Daniel and Kelly’s Extraordinary Universe - Does the Universe need dark matter?

Episode Date: July 9, 2020

Is it possible that dark matter doesn't exist? Could it just be a misunderstanding of gravity? Learn more about your ad-choices at https://www.iheartpodcastnetwork.comSee omnystudio.com/listener for privacy information.

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community. Well, I know you're never wrong, Daniel. That's unthinkable. But, you know, like, has physics ever gotten something about the very nature of the universe? Kind of, uh, not right. You mean, like, what's the best snack food or what color lab coat should we wear? Yeah, you know, like, can I eat antimatter or not? Maybe it's delicious.

Physics could definitely be wrong about snack foods. Well, I mean, think bigger. Like, is it possible that maybe things are not quite what they seem? Wouldn't be the first time, physics is wrong, and I hope it's not the last time. Hi, I'm Horham, a cartoonist and the creator of Ph.D. Comics. Hi, I'm Daniel. I'm a particle physicist, and I've never been wrong about snack foods. And I've never been wrong about being wrong. So I think that means that I'm always wrong.

I don't know. But welcome to our podcast. I'm definitely right about that. This is our podcast, Daniel and Jorge Explain the Universe, a production of IHeart Radio. That's right, our podcast in which we take a mental tour of all the crazy stuff that's out there in the universe and try to bring it into your head.

We try to wrap up the entire universe, all those trillions and trillions of stars and weird blobs of gas and dust and invisible stuff and insert it through a little hole in your ear. into your brain. Yeah, all of the amazing and incredible stuff out there in the universe and also all of the stuff that's normal, you know, and sometimes I think when you examine what appears to be normal in your everyday lives, it turns out to have all kinds of wonders and all kinds of small miracles in it. Are there small miracles in your snack foods? Is that what you're

talking about? Like, oh, look, there's chocolate chips in this trail mix. Every snack is a miracle, Daniel, especially banana snack. But in this podcast, we go beyond the miracles of banana-based snack foods and talk about the incredible things that scientists are trying to figure out. Because contrary to popular perception, scientists don't have it all figured out. There are lots of really big mysteries out there. And wondering about the universe is something that belongs to everybody, including you. Yeah. So they're one of the biggest mysteries out there in the universe and literally sort of like not just like because we don't know it, but also because it's a huge part of the universe is dark matter. Dark matter is a little bit crazy, right? It's like 27% of the

universe, but we have no idea what it is. That's right. Of the energy budget of the universe, it's a pretty big slice. It's like a little bit more than a quarter of all the energy in the universe is this weird stuff. And we've never seen it directly. We're pretty sure it's there. But we don't really know what is it? Is it made out of particles? Is it made out of black holes? Is it made out of lost Sox. It's a really huge mystery in modern physics. In fact, I would say it's one of the biggest open questions in science. I think it's just dark socks, actually. Wouldn't that make sense? I always lose those. How many dark socks have you lost for him? I mean, we're talking a lot of socks. Well, to be honest, I'm a cartoonist. I don't have to wear dark socks very often or socks

at all because I also live in California. But yeah, dark matter, I mean, it's a big deal. There's five times more dark matter than there is regular matter like planets and stars and gas and dust and black holes there's all that stuff there's actually it's only like 20% of the stuff in the universe yeah which means the normal matter the regular matter

the stuff that you're familiar with is not actually regular or normal it's the unusual stuff if you just took like a survey of stuff in the universe most of the stuff is dark matter the things that make up stars and planets and galaxies and hamsters and me and you and bananas

and snack foods, that's unusual in the universe. It's a minority of what's out there in the universe. And yet, we still don't know what this dark matter is. It's a big mystery. And a lot of times, you know, whenever people talk about dark matter, I feel like a common question we get, at least in talks and appearances, is that people ask us like, what if dark matter doesn't exist? Like, what if it's just an error in the equations that you have about physics and the universe?

Like what if we just maybe like misunderstood gravity or haven't counted all the stars in the galaxy or, you know, what if there's something else that is maybe normal, but we just haven't thought about it. Yeah. And it's a great question because most of the evidence we have for dark matter is a little bit indirect. Like because dark matter is so dark and hard to interact with, we don't have clear pictures of it, right? We haven't seen what it's made out of. We've only sort of seen its effects and sometimes secondhand. And so it's tempting. to wonder if it's really there. You know, it's like if you've only seen the footprints of an animal, are you really sure it exists or could it be something else spoofing you? Until you really capture one or see one in the wild, you don't really believe it exists. Right. And dark matter has been eluding our searches for decades and makes people wonder like,

well, maybe you people have it wrong. Yeah, I mean, it's kind of a crazy idea, right, to think that there's that much stuff out there and conveniently it's invisible and you can't see it. You know what I mean? Like, I would be like, maybe you should check your math or, you know, maybe you should have another grad student do the calculations. So which do you think is the conspiracy theory, dark matter? Like, there's so much of it and you can't see it because it's so dark and that proves

that it exists or the anti-dark matter conspiracy theories. Oh, wait, ooh, anti-dark matter. That's another episode right there. Can you have anti-dark matter, Danny? Yeah, no, that's another great question. We think probably not because then it would annihilate it with dark matter and turn into photons, which we would see. What if it turns into dark photons?

Yeah, though, that's actually a thing, dark photons. Is it really? It really isn't there. There goes my double price. You don't get credit for that one. But, you know, which idea sounds more bonkers, right? That the universe is filled with an incredible amount of invisible matter. Nobody had to text it until recently or that it's not.

Yeah, so today on the podcast, we'll be asking the question, Does the universe need dark matter? Is it an essential part of the universe? Like, could you have a universe without dark manner? Would that make sense? Or is it maybe that the universe doesn't need it? And maybe it is kind of an error that we have in our calculations and observations about the universe.

And this kind of skepticism is very healthy. When you have a crazy idea you're trying to accommodate, when you see results in your experiments, you don't understand, And you need to be flexible about the sort of theoretical framework with which you come at the problem. You need to be open to crazy new ideas, but you also need to be open to the fact that maybe you got your measurements wrong. There always can be an alternative explanation. So before you go big and say, wow, we're going to revolutionize our understanding of the universe, you've got to rule out all the more prosaic, basic, simpler explanations.

And so it's always a good idea to keep those ideas in mind. Yeah, so how sure are we that dark matter exists in the universe and could it be something else? So I went out there into the internet and I asked people, can we explain what we see in the universe without dark matter? Are there good alternative theories that don't require a new particle or a new blob of stuff? So before you listen to these answers from the internet, think about it for a second. Do you think dark matter is necessary in the universe or do you think the universe could ignore it or live without it? Here's what people had to say. I would think we would be much more baffled if we didn't have dark matter to explain the expansion of the universe.

Oh, man, this is tough because I still don't have a good handle on what dark matter really is. But I think we really don't know what dark matter is. So I'm going to say, yeah, we could definitely explain what we see without it because we're kind of just making up what it is. I guess dark matter is just a made-up term for. The stuff that's there that we can't explain, so surely any theory is under the umbrella of dark matter? I have a strong suspicion that the thing could be explained without dark matter. I recall Daniel saying that dark matter is just a name that we've come up with for the phenomenon that we can't explain. I don't know what we would see without dark matter.

My understanding is that we do know that. there's dark matter because the math doesn't work out. At some point, we will have to explain the universe without dark matter because like the dark in the matter sort of implies that we don't know what it is. I believe that dark matter is a theory that we came up with to help explain why we couldn't account for all of the gravity that we see in the universe. I think it's a fairly recent discovery. I don't believe Einstein knew about it, so he must have had some other way

to account for all of the gravity. All right, some pretty good answers there. Yeah, I like the people who treat dark matter just sort of like as an umbrella idea for all the things we don't understand. And so whatever we find out there, we just call that dark matter. And I think that really touches on the sense people have

that we don't really have a clue what's out there. We just sort of labeled it dark matter, and we're talking about it as if it's a thing, but it's really just a name we apply to our cluelessness. Interesting. Like if you put an S at the end, it becomes dark matters. And then it's sort of like an umbrella term for things that are dark. That's true.

And, you know, that is definitely true of dark energy. Dark energy is another piece of the universe pie, right? The universe pie is 5% normal matter, 25% dark matter, 70% dark energy. Dark energy definitely in the category of just stuff we don't understand. we gave a fancy sounding name. Dark energy, this stuff that's making the universe expand. Dark matter, on the other hand, is much better understood. It's much more concrete an idea, much more detailed observation.

So they're both called dark, both things we don't understand, but dark energy definitely a label for our cluelessness, while dark matter is a much better-founded, well-described theory. It's less dark, I guess. We're less in the dark about it. Exactly. We are less in the dark. our minds are not quite so filled with dark shadows. All right.

Well, that's the question for today. And that's the question is, you know, do we need dark matter? Like, is it a concept that is totally necessary for the universe to make sense? Or is it just something weird that exists out there and that maybe we could have a totally wrong idea about? Yeah. And are there other theories that are being worked on in the scientific community that might explain it without needing to add some new kind of stuff to the universe. Darker matter. The darkest matter.

And you'll find that in science, there are always competing voices. You know, there's often like a mainstream. Most people think the answer is X, but there's always somebody out there who thinks it's Y and somebody who thinks it's Z. And you've got to give these people room because sometimes they're right. And sometimes their ideas are the ones that turn into the mainstream. That's how the mainstream became mainstream. It used to once be lunatic fringe. Are there alternative physicists?

You know, French physicists? There definitely are. There are people out there once they get tenure. start working on crazy bonkers theories. And sometimes for decades, that nobody pays attention, nobody really reads their papers, that people even laugh behind their hands,

but sometimes they're right, you know? Literally the history of physics is filled with revolutions that started as crazy ideas. And so we definitely got to pour water on some of those seedlings because they could sprout into huge new intellectual trees. Wow. Awesome. Dark physics trees I'm picturing. All right, well, step us through.

Like, what's the main argument for dark trees? matter. Like, what's the main evidence about it and what makes this thing that, you know, it's something new and different as opposed to maybe it's just more stuff out there that we can't see. Yeah. So if you're going to come up with another theory of the universe, another way to explain the way the universe works, you have to explain what we do see, right? I mean, that's the whole idea behind making a theory of physics. So if you're going to come up with your theory, you have to understand what are the observations, what are the experiments reveal that need to be explained that you know what's the motivation for creating this idea of dark matter and the short

version is it's all gravity like everything we see out there in the universe that we need dark matter to explain are weird gravitational effects that we can't explain with all the other stuff just the gas and the dust and the stars right with the universe feels dark matter in terms of gravity like it's there affecting the gravity of other things but you can't see it that's the main evidence for it Yeah, basically there's unexplained gravity. Like we thought we knew where all the stuff was in the universe from the stars and the gas and the dust. And from that, you can calculate how much gravity there should be. And we can see the effects of gravity.

And we'll go through in a list of how we see the effects of gravity. But there's more gravity than we expected. So either there's more stuff, i.e. dark matter, or gravity is weird and different. And so it's all about the gravity then, right? Because that's kind of the basis of the theory about dark matter is that it feels gravity. but not anything else. Exactly. And that's why it's dark

because if it felt electromagnetism, it would reflect light or it would give off light like everything else does than the university glows. Or if it felt the strong force, it would bind with corks

and form nucleons and interact with us. So it doesn't interact with us in any way that we know of other than gravitational. And that's why we call it matter because we think it's something that has new gravity.

But, you know, it could also just be a tweak to the way we understand gravity. But at its core, it's really an observation that our theory of gravity doesn't work either because there's missing mass or the theory is wrong, right? So those are two sides of the coin. Right. Like according to what we know about how gravity works, there's a lot of gravity missing, or there's too much gravity in the universe almost.

Yeah, there's gravity out there and we don't have mass to explain it. And so we have to sort of fill in those gaps. And that's an uncomfortable feeling, right? You're like, well, you can't just like fill in the gaps and assume that you're, your theory is right and add extra stuff to make it work out, you know, that feels uncomfortable. Right. Like there's snack food missing from my fridge. Surely it was my son who got up in the middle of the night to eat it. But you can't, you can't just assume that. You can't just assume that. It could have been your daughter.

Maybe you sleepwalked and ate it yourself, right? And that's when you install a camera in front of your fridge and you get those weird videos of yourself at 4 a.m. stuffing cake in your face. You speak from experience, Daniel. Hypothetically. Hypothetically, right. All right, so we think dark matter is there because of gravity, and there are several ways that we have seen this kind of missing gravity, right?

It's in the galaxy rotations, the way galaxies rotate is kind of one of the first one, maybe even? Yeah, one of the most dramatic and earliest pieces of evidence that there was more gravity in the universe than we expected was looking at how galaxies rotate. And we can add up all the mass of the stars and the stuff in the galaxy and say, okay, we know how much gravity there should be. And then we can calculate how fast those galaxies are rotating. And, you know, for a galaxy when it's rotating, it's trying to push stuff off.

It's trying to, like, throw stuff off the edge. Like if you put ping pong balls on a merry-go-round and spin it, the ping-pong balls fly out. The thing that keeps the galaxy from tearing itself apart from throwing those ping-pong balls out into intergalactic space is the gravity. So you can ask how fast is the galaxy spinning? and is there enough gravity to hold it together? Because the faster it spins, the more gravity you need.

Right. Yeah, it's kind of like our solar system, right? Like, you know, the mass of the sun is what keeps all the planets kind of spinning around it. But what if, like, the plants were spinning faster than what you could explain by the mass of the sun. You would need some other explanation. That's right. You need more force to hold them into their orbits. And so you say, well, maybe there's extra gravity or some other force or something.

And so we know there's something else holding the galaxy together. And so the first explanation is, oh, well, what if there's more invisible mass? There's just some stuff in the galaxy that's providing gravity, and we just can't see it. And you can explain these rotation curves, the way the stars move around the centers of galaxies, if you distribute a bunch of mass sort of smoothly. It's like a big clump in the middle, and then sort of smoothly out past the edge of the galaxy into a big halo that's even actually bigger than the galaxy. That explains that that says that would give you the kind of gravity that we are

Right, because if you look at a galaxy, it doesn't have that enough mass, right? Like there aren't enough stars or planets in it that you can see. That's right. We count up all the visible stuff and we say, how much mass does it have? And that just doesn't give us enough gravity to explain how those galaxies are holding themselves together. And that was the sort of genesis of it. I mean, for a long time, people were like, well, wow, that must be a mistake, you know. Because coming to the idea that there's five times as much mass as you can see is a really big idea.

something people just came to an afternoon and then accepted. Yeah. It doesn't seem likely. It does not seem likely. It seems more likely that you mismeasured something, that you got the velocities wrong, or, you know, your grad student is pranking you or something. It's a big idea. Right.

It's like jumping to the conclusion right away that maybe there are five ghosts in my house, eating my snacks. You know, that's like a big, that's a big leap from like, hey, maybe it's just your son hungry. And so it took other pieces of evidence before dark matter became mainstream. Right. All right, let's get into those other ways that we know dark matter is there. And let's get into whether or not it is there, or maybe we just have gravity wrong. But first, let's take a quick break.

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And also we've sort of kind of can see it, right? We can see it in the way to distort the light from other faraway stars. That's right. These days we have a good handful of ways to indirectly detect dark matter. And one of the coolest is seeing it act like a lens in the sky because dark matter only has gravitational forces, but gravity can bend space, right? It's a bending of space and time. So if you have a big blob of invisible stuff in the sky, it will curve the space it's in so that photons traveling through it will get bent as if they're moving through a huge lens. Right.

So if there's a big blob of dark matter between you and some really far away galaxy, it will distort that galaxy, creating duplicates of it, stretching it, just as if there was a huge lens in the sky. Right. And we can see that. Like if you look at pictures of the sky, you see these kind of distortions, these ripples, these lensing effects. You can sort of see dark matter almost. You can definitely see it and there's sort of two categories. There's strong lensing. Like there's a big, dense blob and it's distorting the galaxies. And it's pretty hard to explain that kind of lensing in any other way. And then there's weak lensing. We just sort of like look at all the galaxies out there. to see, like, are any of them sort of just a little distorted or just looking a little tweaked?

And from that, we can get sort of like a map of where in the sky we think the dark matter is just by looking at small distortions. And that's how we've gotten a pretty good map for where we think this missing mass is, how we know that it's mostly in the center of the galaxy, and how far out past the edge of the visible galaxy the dark matter halo might go. So it's a really powerful technique. Right. And so that's kind of how dark matter entered in kind of our view of the universe, was these first initial ways. But since then,

we sort of have put more nails into sort of the coffin of whether or not dark matter is out there, right? I mean, it's now sort of shows up in pictures of the universe, the early universe. It kind of shows up in our calculations about all of the energy in the universe, right? It's gotten more and more convincing that there's something there. That's right.

It becomes very difficult to explain the universe you see without dark matter. For example, we see the influence of dark matter on the formation of structures in the universe. Like the universe began as a sort of diffuse cloud, you know, just of gas, mostly hydrogen. And then it started clumping together. And that clumping comes from gravity, right? So gravity is a thing that draws these things together

and eventually gives you stars and galaxies and planets and all that cool stuff. If you run a simulation of the universe without any dark matter, then you don't get galaxies in the first 10 billion years. It takes like another 10 or 20 billion years. So it's dark matter that's, that's created these like gravitational wells for stuff to fall into to make the stars in the galaxies and us.

So just like the whole structure of the universe would look very different without some kind of gravitational stuff out there. Maybe we should have just called them dark clumps or dark do it. And even earlier in the universe, like we've talked on this program about the cosmic microwave background radiation. Those are photons from the very, very early universe, 300,000 years. after the universe was created. The first moment when the universe became transparent to light, before that it was thick and soupy and light got reabsorbed. And after that, it was cool enough that light could travel through the universe

without being absorbed. Right. And the shape of that plasma that gave off that light was affected by dark matter and how much dark matter there was and how much normal matter there was. And that stuff like bounced off each other and oscillated and like squeezed and squished that plasma. So the amount of dark matter in that. moment of the universe affects the shape of that light, the currents, the sort of patterns we see

in that light in a very, very precise way. That's hard to describe in any other way other than there's some other kind of stuff out there that's giving us this gravity. Yeah, you can see like it's imprint in the light from the early universe. Like it's visually and like tangibly and like, you know, you can calculate it or it distorts that light. That's right. And so if you just take the universe and you add to it a new particle, a particle that doesn't move really fast, we call it cold and doesn't feel anything but gravity. It explains all of this stuff. It explains why galaxies rotate the way they do, why the cosmic microwave background radiation looks the way it does, why the structure of the universe has this structure at this time in the universe. And also,

there are very specific, awesome experiments that the universe has done to sort of demonstrate dark matter to us. Yeah, I guess, you know, I think maybe a question that a lot of people have and I'm sure physicists had at the beginning was, you know, why does it need to be something special? Like, why does it need to be a new kind of particle or matter? Couldn't it also just be something regular but that you just can't see? Like, you know, what if there's a whole bunch of black asteroids out there that are hard to see or, you know, a lot of small dust that we can see or maybe even like a whole bunch

of little black holes kind of spread around. the universe. Yeah, that's a great question. Like, why do we know it's a new kind of thing? Why can't it just be more of the same, but just kind of dark, right? Yeah, visible. Yeah. Well, we don't really know anything in our current set of particles that is that dark. I mean, other than neutrinos. Everything else has some kind of interaction. Like if it's a black rock or something, well, that does reflect light. It does give off light. So if it's made out of the normal kind of matter, it's going to have the kind of interactions we have, and therefore we would be able to see it,

except, of course, neutrinos. Neutrinos were a candidate for dark matter for a long time. The problem is neutrinos move way too fast. They zoom around the universe because they're so light, so they don't give the same sort of structure to the universe that dark matter does. You need this thing to be sort of slow-moving and cold in order to stick around long enough to give the structure of galaxies.

Couldn't you have cold neutrinos, like slow-moving neutrinos? we talk about that the other in another episode? Could you have cold neutrinos? Yes, but we don't think that neutrinos are that cold. We haven't seen cold neutrinos. Nutrinos are so light. They have almost no mass that they essentially almost always

always travel near the speed of light. They're always in a rush. They're always in a hurry. Yes, neutrinos are hot, as we call them, in particle physics. And literally and figuratively is. It's a pretty big trend right now. That's right. All right, well, I guess if it's not

maybe something we know about, could it be that just maybe we have our theories wrong about how things work? Like, you know, maybe it's not a new particle or a new kind of matter, but maybe we just have gravity wrong. Like maybe gravity doesn't work the way we think it is. And maybe in these larger universe size scales, could gravity, you know, could there be a different theory of gravity that would maybe account for what we think is dark matter? It's definitely something to consider, right? Because all these observations are just observations of gravity. And what we're doing we're saying we assume there's a certain amount of mass out there, we assume we know how gravity works, so we estimate how much gravity there should be based on that mass, right? But there are two

steps in there. There's figure out where the mass is and then calculate how much gravity there is from that mass. If that second step is wrong, right? If the theory of gravity works differently from what we expected, then yeah, that could possibly explain it. Because remember, gravity is very, very weak, which makes it very, very hard to test. Like, it's difficult to measure the force of gravity at the scale of one centimeter between two pebbles because this force there is almost zero. Like, you need to build a very sensitive instrument to measure the gravitational force between anything that's smaller than, you know, planets and moons.

Yeah, because, you know, like our current theory of gravity says that, you know, gravity changes by one over R squared. like the force of gravity kind of depends on the distance between two things squared and then you put that in the denominator and that always seemed kind of almost too simple to me like what are the chances that the universe would pick such a simple little formula to calculate gravity you know why isn't it like one over r squared times r to the 0.75 you know did i mean like it seems so simple maybe what if it's wrong like what if gravity isn't one over r square but maybe changes over distances in a way that could maybe explain dark matter.

Yeah, that's a totally realistic thing to think. Although, you know, there are a lot of these patterns, these one-over-R-squared patterns in physics and in forces, and there is a good reason for it and a fairly simple way to understand it. If you imagine, like, the surface of a sphere surrounding a point, think about like all the gravitational energy coming out of a point. The surface of a sphere surrounding it, all the gravitational energy passes through that sphere.

And then as the radius of that sphere gets larger, what's the surface of that sphere? goes like R squared. And so the power at any point should go like one over R squared. It sort of makes sense geometrically. But what if geometry is wrong? What if geometry is wrong, right?

What if physics is wrong? What if podcasting is wrong? What if one plus one is wrong? Well, kind of, I mean, you know, we talk about sometimes about how space is not, you know, this nice and neat orderly thing and that sometimes you could even like measure triangles in real space that have angles.

that are bigger than 180 degrees, could maybe like space be weird in such a way that it's not really one over R squared? Yeah, absolutely. And there are forces that don't go like one over R squared. Like the strong force doesn't go like one over R squared. At small distances, it gets even stronger as you get further away. So you definitely got to be open to weirdness.

And so around the time when dark matter was sort of coming up in the world as an idea, when it was based mostly on galaxy rotations, people thought, well, how can I tweak gravity? to explain what I'm seeing without dark matter. What would I need to change? What would you have to be like one over R to the third or one of R to the 1.5 or whatever in order to explain these galaxy rotations without dark matter?

And so they came up with a different theory. It's called Mond, M-O-N-D, for modified Newtonian dynamics. Oh, no. I would have just called it dark math. I wish you could get in a time machine and go back and tell them because I hate Monde. Wouldn't that be a lot catchier and fun? Dark math.

Well, Maude, I guess, if you're French, then it's like, oh, yeah, it means the world. I suppose, though. Except it's missing the E, and it commits the terrible acronym crime of taking two letters from one of the words, you know, modified. Jeez, yeah. All right, so there is kind of a theory in physics that says, like, maybe we do have gravity wrong, right? This is like an idea you guys take seriously. Like, maybe there is no dark matter. It's just dark math.

It's definitely an idea that physics. should take seriously in the sense that we should think about alternatives. This one theory in particular doesn't have a lot of supporters, and we'll get into exactly why. But, you know, it's an interesting idea. And it says, like, maybe gravity works differently at very, very large distances. Like, you know, we've tested gravity here on Earth. We know how it works. We've tested gravity in the solar system pretty well.

But, you know, maybe the first time we're looking at gravity on galaxy scales, maybe gravity just works differently over, like, you know, 50,000. light ears, right? We haven't done that experiment before. Maybe it gets going, like, maybe it gets stronger in galaxy size scales. Yeah. And so the idea is actually even weirder than that. It says maybe gravity works differently when you have a very, very small acceleration.

What? Like, when things are not being pulled very hard, maybe gravity works a little bit different. Oh, my God. You just blew my mind. All right, let's get into this crazy idea that gravity depends on how you're moving. maybe, and whether or not that could work. But first, let's take another quick break. A foot washed up a shoe with some bones in it.

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All right, Daniel, we're talking about dark math to maybe explain dark matter. And so there's an. that maybe our formulation of gravity or how we think gravity works could be wrong maybe and it could maybe act very differently over larger scales, like maybe get stronger the galaxy scale or you're telling me it's actually when there's lower accelerations. And this is just an idea again, right? This is just an idea. But hey, everything's just an idea, right? Even lunch.

Lunch is just an idea, remember? Oh, no, Daniel. Lunch is very real to me. Well, the idea is, you know, somehow you have to get gravity to be stronger without breaking things we already know. So what if, you know, affecting things in the edge of the galaxy, you could have gravity instead of going like one over R squared, which would make it very, very weak as R gets large, you can make it go like one over R. It doesn't fall off as quickly. It doesn't get weak as quickly with large distances. Interesting. So like the gravity I feel with another planet on the other side of the Milky Way, maybe it's stronger than I think. Yeah, maybe it's not like one over R squared. It's more like one over R. And R is still really, really large.

So, you know, those planets don't affect you because the gravity from them is still really tiny. But if you're adding up lots of planets and you're trying to calculate like how a whole galaxy spins, then it really does make a big difference to go like one over R instead of one over R squared. But it seems a bit hacky. As opposed to dark matter? Dark matter doesn't seem hacky. Well, dark matter, you know, you can explain a lot of really different things adding just one simple idea like, hey, what if there's a new particle. That's not hacky.

Like, we don't know why there's a certain number of particles in the universe and not more or if there are more. So adding one new particle doesn't feel as hacky is like, let's have gravity, have a knob on it or like a distance above which it starts behaving different rules or, you know, small accelerations below which it starts behaving differently. Really? Why does it need to be low accelerations? How does that work in? It doesn't really work, as I think I heard you react, because like acceleration with respect to what, right? All of a sudden, the gravity you feel depends on your reference. frame and it breaks all sorts of

symmetries. But there is that one idea that accelerations like 1 10 trillionth of the gravity on Earth, if you feel an acceleration less than that, then the gravity on you behaves differently than it does for larger accelerations. Oh, I see. It's not, you're not saying when you're going

at low acceleration, it's just when the effects of gravity are small, maybe it doesn't behave as one over R square. Yeah, and the effects of gravity at very, very far distances are really small. Like, what is the acceleration on some star at the edge of the galaxy due to the black hole of the center of the galaxy. Well, the distance is 50,000 light years. So the acceleration is very small, also because

the masses are large. And so it's just a way to say, let's have it have an effect where we see this weirdness and not have an effect anywhere else. Let's try not to break what we already know and understand and only have this theory turn on in special cases. All right. So that's a possibility, but where did this idea come from? Like, why would we think that maybe gravity is wrong? So for a long time, there wasn't really a good idea. There was just like, well, I don't know, but if I put in this other formula,

I can explain the galaxy rotation curves. Without dark matter. Without dark matter, yeah. So they're like, all right, so either there's missing mass or gravity works this other weird way. And then people started thinking, well, why would gravity work that way? Is there any reason for it?

And that's a totally valid line of inquiry, right? Like, what theory do we have to have to explain the data? And does that theory make sense? and we come up with a reason why maybe that theory is right. A theory about a theory. Underpinnings of the theory, right? We always in physics want to have a microscopic understanding. We don't want to just say gravity just has this number on it.

We want to know why does it? Where does that come from? Why isn't it something else? Just like you were saying earlier, why is it one over R squared, not one over R2.1 or whatever? That's the next version. Upgraded gravity. And so recently there is an idea from a guy in Holland,

Eric Verlinda, and he has this crazy idea of gravity called entropic gravity that might be able to explain why gravity works differently at these distance scales. I feel like maybe he just grabbed two cool-sounding things and put him together.

He sort of did. Tropic, he did sort of like dynamic gravity or something. But it comes from a cool place. You know, it was Stephen Hawking who first connected sort of thermodynamics and gravity. He started thinking about the temperature

of black holes and thinking, you know, if black holes have temperatures, that means they should radiate, oh, wait, and that's hawking radiation. So in the last sort of 40 years, people have been thinking about gravity and thermodynamics as a connection. They're trying to understand like, you know, actually maybe gravity isn't a fundamental force. And maybe, you know, Einstein's idea that gravity is just a curvature space time. Maybe that's also wrong. Maybe instead, gravity is just like an emergent phenomena of thermodynamics. Wow. Maybe it just like comes out of the manipulation and interaction of some tiny little bits of

space in a way that feels like gravity to us. Wow. That's a little bit mind-blowing. Yeah. It's a little bit mind-blowing. This whole idea of emergent phenomena can be hard to get your mind around, but it's actually very familiar, you know, like we can talk about wind, right? Wind is an emergent phenomenon.

It's not a fundamental force in the universe, you know, two particles, interacting don't feel wind. Wind is a combination of lots of other things we do understand on a microscopic level that has a macroscopic effect. Or like economics. There are laws of economics that come from supply and demand or whatever.

It's not a fundamental force in the universe, but still you can have an understanding of it. And so they think maybe gravity is just like a macroscopic effect of something microscopic, maybe the thermodynamics of space time. Interesting. Like there is no gravity.

It's just kind of like how space itself kind of arranges itself. Yeah, because we know that space likes to increase entropy, likes to increase disorder. And so we think entropy always increases in the universe. And so maybe gravity is just an effect of that. You know, the argument goes something like when things fall into a black hole, for example, that increases the entropy of the black hole. Because otherwise it would violate the second law of thermodynamics.

Things can't just disappear into the black hole. But maybe it's the opposite. it. Maybe it's not that gravity pulls things into the black hole and then increases the entropy. Maybe it's entropy that's pushing things into black holes. And that's actually what gravity is. That's just like, you know, the way gas diffuses in a box, you put a blob of gas in the corner of a box and it spreads out into the box. That's entropy. The same way, maybe entropy is stuff falling into itself. Like, you know, having more stuff near itself. Dense blobs of mass increases the temperature, which changes the entropy of the situation.

I mean, it's a complicated argument involving like quantum entangled space time, but I think that's the gist of it. Wow. And all of that kind of crazy idea is just to explain how maybe gravity could not be one over a R square. Yeah. If you have entropic gravity, so maybe if gravity is not a fundamental

force in the universe, but just like an emergent property of quantum space time, and you have dark energy, then Verlinda's theory predicts that space occurs in this way that gravity, the effect, the effective force of gravity goes the way Mond needs it to

explain the galactic rotation curves. So that's kind of cool. Interesting. That's an alternative to dark matter. That is one alternative to dark matter is to say we have this weird gravitational thermodynamics idea that changes the way gravity works that explains why we thought

there was missing stuff. Instead, it's just the gravity works differently than we expected. Interesting. All right, so then I guess the big question is, could that work? Is that a valid or plot? possible theory that maybe explains gravity in a different way that then explains dark matter and so that it's not just another particle or kind of stuff, could that work? Well, it's a cool idea and it does explain galaxy rotation curves and it actually explains

galaxy rotation curves better than dark matter does. What? There's some galaxies out there that we still can't explain using dark matter. Like they rotate in weird ways and we don't understand it and, you know, but hey, galaxies are weird But they sort of fit. And in conclusion, they have their own history. Galaxies are weird. Noble Prize, sir.

Here you go. My theory. That's right. My theory that the universe is just weird explains everything. Oh, man. The whites in theory of everything. The whites in theory of weirdness. But it explains the galaxy rotation curves really nicely.

But there's a big caveat there, which is it was sort of invented to explain those curves. Like, you came up with a theory to fit that data. Right. really a good test of a theory is, does it explain other data? Is it a real general principle about the universe, something which is really deeply true? Or is it just a mathematical tweak to this one plot, this one figure to make it work? Because if it only works to explain one thing, it's kind of suspicious, right? Exactly.

And if it only works to explain the one thing that motivated it, then probably it's not a deep truth of the universe, right? All right. So maybe it doesn't work. Can it explain some of the other things that we know about dark matter? like the lensing and the cosmic microwave background and the structure of the universe, a tweak to our theory of gravity also explain all these things?

In a word, no. It just doesn't work. No, galaxies are just weird. No, galaxies are just weird. No, it does not explain the cosmic microwave background. Like, it's very difficult to explain that using anything else other than some new kind of particle

or primordial black hole or something, some new kind of stuff. You just can't explain it. deviation in gravity because it was on a really small scale this is like really you know things were nearby each other this wasn't galactic distances interesting and it's very difficult to explain the structure of the universe and then there's this one really awesome example of dark matter that's sort of like a smoking gun that makes it almost impossible to explain using anything but some new kind of matter new kind of stuff some new kind of stuff yeah and that's this bullet cluster

there was this collision millions and millions of years ago far far away between two clusters of galaxy that pass through each other. And we thought that those clusters of galaxies, they had normal matter like stars and planets, et cetera, and then also clumps of dark matter. So what happened is they passed through each other and the gas and the dust, it all collided and made big collisions and slowed down and all that stuff. But the dark matter, it doesn't interact with the normal matter and it doesn't interact with itself very much at all either because gravity is so weak. So the dark matter just sort of passed through. So what we see when we look up in the skies we see like a big blob in the center of all the gas and dust that interacted and then

on either side we see the dark matter that passed through right we can see that because of the gravitational lens right but doesn't it all i know we talked about the bullet cluster before but doesn't it still just come down to gravitational lensing like what if gravity can explain gravitational lensing could that also explain the bullet cluster that we see it's very difficult to explain the bullet cluster in any other way because you need to have gravitational lensing in exactly those right spots on opposite sides of this very obvious collision. So it's a very nice explanation to think, oh, there's some invisible matter that passed through and is now sitting there distorting the background galaxy.

Oh, I see. The bullet cluster is like proved that whatever's causing these gravitational things is mobile. Like it can move like stuff. But if it was just a gravitational theory tweak, that wouldn't like keep going or move or change from here to there. Exactly. And it can be separated from the normal. matter. It's not just a gravitational tweak from the normal matter. The normal matter was left

behind in the middle. It's its own kind of stuff. It has its own gravity. And so that's sort of like a bullet in the brain of the, you know, non-dark matter theories. What? The bullet cluster is a bullet? That is also suspicious thing. People really took Mons sort of seriously until then. And then when the bullet cluster was discovered, people were like, oh, well, that's it. Dark matter is real. And Monde is dead. Never Monde. And at that point, people really didn't take Monde seriously. So since then, Monde has been much more of a fringe theory.

I mean, Verlin's idea is more recent and it's sort of cool, but it sort of explains something that nobody really takes seriously anymore. So it's very fringe. In a way, the bullet cluster shows you that dark matter or whatever is causing these gravitational distortions and footprints is mobile. Like it can move like stuff. It moves like stuff. The stuff can move.

Yeah. Yeah. And so again, it's unsatisfying because we don't know what it is. And we've been looking for it for a while. It's not like we're just like, oh, it's some invisible stuff. Let's move on. We've built dedicated experiments to look for it. We've thought several times that we would definitely find it and then haven't seen it.

So there's definitely some tension there. It's not like dark matter is a beautiful theory that's all wrapped up. Like we don't understand why we haven't been able to find the dark matter yet. There's definitely something weird going on there. And it's very healthy to think about. new ideas, but Mond doesn't quite work. There is no other idea out there that explains what we see nearly as well as some new cold particle. Hey, I have an idea. Is it snack based? It's called, hey, maybe dark matter is weird. Yeah, and that's why people are sort of digging further and

further into the barrel of ideas for dark matter. People thought for a long time, okay, dark matter must just be some weekly interacting massive particle, some new blob of stuff. But we haven't seen it. And so then people are like, well, maybe it's axions or maybe it's primordial black holes or and there are some other even crazier ideas like skirmions. What? Like they're uncomfortable in social situations? What do you mean?

Introvertons, yeah. No, squirmions are this crazy idea that, you know, how particles are like excitations of quantum fields. They're like little bundles of energy in a quantum field. People think, well, maybe not all energy in quantum fields are particles. maybe some of them are like more distributed or spread out in weird ways and they found that you can like tangle up quantum fields in these weird ways like make knots in them and that's what they call squirmions and these knots could like could create gravity yeah because any energy density

creates gravity well we'll have to dig into that for another episode sounds pretty yeah for sure it's pretty squirming for sure so there's lots of ideas the whole spectrum right dark matter's not just one idea. The sort of spectrum of ideas all satisfy the condition that it's some new kind of cold object. But what that object is, it could be one particle, could be many kinds of particles, it could be black holes, it could be squirmions, it could be something else we haven't even thought of yet. Whatever it is, we're pretty sure it's stuff. It's something that has gravity. Stuff and it's there. That's right. But I mean, it's really interesting because I feel like we've known dark matters there for a while now and we just can't seem to crack it. We can't seem to

It keeps eluding us, you know, like it just keeps on hiding there and not letting us know what it is. That's right, but we don't know how long the story is. You know, we were looking for the Higgs boson for 50 years and then we found it. We were looking for the top cork for 20 years before we found it. A lot of those things, people thought, oh, we'll find this in the next year or two. And then they were confused and disappointed to not discover it soon. But, you know, eventually we got there and we figured it out. So maybe we're just five years away from discovering dark matter or maybe it's going to be another 100.

Maybe we need a new crazy idea for what dark matter is. But dark matter, I'm pretty sure, is. So all those physicists squirming around, relax. Maybe dark matter is in our future. That's right. I certainly hope it is. It'd be fascinating. And remember that it's most of the stuff out there in the universe.

Like, what a crazy opportunity to learn about the way the universe works. Yeah. You know, to know that 80% of the stuff out there in the universe has been hidden from us. Right. The day we crack that open and get to learn about it, like it could have incredibly complex. structure, it could have interactions and biology and chemistry and all sorts of crazy stuff. Most of the stuff out there in the universe, we haven't yet gotten to play with. And so we're eager, we're desperate to figure out what this stuff is. Yeah, because if you're the scientist that

discovers what dark matter is, I mean, that's like a lot of street credit, you know? It's like, you could say that you discovered 80% of everything in the universe. I think they would have to give you five Nobel prizes, you know, just for that one discovery. They'd have to give you 80% of all the Nobel prizes, I think. Yeah. Yeah, there you go. You get all the Nobel prizes for the next 400 years just to balance it out. Yeah. Yeah. So it's a big question. And who knows? Maybe one of our listeners will be the one who discovers it. That's right. It could be you out there. It could be your kids out there. What we definitely need to do are keep our minds open and come up with new ideas for what dark matter is. Pretty cool. All right. Well, we hope you enjoyed that. View into this

mysterious dark matter and how we know it's there and how we know it's not just a weird fluke of gravitational equations. Thanks for tuning in. See you next time. Get fired up, y'all. Season two of Good Game with Sarah Spain is underway. We just welcomed one of my favorite people, an incomparable soccer icon, Megan Rapino, to the show. And we had a blast. Take a listen. Sue and I were, like, riding the lime bikes the other day.

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