Is dark matter a fudge factor?

24:24Matter gravitates. It should all just attract and collapse. Shoot. And uh-oh, he looked at the universe. He's like, that doesn't seem to be happening. What's going on? And he was living before we even understood the universe is expanding. So he just added a number to his theories, the cosmological constant, which would balance that and which would make the universe not collapse. And it wasn't even a great fudge factor because in his description, the universe would be like balanced on a knife edge. These two numbers had to match exactly perfectly all the time or the universe would tip over and either expand or collapse.

24:56So then he abandoned it because he's like, okay, this is not working. And that's fair to call a fudge factor because it was just to solve that one problem and didn't generalize and didn't explain anything. It's not like summarizing some lower level details we don't have yet. It was just like, this seems wrong. I'm adding in a number. And so I think to sum up for like, what is a fudge factor? Is it fair to criticize it? Is to ask the question, like, does it generalize? Is this an explanation we can use somewhere else? Is it like an approximate description of something real that's happening? Or is it just like, hey, my checkbook didn't balance, I just created a fake expense to explain why I'm missing$5 ,000.

25:35Got it. Okay, well, let's take a break. And when we get back, we're going to see if dark matter meets the criteria that Daniel just set forth for us.

25:51Bill, why is my eye twitching? Does this mean I'm in perimenopause? Maybe I have adult ADHD. I need to look this up. Where's my phone? Juliana, your phone is hot to the touch. I think it's asking you for a break. You know what? Maybe I should just ask ChatGPT. Or maybe we can ask an actual human. Yes, like a couple sex therapist. Whoa, that escalated quickly, but okay. Because I have questions. We have questions. And I bet everyone has questions. Like, is it normal to sleep in separate bedrooms? We do that. How about this one? Is bribing your kids bad parenting or just negotiating? Oh, and I still do need to know, why was my poop green that one time?

26:28Hypothetically, right? Okay, well, instead of letting the internet guess, we've got actual people answering these exact questions. And laughing with us has got to be better than spending three hours down a rabbit hole online. Listen to Bill and Juliana. The podcast. On the iHeartRadio app, Apple Podcasts, or wherever you listen to your podcasts.

26:52From daily news to dating fails, conspiracy theories to cooking with celebrities who can't actually cook, Amazon Music's got the most ad-free top podcasts ready to entertain, included with Prime.

27:08What did Black music, food, and culture teach us about who we were becoming? 2016 was sort of that last era of monoculture, where we still consumed things in community. From Beyonce and Rihanna. Everybody wanted to be Beyonce. I don't think we'll ever see another Rihanna. to soul food, memory, identity, and the stories we carry through Black culture. What does it mean to be Black and eat in America? So we were this group of people who knew how to work the land, who knew how to live with the land. We make it do what it do. Therapy for Black Girls is bringing together the conversation shaping Black life right now.

27:46You will never make me feel bad for being a Black girl, for being a Black American girl, ever. Therapy for Black Girls is bringing it all to the mic. Listen to Therapy for Black Girls on the iHeartRadio app, Apple Podcasts, or wherever you get your podcasts. Talking about guns with others might not always feel comfortable, but it could save a life. Here's a way to start a conversation. Your family is going over to your neighbor's home for dinner for the first time. How would you ask if there are any unlocked guns in the home? Hey! Hey, we're so excited for tonight. Before we come over, though, may I ask if there are any unlocked guns in your home?

28:19Our guns are stored securely, locked in a safe that the kids can't access. Awesome. Learn how to have the conversation at AgreeToAgree.org. Brought to you by the Ad Council.

28:40All right, Daniel, you are in the hot seat now. Now you set up some criteria for how to determine if a number is a useful fudge factor or not. How does dark matter measure up? So let's delineate the number of independent lines of evidence we have for dark matter. Because that was the test, right? If dark matter is a fudge factor, then it only solves this one problem, the galaxy rotation curves. And if it's not, then it solves many problems independently. It generalizes. It's an explanation that seems likely to describe the real universe. not just an addition to our checkbook. Okay. So the first thing I would say is the thing that convinced folks that dark matter is more than just, look, astronomy student got a wrong number, right?

29:25It was not actually an experimental measurement. I think it was theoretical developments. In the late 1970s, people were thinking like, well, could there be more matter out there that's invisible? Like, where would it all come from? And folks were doing calculations for the early universe, like thinking about how the universe formed and how it cooled into protons and electrons, et cetera. And they realized that it actually was totally possible during the initial production of matter, of those quarks and those gluons, to also produce dark matter. And in a fascinating way, as the universe expands, that dark matter would get frozen out.

30:02So the universe expands and everything gets more dilute. And dark matter, if it only interacts very, very weakly, would no longer be able to find itself to like annihilate and turn into anything else. And so if you produced a bunch of dark matter in the early universe, it would still be here. And so that calculation in the seventies convinced people, hmm, this is maybe plausible. And I think those two lines of argument together turn this from like, hmm, there's something missing in galaxies to like, hmm, maybe there's something we've been missing about the universe. And then they started to look for more independent lines of experimental evidence.

30:37So I'm not totally sure I'm following. So it sounds like They were sitting down, and they were like, pass the bowl, pass the banana peel. And then they were like, what? Is that the first step in theoretical physics for you? I mean, I know enough physicists. So they were imagining what it was like before the Big Bang. And they were like, what if there was a lot more matter there? What would have happened to it? And then when they projected forward, they were like, okay, actually, what would have happened to it? It kind of matches what we see happening right now. Whoa. Yeah. Is that fair to say?

31:14Yeah, exactly. The idea is, look, if there's a bunch of additional matter created, why didn't it also like annihilate itself and disappear, right? And this is a way to explain how you could produce dark matter and still have it be around. Okay. Just as a theoretical mechanism for like, how could it exist? And then people started digging into these other lines of evidence. Say like, do we have other evidence that dark matter exists? And then you can go back again to the 1930s. There was a guy, Franz Zwicky, and he was looking not at galaxies, but clusters of galaxies, right? Because beyond just individual galaxies, you find groups of galaxies that are gravitationally bound.

31:53They're orbiting their common center. And then you can play the same game. And you can say, how fast are they moving? How massive are they? Is there enough mass in this cluster to hold it together and to explain the velocities? It's like galactic rotation curves, but now galactic cluster rotation curves. Okay. All right. So he scales up and does he find he's off by about what he expected he'd be off by? So this is 1933. Oh, yeah, this is earlier. This is before Babcock. This is before the idea of invisible matter. Okay. Right. And remember, Babcock saw the galactic rotation curves and didn't even jump to that idea of dark matter.

32:28But Zwicky is looking at the Coma Cluster. This is a cluster of about a thousand galaxies, like 300 million light years from Earth. And it's really cool because it's one of the first galaxy clusters to ever been remarked on in the early astronomical literature. Like Herschel saw it in the 1780s. Anyway, he was looking at this thing and he saw that it was moving way too fast. There was not nearly enough matter to explain how it was holding itself together. And so he invented this phrase, Dunkelmaterie in German, which is dark matter. And he wrote, quote, if this would be confirmed, we would get the surprising result that dark matter is present in much greater amount than luminous matter.

33:08Whoa. This is already back in 1933. This is an analogous piece of evidence, not only within galaxies, but between galaxies. It's almost a bummer we don't still call it dunkel material, because I feel like dunkel even lends itself more to poop jokes. That just sounds like a fecal-related word. But anyway. It does. Like you got to wipe that dunkel away. The dangling dunkel is the thing to worry about. Dunkel berries. There you go. Nobody ever make a dunkel berry pie. Oh, gosh. You got a little dunkel berry on your face there. You got to. Oh, God. Okay, let's move on. Do you remember like 20 minutes ago we challenged ourselves not to make any poop jokes?

33:49We knew. We knew. We knew. False hopes. That's right. False hopes. Yeah. All right. Give me some more evidence. All right. So that still feels like maybe an extension of galaxy rotation curves, very similar. Though it's the kind of thing you should expect to see if there really is a lot of dark matter out there. The second completely independent line of evidence for dark matter is the very early universe. Because if we think dark matter was made in the early universe, we should see evidence for it in the early universe. And we do. If we look at the cosmic microwave background light, remember your history of the early universe.

34:28Stuff formed and we had quarks and gluons and electrons and photons. Then the universe expanded and cooled. Quarks and gluons formed protons. So now we have protons and electrons and photons. And then things cooled even further. And around 380 ,000 years after the Big Bang, the protons and electrons came together to make neutral hydrogen. And then the universe went from being opaque to being transparent. And so any light created at that moment is still around because it's flying through the universe. and we can see it and we do. And it's actually one of the best pieces of evidence we have that the universe used to be much denser.

35:04So that's the CMB light. And it's basically light from the early universe. And it's very cool because we can use it to see patterns and distributions of matter in the early universe. And we once weren't sure if it was pigeon poop or not, right? Is this - That's right. Okay. Yes, exactly. Thank you for adding a completely independent line of poop jokes to the episode. You're welcome. Poop jokes. Also not a fudge factor, right? They're a load-bearing support for the podcast. Okay. All right. But the scientists working on the project decided it wasn't pigeon poop that was messing up their measurements.

35:37It was actual cosmic background radiation. So let's get back on track. And the fascinating thing about the CMB is, of course, that it exists and tells you what the early universe, but there are variations in it. So mostly it's very, very smooth. You look in every direction. It's about just around the same intensity and temperature at frequency, but not perfectly. There are little anisotropies in it, the little hot spots and little cold spots. And this corresponds to fluctuations from the early universe, right? The reason it's not perfectly smooth is that the very early universe was not perfectly smooth.

36:10And people are used to thinking about early universe quantum fluctuations. But what we could actually see in the CMB is something much more interesting, is early universe dynamics. It wasn't just like, okay, this spot was a little denser and that spot was a little less dense. We can see stuff sloshing around in the early universe. Because the early universe was not just made of one thing. We think it had electrons and protons and photons and dark matter. okay and there's like 1 billion photons for every electron and proton in the early universe it's just like mostly photons at that point because of all the matter antimatter annihilation turned into photons okay and so what happens is that the photons which are everywhere they push on matter they speed it up but the dark matter that's there it pulls on the matter because of its gravity and it doesn't get pushed by the photons so you have these three different things all mixing around interacting in complicated ways, and you end up with sloshing.

37:09You end up with this oscillation where things rush back into the dark matter gravity wells, and then they get pushed out by the light. So this is called acoustic oscillations in the early universe because there are pressure waves. So we think about it as like sound. And so these oscillations in the early universe are what we are seeing in the CMB, not just initial primordial quantum fluctuations, but the dynamics, the sloshing itself. And that sloshing is super duper sensitive to exactly how much dark matter did you have to pull the things back into the well versus how many photons did you have to push things out of the well.

37:44All these ratios predict different shaped wiggles, different heights of the wiggles, different widths of the wiggles. So we look at the CMB light and we look at the hotspots and the cold spots. We measure the typical distances, like how far apart are hotspots usually? And then you run the calculations and you say, well, this distribution requires this amount of dark matter and this amount of normal matter. So we can measure the amount of dark matter and normal matter in the early universe just by looking at this pattern, totally separate from galactic rotation curves, from theoretical calculations, from galaxy cluster rotation curves.

38:20This tells us, number one, how much energy there is in the universe, like what is the overall energy density, which tells us the universe is mostly flat. and what fraction of that comes from matter that responds to photons, that's like 5%, and what fraction of it comes to matter that ignores photons but has gravity, and that's like 25%. So those numbers, totally independent, very precise, amazing measurements here. Is this a situation where you had two different lines of evidence where you were like, oh, we don't know what's happening, but now we're going to layer in dark matter, and oh, we get the same value for dark matter?

38:55Or is this a situation where somebody was like, okay, we've thought about dark matter in this other scenario. And so now we're going to look at cosmic background radiation with dark matter in our head and test predictions based on values we already have and see if those values give you the right answer. Yes, it's more like the second. Cool. And I would say even more dramatically, it's like people had this idea of dark matter and then they realized they could probe it in the early universe because these new generations of telescopes and satellites were able to see this kind of stuff. And then they made a prediction.

39:25And they were like, look, if there's no dark matter, it's going to look like this. And if there's a lot of dark matter, it's going to look like that. And they went out and they did the measurement and it came back dark matter. Not just dark matter, but dark matter in the amount that we see from galactic rotation curves. The same number. Okay, that's pretty cool. So you're like, whoa, that's spooky, man. Maybe there really was something, right? We're talking about the same kind of stuff. Some kind of stuff that doesn't interact with photons and yet has gravity. And you have to have it to explain the early universe observations.

39:57They just do not work without some kind of invisible gravitating matter. Okay, that is awesome. It's pretty cool, yeah. And yet that's only the third line of evidence we have for dark matter, right? Galactic rotation curves, galactic cluster rotation curves, and now the CMB. But we have more, right? That's just the beginning. We can look, for example, at what happens as the universe progresses. So you have protons and you have electrons and they've cooled now to make hydrogen. And for a few minutes, like literally minutes, not poetic minutes or something, for a few minutes, the universe is filled with hydrogen at the right density to fuse it.

40:36We usually think about fusion happening in stars because it needs a lot of density and a lot of temperature and a lot of pressure. And that's true. But as the universe cools, it goes through a window when it satisfies those conditions. right now of course out in space it's way too cold and pressure is way too low for fusion but there was a literal a few minutes when the universe could fuse not in stars but way before there were stars wow so this is called big bang nucleosynthesis and hydrogen fused together to make light elements a little bit of helium very small amounts of heavier stuff there wasn't enough time to work all the way up to iron, for example.

41:15But the amount of helium you make and of the heavier stuff is very, very sensitive to the density of matter that we had back then. So if you had a little bit more quarks or a little bit less quarks, you'd get a little bit more helium or a little bit less helium, right? And so we have very precise calculations that tell us exactly how much we should get for various densities of matters. And this doesn't tell us about dark matter. It It gives us a really broad understanding of normal matter. Like some people ask sometimes, couldn't dark matter just be like a bunch of rocks out there you haven't seen?

41:49Because stuff in space is dark sometimes, right? And people look for this, right? And they're called machos, massive astronomical compact halo objects, machos. Nice. And for a while, it was like a really nice counterpart to the WIMP theory, weakly interacting massive particles. I need to know how long somebody worked to come up with macho after the Wimps theory. I bet it was like a week or two. Astronomical acronyms are the worst, slash best, slash best. That's right. That's right. A little bit of both. Anyway, what this tells us is something about basically the total density of normal matter, because we can measure how dense it was because we can tell how much helium was produced.

42:29And so this answers a question like, is there enough normal matter density out there to explain the dark matter? Could it just be a bunch of rocks? And the answer is very definitively no. If you measure the amount of helium and lithium produced in the early universe, you can derive the energy density of normal matter. And it comes out again at about 5 % of the energy density of the universe. Completely independent line of evidence that there is not enough normal matter out there to explain the gravity that we see. This is not directly evidence for dark matter, but this is evidence that it's not atomic matter.

43:05Whatever it is out there, it's not made out of quarks and electrons. All right. Okay, Extraordinaries, we are not done beating the dead horse of dark matter is not a fudge factor. And when we get back from the break, we're going to beat it some more.

43:27Bill, why is my eye twitching? Does this mean I'm in perimenopause? Maybe I have adult ADHD. I need to look this up. Where's my phone? Juliana, your phone is hot to the touch. I think it's asking you for a break. You know what? Maybe I should just ask ChatGPT. Or maybe we can ask an actual human. Yes, like a couple sex therapist. Whoa, that escalated quickly, but okay. Because I have questions. We have questions. And I bet everyone has questions. Like, is it normal to sleep in separate bedrooms? We do that. How about this one? Is bribing your kids bad parenting or just negotiating? Oh, and I still do need to know why was my poop green that one time?

44:04Hypothetically, right? Okay, well, instead of letting the internet guess, we've got actual people answering these exact questions. And laughing with us has got to be better than and spending three hours down a rabbit hole online. Listen to Bill and Juliana. The podcast. On the iHeartRadio app, Apple Podcasts, or wherever you listen to your podcasts.

44:28From daily news to dating fails, conspiracy theories to cooking with celebrities who can't actually cook, Amazon Music's got the most ad-free top podcasts ready to entertain, included with Prime.

44:44What did Black music, food and culture teach us about who we were becoming? 2016 was sort of that last era of monoculture where we still consumed things in community. From Beyonce and Rihanna. Everybody wanted to be Beyonce. I don't think we'll ever see another Rihanna. To soul food, memory, identity and the stories we carry through Black culture. What does it mean to be Black and eat in America? So we were this group of people who knew how to work the land, who knew how to live with the land. We make it do what it do. Therapy for Black Girls is bringing together the conversation shaping Black life right now.

45:22You will never make me feel bad for being a Black girl, for being a Black American girl, ever. Therapy for Black Girls is bringing it all to the mic. Listen to Therapy for Black Girls on the iHeartRadio app, Apple Podcasts, or wherever you get your podcasts. Talking about guns with others might not always feel comfortable, but it could save a life. Here's a way to start a conversation. Your family is going over to your neighbor's home for dinner for the first time. How would you ask if there are any unlocked guns in the home? Hey! Hey, we're so excited for tonight. Before we come over, though, may I ask if there are any unlocked guns in your home?

45:55Our guns are stored securely, locked in a safe that the kids can't access. Awesome. Learn how to have the conversation at AgreeToAgree.org. Brought to you by the Ad Council.

46:16All right, Daniel, you've given us, what, I think we're on four, maybe five lines of evidence showing that dark matter makes predictions that then you can test and they pan out. Give me some more. So another really amazing and mind-blowing prediction is to go back to the density of stuff in the early universe and think about that plasma sloshing around. and the plasma sloshing around tend to happen in rings, like spheres actually, where you have these pressure waves propagating through the early universe. And those pressure waves are like over densities, right? Where matter gets really, really dense and then the photons push it apart and then it sloshes back.

46:51But at the moment that the universe cooled enough to form hydrogen, right? Where the protons and electrons found each other, now they became neutral and they were protected from all those photons. Photons can't push them around anymore because they no longer have a charge. And so they got frozen out. So all of a sudden these dynamics ended and whatever was left just at that moment was stuck. So the structure is like frozen in, right? It can't oscillate in the same way anymore. So what you can do is say, well, do we see evidence for that? Do we see like rings of structure in the universe? And we do.

47:24If there was extra matter concentrated in these massive spheres, then you would expect that to eventually form galaxies. And you should look at the galaxies and say, hey, are galaxies concentrated in rings? And they are. If you measure the mean distance between galaxies and you make a plot of it, there's a huge peak. And that peak is exactly where you would expect it to be if dark matter was there creating these kind of structures. And not only do you see it statistically, like it's cool to see a bump in the galaxy to galaxy mean distance, but also we've recently seen a single individual actual bubble, like a ring structure where you can look at the distribution of galaxies and you're like, why is there this huge bubble there?

48:10And the answer is that that was an acoustic oscillation from the early universe, which left this imprint on the gravitational density of the universe, which created this literal sphere of galaxies. It's really incredible. That's awesome. And we found that out pretty recently. Is that right? Yeah, exactly. This is a recent survey. And this thing is massive. It's one of the largest structures observed in the nearby universe. It's 250 megaparsecs away. It's really massive. It includes the Sloan Great Wall and some other objects. And it's got this totally very easy to pronounce name. Kelly, can you pronounce it for us?

48:46No, no. Nice try. This is your outline. You got us in this situation, in this mess. You try it. It looks something like Ho 'oleilana, maybe. It might be Hawaiian. Anyway, this is called baryon acoustic oscillations. And it's another independent line of evidence that the structure of the early universe deeply influenced by the distribution of dark matter. You have to have dark matter in order to make these rings in the early universe plasma in order to create these superstructures of galaxies. This stuff is so important. Why can't we see it? That's really frustrating. I guess you're seeing it indirectly is maybe the point.

49:27Yeah. And there's lots of stuff in the universe that's out there that we don't see. There's light of all kinds of frequencies that we don't see. There's neutrinos passing through you all the time. Most of the universe is invisible. That's the typical state of affairs, actually. All right. Give me another one, Daniel. So another independent source of evidence for dark matter, I'm losing track of how many we have here, is the current structure of the universe. So the large-scale structure of the universe. As we said, we expect to see these rings, but it's not just the rings of galaxies that we see.

49:59You can ask the question like, well, why do we have galaxies at all? If you run a simulation of the universe without dark matter, then you have photons, you have protons and electrons. The protons and electrons eventually cool to make hydrogen. And you do have over-density there. And those hydrogen atoms can pull themselves together to make stars and galaxies. But unfortunately, it doesn't happen in 14 billion years. It takes like tens of billions of years for that to happen without dark matter. Wow. With dark matter, what happens? Well, remember, dark matter is not getting pushed around by photons.

50:35So it can gather itself together and start making structure almost immediately. It doesn't have to wait 400 ,000 years to cool down enough to form neutral hydrogen. So it starts to form structure much earlier. It's created these gravitational wells that the protons and electrons oscillate in and out of. So by the time they do cool and form hydrogen, there's already gravitational structure there for them to fall into, right? And if you add dark matter to your simulations, then you get structures like galaxies forming about the time we see them. And you get a universe that looks very similar to ours.

51:09Without dark matter as an element of those calculations, you don't predict large-scale structure the way we see it at all. And just to sort of hammer a point home, our age of the universe we calculated without needing to think anything about dark matter, right? Yeah. Yeah, okay. Yeah, absolutely. And not only does it require dark matter, it requires dark matter and normal matter in the proportions of 25 % and 5%. Like without those numbers pretty close to bang on, you don't see the universe evolving the way that we see it. So it's not just today. It's not just in the early universe. We see the evolution of the universe between its birth and the present day requiring dark matter to create all these beautiful galaxies that we see.

51:54So you need dark matter to explain this large scale structure of the universe, right? This is another independent test of dark matter. And not only do you need it, you need it to be what we call cold. We need it to have low temperature. Because if dark matter is really, really hot, meaning like it flies around really fast, then any initial structure gets washed out. So you need it to be fairly slow moving. This is one reason we were able to rule out neutrinos as a candidate for dark matter, because neutrinos, very low mass, very fast moving, they wouldn't be able to create this kind of structure.

52:28Okay, got it. Gosh, I don't mean to be hammering home this negative point, but we've seen it from so many, we've measured it from so many different angles. It's frustrating that that hasn't given us insights into how to actually interact with it or measure it more directly. Well, that's the problem is the only interaction we've ever seen is gravitational and gravity is super duper weak. So that makes it very hard to nail down a lot of details about it. And we'll talk about that at the end. But we have yet more evidence for dark matter. You know, there's the evolution of dark matter through time is really fascinating because as the universe expands, stuff gets diluted.

53:06But depending on the nature of that stuff, it dilutes differently. Matter dilutes in a very intuitive way. Like if you have an M &M in your living room, then you have a certain amount of M &M density. If you like quadruple the size of your living room and don't change the number of M &Ms, then the density drops. And density drops in exactly the way you'd expect because the volume is increased and the matter hasn't. Very simple. And then the mood drops because you're like, I have less M &Ms relative to space and like the sad. Okay. But photons react to expansion differently because as the universe expands, the energy in matter stays.

53:41But as we've mentioned on the podcast, photons get stretched. They get redshifted. So not only do you have more space for every photon, but also the photons have lost energy. So the energy in photons decreases as time goes on. So the energy density of photons drops faster than the energy density of matter. If you have the same amount of energy density in photons and in protons and you expand your universe, then you're now going to have more energy density in protons than photons. Because the photons, not only are they spread out across more space, but they also got red shifted to have less energy.

54:16Okay. So you can tell by how something gets diluted, something about what it is. And dark energy, even weirder because dark energy as the universe expands, doesn't dilute. Like you have four times as much space, you got four times as much dark energy. That's a whole other super weird puzzle. But we have these three different ways we know that things can get diluted, right? Is it like matter? Is it like radiation? Or is it like dark energy? And we can study the distribution of dark matter over time by studying the structure formation. And that requires it to get diluted exactly like matter does.

54:50Not like radiation, not like dark energy. Whatever this stuff is, it moves around and it spreads around and it gets diluted just like matter does. That's a very strong piece of evidence. That's one reason why we call it matter. Okay. Yeah. Because otherwise it was like, what, dark stuff? And then you're like, okay, it's got to be matter because of how it dilutes. Was this part of how you all figured out you were going to call it dark matter? This is many lines of evidence. This is a big problem in astronomy and in physics for decades. Lots of people working on it from lots of different directions because they wanted to see, like, does this hold up?

55:25Is this a fudge factor? Or if we poke it from this other angle, what about this other angle? Ooh, nobody ever thought to do this test. What if that? And remember, everybody here is not just out for confirmation. It's boring if you come up with a new way to say, okay, yeah, dark matter is there. If people had found something that showed dark matter isn't there or didn't work, that would have made their careers. And so everybody's motivated to find something that doesn't work. Creating a puzzle is the way people get faculty positions. If anything, people are incentivized to overhype some new thing that doesn't seem to make sense.

55:58Yeah. Well, surely there's no more lines of evidence, Daniel. We haven't even gotten to the best ones, Kelly. What? But we're almost at the end of the show, Daniel. So let me quickly mention a couple more. So there's like the supernovas, right? We measure that the universe is expanding and that expansion is accelerating. Now we know the total energy density in the universe because we can measure how the universe is curved. The CMB tells us that. How much stuff is there in the universe? And we know there's just enough stuff in the universe for space to be totally flat. So that gives us the total amount of energy.

56:32Then we can say, well, how much energy is there in the expansion? So that's dark energy. We can subtract that away. And so if you do that subtraction, you get 100 % minus 70%, and you end up with 30%. 30 % is exactly how much we think there is dark matter plus normal matter. What? So those numbers line up really nicely. Gold sticker for the physicists. Just like another independent way to say like, look, it seems like 30 % of the universe is some kind of matter. Then there's gravitational lensing, right? Einstein tells us that gravity is not a force between particles. It's a curvature of space.

57:09And light, when it moves through that curved space, will bend. That doesn't happen in Newtonian gravity. It's exclusive to relativity. The amount of lensing depends on the mass. More mass, more curvature, more bending. So you can do this amazing thing where you can say, look, I have a bunch of galaxies. And behind them, I have a really bright source like a quasar. Can I see the light getting bent by the mass, the curvature caused by these galaxies? And you can. This is called strong gravitational lensing. You see crazy distortion and Einstein crosses and curves and all sorts of stuff. And you can use that to measure the mass of that cluster, like how much stuff is in there, because I'm seeing a certain amount of lensing, which means a certain amount of curvature, which means a certain amount of mass.

57:51And again, this measurement is not consistent with a visible matter. It requires way too much mass to cause the lensing that we see. And you can use this to see beyond the edges of galaxies where there are no stars. You still see lensing where there are no stars. And then even beyond galaxy clusters, like if there's a little bit of dark matter out there, it would cause what we call weak lensing, like small distortions in background galaxies. And people have done these huge surveys and measured those distortions and see evidence for distribution of dark matter exactly where we expect it and in the same amount.

58:30So it's mostly clumped around galaxies, as you'd expect. There are these filaments between galaxies that we see, exactly what's predicted by the large-scale structure simulations. We can go out and measure a map of the dark matter in the galaxy, the amount and the distribution, totally independently of every other line of evidence we've been talking about. I am running out of ways to say whoa and awesome, so this time I'm going with gee willikers, Daniel. So then finally, the closest we have to a smoking gun for dark matter is the bullet cluster, which combines so many of these things. This is a cluster of galaxies where two sets of galaxies smashed into each other.

59:07And the gas in the galaxies interacted and heated up and stuck around, and the dark matter passed right through itself. because both clusters had dark matter, but dark matter doesn't interact with itself very well. It just feels gravity, which is super duper weak. So in the center, you have like the interacting matter from the galaxies, and then the dark matter passes through itself and comes out on either side. And you can tell that it's there because you can see it gravitationally lensing the background galaxies. So you can see where the matter is and where you can see where the dark matter is.

59:40So this is an example of the universe separating the matter and the dark matter. So we can tell that dark matter is its own thing. Whatever it is, we think it's matter. We know that it's separate from normal matter. This is like a death blow to theories that like maybe dark matter is just a change in how gravity works, right? It doesn't track the normal matter. It tracks where we think that dark matter is. It's its own separate thing. Take that, detractors. So those are the many lines of evidence we have for dark matter, which in my book means it's unfair to call dark matter a fudge factor. I'll give you that.

1:00:17You've convinced me. I mean, originally it was a puzzle. And if we had only seen it in galactic rotation curves and it didn't explain anything else, and it was in conflict with measurements in other places, then it would be more akin to Einstein's cosmological constant or a fake entry in your checking book. But instead, it's absolutely required to understand the evolution of the universe, the early universe, in so many different ways, we know that dark matter is there. We don't know what it is, right? We just know that it's invisible, that it's cold, and that it doesn't feel anything but gravity as far as we know.

1:00:51We've been looking for the particle that makes up dark matter for decades without any luck, right? Which is a bummer, and it sure would be nice to have, okay, here's what it is. I can show it to you. We just don't know. We don't know if there's one dark matter particle, there's many particles. If it's not even a particle, if it's some other kind of matter, and I don't want to oversell it, there are still some issues with the theory. Like, there's some distributions of dark matter in galaxies that we can't explain. You know, like, in the core of galaxies, we see different distribution than we expect.

1:01:22So there must be something going on there, maybe something weird about galaxy cores. We see a mismatch between the large number of small dark matter subhalos predicted by our simulations and the observation of dwarf galaxies around the Milky Way, though that seems to be closing with recent observations. So it's not perfect. There's definitely things to figure out, and we would love to know what dark matter actually is. But the headline is we have lots of independent lines of evidence. So I think fudge factor is not an honest criticism. I think it's an attempt to dismiss it rather than engage with it.

1:01:55So you and I have done a lot of episodes where at the end, after an hour, I've been like, and so Daniel, thank you for that explanation. How likely do you think it is that what we've talked about is actually happening? And you'll be like, oh, it's not. I really don't think it is. But you sound pretty darn convinced that dark matter is spot on, but there's still a lot left to learn. I think it's pretty convincing that there's something out there that's gravitating and is matter-like. What is it? We don't know. Is it possible we have a totally misunderstanding of general relativity and everything?

1:02:26Yeah, sure. I got a question just this morning from Cole Zurbrig who said, is it possible that what we call dark matter could just be the shape of space itself, which is a question I hear a lot. And it's a fun idea, but the challenge with these alternative ideas of dark matter is that some of them can explain one of these lines of evidence, but explaining all of them is really, really hard. It's a high standard. So somebody could come along with some new idea and topple the concepts of dark matter with some new theory, but it's got to explain everything we've seen, which is a real challenge. But we should hold new theories to the same standards to which we hold old theories.

1:03:03And very quickly, we don't think dark matter could just be the shape of space because we see the distribution of it changing over time, right? It was dense and then it spread out and then it clumped. And so that's not something space does as far as we know. And it only does it in the presence of matter. That's sort of the characteristic of matter. Anyway, so that's my summary of why I think dark matter is not a fudge factor. And so that's two marks against fudge in my book. Oh, ouch. Okay. All right. It's not my favorite dessert either, but I'll note that in a past episode, I asked if dark matter was like a fudge factor, and I will say you have convinced me.

1:03:40I will no longer refer to it as a fudge factor. There's a lot of good evidence for it, but we still have some work to do. We have lots of work to do, and there's lots of stuff still to figure out. And so write to me and let me know if this convinced you or if you still have questions about our understanding of dark matter. questions at danielandkelly.org we can't wait to hear from you go off and enjoy some fudge at least one tiny bite of it

1:04:11thanks everybody for listening please go and do us a favor and rate the show on whatever podcast app you're using it really helps people find us Daniel and Kelly's Extraordinary Universe is edited by the amazing Matt Kesselman he really is a wizard You can also find us online on Blue Sky, Instagram, and XDNKUniverse. Come engage with us. You can email us at questions at danielandkelly.org. We really do want to hear from you. And you can find our website, www.danielandkelly.org, where you'll also find an invitation to join our Discord, where everybody comes and talks about the amazing universe.

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