blobs, so when gravity pulls it together, it sticks together and then that accumulates forms this runaway effect, whereas dark matter is not sticky, and so dark matter halos can pass right through each other without very much distortion. The other question is a cool one, like do stars form clumps of dark matter? And this is something people have studied. They've tried to look to see if there's like an intense blob of dark matter inside the Sun, for example. But remember that there's much more dark matter than there is normal matter, and so dark matter sort of wins the gravitational battles. You would expect it to mostly go the other direction, that dark matter would influence the pattern of normal matter rather than vice versa. But yet it is a tug of warm.
Interesting, you're saying dark matter is ignoring us. Basically, it's ghosting us.
It mostly can. But back to this question of like following the stars, there are some really cool things that we do see inside our galaxy. We can see the remnants of other galaxies that the Milky Way has eaten, little mini galaxies. We call these dwarf galaxies, and some of these are really really interesting because they're super high in dark matter. Like our galaxy has a lot of dark matter, but some of these dwarf galaxies are almost entirely dark matter, and we can tell that they're there because we see stars orbiting these invisible dark matter hates. So there's sort of like many clumps of dark matter within our dark matter halo.
Interesting so it is clumpy, but maybe because we've added the clumps.
Kind of yeah, because we've formed our big halo from a bunch of clumps. So we think these clumps formed initially, each one of these its own galaxy, and then galaxies eventually do merge and collide and form bigger galaxies. And sometimes those dark matter halos don't necessarily spread out and just join like the original creamy blob of the Milky Way. They sort of stay there as chunks, and you can tell they're because the stars swirling around those little dwarf galaxies.
Well, here's the question. Do you think that dark matter in our galaxy is spinning also with the rest of the stars and galaxies, or does it just standing still.
It's almost certainly spinning. That's the reason that it doesn't collapse into the black hole in the center. If it was standing still, then that gravity from that black hole would just suck it up. So it's almost certain that it's rotating. That we can't measure that directly, right, we haven't seen that, but we're fairly certain that it has to be otherwise it would have collapsed.
So through strong gravitational lensing, we can tell that there are some clumps out there, and through some of these absorbed galaxies, we know there are clumps out there, but what else do we know about this clumpiness.
We can also try to measure the clumpiness by looking at the effect of our gravity on things near the galaxy. So sometimes these mini galaxies or these globular clusters get sucked inside the galaxy. Sometimes they're in orbit around the galaxy. So for example, the large Madeleinic cloud is a blob of stuff that's sort of like a satellite galaxy of the Milky Way. And often these galaxies get torn apart, they don't hold themselves together. They turn into these streams. So around the galaxy there are these things called stellar streams, which are these lines of stars moving in a loop sort of around the galaxies. But it's sort of like the galaxy has rings of stars.
Mm interesting like accidents almost.
Yeah, and they're sort of swooped around the galaxy. And those are very sensitive to the distribution of dark matter. So if, for example, the dark matter halo is clumpy, it'll affect how those stars get pulled apart and whether they're like gaps in those streams. So there are people right now studying these stellar streams. They're like probes of that dark matter halo to look to see if there are clumps in the dark matter halo, or to see if it's like perfectly spherical or kind of elliptical. So these are very nice ways to tell how much dark matter they're passing through and how clumpy it is.
Interesting, and so that's one way to sort of know the clumpiness of dark matter, and what have we learned from all of these different ways.
We don't have a great picture of where dark matter is in the galaxy. People often write in and ask, like where is the dark matter? Can we see like planets of dark matter or that kind of stuff. Really, we're not very sensitive to the details. We know that the Milky Way has a big blob of dark matter that it's probably elliptical, you know, it's not totally spherical. We can see some clumps where these faint dwarf galaxies were absorbed, but we don't have a great sense for the structure of the dark matter. It's mostly smooth, but we can't see things smaller than like, you know, ten to the six stars.
I see like the smallest clump. We can sort of tell, right now is ten to the six billions of kilometers.
Maybe ten to the six solar masses equivalents of dark matter is like the smallest chunk of thing we can tell.
WHOA, that's like our best resolution of our picture of dark matter in the galaxy. It's like a pixel to side a million sons.
Yeah, so we're not very sensitive, and that's just because it's mostly smooth. There aren't a lot of features to see, we think, and because we're not very sensitive to it. Again, gravity is very weak and it's our only way of interacting with it, which makes it kind of frustrating.
All right, well, it sounds like it's still yet to be discovered. Who knows. Maybe it's forming giant dark matter squirrels or bananas or girasses out there, but we just can't see it with our current resolution. And so let's get a little bit into what the overall picture of dark matter then is in the universe, and also what's happening between galaxies. But first, let's take another quick break.
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All right, Daniel has lost his dark matter and his mind. Apparently, did you lose your mind looking for the dark matter?
I did. It's driving me crazy. Where are you?
It's avoiding you, to ghosting you. It is a little bit ghosting us as humanity because we know it's there, but it doesn't seem to want to make its presence known to us. In detail, we sort of have a big picture of it that it's in a big clump around the galaxy, mostly concentrated in the middle. We see some clumps out there, but we don't know the exact structure of dark matter, but we sort of do sort of know it's density out there, right, We have some figures for its general density.
Yeah, And it's quite interesting because we think that on average over the universe, dark matter is like eighty percent of the matter of the universe, but in our neighborhood it's actually a little bit different. The Milky Way, for example, we think is ninety five percent dark matter. So our whole galaxy is kind of badly named. It should be called like the dark Way or something.
The chocolate milk Galaxy, the darkly Milky Way.
The dark Chocolate Way or something. So we're like ninety five percent dark matter, which means if we have, like you know, the equivalent of ninety billion times the mass of the Sun in terms of stars and gas and all that kind of stuff, that means that there's like two trillion times the mass of the Sun in dark matter. There's just so much more of it. It's like twenty times as much dark matter in our galaxy as normal matter, which is a bigger ratio than in the rest of the universe.
Well, that's true for regular matter too, right, Like our Milky Way has a higher density of regular matter than the rest of the universe or other parts of the universe.
Right, Yeah, that's true. But on average galaxies have about eighty percent dark matter, and our galaxy is ninety five percent, So there's a big variation galaxy the galaxy and how much dark matter there is.
Oh, you mean our galaxy has more than other galaxies?
Absolutely, Yeah, we are a darker galaxy than.
Most interesting we're more mysterious, I guess.
Yeah. And there's some galaxies out there that are overwhelmingly dark matter, like ninety nine points something percent. And then there are some galaxies that have very little dark matter. We think that might be evidence of collisions where dark matter gets separated from the normal matter. All sorts of crazy stuff. These things tell you the crazy cosmic history of all of these objects.
Interesting. And again you can tell by when you look at these galaxies out there. You can tell that they're holding on together more than they should by the number of stars or the brightness of them. Right.
Yeah, you can tell when a galaxy is overwhelmingly dark matter because its stars are moving super duper fast compared to how bright they are. And so we can see these faint dwarf galaxies, for example, just have a handful of stars, but they're whizzing around in a circle and there's not nearly enough gravity to hold them in place. Just from the stuff that we can see, so it's pretty cool.
But it's so far away. How do you know it's not just like filled with the black holes or something, or rocks dark rocks.
Because we can see light passing through it, right, we can see through it to something else behind it. If there was a black hole there, it would absorb the light. If it was just like a huge death star or something cloaked, then it would absorb that light. So we see it as invisible, not as dark, all.
Right, So then it's sort of denser in our galaxy. What about in our more immediate neighborhood or like a star solar system also extra dark mattery and.
In our neighborhood. Remember that while the Milky Way is ninety five percent dark matter that is spread out throughout the stars, we think, so normal matter clumps up a lot more than dark matter, which means that there isn't that much dark matter in any like cubic light year of space. So in a cubic light year of space, there's less than a one quarter of one to one thousands of the mass of the Sun in a cubic light year of space. That's like a quarter of the mass of Jupiter in a cubic light year of space. That's how much dark matter there is.
If you took Jupiter and spread it over billions of miles, right.
It wouldn't be very much. And you know, if you zoom in, for example, into like a cubic meter, that's like ten to the minus twenty two grams of dark matter in a cubic meter. So you know, if you look at the space around you in your office, for example, then there's just like a super tiny amount of dark matter, almost hard to measure, but some of it's there are few particles. And if you zoom out to like the whole volume of the Earth, there's less than a kilogram of dark matter in the volume of the Earth.
Again, these are sort of approximations, right, because you told me earlier that our ability to resolve or a resolution of dark matter is pretty bad. So how do we know that there isn't sort of clump of dark matter just around us right now?
We don't know. Absolutely, We do not know. We are not sensitive to these things, so it could be a lot clumpier than we think these numbers are, assuming that dark matter is mostly smoothly spread out throughout the galaxy according to the distribution that we've seen from the radius, but we absolutely cannot tell if there's like a huge blob of dark matter that we're sitting in, or if there's almost no dark matter in our neighborhood. And remember that we have experiments underground looking for dark matter particles. They're basically trying to measure how the Earth is moving through this dark matter wind, and they haven't seen anything. And one of my favorite explanations is like, well, maybe we're just happening to be sitting in a bubble that has almost no dark matter in it, which would make it impossible for us to detect that dark matter wind. We just don't know.
It's ghosting us an avoiding us physically at the same time. But it's kind of interesting because I think we talked about earlier how like in the volume of the Earth, there's about one squirrels worth full of dark matter. Like that's not a lot, right, and the whole Earth is pretty big, but you only have one squirrel full of dark matter.
And that's why we can't ever detect it gravitationally. You know, we're literally looking for a squirrel that's hiding inside the Earth, and that's pretty hard to tell the difference. We can't measure the number of squirrels on Earth using ground you'd go nuts.
But yeah, so let's talk about then, now sort of dark matter between galaxies, because you know, there's a lot of space between galaxies and we sort of have a pretty good idea of the structure of the universe, you know, the galaxy clusters and superclusters. Is does dark matter also follow these clusters?
We think that mostly does, And again we think it's sort of the opposite that normal matter follows the path of dark matter. But it's much harder to see the things between galaxies because there's much less light there and there's much less visible objects. Like mostly we have seen where dark matter is within our galaxy by following the path of stars their rotation, their wiggles, their distortions, all that kind of stuff. Well, there are in stars like tracers to tell us where things are between galaxies, so it's much trickier.
Yeah, I guess you can sort of extrapolate right where we can see a little bit around this, then you sort of assume that that's what's happening maybe in the rest of the universe sort of.
But we know that the galaxies are very different from the rest of the universe, Like we know that there's a huge gravitational well that we are sitting in. That's why there's a galaxy right here. There's a big blob of dark matter. What does it look like between our galaxy and Andromeda? You know, are there strands of dark matter? How quickly does it peede out? Are there blobs of dark matter out there without any stars in them at all? And so one way we can try to figure that out is to look at how light from distant galaxies is distorted as it passes through that space.
I see. You can do the gravitational lensing, but with galaxies and look for blobs in between galaxy. But then these blobs would got to be humongous, right.
Those blobs would have to be humongous. And in this case we use a slightly different technique than we do for looking at like one specific blob before. What we were doing is called strong lensing. And that's like, I want to have a blob of dark matter right between me and another galaxy, so I can see like a massive distortion. You can see one galaxy how it's distorted, and you can use that to measure the mass of stuff between you and other galaxies. If instead you think that dark matter's sort of spread out so it doesn't really distort any individual photon that much. You can do something called weak gravitational lensing where you look at lots of galaxies and you look for lots of very small distortions and you sort of add them up statistically to get a map for where the dark matter might be and where it might not be. So you see sort of like fewer generalized distortions over here and more generalized distortions over there. It can tell you sort of where the dark matter is denser and where it's less dense.
Interesting, you're sort of looking for sort of wiggles in the overall picture. But how would you know that is dark matter? Would that be changing or are you assuming that it changes as our view of the universe changes.
Well, we think we know what galaxies should look like when they're not distorted, and so we compare how galaxies look to ideas of how a galaxy should look when it's not distorted and how it should look when it's slightly distorted by dark matter. And so we can use that to estimate like how much distortion galaxies have. But it's very very weak, you know, it's hard to tell the difference in a galaxy that's undistorted and slightly distorted. And that's why we need like thousands of galaxies to add this up statistically to get a sense for where the dark matter is interesting.
And this is like an ongoing thing, right, Like there's people looking for these ripples in our view of the universe.
Yeah, this is recent. Actually there's a program using a huge telescope with a massive camera five hundred and seventy megapixels. It's called the Dark Energy Survey, and this camera is basically built just to do this, just to look at all the galaxies and build a huge map. And they've studied one hundred million galaxies out there, like think about all the crazy stars and planets and everything that's out there, one hundred million of those. And they have built a map of where they think the dark matter is between galaxies using this weak lensing idea.
Because I guess we can tell how old they are the galaxies, right, and how far away they are from us, not just in the night sky, and so we can build this three D map right exactly.
We know where galaxies are because we can look at like Type one A supernova within them, we can measure their brightness, and we can tell how far away they are based on how bright they appear to be here on Earth. So we have this incredible three D map of all the galaxies, and then this thing is taking careful pictures of them to try to estimate how much each one is distorted, and then it's comparing that to our idea for where we think the dark matter should be. We have an idea for where we think dark matter should be based on where all the galaxies are. We can sort of back that up to the early universe and say where would the dark mind have to have been in order to make these galaxies end up here and that galaxy end up there to sort of create the large scale structure that we see, because we think that mostly where galaxies ended up depends on where dark matter was. So we have like a simulation for where we think the dark matter should be on our idea of how it all works, and then we go out and measure it and build a real map of where the dark matter is, and then we can compare the two and that's when the fund starts.
WHOA, So, why have we phound? Do they match? Or are they very different?
They mostly match? Like it mostly makes sense the dark matter is mostly where we expect, but there are some deviations. It looks like dark matter is sort of more spread out than we expected. Instead of being in these like thin strands between galaxies, it tends to be sometimes in places where you don't expect. It's like spread out and globbed out more than we expected, more than our simulations predict. It's just a few percent compared to our predictions, but it's an important deviation. It means that like, maybe something is going on with that dark matter, or maybe dark matter is made out of something weird we didn't understand, or maybe we've made a mistake in building our map of dark matter. But it's sort of like at the edge of science. These are very very recent results.
That dark matter just wants to be alone between the galaxy or maybe it got timed out. But I guess what's surprising is that there is dark matter between galaxies because there's not much matter out there, so why wouldn't this dark matter also accumulate regular matter.
There is actually a good bit of regular matter out there, it's just not glowing, like a huge amount of the atoms in the universe are actually between galaxies, and this intergalactic matter these streams of stuff, and so there's a good bit of stuff out there. There's just not enough density to form stars and planets and galaxies and all kinds of stuff. And so that's why it's not as visible because it doesn't glow interesting.
But then what makes some dark matter have a lot of bright matter in it, and.
The denser blobs of dark matter did form enough stuff to create galaxies and stars. The kind of strands we're talking about between galaxies is a smaller fraction of the amount of dark matter. The density is not there in order to create the gravitational well, you need to attract enough hydrogen to get stars to form. Pretty cool, so the galaxies still show you where like the densest blobs of dark matter are in the universe.
Like the dark matter also varies in density out there.
Absolutely, Yeah, all.
Right, well it sort of sounds like you sort of know where dark matter is, but maybe not to super high resolution where you can tell if it's super clumpy or super smooth. And also we have a pretty good picture of where it is in the whole universe.
Yeah, And scientists are working very hard to build more and more sensitive tools to try to use these little gravitational clues to build as accurate and as localized a map of dark matter as possible, because the better map of dark matter we get in our galaxy, the more we can study what it might be and how it came to be where it is.
Yeah. I guess the big question now is what are you going to do when you find it? Daniel retire. Yeah, like the dog that finally catches up the car and doesn't know what to do with it.
Oh, we'll find some other mystery to focus on.
You'll come up with another cool name darkest matter.
We always have the seventy percent of the universe called dark energy that we haven't even gotten started on.
Yeah, that's another huge mystery, huge hole in our view of the universe, and hopefully you won't lose your mind trying to find it.
I lost it here, all right.
Well, another awesome reminder that there's still a lot of universe out there to be discovered. You know, anyone listening to this could be the person that comes up with the next great idea or the next grade concept that makes sense of all this and helps us find these big mysteries in the universe.
If you're in tranced by the concept of building maps and discovering the way the world is and how everything looks. Remember, the biggest map of the most important stuff in the universe is still being drawn.
You can really lose yourself in that search for lost things.
Come join me in the asylum.
Or the basement Lost and Found Department. Thanks for joining us. We hope you enjoyed that. 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, town and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
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