Daniel and Jorge Explain the UniverseDaniel and Jorge discuss how dark matter shapes our Universe and whether it would be possible to have a galaxy without it.
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Hey Daniel, Most of the stuff in the universe is dark matter, isn't that right?
Yeah, it's about eighty percent of the matter in the universe.
And it's all around us, like right here with us immersed in dark matter.
Yeah, we are swimming in it.
And it's also inside of us, is it?
Yeah, it passes through us, it doesn't bounce off your skin.
Does that mean that we're partly made out of dark matter?
Oh? I guess. So that's kind of dark to think about.
So if I discover myself, I'm discovering dark matter.
Know thyself. Win a Nobel Prize.
Or at least a dark Nobel Prize. Technically all Nobel prizes have dark matter inside of them too.
Maybe they'll give you the cash in dark money.
Ooh, that got dark quick. Hi. I'm jeham and cartoonist and the author of Oliver's Great Big Universe.
Hi. I'm Daniel. I'm a particle physicist at CERN and a professor at UC Irvine. And I'll take your money dark or light.
Really you'll take dark money. Isn't that dangerous for a tenured professor?
No, that's what ten years for man. I can take money from Dunias Visuals.
I guess technically the university takes the money and you get a cut of it.
I never really had to worry about that. Like, nobody who got wealthy with dubious techniques ever offered me a slush fund, So I've never had to really grapple with that. Question.
Wait, how do you know? I mean, you've taken money. Have you done your due diligence? How deep did you go?
You know? Almost all my research money comes from the government, and we all know the various crimes that the government has committed.
Exactly all government money is dark money.
M do you believe taxes are theft? Is that where this is going?
Well, technically funding comes from taxpayers and there must be some sketchy taxpayers out there, so your work is tainted.
So you're saying drug dealers who are paying taxes and indirectly funding my research have made me complicit in their crimes.
That's right. Yes, assuming drug dealers pay taxes, which I guess they do if they're laundering money, I don't know. I feel like we need a whole podcast episode just to cover dark money.
Yeah, and then I suppose everybody's guilty.
That's right, We're all guilty of paying taxes, I guess. But anyways, welcome to our podcast. Daniel and Jorge Explain the Universe, a production of iHeartRadio.
In which we try not to overtax your brain while explaining all of the mysteries of the universe. We want to reveal the light universe and the dark universe, the visible and the invisible. We want to show you how the universe is not just what you see around you and experience day to day, but so much more, so much deeper, so many more mysteries waiting to be solved.
That's why we explore the universe, the dark corner it, the light corners, and the sketchy corners of it, because our understanding of the universe is still a little bit sketchy.
Like the Mafia corners of the universe.
No, like, not clearly drawn.
Oh I see, I thought we're gonna be talking about like the physics of New Jersey or something.
Yeah, there's the only some dark matters going on out there.
How do you eat all those rich cookies and not gain weight? Really, it's amazing.
Yeah, well, I think the secret is dark chocolate. It's lower and fat, isn't it? Is it?
Really?
I don't know, You're like, what, what?
What?
Why aren't I eating more dark chocolate.
I'm not really stopping myself from eating dark chocolate. But that wasn't the reason. I don't see dark chocolate is like diet chocolate.
It's chocolate light. Oh no, wait, it's chocolate dark.
It's dark chocolate exactly. But there are lots of fun questions out there in the universe, not just about how waste management organizations in New Jersey are getting their money and the calories that Dangel's eating. We're wondering about the biggest questions in the universe, like where is all the stuff? What is most of the universe made out of? But how has it shaped the night sky that we see today in the galaxy that we live in.
That's right, because what the universe is made out of is maybe one of the biggest questions we can ask about the universe. What is this whole place made out of? What are we made out of? Are we made out of? Dark matter? Can we exist without dark matter?
And we talk on this podcast a lot about dark matter. We have lots of episodes about what it is and where it is and how it works. And one thing we often stress is that dark matter is part of our galaxy, that most of our galaxy is actually dark matter that has played a big role in the formation of our galaxy. We wouldn't be here without it.
Yeah, we do talk a lot about dark matter. Daniel, what percentage of our episodes would you say we talk about dark matter, is the representative of the amount of dark matter in the universe? Are we ignoring dark matter in a way?
Is dark matter underrepresented matter on podcasts? That's a good question.
Yeah, well, especially our podcast, if we're trying to explain the universe and the universe is twenty seven percent dark matter.
You know, we might reach twenty seven percent of our podcast being about dark matter. It's been a lot of them, and I'm pretty sure we don't have two thirds of them about dark energy. Maybe we should.
Yeah, yeah, that's what everyone is asking for more dark energy episodes. But it is a pretty important part of the universe. It's a pretty important part of our existence because without dark matter, maybe galaxies would not have formed the same way that they formed. The Milky Way might not be the same way it is now.
But we're always tempted to over general life, to say that the way we live in our certain situation over here is the way the whole universe works. It's important to take a step back and to ask whether our way of life and our way of galaxying is the only way that it can be.
And so today, own the podcast, we'll be asking the question are there galaxies without dark matter? Would you call these light galaxies then, or galaxies light diet galaxies?
I call them tragic galaxies because they're probably galaxy where everybody only eats white chocolate or.
Milk chocolate some chocolate snobs. I also call that a.
Tragedy, also known as crimes against chocolate.
Are you saying Hershey's is dark money as well?
Her She's his garbage? Man?
Oh my gosh. And there goes our sponsorship deal with Hershe's. We were so close to funding this thing for the next twenty years, and you had to go and insult them.
Maybe you could hear my hesitation there. I'm being torn between being honest and truthful on a hard science podcast and pandering to our sponsors, and I just had to be honest about it. I passed on hershe'es. I'd rather have no chocolate than Hershe's.
That was pretty harsh to garbage. Oh my gosh. Well, here here's the thing. Like when you eat it. Let's say you eat like a chocolate cake at a restaurant, how do you know they didn't use Hershey's chocolate.
You can taste it, man, that's sour affront to chocolate. It's turned so many people off of chocolate when chocolate is this wonderful, amazing thing. Even milk chocolate can be high quality, can be amazing. What they make in Pennsylvania. O, man, it's a crime.
Now you're insulting the whole state of Pennsylvania.
Let's keep going. See how many people I can offend.
Yeah, yes, let's move. Let's insult the whole universe. Why don't you.
Fortunately, most of the universe is dark matter and therefore appreciates dark chocolate, and so we're good. We have the majority firmly on our side.
But doesn't Hershey's own some fancy brands like Scharfenberger or something like that.
I don't know, I hope not.
Well, maybe you've been eating Hershey's chocolates all this time. A plot twist. Anyways, we're talking about dark matter and galaxies and could there exist galaxies without dark matter? These would be like galaxies that don't have any dark matter in them or around them.
Yeah, exactly, just stars and gas and dust and black holes.
No dark matter. All right, Well, whether they are tragic or not. We were wondering how many people out there had wondered about this question. And if they have any ideas about the answer.
Thanks very much to everybody out there who answers these questions, whether or not they are supporters of Hershey's crimes against Chocolate. We really appreciate everything you do. If you would like to join this group, just write to me two questions at Danielandjorge dot com.
I feel like maybe there's a Hurshi's employee out there who listens to our podcast and it is now very, very sad.
There's an easy fix for that, find a new job or a new podcast. No, you got to go to the root of the problem.
That's right anyway. So think about it for a second. Do you think there can be galaxies without dark matter? Here's what people had to say.
I feel like there probably aren't, only because I know that dark matter, as far as I know, is distributed pretty much evenly throughout the universe. I think it tends to be clustered in galaxies, but I think it tends to be pretty uniform, So I would be surprised if there were galaxies without dark matter.
I'm going to say why not, because, like going back to an episode about uranium on uranus, there could be a tiny bit here and there in a galaxy. So I'm going to say I don't know why there wouldn't be dark matter in a galaxy. I'm also going to say I don't know why there would.
I don't believe there are galaxies without dark matter because dark matter is, in my understanding, a general term for unknown matter, which is this and makes up the overwhelming majority of the universe. Therefore, I think it is not possible for galaxies not to have at least some dark matter in them.
I'm not sure that we've observed any galaxies without dark matter, but I suppose anything could be possible in this crazy universe. It's also possible that all galaxies are without dark matter and we just don't understand gravity all right.
Most people are skeptical about this question.
Yeah, people have the idea that dark matter is everywhere. It's inescapable.
You can't get away from it, it seems in our question, although one person has kind of said why not. That's a good attitude to have.
Yeah, that's the whole attitude about physics, like, well, maybe everything is different from what we'veoughter. Maybe there's something really weird out there that could teach us something new about the universe.
I do feel like that is a guiding question in theoretical physics at least. Why not yes? Why not?
Sure? Maybe everything is just tiny cats at the quantum scale.
That's orry. Maybe everything's just made out of Hersy's charkga.
No no, I got to hop somewhere else in the multiverse.
If that's the case, all right, well let's get going before this podcast gets too dark. Daniel, give us the basics. What is dark matter? For those of us who haven't listened to the twenty seven percent of our podcast.
Episodes, dark matter is fascinating because we simultaneously know a lot about it and very little, Like we know that there's a lot of dark matter in the universe, and we know that it's matter. We know there's something out there that's creating gravity or curvature of space time, but that it's invisible. It doesn't glow, it doesn't give off light, it doesn't reflect light. We sense it only because we see its gravitational effects on stuff. It's curving space, which changes how things are moving through that space. We see galaxies rotating much faster than their gravity would be able to hold together if there wasn't also dark matter in them, holding them together. On the other hand, we don't know what stuff it is. We know that it has gravity, so it's matter, but we don't know it's some weird new kind of neutrino or a totally different kind of particle we've never seen before, or a thousand new kinds of particles, or something that's not even a particle. So we know a lot about it on the sort of cosmological scale, but very very little or almost nothing about it at the particle level. Right.
Dark matter is this kind of mysterious stuff in the universe that we kind of feel its presence. We can see its presence through gravity, but as you said, you can't see it because it doesn't interact with elechormagnetic light or maybe any of the other forces in the universe, and so you can't see it. And that's why you call it dark exactly.
And a bunch of listeners write in with the idea that maybe dark matter is matter in another universe that's somehow leaking in to hours. Remember that dark matter is creating gravity in our universe or changing the curvature of space in our universe, which means that it's in that space, right, it shares that space with us, which kind of means that it's in our universe, And so we really do know that dark matter is something in our universe that's changing the shape of space. We can only see sort of indirectly through gravity, which is really frustrating because gravity is the worst way to see things. It's so weak that it makes it very, very difficult the way.
Couldn't it bend our space and not be part of our space?
That's possible if you overthrow general relativity in our entire understanding of space time. General relativity says that matter tells space how to bend, and space tells matter how to move, and that means matter in our space. Like in general relativity, the curvature or space comes from the energy density in that space. So if you have some like parallel space overlaid on top of it, which can also bend that space, then it seems to me like it'd be part of our space, you know, sort of like the by definition. But yet you could augment or throw out general relativity replace it with something totally different. But the simpler idea is that it's just some kind of mass we can't see. That explains almost everything we see out there in the universe, So it's sort of the best going explanation. You can always make more baroque complicated explanations if you.
Like, Well, it kind of might as well be in another universe, right, because if it doesn't feel a lot of the same force as we don't, it's just kind of like ghostly matter that's kind of living on top of us. There might be beings made out of dark matter, right.
This is call the different sectors of the universe. If you have like two different sets of particles that don't interact at all except for through gravity, then we call those like the lights of the visible sector and the dark sector of the universe. And that's totally possible that you could have a whole complicated physics happening in the dark sector that we can't see now. Mostly we know that dark matter can't interact with itself. If it did interact with itself, it would form all sorts of complicated structure and do all sorts of interesting things. We think that dark matter is pretty spread out. However, there could be a little component dark dark matter. Dark matter may be lots of different kinds of things, and one little component of it might be more complicated and do complicated things like form life or ice cream, cones or cats or good chocolate, without violating what we've seen dark matter do, which is mostly spread out smoothly.
All right, Well, the question here today is can there be galaxies without dark better? Why is this even a question? Like, are most of the galaxies that we see out there? Do they all have dark matter?
Yeah? The galaxies that you see out there in the sky are like tracers. They're basically telling you where the dark matter is in the universe. Remember that dark matter is not something we can see, but it also dominates the universe. Our estimates are that four fifths of the matter in the universe is dark matter. So if you like spin the wheel and pick our random object in the universe, most likely you're going to get dark matter. It's like overwhelmingly dark matter. So when the universe is forming its structure and the gravity that determines like where things are going to be, it's mostly the gravity of dark matter that decides where things are going to clump together and where things are not going to clump together. Remember, the very early universe is mostly smooth, with a few little blobs that are denser than others. The gravity of those over dense pieces pull things together to form structure, and that's where you get like galaxies in one part of space and not galaxies in another part of space. Where you have galaxies is where you had more dark matter to pull that stuff together to form those galaxies.
Yeah, that's pretty wild to think that something we can't see, that is invisible to us basically kind of dictates the entire structure of the universe, right at least at the galaxy level. Does dark matter also dictate things like superclusters and beyond exactly?
And so you can imagine like these invisible wells, like dark matter's curving space, which Shepherd's the other kind of matter together. So every time you look up at the night sky and you see a galaxy, you should imagine there's an invisible blob of dark matter surrounding that galaxy. There's a whole halo that's created the conditions to form that galaxy. We run simulations, for example, of a universe without dark matter, and it doesn't form galaxies after fourteen billion years, so quite literally, we would not be alive without dark matter.
And now is the same true for things like superclusters and those giant bubbles of galaxies out there in the universe. Is that dictated by dark matter as well, or is that more of the quantum fluctuations of the universe.
Well, both, because there's this cosmic web that tells us where dark matter will be denser and where dark matter will not be dense. These filaments, in some places they overlap and you get these wells where things pool together. And so the whole cosmic web is dictated by dark matter and the light matter. The normal matter baryons and quarks and electrons just follows that. And so it's not like a supercluster has a super halo. A supercluster is made of galaxies connected together by these filaments.
So it also has filaments of dark matter.
Absolutely, Yes, there are filaments of dark matter connecting these halos of around each galaxy.
Hmmm.
Interesting. All right, Well, let's dig into the question of whether a galaxy can be clean of dark matter, or whether it's kind of a requirement for a galaxy to form. So let's dig into that. But first let's take a quick break.
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All right, we're asking the question, can you have a galaxy without dark matter? Could you maybe have a scripy little galaxy out there that was like, No, I don't care about dark matter. I'm just going to gather all these all this gas and dust and on my own without any help. That's kind of what we're asking.
Today, right, Yeah, exactly.
And so you talked about how most of the galaxies that we see out there probably have dark matter, right, We think they have dark matter, right, because they couldn't be holding together without dark matter.
That's right. And it's even more than just most galaxies have dark matter. It turns out galaxies are sort of like extra rich in dark matter, Like most galaxies have more dark matter than the average dark matter density in the universe.
Wait, what what do you mean? Like, why are some numbers?
So if you average over the whole universe, like what fraction of matter in the universe is dark matter, that's eighty four percent. And that's not by counting. We don't know how many dark matter particles there are. That's by mass. Like, what fraction of the mass of stuff in the universe is dark matter? That's about eighty four percent. But when you look at galaxies and you ask, like, what fraction of the mass in a galaxy is dark matter, that's more like ninety one percent. So galaxies have like about half as much normal matter as the average normal matter density in the universe. Galaxies are like concentrated blobs of dark matter.
And we get these numbers by measuring how fast the galaxies are rotating and kind of guessing how much dark matter you need to hold it all together.
Yeah, not so much guessing, measuring, right, But you're right, it's looking at how the galaxy rotates. We can measure the speed of those stars in the galaxy as they whizz around the center by looking at their light and seeing how it's red shifted or blue shifted. You're looking at a galaxy, some of the stars will be moving away from you and some moving towards you, so to be red or blue shifted their light from the Doppler shift, so you can measure their velocities. So you can look at the velocity of stars as they get further and further away from the center. And in order to hold a star at a certain velocity a certain radius, you need a gravitational force there, so you can calculate exactly how much gravity is needed to hold a star at a certain radius. They have all these stars at different distances from the center, telling you exactly how much gravity you need to keep those stars going at that speed. And then you can add up how much you can see, like count all the visible stars and the rest you suppose is dark matter. I guess that's what you mean by.
Guessing, Yeah, inferring guessing.
There's a whole field of statistical inference that we should just call guessing.
Yeah, Well, I mean you don't actually know how many stars there are in that galaxy right so far away you can't see the individual stars, so you're also sort of inferring how many stars there are there. You're guessing a little bit, aren't.
You there's always uncertainty in these measurements. Absolutely, and you're right that we cannot resolve individual stars, especially near the center where things get very dense. But we can see the streams, right, we can see streams of stars. We have models for how these galaxies work. But absolutely there's always uncertainty. But the uncertainty in these calculations is tiny compared to the size of dark matter. So there's no uncertainty that there's a lot of dark matter in these galaxies. Because remember the fractions we're talking about here, like ninety percent, which means you're looking at a star, you're measuring its velocity. You figure out how much gravity is needed hold it there so it doesn't fly out into intergalactic space. You add up all the stars, you get like ten percent of the gravity you need. So there's a huge missing chunk.
And you're sure it's not just you know a lot of asteroids or rocks that don't close.
Yeah, that's a great question. Could dark matter just be normal matter that we're not seeing, right, just like dark chunks of matter? So people have looked for that directly. Those are called MACHOs massive compact halo objects, and we think we would see those occasionally, like they would pass in front of stars if there was a lot of them. If they were really big, we would have spotted them. So people have looked for that kind of stuff and not seen it. Plus, we know something about how much normal matter there was in the very early universe because it dictates the fraction of elements that were produced, like the hydrogen and helium and lithium very sensitive to the density of quarks and electrons. In the early universe. We talked about that once Big Bang nucleosynthesis. So we have a pretty good handle on like how much normal matter there was around, and we can explain where most of that is now and the rest of it's got to be dark matter.
And so basically every galaxy out there that we've seen, we see that it's spinning faster than it should, or it's holding together more than it should, so we think it has dark matter. And now is that true for every galaxy we've seen out there?
It turns out there's a pretty wide variety. Like when you look at galaxies out there, some of them have a lot of dark matter and some of them have a huge amount of dark matter. There is a variety. It turns out that smaller galaxies tend to have more dark matter than really massive galaxies.
Not by absolutely amounts, but just relative to how many stars they have.
Yeah, exactly higher dark matter fractions, I should say, And that's because galaxies are better at holding onto their dark matter than they are their normal matter. Galaxies are crazy places. There's winds from all the stars right every star is a fusion furnace and pushing out protons and electrons. These cellar winds are pushing gas out of the galaxies. Then there are supernovas going off all the time, blowing things up and pushing things out. There's radiation really intense from the center of the galaxy that's pushing gas out. So galaxies are basically exploding and they're pushing a lot of their matter out. And so the smaller a galaxy is, the less it's capable of holding onto its normal matter, the less it's capable of resisting these forces that push gas out of the galaxies.
Because the bigger galaxies have more gravity.
Basically right exactly, the bigger galaxies are still doing this, but they have more gravity so they can hold onto their normal matter. So smaller galaxies, which have weaker gravity lose more of this normal matter. So you look out there at dwarf galaxies, the really tiny ones, they can be like ninety nine percent dark matter.
But wouldn't the larger galaxies also be better at holding onto their dark matter? Like wouldn't smaller galaxies lose some dark matter eventually, like it might evaporate or something.
Yeah, that depends on what dark matter does. And in this theory, dark matter does nothing but gravity, and so you can't really lose your dark matter. Like to lose your dark matter, you need some force that's pushing out on it. But gravity is just attractive. So all these forces like the solar winds and the radiation and the supernova basically have no impact on the dark matter. Dark matter just like brushes it right off, Like supernova could happen right next to you, and a dark matter particle would be like whatever, dude.
And so we haven't seen any galaxies without dark matter, So then why are we asking the question are there galaxies without dark matter? Is it more of like, is it possible to have a galaxy without dark matter? Or are we asking like could there be galaxies where we haven't noticed it doesn't have dark matter.
Yeah, so great question. We're curious about this for lots of reasons. Like number one, we have a theory about how the structure of the universe came to be and how it made galaxies, and this nice story we told you about over densities clumping together to form galaxies, et cetera, et cetera. But we'd like to test that. We'd like to make sure that's correct. We're often surprised when we look out in the universe and see how things actually work, and so what we'd like to do is check our predictions about like the dark matter of fractions of galaxies against reality and see is this really the way things work. Also, this really helps us understand what dark matter is, because seeing how dark matter very across the universe can tell us something about the nature of dark matter and help us test various alternative theories about what dark matter might or might not be. But it's not exactly true that we've never seen a galaxy without dark matter. People are out there looking for these and they found some pretty weird cases.
Interesting, all right, what are some of these cases?
So this galaxy group kind of nearby on cosmic scales, that sixty three million light years away. It is called n GC one zero five to two, and basically it's an elliptical galaxy in Nsitis constellation. We've known about it for like two hundred and fifty years or so. But there's actually a little group of galaxies. It's like a major galaxy with a bunch of little galaxies nearby. They call these dwarf galaxies. So it's a whole group, so they call the group of galaxies. And these little galaxies are actually ultra diffuse galaxies. That means there galaxies that are not very bright, they have very few stars in them. And these ultra diffuse galaxies near this NNGC group they think might have no dark matter in them at all. They think that so they look at the rotations of these galaxies and they do that calculation and they estimate zero dark matter. Like every time you're doing this, you're not assuming the dark matter. You're measuring it, and sometimes it comes out ninety percent, sometimes eighty four percent, sometimes ninety nine percent. In this case, it comes out close to zero or consistent with zero. So they think these are little galaxies that have no dark matter in them at all.
So that's pretty wild. That means that you can have a galaxy without dark matter.
Yeah, it's fascinating because remember our theory of galaxy formation is that basically every big galaxy is a merger of a bunch of small galaxies. Big galaxies don't like form all at once in a single collapse. You have a bunch of baby galaxies then merge to make bigger and bigger galaxies, so like a bottoms up approach. And so if your big galaxy ends up with a lot of dark matter in it, that means that the little galaxies that made it should each have their own dark matter. And we look at a dwarf galaxies and we mostly see them having dark matter. In fact, some of them have a lot. So it is really weird to see these little galaxies without any dark matter at all. And the question it is like, did they form this way or did something happen to strip them of their dark matter.
Or maybe they formed later in the universe.
Yeah, exactly, And so that's a fascinating question. And so there's a group that's done a study of these and they have a theory about how these little diffuse galaxies ended up without any dark matter in them.
What's the theory?
So The theory is basically a mini version of the Bullet Cluster. You remember, the Bullet cluster is this famous example that really convinced a lot of people that dark matter was a real thing. It was a cluster of galaxies that collided with another cluster of galaxies, and we saw that what happened to the gas and the dust and the dark matter was very different. So the gas and the dust interacted and created collisions that dark matter passed right through because it doesn't interact at that level. Gravity's not strong enough, so basically separated the dark matter from the normal matter. So the bulletluster, now you have a blob in the middle with a bunch of normal matter in it, and then you have dark matter on both sides, so we can see through gravitational lensing. So they think that might be similar to what happened in this case, that maybe there was a big color between two other objects, and these two things that we're seeing now they call them DF two and DF four are basically the results of that, like chunks of stars and gas and dust that got stripped of their dark matter and a collision and then tossed aside.
Well, why wouldn't some little bit of dark matter go with.
Them, because the dark matter and the normal man have very different experiences in a collision, Like dark matter basically passes right through. There's not really a collision when it comes to dark matter. It's like two ghosts just phasing through each other, whereas two people bumping into a hallway are going to change their direction. So imagine you have like a ghost inside you and somebody else is a ghost inside them, and you have a collision in the hallway. The ghosts just keep on going and the living people bounce off each other. Now you know you're separated from your ghost.
But there's so many that they discovered. Did they all get that way from the collision?
So they've only found these two, and they have this reconstruction of the collision that suggests that these two things happen somewhere near NGC and created its collision, and it should have also created a bunch of other ultra diiffuse galaxies that they should be able to spot that there should be like five or six of these that came out of the collision that also have no dark matter, So they're going to go and look for those.
So these are you said, these are dwarf galaxies.
Yeah, they're ultra diffuse galaxies. They're also dwarf galaxies, so they're small and they're not very bright.
So in a way they kind of got made later or not. Are they as old as the universe?
Well, they think this collision happened about eight billion years ago, and so how you age these things, I guess depends. Like the way they are now started about eight billion years ago. Now, of course, have some progenitor or something that they came from. Right, there was a larger object they were a part of which definitely had dark matter in it, and their dark matter is now sprayed in some other direction. So they become separated from their dark matter. And this must have been a pretty mammoth event. I mean, they reconstruct this thing and it's like a collision at three hundred kilometers per second of these huge cosmic objects. WHOA.
So these are galaxies that had dark but then they got stripped away of their dark matter, and so that shows that, hey, you can have a galaxy without dark matter. But I guess maybe the larger question is can you form a galaxy without dark matter?
Right? And so as you say, that's the deep question about the nature of the formation of structure in the universe, and so far, the answer to that is no, we do not think it's possible to form a galaxy without dark mattery. We think you need that dark matter around to gather enough gas and dust to make stars and to make a galaxy that without dark matter, normal matter doesn't have enough gravity to have formed galaxies this early in the universe. If you had a universe without dark matter, or big section of it without dark matter and just normal matter, it would form galaxies eventually, but it would take a lot longer to do so.
But you know, as I understand it, during the Big Bang, things were really hot and dense, and there were pockets of things, and there were quantum fluctuations which maybe created pockets of extra densities here and there. Couldn't there have been a pocket of extra density of normal stuff but not dark matter. That then when the universe blew, it became a galaxy without dark matter like during the Big Bang. Why does the normal matter have to follow the dark matter?
I guess it does in general because dark matter dominates because there's just so much more of it. So it basically like sets the scene for everything. But you're right, It is theoretically possible to have a downward fluctuation in the dark matter and an upward fluctuation in the normal matter. So you get some region of space where you have like extra super dense normal matter and almost no dark matter. That's possible. Yeah, and so in principle that could happen, and if it had enough matter, then it would form a galaxy on its own. So in principle that's not impossible. But we've never seen that, and I don't know what the chances are of that happening theoretically, Like.
How many galaxies have we've done this calculation to make sure that it has dark matter in it.
Yeah, that's a great question. We've measured the rotation velocity of thousands and thousands of galaxies, but that's a tiny fraction of the number of galaxies that are out there and the number of galaxies we can see. Most of the galaxies we can see, we can't measure the rotation velocity because you're looking at like one or two pixels. You need to be able to sort of resolve the whole galaxies so you can see light from one side versus light from the other side. So it's tricky, but yeah, we haven't looked at that many galaxies as possible. There are galaxies out there that did really form without dark matter.
All right, Well, it seems like we kind of answered the question of the episode, which is, can you have a galaxy without dark matter? The answer is yes, you can have maybe galaxies that had dark matter, but then they lose it or they get left behind by the dark matter and so they're dark matter less. Or maybe they could have formed at the beginning of the universe in theory, but we haven't seen one yet. All right, Well, let's dig into what this all means about our understanding of dark matter and also gravity and whether or not it needs to be overhauled. But first, let's take another quick break.
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All right, we're asking the question, can there be a galaxy with no dark chocolate? And the answer is probably, But that's not a universe Daniel wants to live in.
No, that's right, Transport me somewhere else in the multiverse asap.
What if you end up in a universe where there's only milk or white chocolate?
Just keep smashing that button until I get.
Somewhere good And no, no, you only get one trip. Would you take the risk?
I'm pretty happy with our universe, you know.
It's a pretty good one. We're talking about whether galaxies can exist without dark matter, and the answer is yes, they can be stripped away of their dark matter, or theoretically they could form in the early universe, but we haven't seen one yet and so maybe probably not. What do you think happened? Why haven't we seen any? If they can form without dark matter, why haven't we seen any?
I think that'd be really unlikely. I mean, the kind of fluctuations we're talking about are very very large. In the early universe, you had just sort of like energy and then it decays into matter. As the universe expands in cools, and every kind of matter is sort of made uniformly, so you get more dark matter made and less normal matter. But in order to have no dark matter made, you'd need a really big fluctuation. You expect to get like eighty five percent and you get zero. It's like flipping a coin one hundred times in a row and getting only heads instead of half heads and half tails. It's pretty unlikely. Now the universe is really big, of course, so that means that eventually it's going to happen, especially if the universe is infinite, but it's so unlikely that it's not going to be the first kind of galaxy we see out there, or even in the first trench. Eventually we might spot one.
Well, as you said, it is possible to have a galaxy without dark matter. We've seen it in some ultra diffuse galaxies, and they've done the measurement on these galaxies, right, They've measured how fast it's spinning, and they're pretty sure there's no dark matter in them.
Yeah, there were a series of papers where people said, oh, there's no dark matter, and another group did a different measurement said no, there is some dark matter, and then there were follow up papers arguing, and now they're pretty sure there's no dark matter in these, but there's always somebody out there who disagrees. I mean, it's astronomy after all.
That's right, they're all just guessing.
They're all just doing their best statistical inference.
That's right. That's a great word for guessing. I'm just kidding. A best guessing. How about that best guessing? Nobody has a better guess.
You know, those whole departments of people who do nothing but statistics for a living. I'm trashing Hershey's, but you're trashing statistics. Man.
No, there's nothing wrong with guessing wrong.
How do you know the universe is the way it is. We're just guessing.
A best guess does mean that you're making things up randomly. You're just using the best information you have to make a best estimate or inference, right, I suppose.
So.
I think if it's very well informed, it's not really a guess, you know.
But if you're one hundred percent sure, it's also not a fact.
Yeah, that's true. That's why we use statistics to describe our uncertainties. Anyway, One of the things we are uncertain about is the nature of dark matter. Like a lot of the stuff we talk about for dark matter is kind of unsatisfyingly indirect, and a lot of people out there treat dark matter like it's some placeholder, not a real theory of the universe because we never see it directly. We don't can't really grapple with it and grasp it.
Directly, right right, Well, I think it's interesting that we have or that astronomers have found galaxies without dark matter, because it almost gives you kind of like a test case to confirm that the other galaxies that we have seen with dark matter actually have dark matter and it's not just some weird, you know, fluke or mistake in our theory of gravity.
Mm hmm. Yeah, it's a cool test case. It's like a control right would you see galaxies without dark matter if they were there? So it's nice to have some verification that we're seeing that. It's also, as you say, a great test bed for comparing various theories of dark matter, which make different predictions about what would happen in these scenarios.
M Yeah, different guesses about dark.
Matter, different ideas, different theoretical.
All right, well, how do these galaxies help us decide what dark matter is made out of.
Well, one of the most popular alternatives to dark matter as a theory of matter, some kind of stuff in the universe is an alternative theory of gravity to say, well, there's no other stuff in the universe. We're seeing everything there is. It's just that gravity works differently from what we expected. Because remember, the argument for dark matter is like, we understand gravity, and there's a lot more gravity than we can explain with the visible stuff, So there must be more stuff, but there must be invisible stuff creating that gravity. But what if instead we just don't understand how gravity works and it can be explained by all the visible stuff if you tweak your theory of gravity.
Meaning like, what if gravity just gets stronger the bigger the distances, that might account for why galaxies are holding on together without needing dark matter. That's kind of the idea, right.
That's kind of the idea. More specifically, there's this theory called mond modified Newtonian dynamics that suggests that gravity's mostly like Newton described, but there are some tweaks. It depends on the acceleration of these objects, and for some accelerations, gravity gets stronger or weaker, and you know it's a little baroquely like added these terms and these tweaks basically to explain these rotation curves, say like, oh, these stars are accelerating more than those stars. So if we change the way gravity works, can we describe the rotation curves that we see. The answer is yes, you can devise a theory to describe the rotation curves that explain how these galaxies are rotating without needing dark matter. If you tweak gravity, right, you have to tweak something, either change the amount of matter that's there, or you change the way gravity works.
Well, first of all, you just insulted the whole period of human history, the Baroque period.
I meant that in a positive way, right right.
The second I know that we've talked about before, how you know, maybe Mond modified Newtonian dynamics. It could replace dark matter, but we confirm dark matter in other ways.
Right exactly. So Mon is a success in describing the rotations of galaxies by making these beautiful baroque extensions to Newton's theory. But there's lots of other ways we've seen dark matter, like in the ripples of the cosmic microwave background from the very early universe. We can see how that early universe plasmo sloshing around, and that depends very sensitively on the amount of dark matter, which slashes differently in that plasma than normal matter did, and Mond cannot explain that. Also, the Bullet cluster shows us that dark matter can be separated from normal matter. It's not just a different way that gravity works from normal matter. It really is something else with its own gravity. So Mond really struggles to explain and everything that the theory of dark matter can explain, but it's still a popular alternative. And this is another way. These dark matter free galaxies are another way to draw contrast between what Mond predicts and what dark matter predict because according to Mond, there is no dark matter and gravity only depends on the visible matter, and so these altered diffuse galaxies with no dark matter should behave the same way all the other galaxies do because there's no dark matter in any of them. But we do see a difference. We see that these guys are rotating more slowly, right, and so Mond struggles to explain how slowly rotating these galaxies are without any dark matter, whereas dark matter can explain all of it. It's like, well, this has more dark matter, that has less dark matter. So because dark matter can be variable in the universe, some galaxies have more and some have less, whereas the rules of gravity have to be the same, Mond is sort of hamstrung and can't really explain the variation of all these galaxies.
Well, I feel like Mond was already kind of dead in the water for all these other reasons for a while. But it is kind of interesting that seeing a galaxy without dark matter almost kind of helps prove that it exists.
Yeah, that is really interesting, and I agree with you that Mond is not a theory we should take terribly seriously. In dark matter is overwhelming the better guess for what's going on in the universe.
Yeah, there you go. See I brought you on board.
I'm loving that word.
Now you're like, I guess, I.
Guess so, but you know, full caveats. There are some things that dark matter can't explain. There are a few galaxies out there that don't make any sense that no dark matter can really explain. Some people think that some hybrid like mostly dark matter with a little bit of mind is what we need to explain everything. That's out there in the universe, and so it's best to keep an open mind. It's also always nice to find a new way to test our understanding of dark matter and gravity in general, and so these galaxies without dark matter are a nice test bed for.
That interesting you could call the new theory darkmond All right, well, another interesting example of how the universe just always has surprises. Like you think that maybe you need dark matter to have a galaxy, but only one day you find galaxies without dark matter, and it makes you think, and it actually maybe helps you confirm the existence of something as mysterious as dark matter.
And it goes to show you that the universe does all these experiments for us. We can just look up in the night sky and find the examples of galaxies smashing into other galaxies or black holes colliding. All these things are wonderful experiments that help reveal the nature of the universe, the rules that it follows, and how it all works.
Yeah, I guess, or you guess.
In the end, aren't we all just guessing?
Man?
All right, Well, we hope you enjoyed that. Thanks for joining us. See you next time.
For more science and curiosity. Come find us on social media where we answer questions and post video. We're on Twitter at this word instant and now TikTok and remember that Daniel and Jorge explain the Universe is a production of my Heart Radio. For more podcast from my Heart Radio. Is it the iHeartRadio app, Apple Podcasts or wherever you listen to your favorite shows.
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