Is dark matter a fudge factor?

Daniel and Kelly talk about what it means to be a physics fudge factor and whether DM is fudgy or solid science. See omnystudio.com/listener https://omnystudio.com/listener for privacy information.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Daniel Whiteson?

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• 2:19Daniel Whiteson?

  Physicists are famously not so great at naming stuff. You've got your hadrons, your muons, your fermions. It's a mess. Then if you zoom out to the big categories of the universe, you have matter, dark matter, and dark energy. What's the connection between dark matter and dark energy? What does dark really mean? It gives you the impression that dark matter is somehow like, well, dark. But the truth is that dark matter is invisible matter. So in the end, the name dark matter is probably more misleading than helpful. And more broadly, I think physics has not done a great job in PR for dark matter. Online, you see lots of people arguing that dark matter is a scam.

• 3:04Daniel Whiteson?

  It's a placeholder. It's a fudge factor. It's not actually science. What does that mean? And do those criticisms have any merit? So on today's episode, we're going to dive into the question of fudge factors. What is a fudge factor actually? Does it have to have peanut butter in it? Is it so bad? Does dark matter qualify as a fudge factor? Since we haven't identified what particle it's made out of? Is that a fair criticism of modern physics? Welcome to Daniel and Kelly's extraordinarily fudgy universe.

• 3:49Kelly Weinersmith?

  Hello, I'm Kelly Wiener Smith. I study parasites and space. And today we're talking about fudge factors, but not the kind of fudge that I like.

• 3:58Daniel Whiteson?

  Hi, I'm Daniel. I study particles and aliens, and I don't actually even like fudge.

• 4:03Kelly Weinersmith?

  What? Who are you? Okay, so I understand if you don't like white chocolate fudge. None of us would be surprised. But you don't even like, like, chocolate fudge or peanut butter fudge? Or?

• 4:15Daniel Whiteson?

  I mean, I like going to state fairs and I like tasting fudge. But it's a little bit like eating butter. You know, like, the tiniest little bit in your mouth is all right. But like, nobody wants to like, buy a big chunk of fudge and walk around snacking on it. You know, it just makes me sick.

• 4:31Kelly Weinersmith?

  Speak for yourself. No, no, you're right. You're right. I like fudge in small quantities. But when I was a kid, my mom did find me underneath the table snacking on a stick of butter. So.

• 4:42Kelly Weinersmith?

  I'm already a bit of a weird, weird kid, but the extraordinary should know that beforehand, you and I had a discussion about whether or not the jokes were going to be about fudge or about poop. And we decided to go with food. So everyone should give us a little pat on the back for that.

• 4:57Daniel Whiteson?

  You're saying we're going to totally avoid poop jokes the whole episode? And that's a challenge.

• 5:01Kelly Weinersmith?

  I doubt it, especially because you have enjoyed making dark matter poop jokes in the past. And so

• 5:07Daniel Whiteson?

  They're so easy and fun.

• 5:08Kelly Weinersmith?

  I know, it could be either one of us, really. But okay, so fudge factor made me think fear factor. And so what is one of your irrational fears?

• 5:21Kelly Weinersmith?

  Let it all out there, Daniel.

• 5:23Daniel Whiteson?

  Being trapped in a room and forced to eat fudge.

• 5:26Kelly Weinersmith?

  Oh, what a

• 5:29Kelly Weinersmith?

  Do better. Do better.

• 5:32Daniel Whiteson?

  I have that thing where you're standing on a balcony like 50 stories up and wonder, am I gonna jump?

• 5:38Kelly Weinersmith?

  Yeah.

• 5:39Daniel Whiteson?

  What would it be like to jump? Maybe I should back away from this balcony. What's wrong with me?

• 5:43Kelly Weinersmith?

  Yeah, I don't trust me anymore. I need an adult.

• 5:49Daniel Whiteson?

  Cause it would just take a second, you know? And it's irreversible. And I don't know why my brain goes there when I stand at the edge of anything. But it's weird.

• 5:57Kelly Weinersmith?

  That is weird. That's it. Sounds like you're not afraid. You're more like curious. But maybe you should be afraid.

• 6:03Daniel Whiteson?

  I'm not that curious. I pretty much know what would happen. But somehow I'd just be like myself floating out there. I don't know. It's weird. Anyway, so that's an irrational fear that I'll just like jump off a balcony.

• 6:14Kelly Weinersmith?

  That's yes. Well, please don't do that. And especially don't do that before you have explained to all of us if dark matter is a fudge factor or not, because we're all dying to know.

• 6:26Daniel Whiteson?

  This is a topic you see discussed out there, mostly folks on the internet dismissing dark matter as an idea, calling it a fudge factor or a placeholder. And I get a lot of emails about this. And so I thought, let's dig into this question. What do we mean by a fudge factor? Is it fair to call dark matter a fudge factor? Are fudge factors actually so bad, even if they have chocolate chips in them?

• 6:50Kelly Weinersmith?

  That's right. Or peanut butter. Peanut butter's my thing, really, but.

• 6:53Daniel Whiteson?

  Peanut butter fudge is always improved by chocolate chips.

• 6:56Kelly Weinersmith?

  Yeah. Oh, yeah. I'll give you that. Okay. So, so is this an episode? Is this like a defensive episode? Like, you guys gotta stop picking on dark matter? Or, or what do you think? Because I was going through presidential address for the American Society of Parasitologists, and one of the address was something like parasitology really is a science. And I was like, oh, oh, it's not good to be in the position of needing to argue that.

• 7:17Daniel Whiteson?

  No, this is not defensive. This is a share the joy episode. This is make sure everybody appreciates the incredible science behind dark matter, what we do know about the universe, and also the open question. So we can legitimately discuss where we stand as a science, rather than dismissing it with silly slogans.

• 7:37Kelly Weinersmith?

  All right. Love it. So let's go ahead and see what the extraordinaries know about dark matter and how much wonder they feel about the topic.

• 7:45Speaker 07

  Dark matter is a fudge factor in that it was invented to explain observations, but it seems to be physically real. But is this how we can detect it? Stop trying to see it. Taste it instead. Fudge as in chocolate fudge? I don't know. But I think it's everywhere and it's

• 8:06Speaker 03

  dark. And there is dark chocolate. So maybe yes. I think dark matter is a fudge factor because. Well, it must be because we don't fully understand dark matter currently.

• 8:17Daniel Whiteson?

  Matter, energy, lemonade around the corner, cosmological fudge is made. Until we can find out what it is, it is a fudge factor. I guess dark matter could just be a mathematical artifact, but I wouldn't bet

• 8:29Kelly Weinersmith?

  money on it. I think dark matter is a cosmological fudge factor because it shapes how galaxies form. The term dark matter is in itself a placeholder, because we really, we really don't know what it is.

• 8:42Speaker 03

  I don't think that dark matter is a fudge factor. Be dark matter is forces that are twisted in a different direction. Cosmological fudge factor? Eh, there's something there. I believe that in a way, dark matter is a cosmological fudge factor, but it's more of an unknown. So there could be several contributing factors. Dark matter is more than one thing. For now, it is. I mean, anything we didn't understand first. Becomes a fudge factor like the ether being used to explain things that were not understood. The model seems to be working in all kinds of conditions.

• 9:23Speaker 04

  I think it is, but that's quite normal for science, isn't it? You you find something that you don't understand, give it a name, and then hopefully actually provide evidence to support its existence.

• 9:35Daniel Whiteson?

  There is a hypothesis out there that dark matter doesn't exist and we just understand gravity wrong.

• 9:42Speaker 03

  If its detection requires measurements that can never be performed or perceived, then maybe. But it's been proposed to explain a real observation. So it's not just a matter of being a fudge factor. It just might not be the correct deduction.

• 9:56Daniel Whiteson?

  Well, I'll notice the extraordinaries went for fudge jokes instead of poop jokes. So bonus points for all of you.

• 10:03Kelly Weinersmith?

  Yeah, I guess, but I feel like that's like they're all voting with you and that's not fair.

• 10:10Daniel Whiteson?

  My favorite, as always, are the musical answers to these questions.

• 10:13Kelly Weinersmith?

  Yes, I love when people are like, you know what? I'm gonna put myself out there. I'm gonna make up poems. I'm gonna sing songs and, you know, let's just have some fun.

• 10:21Daniel Whiteson?

  I wonder what Zach thinks about that, you know, since he's so critical.

• 10:24Kelly Weinersmith?

  He's, you know what? You can't stop Zach from judging. It's like you can't stop the sun from shining. And so don't worry about it. Just kidding.

• 10:34Daniel Whiteson?

  Spitting fire tonight. All right.

• 10:36Kelly Weinersmith?

  Zach catching strays.

• 10:39Daniel Whiteson?

  Let's maintain Wiener Smith marital harmony and focus on the science.

• 10:43Kelly Weinersmith?

  Okay, all right. So we are retreating quickly. So Daniel, why is it sometimes called a fudge factor?

• 10:51Daniel Whiteson?

  I think dark matter is called a fudge factor because most people know only one aspect of the dark matter story. It's the historical one and the most famous one. So let's dig into that first. And that is galactic rotation curves, like how galaxies rotate, how fast are they rotating and can we understand it? This is a fascinating bit of astronomical history that goes back almost 100 years.

• 11:15Kelly Weinersmith?

  Wow.

• 11:15Daniel Whiteson?

  It was the late 1930s when people were looking out into the universe and remember. Hubble had only recently discovered that there are things beyond the Milky Way, that there are more galaxies out there. Until Hubble's work, we thought that everything was just one galaxy. And those smudges we saw, which we now know are other galaxies, were just nebula, just blobs in our galaxy. And then people started looking out into space and looking at other galaxies and looking at galaxy clusters. But it was Babcock in 1939, who was studying Andromeda. And he was looking at how stars rotate in Andromeda and trying to understand the connection between the rotation speed and the amount of stuff in Andromeda.

• 11:56Kelly Weinersmith?

  Your 100 years things makes me wish that I could like, I would like a list of like, things that happened with 100 years between them, just to think about what it would be like to be a person who lived to be 100 and imagine like, I don't know, the telegraph to the cellphone. And I probably have insulted somebody. It's probably, anyway, let's move on.

• 12:13Daniel Whiteson?

  So the connection here, the basic physics is that if you are moving in a circle, you need some force to hold you in, right? Like, if you're on a merry-go-round and somebody spins it, you've got to hold on, otherwise you're going to get thrown off the merry-go-round. And the faster they spin it, the harder you have to hold on, right? And eventually they spin it fast enough, you lose your grip, boom, you're flying off the merry-go-round. We've all seen that epic fail video. It's hilarious, but dangerous.

• 12:39Kelly Weinersmith?

  I actually have no idea what video you're talking about, but, but I'll go look it up.

• 12:44Daniel Whiteson?

  So the idea is you can also invert that. You can measure the velocity that things are rotating around the center and use that to infer, how strong is the force holding them in place?

• 12:54Kelly Weinersmith?

  Okay.

• 12:54Daniel Whiteson?

  Right? Which means that you can measure the mass. So just by seeing how stars are moving around a galaxy, because they are being held in place for the most part, you can tell how much mass is there holding them in.

• 13:06Kelly Weinersmith?

  Okay, but that's got to be hard to tell from the kind of microscope you had in 1939.

• 13:12Daniel Whiteson?

  Well, if you're looking at stars through the microscope, you're probably not going to learn anything.

• 13:15Kelly Weinersmith?

  Telescope, telescope.

• 13:16Daniel Whiteson?

  Instead, you would use a telescope.

• 13:19Kelly Weinersmith?

  You got me. Okay. The point stands, Webster. But yeah, I mean, it's going to be hard to get information from a telescope from 1939, right?

• 13:29Daniel Whiteson?

  Well, all you need to do is measure the velocity of those stars, which means you're measuring the redshift. Because we're not watching these galaxies long enough to actually see them move and calculate the velocity based on changing distance. We're measuring their velocity based on the light that comes to us being redshifted or blueshifted. And you can definitely do that with these telescopes. I mean, that was Hubble's discovery a decade before Babcock is measuring the redshift of all these galaxies. So that's definitely something that we could do back then.

• 13:57Kelly Weinersmith?

  Okay.

• 13:58Daniel Whiteson?

  So you have now the velocity of the stars. You can tell how much mass there has to be, and not just like the total amount of mass in the galaxy, but because you can measure the velocity of stars close to the center and further from the center and yet further from the center, you can tell how much mass there has to be in each of those enclosed shells, which means you can tell the mass distribution in the galaxy. It's really amazing, just from the velocity. You can tell not only how much mass there is total in the galaxy, but also how it's spread radially.

• 14:26Kelly Weinersmith?

  Holy cow.

• 14:27Daniel Whiteson?

  And then you can say, well, can I explain that mass? Do I know what that mass is? And if you look at the distribution of stars, you can tell, well, there's a big blob of gas here and a big blob of gas there. And the problem, the galaxy rotation puzzle was there wasn't nearly enough stars to explain all the mass to give the gravity to hold the galaxy in place.

• 14:46Kelly Weinersmith?

  And so it's got a, here's Kelly being a skeptical jerk again, but like, how can you tell from so far away how big the stars are? How could you be like, oh, no, you couldn't possibly weigh that much? Like, I'm not good at guessing people's weight. How is Babcock guessing the weight of a star from another galaxy in 1939?

• 15:05Daniel Whiteson?

  Yeah, totally fair. It's hard, actually, to measure the mass of a distant star. All we can see is the light from it. And that's like where the whole other episode of how we know the connection between the brightness of a star and its mass. There's a lot of complicated science there. And you're right, there are uncertainties. But even with In those uncertainties, right? Even if those uncertainties are a hundred percent, that's not enough to explain this discrepancy.

• 15:29Kelly Weinersmith?

  Oh, wow.

• 15:29Daniel Whiteson?

  And it's not just the amount, but the distribution of the mass. If it was just stars, then as you got further out towards the edge of the galaxy, then you expect the gravity to get weaker and weaker because you're getting further from the center, the cluster of stars. And yet it doesn't. The velocity of stars near the edge of the galaxies is pretty high. It just stays high. It doesn't fall like you would expect. And that suggests that there's a lot of mass further out as well. So it's not just the amount, but it's the distribution. And so Babcock saw this in 1939 and he said, well, there must be something absorbing the light. Maybe I'm not seeing all of it or something. He didn't jump to this conclusion of unseen matter. But we saw this already in Andromeda in 1939, but it was just sort of like a puzzle like,

• 16:12Daniel Whiteson?

  hmm, that's weird, but you know, there's lots of things in the sky that don't quite make sense. You don't jump from that to like, maybe 80% of the matter in the universe is invisible, actually.

• 16:22Kelly Weinersmith?

  Right, right. That does seem crazy, Daniel.

• 16:24Daniel Whiteson?

  Yes, and it's not like astronomers went straight to that answer, right? They were like, let's think about other things and all sorts of stuff.

• 16:30Kelly Weinersmith?

  So wait, he just threw up his arms and he was like, oh, that was totally off. Those calculations were nowhere near what I thought they were going to be.

• 16:37Daniel Whiteson?

  Yeah, and I think people underestimate how often that happens in astronomy and how often it used to happen. And there was lots of times people were like, well, this is weird, shrug. And then, you know, let's keep studying it. This is a puzzle. That's the joy of astronomy, actually, is that every time you look at into the universe, there's something out there that doesn't make sense. And many of them have boring explanations like, oh, there's dust cloud absorbing XYZ, but some of them don't, you know, and, and those are the wonderful ones. So that's where it stood for about 30 years until the 1970s. And in the seventies, it was a revolution in how well we could measure these galaxies. So Vera Rubin, who died before she could be awarded a Nobel Prize,

• 17:17Daniel Whiteson?

  and Kent Ford measured this very accurately for a bunch of spiral galaxies using new technology. So they used their super fancy astronomical microscopes and they pointed them up at the sky and they measured this.

• 17:30Kelly Weinersmith?

  Misspeak once and it becomes a thing. All right.

• 17:33Daniel Whiteson?

  Welcome to the podcast.

• 17:36Kelly Weinersmith?

  Where have you been, Kelly?

• 17:38Daniel Whiteson?

  Exactly. And so they repeat this for lots and lots of galaxies and they see the same kind of pattern. And then radio astronomers, radio astronomy having been developed basically at the advent of World War II, use it to study the velocity of hydrogen. So Vera Rubin and folks are looking at stars, but they can look at emission from hydrogen. And hydrogen is like this huge cloud of gas that extends well beyond the visible part of the galaxy. So it's like a new kind of tracer that lets you measure the velocity past the edge of the stars. So now we can measure much further out, which gives you a stronger lever arm. So by the eighties and nineties, we had seen similar behavior in like thousands of galaxies. This is the kind of thing we saw everywhere.

• 18:21Kelly Weinersmith?

  And was it always off by like the same factor? Like it was always 50% off of what they expected?

• 18:27Daniel Whiteson?

  So there turns out to be a significant variation galaxy to galaxy. Some galaxies have more, some galaxies have less, but overall the number is about five to one. To explain this using invisible matter, now we call it dark matter, you would need invisible matter in the amount of five times the matter in stars and gas and dust. So like not a small hypothesis, right?

• 18:49Kelly Weinersmith?

  Yeah.

• 18:50Daniel Whiteson?

  And I think this is the origin of the fudge factor or the placeholder because dark matter wasn't something directly observed like, oh, look, we found this thing. What is it? There was a discrepancy between two measurements and like, yeah, you could add something to your theory to make those measurements fit. And so that's why I think it's called a fudge factor or a placeholder or this kind of stuff because it was introduced to explain a discrepancy between expectations and observations.

• 19:19Kelly Weinersmith?

  So it's not because we thought it was a load of poop. We, it's a discrepancy.

• 19:24Daniel Whiteson?

  Or because Kelly got caught under the table snacking on it all day long.

• 19:31Kelly Weinersmith?

  And so there's no, there's no way that it's like, we just didn't realize that suns are made out of something like much heavier. It's not that we just are totally blowing it on the mass measurements. It's gotta be something totally different because this, this fudge factor is so big.

• 19:46Daniel Whiteson?

  Yeah, the fudge factor is big. And again, the distribution doesn't match the distribution from stars. And we've done a lot of work to understand the masses of stars. And we think we do, like, we see binary stars out there, and we can see them orbiting each other. And from that, you can measure their masses. So it doesn't make sense for it to be from stars. Either it's a misunderstanding of gravity, or something else, or invisible matter, but it's definitely not from stars.

• 20:12Kelly Weinersmith?

  Okay, so you all have gotten away with having something like a fudge factor for a long time. What are some other fields that are trying to get away with having fudge factors? Are we okay with this in other domains?

• 20:25Daniel Whiteson?

  Yeah, I think that fudge factors, you know, as delicious as they sound, often get a bad rap, right? Because often, they're very useful to sum up a lot of details that you don't yet understand, or maybe don't care about, in a single number. And we do this all the time in physics. We say, look, there's a lot of complicated stuff going on here. We can't model all of it. Can we just approximate it with a single number in a simplified theory? For example, people who do introductory physics learn about the coefficient of friction, right? Like, a box is sliding down a plane, right? Classic problem. And it doesn't slide smoothly. There's friction between them. And you can describe that friction with a single number. What's really going on in the details is really

• 21:09Daniel Whiteson?

  complicated. You have these two surfaces that are grabbing at each other. But incredibly, you can summarize all of that basically with a single number. And different surfaces have a different number, and you can measure it for each surface. And that's a pretty good model of friction. It ignores, or summarizes essentially, a lot of tiny little details. Why Teflon is slippery, why concrete is not, etc. But it's useful. So fudge factors, you know, let you skip over the details that you're not interested in.

• 21:37Kelly Weinersmith?

  But there, you know, like, it's, you know, one molecule hits another molecule, and it slows them down, or something like that. Like, you've got like, a general sense of what it is that you're summarizing, and you're making measurements to get that quote unquote, fudge factor. That doesn't feel fudgy to me, I guess.

• 21:55Daniel Whiteson?

  Well, you're right. You'd like to understand it. You always want to, but it's actually really important. Then in physics, we don't have to, right? There's lots of things we don't understand about the microphysics. Why does the electron have the mass that it does? We didn't understand the mechanisms there until a few years ago when we discovered the Higgs boson, but we could still build cell phones and we could still do all sorts of stuff. And so the fact that we're not paralyzed by understanding the microphysics at every single stage allows us to make progress. And yes, you're right. We always want an understanding. The best case scenario is like, okay, later we came along, we figured it out, and we can predict or derive those numbers we were using earlier. That would be awesome, right? But it's like a good first step, right? Until you do understand it. So I'm not saying you should always use fudge factors.

• 22:38Daniel Whiteson?

  I'm saying sometimes fudge factors are actually very useful as a first step towards deeper understanding.

• 22:43Kelly Weinersmith?

  Yeah, sure. I guess the first person who, what, used Willow to get the aspirin to feel better was like, I don't care why this works, but I feel better now. And so, yeah, I'm not up here on my high horse.

• 22:56Daniel Whiteson?

  And do we now understand how aspirin works?

• 22:57Kelly Weinersmith?

  No, I don't think so. I doubt it.

• 22:59Daniel Whiteson?

  Hmm, but we still take it, right?

• 23:01Kelly Weinersmith?

  That's right. Yeah, we sure do. Lots of drugs we don't understand, but we pop in there anyway.

• 23:05Daniel Whiteson?

  Exactly. The criticism, of course, and we shouldn't avoid that, is when you just sort of add it to the equations to make it work without any sort of science or maybe even interest in figuring it out, right? Because it's easy to do that. In some cases, you can just measure how far your theories are off and then just stick in a number and then everything works. But it's explanatory, it's descriptive, it's not predictive. It doesn't generalize, and so it's not likely to be real. You know, the fudge factors I was talking about earlier, like the coefficient of friction, I can not care about the microphysics and I can measure this, and then I can go off and use it in other scenarios, right? It works as a theory. It is generalizable. Even if it is ignoring a lot of details,

• 23:47Daniel Whiteson?

  it doesn't just apply to that one experiment I was doing. It works anytime wood comes into contact with concrete, mostly. So a real fudge factor is when you're like, okay, this isn't working, and I'm just going to fix this one thing, and it's not going to generalize very well. And we do have examples. Of that in physics like Einstein and the cosmological constant. You know, Einstein was developing his theory of gravity and his nascent theory predicted that the universe should collapse in on itself, like the universe was filled with matter, matter gravitates, it should all just attract and collapse.

• 24:20Kelly Weinersmith?

  Shoot.

• 24:20Daniel Whiteson?

  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. So then he abandoned it because he was 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.

• 25:03Daniel Whiteson?

  It was just like, this seems wrong and adding in a number.

• 25:06Kelly Weinersmith?

  Okay.

• 25:07Daniel Whiteson?

  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.

• 25:28Kelly Weinersmith?

  Got 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.

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  What's that? My fresh detector. Uh, your what? There it is. What is it detecting? Fresh produce? Fresh meat? Okay, what happens if it detects something really fresh?

• 27:07Speaker 03

  Apparently, we celebrate. At Food Lion, you can count on fresh food at fresher prices when your fresh detector is going off. Head to your local Food Lion or shop at foodlion.com. ChatGPT for Teens is designed to help teens learn, think critically, and grow their skills. ChatGPT for Teens supports active learning by breaking concepts down. Rather than giving teens the answer, study mode helps teens work through problems and interactive quizzes test their knowledge and provide immediate feedback. Safety protections are on by default, and parental controls offer families additional options, like setting study hours and quiet hours. Visit chatgpt.com/parents to learn more.

• 27:58Kelly Weinersmith?

  All right, Daniel, you are in the hot seat 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?

• 28:09Daniel Whiteson?

  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.

• 28:32Kelly Weinersmith?

  Okay.

• 28:33Daniel Whiteson?

  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? 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,

• 29:17Daniel Whiteson?

  that dark matter would get frozen out. So 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 70s convinced people, hmm, this is maybe plausible. And I think those two lines of argument together turned 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.

• 29:55Kelly Weinersmith?

  So I'm not totally sure I'm following. So it sounds like they were like, they were sitting down and they were like, pass the bowl, pass the banana peel. And, and then they were like, what?

• 30:04Daniel Whiteson?

  Is that the first step in theoretical physics for you?

• 30:07Kelly Weinersmith?

  I mean, I know enough physicists. So they were, and so they, they were imagining what it was like at the beginning of, 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? Kind of matches what we see happening right now. Whoa.

• 30:30Daniel Whiteson?

  Yeah.

• 30:30Kelly Weinersmith?

  Is that fair to say?

• 30:31Daniel Whiteson?

  Yeah, exactly.

• 30:32Kelly Weinersmith?

  Okay.

• 30:33Daniel Whiteson?

  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.

• 30:44Kelly Weinersmith?

  Okay.

• 30:45Daniel Whiteson?

  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. They'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.

• 31:29Kelly Weinersmith?

  Okay, all right. So he scales up and does he find he's off by about what he expected he'd be off by?

• 31:34Daniel Whiteson?

  So this is 1933.

• 31:36Kelly Weinersmith?

  Oh, yeah, this is earlier.

• 31:37Daniel Whiteson?

  This is before Babcock. This is before the idea of invisible matter.

• 31:41Kelly Weinersmith?

  Okay.

• 31:41Daniel Whiteson?

  Right? And remember Babcock saw the galactic rotation curves and didn't even jump to that idea of dark matter. But Zwicky is looking at the Koma Cluster. This is a cluster of about 1,000 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 like 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."

• 32:25Kelly Weinersmith?

  Whoa.

• 32:26Daniel Whiteson?

  So this is already back in 1933. This is an analogous piece of evidence, not only within galaxies, but between galaxies.

• 32:34Kelly Weinersmith?

  Heck, it's almost a bummer. We don't still call it dunkelmaterie. Because I feel like dunkel even lends itself more to poop jokes. It's, that just sounds like a fecal related word, but anyway.

• 32:45Daniel Whiteson?

  It does, like you gotta wipe that dunkel away there.

• 32:47Kelly Weinersmith?

  That's right.

• 32:49Daniel Whiteson?

  The dangling dunkel is the thing to worry about.

• 32:51Kelly Weinersmith?

  Dunkelberries.

• 32:55Daniel Whiteson?

  Nobody ever make a dunkelberry pie.

• 32:57Kelly Weinersmith?

  Oh, gosh.

• 32:59Kelly Weinersmith?

  You've got a little Dunkleberry on your face there. You gotta, okay, let's move on.

• 33:03Daniel Whiteson?

  Do you remember like 20 minutes ago, we challenged ourselves not to make any poop jokes?

• 33:08Kelly Weinersmith?

  We do, we do, we do.

• 33:11Daniel Whiteson?

  False hopes.

• 33:11Kelly Weinersmith?

  That's right. Yeah. All right. Give me some more evidence.

• 33:15Daniel Whiteson?

  All right. So that still feels like maybe an extension of galaxy rotation curves, very similar. Though, you know, 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? Stuff 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.

• 33:57Daniel Whiteson?

  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. So 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.

• 34:33Kelly Weinersmith?

  And we once weren't sure if it was pigeon poop or not, right? Is this?

• 34:36Daniel Whiteson?

  That's right. Yes, exactly. Thank you for adding a completely independent line of poop jokes to the episode.

• 34:42Kelly Weinersmith?

  You're welcome.

• 34:42Daniel Whiteson?

  Poop jokes, also not a fudge factor, right? They're a load bearing support for the podcast.

• 34:49Kelly Weinersmith?

  Okay. All right. But, but the scientists working on the project decided it wasn't pigeon poop that was messing up their measurements. It was actual cosmic background radiation. So let's get back on track.

• 35:00Daniel Whiteson?

  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 hotspots 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. And 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

• 35:43Daniel Whiteson?

  less dense. We can see stuff sloshing around in the early universe. Well. 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 one 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.

• 36:27Daniel Whiteson?

  You 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? All these ratios predict different shaped wiggles, different heights of the wiggles, different widths of the wiggles. So we look at the And we look at the hot spots and the cold spots.

• 37:13Daniel Whiteson?

  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. This 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%.

• 37:57Daniel Whiteson?

  So those numbers, totally independent, very precise, amazing measurements here.

• 38:03Kelly Weinersmith?

  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? Or 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?

• 38:28Daniel Whiteson?

  Yes, it's more like the second.

• 38:29Kelly Weinersmith?

  Cool.

• 38:29Daniel Whiteson?

  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 and 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.

• 38:59Kelly Weinersmith?

  Okay, that's pretty cool.

• 39:01Daniel Whiteson?

  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. They just do not work without some kind of invisible gravitating matter.

• 39:20Kelly Weinersmith?

  Okay, that is awesome.

• 39:22Daniel Whiteson?

  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, 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. We 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?

• 40:06Daniel Whiteson?

  Now, of course, out in space, it's way too cold and pressure is way too low for fusion. But there was a literal few minutes when the universe could fuse, not in stars, but way before there were stars.

• 40:18Kelly Weinersmith?

  Wow.

• 40:18Daniel Whiteson?

  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 like work all the way up to iron, for example. But 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 gives us a really broad understanding of normal

• 41:00Daniel Whiteson?

  matter. Like, some people ask sometimes, couldn't dark matter just be like a bunch of rocks out there you haven't seen? Because stuff in space is dark sometimes, right? And people look for this, right? And they're called Machos, massive astronomical, compact halo objects, Machos.

• 41:16Kelly Weinersmith?

  Nice.

• 41:17Daniel Whiteson?

  And for a while, it was like a really nice counterpart to the WIMP theory, weakly interacting massive particles.

• 41:23Kelly Weinersmith?

  I need to know how long somebody worked to come up with Macho after the WIMP theory. I bet it was like a week or two.

• 41:29Daniel Whiteson?

  Astronomical acronyms are the worst. Slash best, slash best.

• 41:33Kelly Weinersmith?

  That's right. That's right. A little bit of both.

• 41:35Daniel Whiteson?

  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. And 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,

• 42:20Daniel Whiteson?

  but this is evidence that it's not atomic matter. Whatever it is out there, it's not made out of quarks and electrons.

• 42:26Kelly Weinersmith?

  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 gonna beat it some more.

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• 43:15Kelly Weinersmith?

  Hey, this is Mario Lopez from all Mario Lopez Daily Podcast. Buy smart, drive for less, and trade for more. That's Toyota affordability. And it starts with a great deal on an all-wheel-drive Camry, Corolla Cross, or a Corolla sedan with the latest features in tech. Test drive a spacious Grand Highlander or adventure-ready RAV4. And with so many hybrid models available, you can enjoy longer trips without all the stops. While Toyota's legendary resale value means you could get more at trade-in. So get a great deal today and great value tomorrow. Shop toyota.com for deals and details. Toyota, let's go places. What's that? My fresh detector. Uh, your what? There it is. What is it detecting?

• 43:59Kelly Weinersmith?

  Fresh produce, fresh meat.

• 44:01Speaker 03

  Okay, what happens if it detects something really fresh?

• 44:07Kelly Weinersmith?

  Apparently, we celebrate.

• 44:09Speaker 03

  At Food Lion, you can count on fresh food at fresher prices when your fresh detector is going off. Head to your local Food Lion or shop at foodlion.com. ChatGPT for Teens is designed to help teens learn, think critically, and grow their skills. ChatGPT for Teens supports active learning by breaking concepts down. Rather than giving teens the answer, study mode helps teens work through problems, and interactive quizzes test their knowledge and provide immediate feedback. Safety protections are on by default, and parental controls offer families additional options, like setting study hours and quiet hours. Visit chatgpt.comPARENTS to learn more.

• 44:57Kelly Weinersmith?

  All 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.

• 45:08Daniel Whiteson?

  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. But at the moment that the universe cooled enough to form hydrogen, right? Where the protons and electrons found each other, now that it became neutral, and they were protecting. 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

• 45:50Daniel Whiteson?

  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. If 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 like 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 you not only do you see it statistically, like it's cool to see

• 46:35Daniel Whiteson?

  a bump in the galaxy to galaxy mean distance, but also, we've recently seen a single individual, like actual bubble, like a ring structure where you can like, look at the distribution of galaxies and you're like, why is there this huge bubble there? And 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.

• 47:04Kelly Weinersmith?

  That's awesome. And you said, we found that out pretty recently? Is that right?

• 47:07Daniel Whiteson?

  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 mega parsecs 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?

• 47:27Kelly Weinersmith?

  No, no, nice try. This is your outline. You got us in this situation, in this mess. You try it.

• 47:34Daniel Whiteson?

  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.

• 48:00Kelly Weinersmith?

  This stuff is so important. Why can't we see it? That's really frustrating.

• 48:06Kelly Weinersmith?

  I guess you're seeing it indirectly is maybe the point.

• 48:08Daniel Whiteson?

  Yeah. And you know, 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.

• 48:22Kelly Weinersmith?

  All right, give me another one, Daniel.

• 48:24Daniel Whiteson?

  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. You 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.

• 49:05Daniel Whiteson?

  It takes like tens of billions of years for that to happen without dark matter.

• 49:10Kelly Weinersmith?

  Wow.

• 49:10Daniel Whiteson?

  With dark matter, what happens? Well, remember, dark matter is not getting pushed around by photons. So 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. Without dark matter as an element of those calculations,

• 49:54Daniel Whiteson?

  you don't predict large scale structure the way we see it at all.

• 49:58Kelly Weinersmith?

  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?

• 50:05Daniel Whiteson?

  Yeah.

• 50:05Kelly Weinersmith?

  Yeah, okay.

• 50:06Daniel Whiteson?

  Yeah, absolutely. And not only does it require dark matter, it requires dark matter and normal matter in the proportions of 25% and 5%. Five percent, 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. So 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

• 50:48Daniel Whiteson?

  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.

• 51:09Kelly Weinersmith?

  Okay, got it. Gosh, it's, I don't mean to be hammering home this negative point, but we're like, 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 like, interact with it or measure it more directly.

• 51:24Daniel Whiteson?

  Well, that's the problem is the only interaction we've ever seen is gravitational and gravity's 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. But depending on the nature of that stuff, it dilutes differently. Matter dilutes in a very intuitive way. Like if you have an M and M in your living room, then you have a certain amount of M and M density. If you like quadruple the size of your living room and don't change the number of M and 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.

• 52:09Kelly Weinersmith?

  And then the mood drops because you're like, I have less M and Ms relative to space and life is sad. And okay.

• 52:17Daniel Whiteson?

  But photons react to expansion differently because as the universe expands, the energy in matter stays. But 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 redshifted to have less energy.

• 52:57Kelly Weinersmith?

  Okay.

• 52:58Daniel Whiteson?

  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. Not 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

• 53:43Daniel Whiteson?

  of evidence. That's one reason why we call it matter.

• 53:45Kelly Weinersmith?

  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. Is that, was this part of how you all figured out you were going to call it dark matter?

• 53:56Daniel Whiteson?

  This is, you know, many lines of evidence. It's 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? Is 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, hmm, 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. Like, creating a puzzle is the way people get faculty positions. If anything, people are incentivized to, like, overhype some new thing that doesn't seem to make sense.

• 54:39Kelly Weinersmith?

  Yeah. Well, surely, there's no more lines of evidence, Daniel.

• 54:43Daniel Whiteson?

  We haven't even gotten to the best ones, Kelly.

• 54:45Kelly Weinersmith?

  What? But we're almost at the end of the show, Daniel.

• 54:50Daniel Whiteson?

  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. Then 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.

• 55:30Kelly Weinersmith?

  What?

• 55:30Daniel Whiteson?

  So those numbers line up really nicely.

• 55:33Kelly Weinersmith?

  Gold sticker for the physicists.

• 55:37Daniel Whiteson?

  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. And 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.

• 56:19Daniel Whiteson?

  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. And again, this measurement is not consistent with the 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.

• 56:50Kelly Weinersmith?

  Wow.

• 56:50Daniel Whiteson?

  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 a distribution of dark matter exactly where we expect it and in the same amount. So 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.

• 57:31Kelly Weinersmith?

  I am running out of ways to say whoa and awesome. So this time I'm going with gee whillikers, Daniel.

• 57:37Daniel Whiteson?

  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. And 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 you can see where the dark matter is.

• 58:21Daniel Whiteson?

  So 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 of 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 the dark matter is. It's its own separate thing.

• 58:46Kelly Weinersmith?

  Take that detractors.

• 58:49Daniel Whiteson?

  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.

• 58:57Kelly Weinersmith?

  I'll give you that. You've convinced me.

• 58:59Daniel Whiteson?

  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. We'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.

• 59:44Daniel Whiteson?

  We don't know if there's one dark matter particle, if 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. So 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 observations 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,

• 1:00:27Daniel Whiteson?

  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.

• 1:00:37Kelly Weinersmith?

  So 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.

• 1:00:56Daniel Whiteson?

  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? Yeah, sure. I got a question just this morning from Cole Zerbrig, 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.

• 1:01:39Daniel Whiteson?

  But, you know, we should hold new theories to the same standards to which we hold old theories. And very quickly, like, 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 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.

• 1:02:02Kelly Weinersmith?

  Got it.

• 1:02:03Daniel Whiteson?

  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.

• 1:02:10Kelly Weinersmith?

  Oh, ouch. Okay. All right. It's not my favorite dessert either, but, 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. It is, uh, I 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.

• 1:02:29Daniel Whiteson?

  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, if you still have questions about our understanding of dark matter.

• 1:02:39Kelly Weinersmith?

  Questions at danielandkelly.org. We can't wait to hear from you.

• 1:02:43Daniel Whiteson?

  Go off and enjoy some fudge, at least one tiny bite of it.

• 1:02:53Daniel Whiteson?

  Thanks, 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.

• 1:03:01Kelly Weinersmith?

  Daniel and Kelly's Extraordinary Universe is edited by the amazing Matt Kesselman.

• 1:03:05Daniel Whiteson?

  He really is a wizard. You can also find us online on Blue Sky, Instagram, and X, D and K Universe. Come engage with us.

• 1:03:15Kelly Weinersmith?

  You can email us at [email protected]. We really do want to hear from you.

• 1:03:21Daniel Whiteson?

  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.

• 1:03:32Kelly Weinersmith?

  And we also have the most amazing moderators. This is an iHeart Podcast. Thanks for joining us.

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