Speaker 1: Physicists are famously not so great at naming stuff. You've
Speaker 1: got your hadrons, your muons, your fermions, it's a mess.
Speaker 1: Then if you zoom out to the big categories of
Speaker 1: the universe, you have matter, dark matter, and dark energy.
Speaker 1: What's the connection between dark matter and dark energy? What
Speaker 1: does dark really mean? It gives you the impression that
Speaker 1: dark matter is somehow like well dark, but the truth
Speaker 1: is that dark matter is invisible matter. So in the end,
Speaker 1: the name dark matter is probably more misleading than helpful.
Speaker 1: And more broadly, I think physics has not done a
Speaker 1: great job in pr for dark matter. Online you see
Speaker 1: lots of people arguing that dark matter is a scam,
Speaker 1: it's a placeholder, it's a fudge factor, it's not actually science.
Speaker 1: What does that mean and do those criticisms have any merit?
Speaker 1: So on today's episode, we're going to dive into the
Speaker 1: question of fudge factors. What is a fudge factor? Actually?
Speaker 1: Does it have to have peanut butter in it? Is
Speaker 1: it so bad? Does dark matter qualify as a fudge
Speaker 1: factor since we haven't identified what particle it's made out of.
Speaker 1: Is that a fair criticism of modern physics? Welcome to
Speaker 1: Daniel and Kelly's Extraordinarily Fudge Universe.
Speaker 2: Hello, I'm Kelly Windersmith.
Speaker 3: I study parasites and space, and today we're talking about
Speaker 3: fudge factors, but not the kind of.
Speaker 2: Fudge that I like.
Speaker 1: Hi, I'm Daniel. I study particles and aliens, and I
Speaker 1: don't actually even like fudge.
Speaker 2: What who are you? Okay?
Speaker 3: So I understand if you don't like white chocolate fudge,
Speaker 3: none of us would be surprised. But you don't even
Speaker 3: like like chocolate fudge or peanut butter fudge or.
Speaker 1: I mean, I like going to state fairs and I
Speaker 1: like tasting fudge, but it's a little bit like eating butter,
Speaker 1: you know, like the tiniest little bit in your mouth
Speaker 1: is all right, but like nobody wants to like buy
Speaker 1: a big chunk of fudge and walk around snacking on it.
Speaker 1: You know, it just makes me sick.
Speaker 3: Speak for yourself. No, No, you're right, You're right. I
Speaker 3: like fudge in small quantities. But when I was a kid,
Speaker 3: my mom did find me underneath the table snacking on
Speaker 3: a stick of butter. So I'm already a bit of
Speaker 3: a weird, weird kid. But the extraordinary should know that beforehand.
Speaker 3: You and I had a discussion about whether or not
Speaker 3: the jokes were going to be about fudge or about poop,
Speaker 3: and we decided to go with food.
Speaker 2: So everyone should give us a little pet on the
Speaker 2: back for that.
Speaker 1: You're saying we're going to totally avoid poop jokes the
Speaker 1: whole episode, and that's a challenge.
Speaker 3: I doubt it, especially because you have enjoyed making Dark
Speaker 3: Matter poop jokes in the past, and so.
Speaker 1: They're so easy and fun.
Speaker 3: I know it could be either one of us, really,
Speaker 3: but okay, so fudge factor made me think fear factor,
Speaker 3: and so what is one of your irrational fears?
Speaker 2: Let it all out there, Daniel.
Speaker 1: Being trapped in a room and forced to eat fudge?
Speaker 2: Do better? Do better?
Speaker 1: I have a thing where you're standing on a balcony
Speaker 1: like fifty stories up and wonder am I gonna jump?
Speaker 1: Would you like to jump? Maybe I should back away
Speaker 1: from this balcony. What's wrong with me?
Speaker 4: Yeah?
Speaker 2: I don't trust me anymore. I need an adult.
Speaker 1: Because it would just take a second, you know, and
Speaker 1: it's irreversible. And I don't know why my brain goes
Speaker 1: there when I stand at the edge of anything, but
Speaker 1: it's weird.
Speaker 2: That is weird.
Speaker 3: It sounds like you're not afraid, you're more like curious,
Speaker 3: but maybe you should be afraid.
Speaker 1: I'm not that curious. I pretty much know what would happen,
Speaker 1: but somehow I just feel like myself floating out there.
Speaker 1: I don't know. It's weird. So that's an irrational beer
Speaker 1: that I'll just like jump off of alchemy.
Speaker 3: That's yes, Well, please don't do that, and especially don't
Speaker 3: do that before you have explained to all of us
Speaker 3: if dark matter is a fudge factor or not, because
Speaker 3: we're all dying to know.
Speaker 1: This is a topic you see discussed out there, mostly
Speaker 1: folks on the internet dismissing dark matter is an idea,
Speaker 1: calling it a fudge factor or a placeholder. And I
Speaker 1: get a lot of emails about this, and so I thought,
Speaker 1: let's dig into this question. What do we mean by
Speaker 1: a fudge factor? Is it fair to call dark matter
Speaker 1: a fudge factor? Our fudge factors actually so bad even
Speaker 1: if they have chocolate chips.
Speaker 2: In them, that's sorry, or peanut butter. Peanut butter's by thing, really, but.
Speaker 1: Peanut butter fudge is always improved by chocolate chips.
Speaker 3: Yeah, oh yeah, I'll give you that. Okay, So is
Speaker 3: this an episode? There's this like a defensive episode, like
Speaker 3: you guys got to stop picking up dark matter or
Speaker 3: what do you think? Because I was going through presidential
Speaker 3: address for the American Society of Parasitologists and one of
Speaker 3: the address was something like hersitology really is a science,
Speaker 3: and I was like, oh.
Speaker 2: Oh, it's not good to be in the position of
Speaker 2: needing to argue that.
Speaker 1: No, this is not defensive. This is a share the
Speaker 1: joy episode. This is make sure everybody appreciates the incredible
Speaker 1: science behind dark matter, what we do know about the universe,
Speaker 1: and also the open question so we can legitimately discuss
Speaker 1: where we stand as a science, rather than dismissing it
Speaker 1: with silly slogans.
Speaker 2: All right, love it.
Speaker 3: So let's go ahead and see what the extraordinaries know
Speaker 3: about dark matter and how much wonder they feel about
Speaker 3: the topic.
Speaker 5: Dark matter is a fudge factor and that it was
Speaker 5: invented to explain observations. But it seems to be physically real.
Speaker 5: But is this how we can detect it? Stop trying
Speaker 5: to see it, taste it instead.
Speaker 1: Fudge efine chocolate foodge.
Speaker 3: I don't know, but I think it's everywhere, and it's dark,
Speaker 3: and there's dark chocolate, so maybe yes.
Speaker 4: I think dark matter is a fudge factor, because well it.
Speaker 6: Must be, because we don't fully understand dark matter.
Speaker 1: Currently, matter energy lemonade around the corner. Cosmological fudge is.
Speaker 6: Made until we can find out what it is. It
Speaker 6: is a fudge factor.
Speaker 7: I guess dark matter could just be a mathematical artifact,
Speaker 7: but I wouldn't bet money on it.
Speaker 6: I think dark matter is a cosmological fludge factor because
Speaker 6: it shapes our galaxies form.
Speaker 1: The term dark matter is in itself a placeholder because
Speaker 1: we really, we really don't know what it is.
Speaker 5: I don't think that dark matter is a fudge factor.
Speaker 1: Be dark matter is forces that are twisted in a
Speaker 1: different direction. Cosmological fudge factor.
Speaker 4: There's something there.
Speaker 7: I believe that in a way, dark matter is a
Speaker 7: cosmological fudge factor, but it's more of an unknown so
Speaker 7: there could be several contributing factors.
Speaker 6: The matter is more than one thing.
Speaker 4: For now it is, I mean, anything we didn't understand
Speaker 4: first we come a fudge factor, like the ether being
Speaker 4: used to explain things that were not understood.
Speaker 6: The model seems to be working in all kinds of
Speaker 6: good conditions.
Speaker 7: I think it is. But that's quite normal for science,
Speaker 7: isn't it. You find something that you don't understand, give
Speaker 7: it a name, and then hopefully actually provide evidence to
Speaker 7: support its existence.
Speaker 1: There is a hypothesis out there that dark matter doesn't
Speaker 1: exist and we just understand gravity wrong.
Speaker 6: If it's detection requires measurements that can never be performed
Speaker 6: or perceived, then maybe, But it's been proposed to explain
Speaker 6: a real observation, so it's not just a matter of
Speaker 6: being a fudge factor. It just might not be the
Speaker 6: correct deduction.
Speaker 1: Well, I'll notice the extraordinaries went for fudge jokes instead
Speaker 1: of poop jokes. So bonus points for all of you.
Speaker 3: Yeah, I guess, But I feel like that's like they're
Speaker 3: all voting with you, and that's not fair.
Speaker 1: My favorite, as always, the musical answers to these questions.
Speaker 3: Yes, yeah, I love when people are like, you know what,
Speaker 3: I'm gonna put myself out there, I'm gonna make.
Speaker 2: Up poems, I'm gonna sing songs, and you know, let's
Speaker 2: just have some fun.
Speaker 1: I want to what Zach thinks about that, you know,
Speaker 1: since he's.
Speaker 3: So critical, he's you know, what you can't stop Zach
Speaker 3: from judging. It's like you can't stop the sun from shining,
Speaker 3: and so wow.
Speaker 1: Wow, we're spitting fire tonight.
Speaker 2: All right, Zach catching strays.
Speaker 1: Let's maintain Wiener Smith in marital harmony and focus on
Speaker 1: the science.
Speaker 3: Okay, all right, so we were retreating quickly. So, Daniel,
Speaker 3: why is it sometimes called a fudge factor?
Speaker 1: I think dark matter is called a fudge factor because
Speaker 1: most people know only one aspect of the dark matter story.
Speaker 1: It's the historical one and the most famous one. So
Speaker 1: let's dig into that first, and that is galactic rotation curves,
Speaker 1: like how galaxies rotate? How fast are they rotating? And
Speaker 1: can we understand it? This is a fast and any
Speaker 1: bit of astronomical history that goes back almost one hundred years.
Speaker 2: Wow.
Speaker 1: It was the late nineteen thirties when people were looking
Speaker 1: out in the universe. And remember Hubble had only recently
Speaker 1: discovered that there are things beyond the Milky Way, that
Speaker 1: there are more galaxies out there. Until Hubble's work, we
Speaker 1: thought that everything was just one galaxy, and those smudges
Speaker 1: we saw, which we now know are other galaxies were
Speaker 1: just nebula, just blobs in our galaxy, and then people
Speaker 1: started looking out into space and looking at other galaxies
Speaker 1: and looking at galaxy clusters. But it was Babcock in
Speaker 1: nineteen thirty nine who was studying Andromeda, and he was
Speaker 1: looking at how stars rotate in Andromeda and trying to
Speaker 1: understand the connection between the rotation speed and the amount
Speaker 1: of stuff in Andromeda.
Speaker 3: You're one hundred years things makes me wish that I could, like,
Speaker 3: I would like a list of like things that happened
Speaker 3: with one hundred years between them, just to think about
Speaker 3: what it would be like to be a person who
Speaker 3: lived to be one hundred and imagine like I don't
Speaker 3: know the telegraph to the cell phone, and I probably
Speaker 3: have insulted somebody.
Speaker 2: It's anyway, let's move on.
Speaker 1: So the connection here, the basic physics is that if
Speaker 1: you are moving in a circle, you need some force
Speaker 1: to hold you in right, Like if you're on a
Speaker 1: merry go round and somebody spins it, You've got to
Speaker 1: hold on otherwise you're going to get thrown off the
Speaker 1: merry go round. And the faster they spin it, the
Speaker 1: harder you have to hold on right And eventually they
Speaker 1: spin it fast enough you lose your grip. Boom, you're
Speaker 1: flying off the Merry Go Round. We've all seen that
Speaker 1: epic fail video. It's hilarious but dangerous.
Speaker 2: I actually have no idea what video you're talking about,
Speaker 2: but I'll go look it up.
Speaker 1: So the idea is you can also invert that. You
Speaker 1: can measure the velocity that things are rotating around the
Speaker 1: center and use that to infer how strong is the
Speaker 1: force holding them in place? Okay, right, which means that
Speaker 1: you can measure the mass. So just by seeing how
Speaker 1: stars are moving around a galaxy, because they are being
Speaker 1: held in place for the most part, you can tell
Speaker 1: how much mass is they're holding them in.
Speaker 3: Okay, but that's got to be hard to tell from
Speaker 3: the kind of microscope you had in Night teen thirty nine.
Speaker 1: Well, if you're looking at stars through the microscope, you're probably.
Speaker 2: Not going to learn anything telescope.
Speaker 8: You can use a telescope, you got me, Okay, So
Speaker 8: the point stands Webster, But yeah, I mean it's going
Speaker 8: to be hard to get information from a telescope from
Speaker 8: nineteen thirty nine.
Speaker 1: Right, Well, all you need to do is measure the
Speaker 1: velocity of those stars, which means you're measuring the red shift.
Speaker 1: Because we're not watching these galaxies long enough to actually
Speaker 1: see the move and calculate the velocity based on changing distance.
Speaker 1: We're measuring their velocity based on the light that comes
Speaker 1: to us being red shifted or blue shifted. And you
Speaker 1: can definitely do that with these telescopes. I mean that
Speaker 1: was Hubble's discovery a decade before Babcock is measuring the
Speaker 1: red shift of all these galaxies. So that's definitely something
Speaker 1: that we could do back then. So you have now
Speaker 1: the velocity of the stars. You can tell how much
Speaker 1: mass there has to be, and not just like the
Speaker 1: total amount of mass in the galaxy, but because you
Speaker 1: can measure the velocity stars close to the center and
Speaker 1: further from the center, and yet further from the center,
Speaker 1: you can tell how much mass there has to be
Speaker 1: in each of those enclosed shells, which means you can
Speaker 1: tell the mass distribution in the galaxy. It's really amazing.
Speaker 1: Just from the velocity, you can tell not only how
Speaker 1: much mass there is totally in the galaxy, but also
Speaker 1: how it's spread radially.
Speaker 2: Holy g.
Speaker 1: And then you can say, well can I explain that mass?
Speaker 1: Do I know what that mass is? And if you
Speaker 1: look at the distribution of stars you can tell, well,
Speaker 1: there's a big blob of gas here and a big
Speaker 1: blob of gas there. And the problem the galaxy rotation
Speaker 1: puzzle was there wasn't nearly enough stars to explain all
Speaker 1: the mass to give the gravity to hold the galaxy
Speaker 1: in place.
Speaker 3: Yeah, so it's got a here's Kelly being a skeptical
Speaker 3: jerk again, but like, how can you tell from so
Speaker 3: far away how big the stars are? How could you
Speaker 3: be like, ah, no, you couldn't possibly weigh that much,
Speaker 3: Like I'm not good at guessing people's weight. How is
Speaker 3: Babcock guessing the weight of a star from another galaxy
Speaker 3: in nineteen thirty nine?
Speaker 1: Yeah, totally fair. It's hard actually to measure the mass
Speaker 1: of a distant star. We sees the light from it,
Speaker 1: and that's like where the whole other episode of how
Speaker 1: we know the connection between the brightness of a star
Speaker 1: and its mass. There's a lot of complicated science there,
Speaker 1: and you're right, there are uncertainties. But even within those uncertainties, right,
Speaker 1: even if those uncertainties are one hundred percent, that's not
Speaker 1: enough to explain this discrepancy. And it's not just the
Speaker 1: amount but the distribution of the mass if it was
Speaker 1: just stars, then as you got further out towards the
Speaker 1: edge of the galaxy, then you expect the gravity to
Speaker 1: get weaker and weaker because you're getting further from the
Speaker 1: center the cluster of stars. And yet it doesn't. The
Speaker 1: velocity of stars near the edge of the galaxies is
Speaker 1: pretty high. It just stays high. It doesn't fall like
Speaker 1: you would expect. And that suggests that there's a lot
Speaker 1: of mass further out as well. So it's not just
Speaker 1: the amount, but it's the distribution. And so Babcock saw
Speaker 1: this in nineteen thirty nine and he said, well, there
Speaker 1: must be something absorbing the light. Maybe I'm not seeing
Speaker 1: all of it or something. He didn't jump to this
Speaker 1: conclusion of unseen matter. But we saw this already in
Speaker 1: Andromeda in nineteen thirty nine. But it was just like
Speaker 1: a puzzle, like, hmm, that's weird. But you know, there's
Speaker 1: lots of things in the sky that don't quite make sense.
Speaker 1: You don't jump from that to like maybe eighty percent
Speaker 1: of the matter in the universe is invisible, actually, right, right?
Speaker 2: That does seem crazy, Daniel.
Speaker 1: Yes, And it's not like astronomers went straight to that answer, right,
Speaker 1: They were like, let's think about other things and all
Speaker 1: sorts of stuff.
Speaker 3: So wait, he just threw up his arms and he
Speaker 3: was like, oh, that was totally off. Those calculations were
Speaker 3: no wordy or what I thought.
Speaker 2: They were going to be.
Speaker 1: Yeah, and I think people underestimate how often that happens
Speaker 1: in astronomy and how often it used to happen, And
Speaker 1: there's lots of times people were like, well, this is
Speaker 1: weird shrug, and then you know, let's keep studying it.
Speaker 1: This is a puzzle. That's the joy of astronomy actually,
Speaker 1: is that every time you look at into the universe,
Speaker 1: there's something out there that doesn't make sense. And many
Speaker 1: of them have boring explanations like oh, there's dust cloud
Speaker 1: absorbing XYZ. But some of them don't, you know, and
Speaker 1: those are the wonderful ones. So that's where it stood
Speaker 1: for about thirty years until the nineteen seventies. And in
Speaker 1: the seventies it was a revolution and how well be
Speaker 1: could measure these galaxies? So Vera Rubin, who died before
Speaker 1: she could be awarded a Nobel prize, and Kent Ford
Speaker 1: measured this very accurately for a bunch of spiral galaxies
Speaker 1: using new technology. So they use their super fancy astronomical
Speaker 1: microscopes and they pointed them up at the sky and they.
Speaker 2: Measured this speak once, all right, welcome to.
Speaker 3: The podcast, where have you been killing?
Speaker 1: And so they repeat this for lots and lots of
Speaker 1: galaxies and they see the same kind of pattern. And
Speaker 1: then radio astronomers radio astronomy, having been developed basically at
Speaker 1: the advent of World War two, use it to study
Speaker 1: the velocity of hydrogen. So Vera Ruben and folks are
Speaker 1: looking at stars, but they can look at emission from hydrogen,
Speaker 1: and hydrogen is like this huge cloud of gas that
Speaker 1: extends well beyond the visible part of the galaxy. So
Speaker 1: it's like a new kind of tracer that lets you
Speaker 1: measure the velocity past the edge of the stars. So
Speaker 1: now we can measure much further out, which gives you
Speaker 1: a longer lever arm. So by the eighties and nineties
Speaker 1: we had seen similar behavior in like thousands of galaxies.
Speaker 1: This is the kind of thing we saw.
Speaker 3: Everywhere, and was it always off by like the same factor,
Speaker 3: Like it was always fifty percent off of what they expected.
Speaker 1: So that turns out to be a significant variation galaxy
Speaker 1: to galaxy some galaxies have more, some galaxies have less,
Speaker 1: but overall the number is about five to one. To
Speaker 1: explain this using invisible matter, now we call it dark matter,
Speaker 1: you would need invisible matter in the amount of five
Speaker 1: times the matter in stars and gas and dust. So
Speaker 1: like not a small hypothesis, right, And I think this
Speaker 1: is the origin of the fudge factor or the placeholder,
Speaker 1: because dark matter wasn't something directly observed, like, oh, look,
Speaker 1: we found this thing, what is it? There was a
Speaker 1: discrepancy between two measurements, and like, yeah, you could add
Speaker 1: something to your theory to make those measurements fit. And
Speaker 1: so that's why I think it's called a fudge factor
Speaker 1: or a placeholder or this kind of stuff because it
Speaker 1: was introduced to explain a discrepancy between expectations and observations.
Speaker 3: So it's not because we thought it was a load
Speaker 3: of poop, wait, it's a discrepancy.
Speaker 1: Or because Kelly got caught under the table snacking on it.
Speaker 3: All day long, and so there's no there's no way
Speaker 3: that it's like we just didn't realize that suns are
Speaker 3: made out of something like much heavier. It's not that
Speaker 3: we just are totally blowing it on the mass measurements.
Speaker 3: It's got to be something totally different because this fudge
Speaker 3: factor is so big.
Speaker 1: Yeah, the fudge factor is big, and again the distribution
Speaker 1: doesn't match the distribution from stars. And we've done a
Speaker 1: lot of work to understand the masses of stars, and
Speaker 1: we think we do, like, we see binary stars out
Speaker 1: there and we can see them orbiting each other and
Speaker 1: from that you can measure their masses. So it doesn't
Speaker 1: make sense for it to be front stars. Either it's
Speaker 1: a misunderstanding of gravity or something else or in visual matter,
Speaker 1: but it's definitely not.
Speaker 3: From Okay, so you all have gotten away with having
Speaker 3: something like a fudge factor for a long time. What
Speaker 3: are some other fields that are trying to get away
Speaker 3: with having fudge factors?
Speaker 2: Are we okay with this in other domains?
Speaker 1: Yeah? I think the fudge factors, you know, as delicious
Speaker 1: as they sound, often get a bad rap, right because
Speaker 1: often they're very useful to sum up a lot of
Speaker 1: details that you don't yet understand or maybe don't care
Speaker 1: about in a single number. And we do this all
Speaker 1: the time in physics. We say, look, there's a lot
Speaker 1: of complicated stuff going on here. We can't moddel all
Speaker 1: of it. Can we just approximate it with a single
Speaker 1: number and a simplified theory. For example, people who do
Speaker 1: introductory physics learn about the coefficient of friction. Right, Like
Speaker 1: a box is sliding down a plane, right, classic problem,
Speaker 1: and it doesn't slide smoothly. There's friction between them, and
Speaker 1: you can describe that friction with a single number. What's
Speaker 1: really going on in the details is really complicated. You
Speaker 1: have these two surfaces that are grabbing in each other.
Speaker 1: But incredibly you can summarize all of that basically with
Speaker 1: the single number, and different surfaces have a different number,
Speaker 1: and you can measure it for each surface, and that's
Speaker 1: a pretty good model of friction. It ignores or summarizes
Speaker 1: essentially a lot of tiny little details why teflon is slippery,
Speaker 1: why concrete is not, et cetera. But it's useful, so
Speaker 1: fudge factors, you know, let you skip over the details
Speaker 1: that you're not interested in.
Speaker 3: But there you know, like it's you know, one molecule
Speaker 3: hits another molecule and it slows them down or something
Speaker 3: like that. Like you've got like a general sense of
Speaker 3: what it is that you're summarizing, and you're making measurements
Speaker 3: to get that quote unquote fudge factor.
Speaker 2: That doesn't feel fudgy to me.
Speaker 1: I guess, well, you're right, you'd like to understand it.
Speaker 1: You always want to, but it's actually really important than
Speaker 1: in physics, we don't have to, right, there's lots of
Speaker 1: things we don't understand it by the microphysics. Why does
Speaker 1: the electron have the mass that it does. We didn't
Speaker 1: understand the mechanisms there until a few years ago when
Speaker 1: we discovered the Higgs boson. But we could still build
Speaker 1: cell phones and we could still do all sorts of stuff.
Speaker 1: And so the fact that we're not like paralyzed by
Speaker 1: understanding the microphysics at every single stage allows us to
Speaker 1: make progress. And yes, you're right, we always want an understanding.
Speaker 1: The best case scenario is like, okay, later we came along,
Speaker 1: we've figured it out and we can predict or derive
Speaker 1: those numbers we were using earlier. That would be awesome, right,
Speaker 1: But it's like a good first step, right until you
Speaker 1: do understand it. So I'm not saying you should always
Speaker 1: use fudge factors. I'm saying sometimes fudge factors are actually
Speaker 1: very useful as a first step towards deeper understanding.
Speaker 3: Yeah, sure, I guess the first person who used willow
Speaker 3: to get the aspirint to feel better was like, I
Speaker 3: don't care why this works, but I feel better now,
Speaker 3: and so yeah, I'm not up here on my high horse.
Speaker 1: And do we now understand how aspirin works?
Speaker 2: No, I don't think so. I doubt it, but we
Speaker 2: still take it right, Yeah, we sure do.
Speaker 3: Lots of drugs we don't understand, but we pop in
Speaker 3: there anyway exactly.
Speaker 1: The criticism, of course, and we shouldn't avoid that, is
Speaker 1: when you just sort of added to the equations to
Speaker 1: make it work without any sort of science or maybe
Speaker 1: even interest in figuring it out right, because it's easy
Speaker 1: to do that. In some cases. You can just measure
Speaker 1: how far your theories are off and then just stick
Speaker 1: in a number and then everything works. But it's explanatory,
Speaker 1: it's descriptive, it's not predictive. It doesn't generalize, and so
Speaker 1: it's not likely to be real. You know, the fudge
Speaker 1: factors I was talking about earlier, like the coefficient of friction.
Speaker 1: I cannot care about the microphysics, and I can measure
Speaker 1: this and then I can go off and use it
Speaker 1: in other scenarios. Right, it works as a theory. It
Speaker 1: is generalizable, even if it is ignoring a lot of details.
Speaker 1: It doesn't just apply to that one experiment I was doing.
Speaker 1: It works anytime wood comes into contact with concrete. Mostly.
Speaker 1: So a real fudge factor is when you're like, Okay,
Speaker 1: this isn't working and I'm just going to fix this
Speaker 1: one thing, and it's not going to generalize very well.
Speaker 1: And we do have examples of that in physics, like
Speaker 1: Einstein and the cosmological constant. You know, Einstein was developing
Speaker 1: his theory of gravity, and his nascent theory predicted that
Speaker 1: the universe should collapse in on itself, like the universe
Speaker 1: was filled with matter, matter, gravitates, it should all just
Speaker 1: attract and collapse. And uh oh, he looked at in
Speaker 1: the universes like that doesn't seem to be happening. What's
Speaker 1: going on? And he was living before we even understood
Speaker 1: the universe is expanding. So he just added a number
Speaker 1: to his theories, the cosmological constant, which would balance that,
Speaker 1: which would make the universe not collapse. And it wasn't
Speaker 1: even a great fudge factor because in his description, the
Speaker 1: universe would be like balanced on a knife edge. These
Speaker 1: two numbers had to match exactly perfectly all the time,
Speaker 1: or the universe would tip over and either expand or collapse.
Speaker 1: So then he abandoned it because he's like, Okay, this
Speaker 1: is not working, and that's fair to call a fudge
Speaker 1: factor because it was just to solve that one problem
Speaker 1: and didn't generalize and didn't explain anything. It's not like
Speaker 1: summarizing some lower level details we don't have yet. It
Speaker 1: was just like this seems wrong and adding in a number, okay,
Speaker 1: And so I think it's to sum up for like,
Speaker 1: what is a fudge factor? Is it fair to criticize?
Speaker 1: It is to ask the question like does it generalize?
Speaker 1: Is this an explanation we can use somewhere else, Is
Speaker 1: it like an approximate description of something real that's happening,
Speaker 1: or is it just like, hey, my checkbook didn't balance.
Speaker 1: I just created a fake expense to explain why I'm
Speaker 1: missing five thousand dollars?
Speaker 2: Got it?
Speaker 3: Okay, Well, let's take a break, and when we get back,
Speaker 3: we're gonna see if dark matter meets the criteria that
Speaker 3: Daniel just set forth for us. All right, Daniel, you
Speaker 3: are in the hot seat now you set up some
Speaker 3: criteria for how to determine if a number is a
Speaker 3: useful fudge factor or not. How does dark matter measure up?
Speaker 1: So let's delineate the number of independent lines of evidence
Speaker 1: we have for dark because that was the test. Right.
Speaker 1: If dark matter is a fudge factor, then it only
Speaker 1: solves this one problem, the galaxy rotation curves. And if
Speaker 1: it's not, then it solves many problems independently. It generalizes.
Speaker 1: It's an explanation that seems likely to describe the real universe,
Speaker 1: not just an addition to our checkbook. Okay, So the
Speaker 1: first thing I would say is the thing that convinced
Speaker 1: folks that dark matter is more than just look, astronomy
Speaker 1: student got a wrong number, right, was not actually an
Speaker 1: experimental measurement. I think it was theoretical developments in the
Speaker 1: late nineteen seventies. People were thinking like, well, could there
Speaker 1: be more matter out there that's invisible? Like where where
Speaker 1: did it all come from? And folks were doing calculations
Speaker 1: for the early universe, like thinking about how the universe
Speaker 1: formed and how it cooled into protons and electrons, et cetera,
Speaker 1: and they realized that it actually was totally possible. During
Speaker 1: the initial production of matter of those quirks and those
Speaker 1: gluons to also produce dark matter, and in a fascinating way,
Speaker 1: as the universe expands, that dark matter would get frozen out.
Speaker 1: So the universe expands and everything gets more dilute, and
Speaker 1: dark matter, if it only interacts very very weakly, would
Speaker 1: no longer be able to find itself to like annihilate
Speaker 1: and turn into anything else. And so if you produced
Speaker 1: a bunch of dark matter in the early universe, it
Speaker 1: would still be here. And so that calculation in the
Speaker 1: seventies convinced people, Hmm, this is maybe plausible. And I
Speaker 1: think those two lines of argument together turned this from like, hm,
Speaker 1: there's something missing in galaxy to like, hmm, maybe there's
Speaker 1: something we've been missing about the universe. And then they
Speaker 1: started to look for more independent lines of experimental evidence.
Speaker 2: So I'm not totally sure I'm following.
Speaker 3: So it sounds like they were like they were sitting
Speaker 3: down and they were like, pass the bowl, pass the
Speaker 3: banana peel.
Speaker 2: And then they were like, what is.
Speaker 1: That the first step in theoretical physics for you?
Speaker 3: I mean, I know enough physicists, and so they were
Speaker 3: so they looked at they were imagining what it was
Speaker 3: like at the beginning of before the Big Bang, and
Speaker 3: they were like, what if there was a lot more
Speaker 3: matter there, what would have happened to it? And then
Speaker 3: when they projected forward, they were like, Okay, actually, what
Speaker 3: would have happened to it kind of matches what we
Speaker 3: see happening right now. WHOA yeah, is that fair to say?
Speaker 1: Yeah? Exactly. The idea is, look, if there's a bunch
Speaker 1: of additional matter created, why didn't it also like annihilate
Speaker 1: itself and disappear? Right, And this is a way to
Speaker 1: explain how you could produce dark matter and still have
Speaker 1: it be around, Okay, just as a theoretical mechanism for
Speaker 1: like how could it exist? And then people started digging
Speaker 1: into these other lines of evidence, say like, do we
Speaker 1: have other evidence that dark matter exists? And then you
Speaker 1: can go back again to the nineteen thirties. There was
Speaker 1: a guy Franz Zwicki, and he was looking not at
Speaker 1: galaxies but clusters of galaxies, right, because beyond just individual galaxies,
Speaker 1: you find groups of galaxies that are gravitationally bound, they're
Speaker 1: orbiting their common center. And then you can play the
Speaker 1: same game and you can say how fast are they moving?
Speaker 1: How massive are they? Is there enough mass in this
Speaker 1: cluster to hold it together and to explain the velocities.
Speaker 1: It's like galactic rotation curves, but now galactic cluster rotation curves.
Speaker 3: Okay, all right, So he scales up and does he
Speaker 3: find he's off by about what he expected he'd be
Speaker 3: off by.
Speaker 1: So this is nineteen thirty three. This is before Babcock.
Speaker 1: This is before the idea of invisible matter. Okay, right,
Speaker 1: I remember Babcock saw the galactic rotation curves and didn't
Speaker 1: even jump to that idea of dark matter. But Zuicki
Speaker 1: is looking at the Coma cluster. This is a cluster
Speaker 1: of about a thousand galaxies. It's like three hundred million light
Speaker 1: years from Earth, and it's really cool because it's one
Speaker 1: of the first galaxy clusters to ever been like remarked
Speaker 1: on in the early astronomical literature, like Herschel saw it
Speaker 1: in the seventeen eighties. Anyway, he was looking at this
Speaker 1: thing and he saw that it was moving way too fast.
Speaker 1: There was not nearly enough matter to explain how it
Speaker 1: was holding itself together, and so he invented this phrase.
Speaker 1: Dunkel Matiri in German, which is dark matter, And he
Speaker 1: wrote quote, if this would be confirmed, we would get
Speaker 1: the surprising result that dark matter is present in much
Speaker 1: greater amount than luminous matter. WHOA, this is already back
Speaker 1: in nineteen thirty three. This is an analogous piece of
Speaker 1: evidence not only within galaxies but between galaxies.
Speaker 2: It's almost a bummer.
Speaker 3: We don't still call it dunkel material because I feel
Speaker 3: like dunkele even lends itself more to poop jokes. That
Speaker 3: just sounds like a fecal related word.
Speaker 1: But anyway, it does like you gotta wipe that dunkel away, right,
Speaker 1: The dangling dunkel is the thing duncleberry. Nobody ever make
Speaker 1: a dunkleberry pipe.
Speaker 3: Oh gosh, you got a little dunkleberry on your face there,
Speaker 3: you gottah God, Okay, let's move on.
Speaker 1: Do you remember, like twenty minutes ago, we challenged ourselves
Speaker 1: not to make any poop you we we knew false hopes.
Speaker 2: Yeah, all right, give me some more evidence.
Speaker 1: All right? So that still feels like maybe an extension
Speaker 1: of galaxy rotation curves very similar though, you know, it's
Speaker 1: the kind of thing you should expect to see if
Speaker 1: there really is a lot of dark matter out there.
Speaker 1: The second completely independent line of evidence for dark matter
Speaker 1: is the very early universe, because if we think dark
Speaker 1: matter was made in the early universe, we should see
Speaker 1: evidence for it in the early universe, and we do
Speaker 1: if we look at the cosmic microwave background light. Remember
Speaker 1: your history of the early universe. Stuff formed, and we
Speaker 1: had quarks and gluons and electrons and photons. Then the
Speaker 1: universe expanded and cooled, quarks and gluons formed protons, so
Speaker 1: now we have protons and electrons and photons, and then
Speaker 1: things cooled even further, and around three hundred and eighty
Speaker 1: thousand years after the Big Bang, the protons and electrons
Speaker 1: came together to make neutral hydrogen, and then the universe
Speaker 1: went from being opaque to being transparent, and so any
Speaker 1: light created at that moment is still around because it's
Speaker 1: flying through universe and we can see it, and we do,
Speaker 1: and it's actually one of the best pieces of evidence
Speaker 1: we have that the universe used to be much denser.
Speaker 1: So that's the CMB light, and it's basically light from
Speaker 1: the early universe, and it's very cool because we can
Speaker 1: use it to see patterns and distributions of matter in
Speaker 1: the early.
Speaker 3: Universe, and we once weren't sure if it was pigeon
Speaker 3: poop or not. Right, is this that's right?
Speaker 1: Okay, yes, exactly. Thank you for adding a completely independent
Speaker 1: line of poop jokes to the episode. Poop jokes also
Speaker 1: not a fudge factor, right, they're load bearing support for
Speaker 1: the podcast.
Speaker 3: Okay, all right, But the scientist working on the project
Speaker 3: decided it wasn't pigeon poop that was messing up their measurements.
Speaker 2: It was an actual cosmic background radiation. So let's get
Speaker 2: back on track.
Speaker 1: And the fascinating thing about the CMB is, of course
Speaker 1: that it exists and tells you with the early universe.
Speaker 1: But there are variations in it. So mostly it's very
Speaker 1: very smooth. You look in every direction, it's about just
Speaker 1: around the same intensity and temperature at frequency, but not perfectly.
Speaker 1: There are little anisotropies in it, the little hot spots
Speaker 1: and little cold spots, and this corresponds to fluctuations from
Speaker 1: the early universe. Right. The reason it's not perfectly smooth
Speaker 1: is that the very early universe was not perfectly smooth,
Speaker 1: and people are used to thinking about early universe quantum fluctuations.
Speaker 1: But what we could actually see in the CMB is
Speaker 1: something much more interesting is early universe dynamics. It wasn't
Speaker 1: just like, okay, this spot was a little denser and
Speaker 1: that spot was a little less dense. We can see
Speaker 1: stuff slashing around in the early universe because the early
Speaker 1: universe was not just made of one thing. We think.
Speaker 1: It had electrons and protons and photons and dark matter. Okay,
Speaker 1: and there's like one billion photons for every electron and
Speaker 1: proton in the early universe. It's just like mostly photons
Speaker 1: at that point because of all the matter antimatter annihilation
Speaker 1: turned into photons. And so what happens is that the photons,
Speaker 1: which are everywhere, they push on matter, they speed it up.
Speaker 1: But the dark matter that's there, it pulls on the
Speaker 1: matter because of its gravity, and it doesn't get pushed
Speaker 1: by the photons. So you have these three different things
Speaker 1: all mixing around, interacting in complicated ways, and you end
Speaker 1: up with slashing. You end up with this oscillation where
Speaker 1: things rush back into the dark matter gravity wells and
Speaker 1: then they get pushed out by the light. So this
Speaker 1: is called acoustic oscillations in the early universe. Because there
Speaker 1: are pressure waves, So we think about it, it's like sound.
Speaker 1: And so these oscillations in the early universe are what
Speaker 1: we are seeing in the CMB, not just initial primordial
Speaker 1: quantum fluctuations, but the dynamics the slashing itself, and that
Speaker 1: slashing is super duper sensitive to exactly how much dark
Speaker 1: matter did you have to pull the things back into
Speaker 1: the well versus how many photons did you have to
Speaker 1: push things out of the well. All these ratios predict
Speaker 1: different shaped wiggles, different heights of the wiggles, different widths
Speaker 1: of the wiggles. So we look at the CMB light
Speaker 1: and we look at the hot spots and the cold spots.
Speaker 1: We measure the typical distances like how far apart are
Speaker 1: hotspots usually, and then you run the calculations and you say, well,
Speaker 1: this distribution requires this amount of dark matter and this
Speaker 1: amount of normal matter. So we can measure the amount
Speaker 1: of dark matter and normal matter in the early universe
Speaker 1: just by looking at this pattern, totally separate from galactic
Speaker 1: rotation curves, from theoretical calculations from galaxy cluster rotation curves.
Speaker 1: This tells us number one, how much energy there is
Speaker 1: in the universe, Like what is the overall energy density
Speaker 1: which tells us the universe is mostly flat, and what
Speaker 1: fraction of that comes from matter that responds to photons,
Speaker 1: that's like five percent. And what fraction comes to matter
Speaker 1: that ignores photons but has gravity, and that's like twenty
Speaker 1: five percent. So those numbers totally independent, very precise, amazing measurements.
Speaker 6: Here.
Speaker 3: Is this a situation where you had two different lines
Speaker 3: of evidence where you were like, Oh, we don't know
Speaker 3: what's happening, but now we're going to layer in dark
Speaker 3: matter and oh, we get the same value for dark matter,
Speaker 3: or is this a situation and where somebody was like, Okay,
Speaker 3: we thought about dark matter in this other scenario, and
Speaker 3: so now we're going to look at cosmic background radiation
Speaker 3: with dark matter in our head and test predictions based
Speaker 3: on values we already have, and see if those values
Speaker 3: give you the right answer.
Speaker 1: Yes, it's more like the second cool And I would
Speaker 1: say even more dramatically, it's like people had this idea
Speaker 1: of dark matter and then they realized they could probe
Speaker 1: it in their early universe because these new generations of
Speaker 1: telescopes and satellites were able to see this kind of stuff.
Speaker 1: And then they made a prediction and they were like, look,
Speaker 1: if there's no dark matter, it's going to look like this,
Speaker 1: and if there's a lot of dark matter, there's going
Speaker 1: to look like that. And they went out and they
Speaker 1: did the measurement and it came back dark matter, not
Speaker 1: just dark matter, but dark matter in the amount that
Speaker 1: we see from galactic rotation curves the same number.
Speaker 2: Okay, that's pretty cool.
Speaker 1: So you're like, whoa, that's spooky, man, Maybe there really
Speaker 1: was something. Right, we're talking about the same kind of stuff,
Speaker 1: some kind of stuff that doesn't interact with photons and
Speaker 1: yet has gravity, and you have to have it to
Speaker 1: explain the early verse observations. They just do not work
Speaker 1: without some kind of invisible gravitating matter.
Speaker 2: Okay, that is awesome.
Speaker 1: It's pretty cool. Yeah, And yet that's only the third
Speaker 1: line of evidence we have for dark matter, right, galactic
Speaker 1: rotation curves, galactic cluster rotation curves, and now the CMB.
Speaker 1: But we have more. Right, that's just the beginning. We
Speaker 1: can look, for example, what happens as the universe progresses.
Speaker 1: So you have protons and you have electrons and they've
Speaker 1: cooled now to make hydrogen, and for a few minutes,
Speaker 1: like literally minutes, not you know, poetic minutes or something.
Speaker 1: For a few minutes, the universe is filled with hydrogen
Speaker 1: at the right density to fuse it. We usually think
Speaker 1: about fusion happening in stars because it needs a lot
Speaker 1: of density and a lot of temperature and a lot
Speaker 1: of pressure. And that's true, but as the universe cools,
Speaker 1: it goes through a window when it satisfies those conditions.
Speaker 1: Right now, of course, out in space it's way too
Speaker 1: cold and pressure is way too low for fusion. But
Speaker 1: there was a literal few minutes and the universe could fuse,
Speaker 1: not in stars, but way before there were stars. So
Speaker 1: this is called Big Bang. Nucleosynthesis and hydrogen fuse together
Speaker 1: to make light elements, a little bit of helium, very
Speaker 1: small amounts of heavier stuff. There wasn't enough time to
Speaker 1: like work all the way up to iron, for example.
Speaker 1: But the amount of helium you make and of the
Speaker 1: heavier stuff is very, very sensitive to the density of
Speaker 1: matter that we had back then. So if you had
Speaker 1: a little bit more quarks or a little bit less quarks,
Speaker 1: you'd get a little bit more helium or a little
Speaker 1: bit less helium. Right, And so we have very precise
Speaker 1: calculations that tell us exactly how much we should get
Speaker 1: for various densities of matters. And this doesn't tell us
Speaker 1: about dark matter. It gives us a really broad understanding
Speaker 1: of normal matter. Like some people ask sometimes, couldn't dark
Speaker 1: matter just be like a bunch of rocks out there
Speaker 1: you haven't seen because stuff in space is dark sometimes? Right?
Speaker 1: And people look for this right, and they're called MACHOs
Speaker 1: massive astronomyical compact halo objects MACHOs nice, And for a
Speaker 1: while it was like a really nice counterpart to the
Speaker 1: WHIMP theory weakly interacting massive particles.
Speaker 3: I need to know how long somebody worked to come
Speaker 3: up with MACHO after the whimb's theory.
Speaker 2: I bet it was like a week or two.
Speaker 1: Astronomical acronyms are the worst slash best slash best Anyway,
Speaker 1: what this tells us is something about basically the total
Speaker 1: density of normal matter, because we can measure how dense
Speaker 1: it was, because we can tell how much helium was produced.
Speaker 1: And so this answers a question like is there enough
Speaker 1: normal matter density out there to explain the dark matter?
Speaker 1: Could it just be a bunch of rocks? And the
Speaker 1: answer is very definitively.
Speaker 3: No.
Speaker 1: If you measure the amount of helium and lithium produced
Speaker 1: in the early universe, you can derive the energy density
Speaker 1: of normal matter, and it comes out again at about
Speaker 1: five percent of the energy density of the universe. Completely
Speaker 1: independent line of evidence that there is not enough normal
Speaker 1: matter are out there to explain the gravity that we see.
Speaker 1: This is not directly evidence for dark matter, but this
Speaker 1: is evidence that it's not atomic matter. Whatever it is
Speaker 1: out there, it's not made out of quarks and electrons.
Speaker 3: All right, Okay, extraordinaries, we are not done beating the
Speaker 3: dead horse of dark matter is not a fudge factor,
Speaker 3: and when we get back from the break, we're gonna
Speaker 3: beat it some more. All right, Daniel, you've given us.
Speaker 3: I think we're on four, maybe five lines of evidence
Speaker 3: showing that dark matter makes predictions that then you can
Speaker 3: test and they pan out.
Speaker 2: Give me some more.
Speaker 1: So another really amazing and mind blowing prediction is to
Speaker 1: go back to the density of stuff in the early
Speaker 1: universe and think about that plasma slosh around, and the
Speaker 1: plasma slashing around tend to happen in rings like spheres actually,
Speaker 1: where you have these pressure waves propagating through the early universe,
Speaker 1: and those pressure waves are like over densities, right where
Speaker 1: matter gets really really dense, and then the photons push
Speaker 1: it apart and then it slashes back. But at the
Speaker 1: moment that the universe cooled enough to form hydrogen, right
Speaker 1: where the protons and electrons found each other. Now it
Speaker 1: became neutral and they were protected from all those photons.
Speaker 1: Photons can't push them around anymore because they no longer
Speaker 1: have a charge, and so they got frozen out. So
Speaker 1: all of a sudden, these dynamics ended and whatever was
Speaker 1: left just at that moment was stuck. So the structure
Speaker 1: is like frozen in, right, It can't oscillate in the
Speaker 1: same way anymore. So what you can do is say, well,
Speaker 1: do we see evidence for that? Do we see like
Speaker 1: rings of structure in the universe? And we do. If
Speaker 1: there was extra matter concentrated in these massive spheres, then
Speaker 1: you would expect that to eventually form galaxies. And you
Speaker 1: should look at the galaxies and say, hey, our galaxies
Speaker 1: constant traded in rings, and they are. If you measure
Speaker 1: like the mean distance between galaxies and you make a
Speaker 1: plot of it, there's a huge peak and that peak
Speaker 1: is exactly where you would expect it to be if
Speaker 1: dark matter was there creating these kind of structures. And
Speaker 1: not only do you see it statistically, like it's cool
Speaker 1: to see a bump in the galaxy at the galaxy
Speaker 1: mean distance, but also we've recently seen a single individual
Speaker 1: like actual bubble, like a ring structure where you can
Speaker 1: like look at the distribution of galaxies and you're like,
Speaker 1: why is there this huge bubble there? And the answer
Speaker 1: is that that was an acoustic oscillation from the early
Speaker 1: universe which left this imprint on the gravitational density of
Speaker 1: the universe, which created this literal sphere of galaxies. It's
Speaker 1: really incredible.
Speaker 2: That's awesome. And you said we found that out pretty recently.
Speaker 2: Is that right?
Speaker 1: Yeah, exactly. This is a recent survey. And this thing
Speaker 1: is massive. It's one of the largest structures observed in
Speaker 1: the nearby universe. It's two hundred and fifty mega parsex away.
Speaker 1: It's really massive. It includes the slown Great Wall and
Speaker 1: some other objects, and it's got this totally very easy
Speaker 1: to pronounce a name, Kelly, can you pronounce it for us?
Speaker 2: Nope? No, nice, try this is your outline.
Speaker 3: You got us in this situation, in this mess you
Speaker 3: try it.
Speaker 1: It looks something like ho O Leilana. Maybe it might
Speaker 1: be Hawaiian. Anyway, This is called baryon acoustic oscillations, and
Speaker 1: it's another independent line of evidence that the structure of
Speaker 1: the early universe deeply influenced by the distribution of dark matter.
Speaker 1: You have to have dark matter in order to make
Speaker 1: these rings in the early universe plasma, in order to
Speaker 1: create these superstructures of galaxies.
Speaker 2: This stuff is so important. Why can't we see it?
Speaker 3: That's really frustrating. I guess you're seeing it indirectly, is
Speaker 3: maybe the point.
Speaker 1: Yeah, And you know, there's lots of stuff in the
Speaker 1: universe that's out there that we don't see. There's light
Speaker 1: of all kinds of frequencies that we don't see. There's
Speaker 1: neutrinos passing through you all the time. Most of the
Speaker 1: universe is invisible. That's the typical state of affairs.
Speaker 3: Actually, all right, give me another one, Daniel.
Speaker 1: So another independent source of evidence for dark matter. I'm
Speaker 1: losing track of how many we have here is the
Speaker 1: current structure of the universe. So the large scale structure
Speaker 1: of the universe, as we said, we expect to see
Speaker 1: these rings, but it's not just the rings of galaxies
Speaker 1: that we see. You can ask the question like, well,
Speaker 1: why do we have galaxies at all? If you run
Speaker 1: a simulation of the universe without dark matter, then you
Speaker 1: have photons, you have protons and electrons. The protons on
Speaker 1: electrons eventually cool to make hydrogen, and you do have
Speaker 1: over density there, and those hydrogen atoms can pull themselves
Speaker 1: together to make stars and galaxies. But unfortunately it doesn't
Speaker 1: happen in fourteen billion years. It takes like tens of
Speaker 1: billions of years for that to happen without dark matter.
Speaker 1: Wo with dark matter, what happens, Well, remember dark matter
Speaker 1: is not getting pushed around by photons, so it can
Speaker 1: gather itself together and start making structure almost immediately. It
Speaker 1: doesn't have to wait four hundred thousand years to cool
Speaker 1: down enough to form neutral hydrogen, so it starts to
Speaker 1: form structure much earlier. It's created these gravitational wells that
Speaker 1: the protons and electrons oscillate in and out of, so
Speaker 1: by the time they do cool and form hydrogen, there's
Speaker 1: already gravitational structure there for them to fall into. Right,
Speaker 1: And if you add dark matter to your simulations, then
Speaker 1: you get structures like galaxies forming about the time we
Speaker 1: see them, and you get a universe that looks very
Speaker 1: similar to ours. Without dark matter as an element of
Speaker 1: those calculations, you don't predict large scale structure the way
Speaker 1: we see it at all.
Speaker 3: And just to sort of hammer a point home, our
Speaker 3: age of the universe we calculated without needing to think
Speaker 3: anything about dark matter, right.
Speaker 1: Yeah, yeah, okay, yeah, absolutely, And not only does it
Speaker 1: require dark matter, it requires dark matter and normal matter
Speaker 1: in the proportions of twenty five percent and five percent.
Speaker 1: With those numbers pretty close to bang on. You don't
Speaker 1: see the universe evolving the way that we see it.
Speaker 1: So it's not just today, it's not just in the
Speaker 1: early universe. We see the evolution of the universe between
Speaker 1: its birth and the present day, requiring dark matter to
Speaker 1: create all these beautiful galaxies that we see. So you
Speaker 1: need dark matter to explain this large scale structure of
Speaker 1: the universe. Right, This is another independent test of dark matter.
Speaker 1: And not only do you need it, you need it
Speaker 1: to be what we call cold. We needed to have
Speaker 1: low temperature because if dark matter is really really hot,
Speaker 1: meaning like it flies around really fast, then any initial
Speaker 1: structure gets washed out so you need to be fairly
Speaker 1: slow moving. This is one reason we were able to
Speaker 1: rule out neutrinos as a candidate for dark matter, because
Speaker 1: neutrino's very low mass, very fast moving, they wouldn't be
Speaker 1: able to create this kind of structure.
Speaker 2: Okay, got it.
Speaker 3: Gosh, it's I don't mean to be hammering home this
Speaker 3: negative point, but well, like we've seen it from so
Speaker 3: many we've measured it from so many different angles. It's
Speaker 3: frustrating that that hasn't given us insights into how to actually, like, yeah,
Speaker 3: interact with it or measure it more directly.
Speaker 1: Well, that's the problem is that the only interaction we've
Speaker 1: ever seen is gravitational, and gravity is super duper weak.
Speaker 1: So that makes it very hard to nail down a
Speaker 1: lot of details about it. And we'll talk about that
Speaker 1: at the end. But we have yet more evidence for
Speaker 1: dark matter, you know, there's the evolution of dark matter
Speaker 1: through time is really fascinating because as the universe expands,
Speaker 1: stuff gets diluted, but depending on the nature of that stuff,
Speaker 1: it dilutes differently. Matter dilutes in a very intuitive way,
Speaker 1: like if you have an eminem in your living room,
Speaker 1: then you have a certain amount of eminem density. If
Speaker 1: you like, quadruple the size of your living room and
Speaker 1: don't change the number of M and ms, then the
Speaker 1: density drops, and density drops in exactly the way you'd
Speaker 1: expect because the volume is increased and the matter hasn't.
Speaker 3: Very simple, and then the mood drops because you're like,
Speaker 3: I have less eminem's relative to space and like the said, okay.
Speaker 1: But photons react to expansion differently because as the universe expands,
Speaker 1: the energy in matter stays. But as we've mentioned on
Speaker 1: the podcast, photons get stretched, they get red shifted, so
Speaker 1: not only do you have more space for every photon,
Speaker 1: but also the photons have lost energy, so the energy
Speaker 1: in photons decreases as time goes on, so the energy
Speaker 1: density of photons drops faster than the energy density of matter.
Speaker 1: If you have the same amount of energy density in
Speaker 1: photons and in protons and you expand your universe, then
Speaker 1: you're now going to have more energy density in protons
Speaker 1: than photons because the photons not only are they spread
Speaker 1: out across more space, but they also got red shifted
Speaker 1: to have less energy. So you can tell by how
Speaker 1: something gets diluted something about what it is. And dark
Speaker 1: energy even weirder because dark energy, as the universe expands,
Speaker 1: doesn't dilute like you have four times as much space,
Speaker 1: you have four times as much dark energy. That's a
Speaker 1: whole other, super weird puzzle. But we have these three
Speaker 1: different ways we know that things can get diluted. Right
Speaker 1: is it like matter? Is it like radiation, or is
Speaker 1: it like dark energy? And we can study the distribution
Speaker 1: of dark matter over time by studying this structure formation,
Speaker 1: and that requires it to get diluted exactly like matter does,
Speaker 1: not like radiation, not like dark energy. Whatever this stuff is,
Speaker 1: it moves around and it spreads around, and it gets
Speaker 1: diluted just like matter does. That's a very strong piece
Speaker 1: of evidence. That's one reason why we call it matter.
Speaker 3: Okay, yeah, because otherwise it was like what dark stuff?
Speaker 3: And then you're like, okay, it's got to be matter
Speaker 3: because of how it dilutes. Is that was this part
Speaker 3: of how you all figured out you were gonna call
Speaker 3: it dark matter.
Speaker 1: This is you know, many lines of evidence. Is a
Speaker 1: big problem in astronomy and in physics for decades, lots
Speaker 1: of people working on it from lots of different directions
Speaker 1: because they wanted to see, like does this hold up,
Speaker 1: Is this a fudge factor? Or if we poke it
Speaker 1: from this other angle? What about this other angle? Oh,
Speaker 1: nobody ever thought to do this test? What if that?
Speaker 1: And remember everybody here is not just out for confirmation.
Speaker 1: It's boring if you come up with a new way
Speaker 1: to say, Okay, yeah, dark matter is there. If people
Speaker 1: have found something that showed dark matter isn't there or
Speaker 1: didn't work, that would have made their careers. And so
Speaker 1: everybody's motivated to find something that doesn't work. Like creating
Speaker 1: a puzzle is the way people get faculty positions. If anything,
Speaker 1: people are incentivized to like overhype some new thing that
Speaker 1: doesn't seem to make sense.
Speaker 3: Yeah, well, surely there's no more lines of evidence, Daniel.
Speaker 1: We haven't even gotten to the best ones, Kelly.
Speaker 2: What but we're almost at the end of the show, Daniel.
Speaker 1: So let me quickly mention a couple more.
Speaker 6: So.
Speaker 1: There's like the supernovas, right, we measure that the universe
Speaker 1: is expanding, and that expansion is accelerating. Now we know
Speaker 1: the total energy density in the universe because we can
Speaker 1: measure how the universe is curved. The CMB tells us
Speaker 1: that how much stuff is there in the universe, and
Speaker 1: we know there's just enough stuff in the universe for
Speaker 1: a space to be totally flat, so that gives us
Speaker 1: the total amount of energy. Then we can say, well,
Speaker 1: how much energy is there in the expansion, so that's
Speaker 1: dark energy. We can subtract that away. And so if
Speaker 1: you do that subtraction, you get one hundred percent minus
Speaker 1: seventy percent and you end up with thirty percent. Thirty
Speaker 1: percent is exactly how much we think there is dark
Speaker 1: matter plus normal matter, So those numbers line up really nicely.
Speaker 2: Gold Sticker for the physicis.
Speaker 1: Just like another independent way to say, like, look, it
Speaker 1: seems like thirty percent of the universe is some kind
Speaker 1: of matter. Then there's gravitational lensing, right. Einstein tells us
Speaker 1: that gravity is not a force between particles. It's a
Speaker 1: curvature of space, and light when it moves through that
Speaker 1: curved space will bend. That doesn't happen in Newtonian gravity.
Speaker 1: It's exclusive to relativity. The amount of lensing depends on
Speaker 1: the mass. More mass, more curvature, more bending. So you
Speaker 1: can do this amazing thing where you can say, look,
Speaker 1: I have a bunch of galaxies and behind them, I
Speaker 1: have a really bright source like a quasar. Can I
Speaker 1: see the light getting bent by the mass the curvature
Speaker 1: caused by these galaxies and you can This is called
Speaker 1: strong gravitational lensing. You see crazy distortion and Einstein crosses
Speaker 1: and curves and all sorts of stuff, and you can
Speaker 1: use that to measure the mass of that cluster, like
Speaker 1: how much stuff is in there, because I'm seeing a
Speaker 1: certain amount of lensing which made a certain amount of curvature,
Speaker 1: which means a certain amount of mass. And again this
Speaker 1: measurement is not consistent with a visible matter. It requires
Speaker 1: way too much mass to cause the lensing that we see.
Speaker 1: And you can use this to see beyond the edges
Speaker 1: of galaxies where there are no stars. You still see
Speaker 1: lensing where there are no stars, and then even beyond
Speaker 1: galaxy clusters, like if there's a little bit of dark
Speaker 1: matter out there, it would cause what we call weak lensing,
Speaker 1: like small distortions in background galaxies. And people have done
Speaker 1: these huge surveys and measured those distortions and see evidence
Speaker 1: for a distribution of dark matter exactly where we expect
Speaker 1: it and in the same amount. So it's mostly clumped
Speaker 1: around galaxies as you'd expect there are these filaments between
Speaker 1: galaxies that we see exactly what's predicted by the large
Speaker 1: scale structure simulations. We can go out and measure a
Speaker 1: map of the dark matter in the galaxy, the amount
Speaker 1: and the distribution, totally independently of every other line of
Speaker 1: evidence we've been talking about.
Speaker 3: I am running out of ways to say woe and awesome,
Speaker 3: so this time I'm going with two Daniel.
Speaker 1: So then finally, the closest we have to a smoking
Speaker 1: gun for dark matter is the Bullet Cluster, which combines
Speaker 1: so many of these things. This is a cluster of
Speaker 1: galaxies where two sets of galaxies smashed into each other
Speaker 1: and the gas and the galaxies interacted and heated up
Speaker 1: and stuck around, and the dark matter passed right through
Speaker 1: itself because both clusters had dark matter. But dark matter
Speaker 1: doesn't interact with itself very well. It just feels gravity,
Speaker 1: which is super duper weak. So in the center you
Speaker 1: have like the interacting matter from the galaxies, and then
Speaker 1: the dark matter passes through itself and comes out on
Speaker 1: either side, and you can tell that it's there because
Speaker 1: you can see it gravitationally lensing the background alexies, so
Speaker 1: you can see where the matter is, and way you
Speaker 1: can see where the dark matter is. So this is
Speaker 1: an example of the universe separating the matter and the
Speaker 1: dark matter. So we can tell that dark matter is
Speaker 1: its own thing, whatever it is. We think it's matter.
Speaker 1: We know that it's separate from normal matter. This is
Speaker 1: like a death blow to theories that like maybe dark
Speaker 1: matter is just to change in how gravity works. Right,
Speaker 1: It doesn't track the normal matter. It tracks where we
Speaker 1: think the dark matter is. It's its own separate thing.
Speaker 2: Take that, detractors.
Speaker 1: So those are the many lines of evidence we have
Speaker 1: for dark matter, which in my book means it's unfair
Speaker 1: to call it dark matter. A fudge factor.
Speaker 2: I'll give you that you've convinced me.
Speaker 1: I mean, originally it was a puzzle, and if we
Speaker 1: had only seen it in galactic rotation curves and it
Speaker 1: didn't explain anything else, and it was in conflict with
Speaker 1: measurements in other places, then it would be more akin
Speaker 1: to Einstein's cosmological constant or a fake entry in your
Speaker 1: checking book. But instead, it's absolutely required to understand the
Speaker 1: evolution of the universe, the early universe, in so many
Speaker 1: different ways, we know that dark matter is there We
Speaker 1: don't know what it is, right, We just know that
Speaker 1: it's invisible, that it's cold, and that it doesn't feel
Speaker 1: anything but gravity. As far as we know. We've been
Speaker 1: looking for the particle that makes up dark matter for
Speaker 1: decades without any luck, right, which is a bummer, and
Speaker 1: sure it would be nice to have. Okay, here's what
Speaker 1: it is that can show it to you. We just
Speaker 1: don't know. We don't know if there's one dark matter particle,
Speaker 1: if there's many particles, if it's not even a particle,
Speaker 1: if it's some other kind of matter. And I don't
Speaker 1: want to oversell it. There are still some issues with
Speaker 1: the theory, like there's some distributions of dark matter and
Speaker 1: galaxies that we can't explain, you know, like in the
Speaker 1: core of galaxies we see different distribution than we expect,
Speaker 1: so there must be something going on there, maybe something
Speaker 1: weird about galaxy cores. We see a mismatch between the
Speaker 1: large number of small dark matter sub halos predicted by
Speaker 1: our simulations and the observations of dwarf galaxy. He's around
Speaker 1: the Milky Way, though that seems to be closing with
Speaker 1: recent observations, so it's not perfect. There's definitely things to
Speaker 1: figure out, and we would love to know what dark
Speaker 1: matter actually is. But the headline is we have lots
Speaker 1: of independent lines of evidence. So I think fudge factor
Speaker 1: is not an honest criticism. I think it's an attempt
Speaker 1: to dismiss it rather than engage with it.
Speaker 3: So you and I have done a lot of episodes
Speaker 3: where at the end, after an hour, I've been like,
Speaker 3: and so, Daniel, thank you for that explanation. How likely
Speaker 3: do you think it is that what we've talked about
Speaker 3: is actually happening?
Speaker 2: And you'll be like, Oh, it's not. I really don't
Speaker 2: think it is.
Speaker 3: But you sound pretty darn convinced that dark matter is
Speaker 3: spot on, But there's still a lot left to learn.
Speaker 1: I think it's pretty convincing that there's something out there
Speaker 1: that's gravitating and is mattered, Like what is it? We
Speaker 1: don't know. Is it possible we have a totally misunderstanding
Speaker 1: of general relativity and everything. Yeah? Sure. I've got a
Speaker 1: question just this morning from Cole Zurbrigg who said, is
Speaker 1: it possible that what we call dark matter could just
Speaker 1: be the shape of space itself? Which is a question
Speaker 1: I hear a lot, and it's a fun idea. But
Speaker 1: the challenge with these alternative ideas of dark matter is
Speaker 1: that some of them can explain one of these lines
Speaker 1: of evidence, but explaining all of them is really really hard.
Speaker 1: It's a high standard. So somebody could come along with
Speaker 1: some new idea and topple the concepts of dark matter
Speaker 1: with some new theory, but it's got to explain everything
Speaker 1: we've seen, which is a real challenge. But you know,
Speaker 1: we should hold new theories to the same standards to
Speaker 1: which we hold old theories, and very quickly, Like, we
Speaker 1: don't think dark matter could just be the shape of space,
Speaker 1: because we see the distribution of a changing over time. Right,
Speaker 1: it was dense and then it spread out and it clumped,
Speaker 1: and so that's not something space does as far as
Speaker 1: we know, and it only does it in the presence
Speaker 1: of matter. That's sort of the characteristic of matter anyway.
Speaker 1: So that's my summary of why I think dark matter
Speaker 1: is not a fudge factor. And so that's two marks
Speaker 1: against fudge in my book.
Speaker 3: Oh ouch, okay, all right, it's not my favorite dessert either.
Speaker 3: But I'll note that in a past episode I asked
Speaker 3: if dark matter was like a fudge factor, and I
Speaker 3: will say you have convinced me it is uh. I
Speaker 3: will no longer refer to it as a fudge factor.
Speaker 3: There's a lot of good evidence for it.
Speaker 2: But we still have some.
Speaker 1: Work to do, and we have lots of work to do,
Speaker 1: and there's lots of stuff still to figure out. And
Speaker 1: so write to me and let me know if this
Speaker 1: convinced you, If you still have questions about our understanding
Speaker 1: of dark.
Speaker 2: Matter, questions at danieland Kelly dot org. We can't wait
Speaker 2: to hear from you.
Speaker 1: Go off and enjoy some fudge, at least one tiny
Speaker 1: bite of it. Thanks everybody for listening. Please go and
Speaker 1: do us a favor and rate the show on whatever
Speaker 1: podcast app you're using. It really helps people find us.
Speaker 3: Daniel and Kelly's Extraordinary Universe is edited by the amazing
Speaker 3: Matt Kesselman.
Speaker 1: He really is a wizard. You can also find us
Speaker 1: online on Blue Sky, Instagram, and x D and K Universe.
Speaker 1: Come engage with us.
Speaker 3: You can email us at questions at Danielankelly dot org.
Speaker 3: We really do want to hear from you, and.
Speaker 1: You can find our website www dot danieland Kelly dot org,
Speaker 1: where you'll also find an invitation to join our discord
Speaker 1: where everybody comes and talks about the amazing universe.
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