Daniel and Kelly’s Extraordinary Universe - What is the dark flow?

Episode Date: January 25, 2024

Daniel and Jorge talk about whether the whole Universe is in motion, and what could be pulling on itSee omnystudio.com/listener for privacy information.

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Hey, Jorge, do you think stuff in science is typically well-named or sometimes misleading? Depends on which science. Do you mean physics? Then no. I mean, sometimes we do a good job, right? Like black hole. Yeah, yeah. Those are black and their holes in space. So I guess you did good on that one. But in other cases, you're saying we didn't do as well? Well, let's see. How about quark flavors, which don't have flavor? How about quantum colors? Don't have color. All right.

That's fair. So let's do a little experiment. I'm going to give you a physics name, and you're going to guess what it means. All right. Go for it. It's called the dark flow. That sounds like maybe a plumbing product to get your toilet moving again. Nice.

Did I get that right? Stay tuned and find out. You have to go with the flow. Is that what you're saying? Hi, I'm Jorge McCartoonist and the author of Oliver's Great Big Universe. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine and I hate plumbing projects. Who loves plumbing projects? I guess plumbers. Plumbers? Yeah. Maybe.

I mean, I hope they love plumbing projects. Otherwise, it's kind of sad. It's probably interesting to them. Sure. But how many plumbing projects have you worked on recently? Oh, too many. You know, anytime you have a plumbing project, that mean bad news something's leaking somewhere something's broken somewhere you know the joys of home ownership why don't you just call a plumber yeah that's like paying somebody to deliver the bad news to you and then they then hopefully they fix it for you what kind of plumbers are you calling yeah they fix it we had a leak in our upstairs bathroom recently and they had to tear out and rehab all the walls in our garage it was a big pain to the butt oh boy what happened the tub overflowed or something it was a dark flow

You took too long of a bath there thinking about physics. I was dunking stuff in the bathtub, hoping for a eureka moment, and it just didn't happen. It just didn't happen. You didn't figure out how buoyancy works. No, it's too complicated for a particle physicist. Too many particles in a tub of water. But anyways, welcome to our podcast, Daniel and Jorge, Explain the Universe, a production of iHeartRadio. In which we teach you to think like a physicist instead of a plumber.

Don't dunk stuff in a tub of water, but immerse your brain in the mysteries of the universe. and try to learn how the universe works by osmosis hanging out with us as we talk and joke about everything that's out there in the universe and try to explain all of it to you. That's right. We try to fill the tub in your mind of knowledge and wonder until it overflows and hopefully maybe leaks into your garage a little bit. You know, in that scenario, maybe a plumbing project is good news because you got to rip everything out and build up a new understanding of the universe.

That's what I'm always saying I want to do. Wait, wait. You're saying the universe has plumbing? I'm saying our mental understanding of how the universe works has plumbing. When you've got to tear all that out and rebuild it, that means you're coming up with a new idea about how the universe works, and that's a revolution. But wait, what's the plumbing for? Ideas? Also? And what do you flush down the toilet of physics? Imagine the inside of a physicist's mind and model it as a bunch of pipes with ideas flushing and flowing and swirling. I see. I always love physicists as sticky pipes themselves in their brains. But aren't physicists as a sticky pipes.

sort of the plumbers of the universe, kind of you're trying to figure out how the plumbing of the universe works. Yeah, everything in the universe is out there sloshing around, banging into itself. It's not like a stable situation. You look up at the night sky and you might think, all everything's just sort of hanging out. But that's just because we live for such brief moments on cosmic scales. If you looked at the universe over millions or billions of years, you would see things expanding and exploding and smashing and flowing. Well, usually pipes expanding is not a good sign. Like if your pipe expands, it's probably going to burst. Yes, that's true. Dark energy would be bad news for your plumbing. Yeah, better flush it down the toilet or call a real professional.

Someone who has actually useful skills, you mean? Yeah, someone who can fix a leak. Someone who knows how to use a wrench. You mean not a physicist and not a cartoonist. No, but an engineer, maybe. I'm also an engineer. I have that skill. So do you tackle the planning projects in your house or do you call a professional? Sometimes I do fix it. Yeah, I fixed my washing machine the other day. I was pretty proud of myself. By kicking it and swearing at it? No, by looking it up on YouTube

and looking at a video of how to replace the little valve there inside. We're all YouTube engineers. Did you type how to replace that little valve there into Google and just follow the instructions? No, I think I just typed the model of my dishwasher and then, you know, helpfully, a lot of people out there have posted videos about it. I had to order apart, pulled it apart. surprisingly nothing exploded so far that's right so far

though Jorge has mastered his washing machine in the plumbing of his house physicists are still trying to figure out how the universe out there works yeah because it is an amazing universe full of what seems like internal plumbing where forces and things like particles and quantum fields all flow around and flush together and get mixed up and flow from one side to the other there's all sorts of stuff involved there's normal matter there's dark matter there's dark energy. There's the expansion of the universe. There's the shape of the universe and the size of the universe, all of which play a role in what's going on out there. Yeah, we've learned a lot

about the universe, but there's still a lot we don't know, big giant concepts that are still a big mystery. So today on the podcast, we'll be tackling the question. What is the dark flow? Now, this is not related to aunt flow, is it? Well, there are a lot of bodily. functions that do flow, but we are strictly a physics podcast, not biology. Although putting the word dark in front of it does make me think of toilet functions. There are a lot of dark matters that get flushed down the toilet. That's true. Yeah, a lot of dark matter that requires a lot of dark energy, especially if you haven't been eating fiber. When your plumber is telling you you need a new diet, then you definitely

have a problem. Yeah. Aren't doctors sort of like body plumbers? Yeah, exactly. Yeah, the digestive system sometimes needs a new valve. But this is an intriguing name for a concept, the dark flow. And so as usually we were wondering how many people out there had heard of this concept or had any idea what it could meet. Thanks to all the volunteers who answered this question. If it sounds like fun to you to get a dose of random physics questions in your inbox every week,

write to me to questions at danielandhorpe.com. So think about it for a second. What do you think is the dark flow? Here's what people had to say. That sounds like it has to do with dark energy and or dark matter and its relative distribution through the cosmos over time. I want to say it has something to do with like dark matter flowing between stars or galaxies or like how we have a solar wind.

So like a dark matter wind. All right. I'm going to go with Batman as well. It's like when Batman's in a state where he's totally concentrated in his crime fighting and just, you know, having the artistic flow of fighting crime. At least it would be something new in the Batman story. I feel like Batman is just the same story over and over again. Oh, boy, are we going to get into Batman in this episode?

How long is this episode? It sounds like you've thought about Batman a lot. I just saw like Batman Year 1 rebooted. It's like how many times can you go back and tell the same story about his parents getting shot and he's a dark soul dot, dot, dot, dot. Like come up with something new people. Wait, you mean, like changes? origin story, then he wouldn't be bad man.

Tell us a story about somebody else. Don't just keep rehashing the same IP. Tell us about possum man. Yeah, exactly. And squirrel man. Squirrel girl is a thing, right? Ryan North writes that. It's great. Yeah, yeah.

But anyways, without getting too dark here about comic books, this is apparently a real physics concept, dark flow. It is a real physics concept. and it's kind of connected to dark matter and dark energy, but it's neither of those things. It's more related to the overall expansion and flow of galaxies in the universe. I wonder if maybe you've overused the word dark in physics. That's a dark thought.

Yeah, because people think it's just all connected. If you use the same word for different things. You're absolutely right. People confuse dark matter and dark energy all the time because they're both called dark. Really what we mean by dark is we don't understand this. It's mysterious and unbearable. known and invisible right like you can't see it in space yeah exactly all right well daniel step us to do

this uh what is the dark flow what's exactly flowing in the universe and where is it flowing to or down to that's part of the question how is the universe flowing and where is it flowing to so to understand the dark flow we have to think about the expansion of the universe like where all the galaxies are going because the dark flow is a mystery that sort of sits on top of our current understanding of the expansion of the universe, how all the galaxies are moving away from each other. And by expansion, you mean like the expansion of space itself, like the space where all the galaxies sit in is expanding. It's getting bigger and bigger. Exactly. Space is being created between galaxies, which effectively increases the distance

between those galaxies. That's the expansion of the universe, which has a really interesting and fascinating history. We know that the universe expanded very quickly early on. That's the big bang, right? In inflation? Yeah, loosely speaking, though we don't understand what caused that initial expansion. And then after that period, things were still expanding, but the signs sort of flipped. The universe took his foot off the gas and started hitting the brake. So it was still expanding, but now it was decelerating. There was all this mass in the universe that was starting to slow down the expansion.

So universe still getting bigger, space still getting created between the galaxies, but at a lower rate. Now, meaning like the mass of the stuff in the universe was somehow slowing down the expansion. Why was that? Was it like the gravity of the stuff or just that having stuff in space makes it not expand? That's just the gravity, right? What determines whether space is expanding or contracting and whether it's accelerating or decelerating is the amount of stuff in space like the matter density and that sends to pull stuff together and then also the shape of the universe. Is it flat? Is it negative? Is it open? And the amount of dark energy, which is pushing things out and accelerating things. Early on in the universe, there really wasn't a whole lot of dark energy.

And so matter dominated and it was slowing down that expansion. But because it was still expanding and as the universe expands, it makes more dark energy because dark energy doesn't get diluted. And then dark energy makes the universe expand more that continuing expansion, even though it was decelerating, made more dark energy, which turned things around again back to acceleration. So we had constant expansion, but we had like initial acceleration, then deceleration, and now for the last 5 or 6 billion years, we've had acceleration again. That's dark energy. It's been sort of a roller coaster ride for the universe, right? Like it's stretched out really fast, and then it's slowed down, and now it's picking up speed

again. And potentially, it's going to keep picking up speed until it grows at a super duper fast rate. Yeah, we don't know what the future holds because we don't really understand dark energy like at all. If this simple model of dark energy is just like some energy that permeates space and causes the expansion of the universe to accelerate is valid, then yeah, I'll just keep going forever and things will get more and more distant. Space will continue to expand between galaxies creating vaster and vaster distances between superclusters, which will collapse into super massive black holes. And that's sort of the far future of the universe. And we're talking about this today because this expansion of the universe, this creation of space between galaxies is sometimes

called by astronomers the Hubble Flow. Now, first of all, I guess this expansion of space is happening everywhere, all at once, the same everywhere, or does it happen more in certain spots? We think it happens the same everywhere. Even the spots with lots of stuff in it. Yeah, that's the current idea. And it's sort of a theory and an observation. It's sort of the simplest idea you could have. You don't know what dark energy is. You postulate something like a cosmological constant, some energy in space that causes this to happen. The simplest thing to do is to say, oh, that's just a number. It's the same everywhere. We don't see any evidence for it being different in different parts of the universe. But, you know, that's a tricky because we can't

see it in different parts of the universe at the same time. We can see the expansion happening locally, nowish. We can see the expansion happening far away like a billion years ago. It's hard to compare point to point. But didn't you say stuff slows down the expansion of the universe? So I wonder, if in this bus where there's a lot of stuff in it, maybe it's expanding less, less fast. Stuff does slow down the expansion of the universe. That sort of contributes overall, but everything is actually balanced so that overall the universe is flat.

But yeah, we don't know how stuff and dark energy interact with each other. Dark energy is so dilute and so actually weak that anywhere there is stuff, you can basically just ignore the dark energy and just treat it as if there was gravity there just like from its stuff. But I guess as we look around, we see the whole of the universe expanding evenly in all directions. Exactly. And the idea of the Hubble flow, I think is kind of helpful visually for you to think about this because we tend to think about this in terms of velocities, like those galaxies are moving away from us at some speed. And we measure those velocities in terms of redshift, how the light from those galaxies is getting stretched out because those galaxies are moving away from us. And this is how Hubble sort of initially described it, you know, the velocity. velocity of these galaxies.

It makes more sense. It's sort of more natural to think about it in terms of general relativity as expansion of space itself. That those galaxies aren't accelerating away from us, but space is being created between us. And so like there is an increased distance between us and those galaxies,

but you're not feeling any acceleration. And you said this motion is called the Hubble Flow. What do you mean motion? And why is it called a flow? So motion there is a bit misleading. The distance between the galaxies is increased. And so for nearby galaxies, you can measure their velocity and say, oh, that galaxy has a certain velocity in our reference frame. But across really, really big distances, it doesn't make sense to talk about relative velocity for objects that are super far away from each other and have different non-inertial frames.

So astronomers sometimes use velocities as a sort of way to talk about those objects, but it doesn't actually make any sense in terms of general relativity. And why it's called the Hubble flow? Yeah, that's a good question. think it just helps you visualize how like space is being created everywhere and things are being carried along by that stream. Okay. So the Hubble flow and then is I guess maybe it's sort of like the flow of space being created pushing everything apart in the universe. Yeah, exactly. And so you can think about those galaxies as sort of like sitting in that Hubble flow. Space is being created. Everything is moving further and further apart. But there's a wrinkle on top of that motion, right? The Hubble flow is not the only thing that's happening in the universe. Each of these galaxies is also moving relative

to the Hubble flow. Like, people often write in and ask, hey, if galaxies are being pushed apart because of dark energy, why is the Milky Way going to slam into Andromeda? And the answer is that these things are moving because of gravity. Right. We talked earlier about how dark energy sort of winds over really large distances, but for short distances, it's too weak.

Well, gravity is the opposite. Gravity gets really weak at large distances. So like between super clusters, you hardly feel anything. But short distances, like just between two galaxies or between the sun and the earth, gravity totally dominates dark energy. Right. Like right now, even where I'm sitting, space is expanding, right? Like if I look at my hand in front of me, the space that my hand is in is technically

expanding just like the rest of the universe. It's just expanding so little that the force is keeping my hand together when over the expansion of the space. Yeah, that's exactly right. And in the case of your body and your hand, the forces are electromagnetic. Those chemical bonds are tying everything together. So the Hubble flow is like the gentlest little current that your body can overpower. Even on the scale of the solar system, gravity wins and the motion of the

Earth around the Sun is totally dominated just by gravity. Like if it wasn't, we might have discovered dark energy much, much earlier because we wouldn't have understood the orbits of the planets without it. And we have very precise measurements of things happening in the solar system. Any little deviation from gravity, we would have noticed if it was at all measurable. But dark energy is not even measurable on the scale of our solar system. You have to go out to between clusters of galaxy in order to see it. So on the local scale, dark energy is basically irrelevant, and it's gravity that winds over. And so these galaxies are moving in random directions and feeling tugs from gravity and having all sorts of dynamics. We talked about how

galaxies merge and form together and make super clusters. So this motion relative to the Hubble flow, that's called the peculiar motion of a galaxy. It just means how it's moving relative to the flow. I see. So like, for example, in our solar system, the space is expanding, but our planet is tied to the sun, just like we're tied to the earth. So like me sitting here on Earth, and not moving relative to the Earth, there's sort of almost like space flowing through me as it expands through me. Mm-hmm, exactly.

So your peculiar motion is towards the Earth, and the Earth's peculiar motion is towards you. And then on the scale of galaxies, like the Milky Way's peculiar motion is relative to Indromeda, and Indromeda's peculiar motion is relative to the Milky Way. If we only were following the Hubble flow and had no peculiar motion, then the space between us would be expanding, right?

The distance would be growing, But it's not. Milky Wayan Andromeda are coming towards each other. And that's the case for lots of galaxies. Even the ones that aren't going to smash into each other, and they're all basically pointing in random directions. Very fewer, like, at rest relative to the Hubble flow. So, like, our galaxy and Andromeda galaxy are moving relative to each other. And there's two components to that, you're saying, right? Like, there's how much we're moving because the space is expanding between us. And there's also how much we're moving if space wasn't expanding between us.

Yeah, that's exactly right. You can think about it either just from the reference frame of the Milky Way and say, look, everything is moving away from us. And then just break that motion into two components, say one of that is due to dark energy and the other is everything else relative to that. And then a more natural way to think about it is instead of just having one reference frame centered at our heads, think about dark energy is just creating this like expanding frame. And then think about the motion relative to those frames.

The Hubble flow is sort of the most natural way to think about the expanding universe. and in that frame, they're called co-moving frames. The only motion is the peculiar motion because you just sort of set the expansion of the universe as like the baseline thing. Right. It's sort of like maybe like the river analogy you just brought up earlier.

Like we're all moving in a river and the flow of the river is the Hubble flow or like the expansion of the universe. But, you know, within my little raft, I can have something moving relative to each other or I can try like rowing towards your raft and then there would be some peculiar motion between our rafts. Yeah, exactly right. Maybe we have engines and we're trying to push towards each other against the current

or trying to get away from each other or whatever. So that's our peculiar motion. All right. Well, these are peculiar concepts. And so let's dig into now what is the dark flow of the universe? Why is it dark? Where is it flowing? And is it something we want to touch or take a bath in? So let's dig into that. But first, let's take a quick break. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then, at 6.33 p.m., everything changed. There's been a bombing at the TWA terminal. Apparently, the explosion actually impelled metal, glass.

The injured were being loaded into ambulances, just, a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, and it was here to stay. Terrorism. Law and Order Criminal Justice System is back. In Season 2, we're turning our focus to a threat that hides in plain sight. That's harder to predict and even harder to stop. Listen to the new season of Law and Order Criminal Justice System on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts.

My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Well, wait a minute, Sam. Maybe her boyfriend's just looking for extra credit. Well, Dakota, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now, he's insisting we get to know each other, but I just want her gone. Now, hold up.

Isn't that against school policy? That sounds totally inappropriate. Well, according to this person, this is her boyfriend's former professor, and they're the same age. It's even more likely that they're cheating. He insists there's nothing between them. I mean, do you believe him? Well, he's certainly trying to get this person to believe him because he now wants them both to meet. So, do we find out if this person's boyfriend really cheated with his professor or not?

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All right, we're talking about the dark flow of the universe, which sounds a little bit sinister, I feel. Like there's something going on under the surface that is not light. Sort of like a Batman-tinged story, The Dark Knights of Gotham. Like an underplot, yes. Like there's something going on. going on in the streets of Gotham. Exactly.

Is it a flow of villainy in the sewers? Yeah, or maybe it's financial plumbing, right? It's a flow of dark money through politics. Oh, there you go. Well, again, maybe you should have called it something different and not dark and confuse everybody. You there means the entire physics community, right? Because I certainly didn't call this dark flow.

But you keep using it, though. So you're complicit. Yes, I am complicit. That's true. I will admit to that. Toss me in physics naming jail and throw away the key. Yes, we'll lock you up in Arkham Asylum with the rest of the Batman villains.

All right, we're talking about the dark flow of the universe, and we talked about how the universe is expanding, and it's because space is expanding, all of space and the universe is expanding, and that flow, that flow of new space, flowing everywhere, being created, everywhere, stretching things out. They're in space, we call that the Hubble flow. Now, is that the same as the dark flow?

So that's the Hubble flow. The dark flow is a question about this peculiar motion. Are there any patterns in the peculiar motion? Like we expect that the universe is the same everywhere and it's all just random. There's nothing special. And so that if you look at the directions and the magnitudes of these peculiar motions, like where's every galaxy going and you add them all up? They should basically add up to zero. There should be no preferred direction in the universe.

Meaning like let's say we ignore the expansion of the universe or like take that into account or you subtract the expansion of the universe from the motion of everything. How is everything moving? How are all those superclusters of galaxies and those galaxies and stars? How is it all moving if the universe wasn't expanding? That's what you mean. Yeah, essentially. Relative to the expansion, how is everything moving? Does it point in some particular direction?

Does it average out to zero? we expect, based on like very simple, kind of naive but powerful arguments that it should average out to zero. We think the universe is the same everywhere and the Big Bang happened at every point in the universe the same time and there was no global over density anywhere. And so we expect it to basically average out to zero. Now, what do you mean by average out to zero? Like why should that be the case? Like the motion of our solar system, does it add up to zero?

So our solar system reflects the spin of the initial blob of gas that formed it. right? That blob of gas had a bunch of particles heading in random directions and it was regularly shaped and it was spinning. And so the spinning the motion of our solar system, including the motion of the solar system around the center of the galaxy does reflect the motion of that initial blob. And we expect all those blobs were basically created equal. And so you'll get some spinning one way and some spinning another way and some moving this way and some moving that way. And you do see a big variation of like directions of stars, etc. I see. It's sort of like maybe if you had like a gas canister full of gas molecules, you sort of expect all the gas

molecules, all the motions of all those particles and molecules to average out to zero, right? Because it's all sort of random and the canister's not going anywhere and it's had some time to like diffuse and even out. Exactly. Unless there was something outside that canister pulling on them or something pushing on the whole canister or something else acting that you weren't aware of, the motion of all the molecules should add up to zero. So that's the question.

And so that's kind of what you were saying. we should expect from the universe, like if you subtract the expansion of the universe from the motion of everything, then basically all the stars and galaxies out there should sort of look like a canister of gas where everything's just moving in random directions, but it should all add up to zero. Exactly. And there's one more little bit of trickiness that we need to think about before we're ready to actually look at those galaxies and answer the question. And that's the frame of reference. When we talked about subtracting out the expansion of the universe, and that's helpful for like removing something we already understand that's causing everything to get away

from each other, but we still need to pick a frame of reference in order to calculate the velocity of a galaxy because galaxies don't have velocity relative to space, they only have velocity in some reference frame, like put a reference frame on one galaxy, measure the velocity of another one relative to that, or your spaceship or something. Velocities are not a property of objects, they're properties of pairs of objects. So you basically have to pick a reference frame in which you're going to measure the velocity of all these objects. And is it going to change? Have you changed the point of reference?

Like, shouldn't it add up to zero no matter what? Yeah, this is a really interesting and subtle point. Like space itself has no frame of reference. It doesn't prefer any frame of reference. And so it's often said like the universe has no preferred frame. And that's mostly true. But the stuff in the universe definitely does have a frame of reference. Like if you add up the velocities of all the stuff in the universe, that has a frame

of reference, right? that has a location that has an average velocity. And so the stuff in the universe has a frame. You can sort of pick that frame of reference just by looking at the cosmic microwave background radiation. But wait, I thought all the motion of all the stuff out there should add up to zero. Are you saying it doesn't or it may not? Like if you calculate the average velocity of all the galaxies in the universe, observable universe, shouldn't that be zero?

It should be if you pick the right reference frame, right? And you understand everything that's happening. So that's why the answer depends a little bit. on the reference frame. So we go back to the very early universe and say, well, what was the reference frame of all the stuff in the early universe, like when things were still hot and dense and a big plasma? We can actually measure that because we can look at the cosmic microwave background radiation and the light from that plasma.

And we can look at like whether it's bluer or redder in one direction. So we can measure our motion through this cosmic microwave background radiation. And that tells us what the frame of the universe was like 14 billion years ago. And so it gives us a frame, then we can ask like, are the galaxies now moving relative to the cosmic microwave background radiation? We expect that to be no because we think the galaxies came from the same stuff that formed the CMB. So it would be weird if that stuff was moving relative to the CMB. Wait, wait, wait.

Are you saying the cosmic microwave background radiation is not the same in all directions? You're saying like it's red or to the right than to our left, kind of? Yeah, because we're moving through the CMB. We have a non-zero velocity relative to the CMB. If you just measure the CMB overall, it's very obviously redder in one direction and bluer in the other. And then typically you see these maps of the CMB, but they've already subtracted that out. They've already measured our overall motion relative to the CMB and subtracted that out. And then they're looking for like tiny little wiggles in the CMB on top of that.

Oh, I see what it's saying. So like if you look at the cosmic microwave background, it has a certain velocity or motion to it. And so you have to sort of assume that's like the motion. of the universe, kind of. That's like the home base of the universe. Yeah, exactly. The zero of the universe. That's really the only thing we can compare to, right?

You can't just measure velocities relative to space. That doesn't have any meaning. You have to measure relative to something. So you need like a baseline. So we go back to the early universe and take the frame of the CMB and say, our galaxies overall moving in some direction relative to the motion of the CMB. That would be weird. It's not weird for one galaxy to be moving relative.

to the CMB, like stuff happens, it gets pulled in some direction. The Earth is moving relative to the CMB, no big deal. But if you add up everything relative to the CMB after subtracting the expansion, also, then the question is, where is everything going? Okay, I see. So we've measured the velocities of all the galaxies and superclusters and stars out there in space, and we've measured how they move relative to the cosmic microve background radiation. And you're saying it should be zero if you think about it, because it should average out.

But is it? It does not average out to zero. And that's the dark flow. The dark flow is this extra unexplained velocity of all of these galaxies relative to the C&B after subtracting out the expansion. Turns out the galaxies are pointing in a certain direction. Wait, what? Where are they going? So they did this really cool study where they measured the velocities of a bunch of different galaxies.

And this is, you measured like everything, like the whole observable universe that we can see. No, definitely not. That would take way too long. What they did is they found like 700 clusters out there in the universe, and they measure the velocities of those. So again, not even individual galaxies, like galaxy clusters. They wanted to like scan as far as they could across the observable universe. But, you know, there's zillions and zillions of galaxies. If you did the whole project, it would take forever.

So then isn't that a really tiny sample of the whole universe? Absolutely. It's a tiny sample. But they measured it across the universe. So they hope there's no bias. and they know how much data they have so they can measure the statistical uncertainty there. So if they measure velocity that's smaller than their uncertainty,

then they say, okay, it's consistent with zero. If they measure something much bigger than their uncertainty, they can still tell that it's happening. Like if you only measure the speed of two cars zooming by where they're going 100 miles an hour, then it's very likely the average speed of your road is not zero. It's closer to 100. Okay, so that's a big mystery.

like you've measured the velocity of all the superclusters out there or at least a sample of them and they don't see to be standing still relative to the background of the universe and they use this very cool technique to measure the velocities of these galaxies because these clusters can be hard to see

and so what they did is they looked at C&B photons passing through these clouds of hot gas as the C&B photons pass through them they get a little bit of boost of energy from interacting with these clouds You can use this to see distant galaxies just by like their effect on the CMB. Like if those photons have passed through the blob of gas, then you can tell and you can tell the velocity of that blob of gas. There's like an extra little Doppler shift there.

Okay, so everything seems to be moving relative to the background of the universe by a lot or is it like a little tiny drift? Well, it depends on your scale of reference, but like it's moving at 800 to 1,000 kilometers per second. You add it all up. It's really a non-zero number. It's pointing sort of in the direction of the Centaurus and Hydra constellations, which listeners might remember is sort of in the same direction as what we call the great attractor, which is behind the zone of avoidance in our galaxy. Like our galaxy is a big disk, right?

And so you can look up above the disc or down below the disc or sort of out away from the center of the galaxy. Those directions are pretty easy to see because you're not looking through a lot of dense galaxy. But if you try to look through the center of the disc itself or, even through the center of the galaxy, it's a lot of stuff there, a lot of gas and dust and other stars and black holes that block our view. And in that direction, there also tends to be like a lot of gravitational motion within the observable universe. So it's called the Great Attractor. We

don't know what's there exactly because we can't see in that direction very well, but they're already, we thought there was a lot of local gravitational motion in that direction towards sort of the center of the Lanakia supercluster. And now it looks like the whole universe is also moving in that direction. Like you add up all the galaxies in the universe, their velocity on average points in the same direction as our motion towards the great attractor. And by all the galaxies in the universe, you mean all the galaxies we can see? I mean, only these 700 clusters that were measured by this one study. Right. Well, as a sample of all of them, maybe.

Yeah, exactly. A tiny, tiny sample. But yes, statistically significant sample of all of them. And they measured a big velocity. This is a really surprising result. So they've measured how things are moving, and you're saying they're moving in a particular direction, which seems to be in the direction of something really, really extra big compared to the size of the observable universe. Yeah, and I don't want to confuse people because there's two different motions we're talking about here. Like one is our particular peculiar motion, the motion of Andromeda and the Milky Way in our little cluster, is being pulled towards the center of the Lankea super cluster by some big mass that's within our universe. right that's our sort of local peculiar motion but then you add up all the peculiar motion of the

whole universe and that also seems to be headed coincidentally maybe i don't know in the same direction so like the whole universe is also moving and its arrow is in the same direction as our local motion towards the great attractor so sort of like back to the analogy of boats in the river we discover that oh our boat is moving in some direction relative to the river and then we add up all the boats and like oh my gosh everybody's moving in that direction What's going on? It's something much bigger than just the great attractor. The great attractor is within our super cluster of galaxies or it's outside of it?

We don't know what the great attractor is. We think it might be something supermassive at the core of our supercluster. So yeah, probably it's within our supercluster. But the dark flow is the motion of everything in the universe. Coincidentally or not, I don't know, in the same direction as our motion towards our supercluster. Well, like we're next to this great attractor and we're moving towards it. Are you saying then that when you measure out the whole universe, even the stuff behind the great attractor is moving towards the greater tractor,

meaning towards us? No, the stuff behind the greater tractor is moving in the same direction as us, meaning away from the great attractor. So the overall motion is in the same direction, not towards the great attractor, but in the same direction as our motion towards the great attractor. Oh, so the great attractor is just maybe a red herring. It's not really doing anything.

Maybe. I don't know. It's sort of weird that it would be a coincidence. but it might just be yes. All right, well, let's dig into what that could mean, what could be causing the dark flow of the universe, and whether or not we do have to call the plumbers to fix it or not. So let's dig into that. But first, let's take another quick break. December 29th, 1975, LaGuardia Airport.

The holiday rush. parents hauling luggage, kids gripping their new Christmas toys. Then, at 6.33 p.m., everything changed. There's been a bombing at the TWA terminal. Apparently, the explosion actually impelled metal glass. The injured were being loaded into ambulances. Just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, and it was here to stay.

Terrorism. Law and order criminal justice system is back. In season two, we're turning our focus to a threat that hides in plain sight. That's harder to predict and even harder to stop. Listen to the new season of Law and Order Criminal Justice System on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Well, wait a minute, Sam, maybe her boyfriend's just looking for extra credit. Well, Dakota, it's back to school week on the OK Storytime podcast, so we'll find out soon.

This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now, he's insisting we get to know each other, but I just want her gone. Now, hold up. Isn't that against school policy? That sounds totally inappropriate. Well, according to this person, this is her boyfriend's former professor, and they're the same age. And it's even more likely that they're cheating.

He insists there's nothing between them. I mean, do you believe him? Well, he's certainly trying to get this person to believe him because he now wants them both to meet. So, do we find out if this person's boyfriend really cheated with his professor or not? To hear the explosive finale, listen to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. Your entire identity has been fabricated.

Your beloved brother goes missing without a trace. You discover the depths of your mother's illness, the way it has echoed and reverberated throughout your life, impacting your very legacy. Hi, I'm Danny Shapiro. And these are just a few of the profound and powerful stories I'll be mining on our 12th season of Family Secrets. With over 37 million downloads,

we continue to be moved and inspired by our guests and their courageously told stories. I can't wait to share 10 powerful new episodes with you, stories of tangled up identities, concealed truths, and the way in which family secrets almost always need to be told. I hope you'll join me and my extraordinary guests for this new season of Family Secrets. Listen to Family Secrets Season 12 on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. I had this overwhelming sensation that I had to call her right then.

And I just hit call, said, you know, hey, I'm Jacob Schick. I'm the CEO of One Tribe Foundation. And I just wanted to call on and let her know. There's a lot of people battling some of the very same things you're battling. And there is help out there. The Good Stuff podcast, Season 2, takes a deep look into One Tribe Foundation, a non-profit fighting suicide in the veteran community. September is National Suicide Prevention Month,

so join host Jacob and Ashley Schick as they bring you to the front lines of One Tribe's mission. I was married to a combat army veteran, and he actually took his own life to suicide. One Tribe saved my life twice. There's a lot of love that flows through this place, and it's sincere. Now it's a personal mission. I wouldn't have to go to any more funerals, you know. I got blown up on a React mission.

I ended up having amputation below the knee of my right leg and a traumatic brain injury because I landed on my head. Welcome to Season 2 of the Good Stuff. Listen to the Good Stuff podcast on the Iheart Radio app, Apple Podcasts, or wherever you get your podcast. All right, we're talking about the dark flow of the universe, which is this idea that it seems like everything in the universe is moving

in a particular direction, and we don't know why. We definitely don't know why, which is why we slapped our familiar dark label on this thing. Or I guess it's moving relative to the cosmic microwave background. Could it be that the microwave background is the one that's moving? Or has a bias? Well, the motion is relative. So I'm not sure what it means to say the C&B is the one that's moving,

like relative to what? Relative to all the galaxies? Yes, but the motion is relative. So you can't just like a scribe. it to one and not the other. But it's the relative motion that's curious, right? Because we think that the stuff that made the galaxies is the same stuff that made that CMB light.

So that plasma that made the CMB light, its overall motion should be the same as the galaxies because the plasma is what turned into the galaxies. Unless there was something else involved, unless there's something else pulling on these galaxies that didn't pull on the CMB. Meaning like if you zoom out of where we are, our super cluster. our galaxies, our superclusters, maybe we're orbiting around something bigger than maybe even the bigger than the observable universe. Exactly.

And we often talk about the observable universe as our limit of the things that we can see, but there's an important subtlety there. The observable universe is what we can see now, what we can interact with now. But the universe is expanding, right? And things used to be closer to us. So there are things that used to be in our observable universe that we used to be able to interact with us, that used to be able to affect us with no longer can. So it might be that by looking deep into our past

and understanding how the universe has been affected, we can see hints of things that affected us which have now left our observable universe. This is sort of like a way to sneak into seeing things that are now outside the observable universe. Wait, what? Because of this motion, like maybe tells us where it's been? Yeah, exactly.

What if there was something really massive something crazy massive in our universe early on, and it created this gravitational tug towards it. And now it's left our observable universe. So it's not in our universe anymore. If you then looked at the motion of just the stuff in our observable universe, you would see it all moving towards this mysterious source of gravity and attraction, even though you wouldn't see that thing itself because it's now left our observable universe. So if something that's now outside our observable universe was once inside of it, right, then it might have still left an imprint on the motion of galaxies. Meaning like maybe we could deduce where the sun is and what the sun is just by

looking at the orbit of the earth without having to actually look at the sun. Exactly. You don't need to see the sun in order to know that the sun is there. And so even though this thing is outside our observable universe, it's within our past light cone. Our past light cone is all the stuff that we have interacted with in the past that could have affected us in the past and the expansion of the universe is pulling things outside of our light cone making it impossible to interact with that stuff because space is expanding faster than the speed of light but it is possible that there was once something very dense some intense mass some source of incredible gravity that affected the formation of the whole universe that we now can't see I see it's sort of like you know how we found out

that the Earth is just orbiting around the sun and then eventually we just found out that the solar system, the sun, is just kind of in a corner of the Milky Way galaxy orbiting around the center of the Milky Way galaxy, not even close to the center of it and now maybe we're finding out that our whole

observable universe, everything that we can see all those bazillions of stars, maybe we're just like at the corner or at the edge of some sort of bigger mass of stuff in the universe. Exactly. And we like to imagine that the whole universe, if it's infinite, is filled with the same kind of stuff and that our chunk in the observable universe we happen to live in is probably an average chunk.

And any chunk, it would all average out to zero. But it might not be the case, right? It might be that there is some larger structure. There are like things that are denser and heavier and so that if you take a random slice of the universe, you don't on average get zero. It gives you a picture as to like, what is that nearby larger structure? I think it's super cool that we could like make measurements in our observable universe. and get glimpses for what's beyond, right?

Which sometimes felt like an impenetrable wall, like a dark wall beyond which we couldn't see. But we can dig out clues from the history of the universe to figure out what has happened that we can no longer see. Well, it's a little bit sort of like the picture we have of the Milky Way galaxy where we live. Like we really can't see it. We're in the middle of it,

but we can sort of reconstruct what it might be or what it is just by looking at the immediate things around us. Yeah, that's true. The things that are obscuring our vision are different And in the case of the Milky Way galaxy, it's like the gas and the dust and other sort of practical stuff. And in the case of the observable universe, it's the speed of light. But yeah, it's a good analogy. However, of course, there's controversy about this.

Is it about the name? Beyond the name. Because they're calling it a dark flow was maybe a terrible idea because it's not really dark. It does kind of flow, though. No, the controversy is about whether it exists. So this measurement was made originally with like CMB data from the WMAP satellite, sort of an intermediate satellite, not the most precise data we have about the cosmic microwave background radiation.

Wait, wait, what is the WMAP? I think it's named after Wilkinson. It's a satellite out in space that picks up these CMB photons. They're very, very low energy photons, super long wavelengths. You need a very sort of specialized equipment to pick them up. The WMAP satellite is part of a long history of these satellites. There was Kobe was the first one, then WMAP, and then Plunk. These more and more precise satellites.

It's a space telescope. Yeah, it's a space telescope. It's an instrument out in space that picks up these photons. And this one is specialized on the C&B. And then what we also used to measure the expansion and the motion of these galaxies. Yeah, you can learn so much from the C&B. Absolutely. Very, very general, very powerful.

That's why WMAP is such an important thing and why the Kobe satellite folks won a Nobel Prize. And why Plunk was such an important thing. Plunk is the follow-up to W-Map. Anyway, these results came from an analysis of 700 clusters. with the WMAP data. And then they reanalyze this using the Planck data. So more precise, more recent data, larger data set. And there's a disagreement about the results there.

Like one group says, yes, we see the dark flow in the Planck data. Another group says, no, we analyze that same data. We don't see anything. What? Couldn't they just cross-track and figure out why they're different? They're working on that. But it's complicated because when you do these analysis, there's so many assumptions and two different groups are going to make different assumptions.

And those assumptions are sometimes hard to spot. Like, what kind of assumptions? Well, there's all sorts of details you need to understand about how the C&B photons are boosted as they pass through this hot gas. So people have a model of that. Nobody's modeling all of the details of every individual photon down to all the microphysics. It's always a simplification and how those simplifications are made and whether they're valid and whether the simplifications introduce errors and whether those errors are important. It's a long series of decisions people make when they analyze these data.

That's why it's important to have crosschecks because it helps you. reveal where those decisions could be biasing your results. So what that tells us is like there's something funky in one of these analyses and you're right, they need to cross check and drill down. But it's not trivial. It's not like they're doing the same calculation and expect to get exactly the same number. They're probing

the same physical thing, but they're doing the calculation in very different ways. I mean, couldn't they just like go to a meeting together and figure it out? Like, I'm doing this. What are you doing? Oh, you're doing that? Oh, wait, that doesn't make any sense. What have you tried that?

Yeah, I think they are working on that, but I think there's also a little bit of acrimony between these two groups. I'm not sure it's always been a friendly disagreement. Yes, exactly. Is it dark drama? I mean, nobody's like murdering other people's parents in the alleyway and leading them to become the dark night of justice or anything. It's not that dark. Not yet, apparently. Stay tuned.

I mean, there's a Nobel Prize at stake. Things will get ugly. But what lies in the future is a deeper analysis of this same day. and we hope maybe even more refined data. Future measurements of the CMB can give us an even clearer picture of what's out there, the motion of the CMB, and the motion of all these blobs of gas and galaxies relative to the CMB. So these folks are talking and what are they saying is maybe the probable cause? If there is a relative motion to all these stars and galaxies around us, what could be the cause of it?

So the only idea that's out there if this thing is real is what we talked about earlier, some big blob of stuff that's out there beyond our observable universe, past our horizon, something very dense that once pulled on all of us, so we all have this overall bias in our peculiar motion. What that could be, nobody knows. Wait, are you saying that maybe there was something bigger than the observable universe that was pulling on all of our galaxies that we can see, but now that it's so far away from us, it doesn't affect us anymore or can't affect us? Yeah, that's exactly right, because the expansion of the universe is,

faster than light. This expansion increases as distance grows. And so things fall off the edge of our horizon. There are some things which could interact with us, could send us photons or pull on us, which now can no longer. Like things near the edge of the observable universe, we see photons from them, but some of them are now past the edge of the observable universe. And if they send us photons, those photons will never arrive because space is expanding between us and those galaxies is faster than the speed of light. Sort of mind-boggling to think about. Right, because even gravity can only go as fast as the speed of light, right?

So if something's moving or being expanded away from us faster than the speed of light, then even we'll never feel its gravity. Exactly. Like if they shine a flashlight at you, you'll never see it no matter how long you wait, which also means you'll never feel its gravity. You might have seen that flashlight earlier on. You might have felt their gravity early on, but no longer. So again, this is like a way to probe things that we might.

have interacted with earlier in the history of the universe that we can no longer see. You're saying we'll see it like the spin it gave us or the orbit it gave us. Yeah, exactly. It's sort of like what happens if you walk into a party five seconds after some celebrity does, right? And everybody's looking in the same direction. And you're like, what's going on? Who was just here?

Who was that? Was that Batman? Or was that Kim Kardashian? Or is Kim Kardashian actually Batman? And you see the effect on the conversations and everybody's head is pointing in some direction. You don't see the person, but you see the effect they left on the room. So we think, right? We still have to confirm

these measurements. Yeah, exactly. These are very difficult measurements to make. And as you hear, there's not a consensus about whether the dark flow is even a thing. Oh, boy. Maybe we should give it a different name until then. What would you call it? What would you call it? The Dark Kardashian? Well, I feel like the word dark,

didn't that originate from the being invisible? Like dark matter, you can't see it because it doesn't interact with the electromagnetic light. Because if you're going to call anything that's mysterious dark, then that's everything, isn't it? The whole universe is kind of mysterious. I think that dark means mysterious. It doesn't mean invisible because dark it doesn't actually mean invisible, right? You can have invisible things in bright light also.

Dark corners mean dark corners are things that are like obscured. Right, right. But I guess what I mean is if you start calling everything that's mysterious dark, then that's just going to confuse everybody. Yeah, that's probably true. There's dark particles. There's, you know, dark things in my fridge. You know, people are going to think they're related to dark matter and dark energy in the dark night. Well, they're all mysterious, so they are all related in that sense.

In the dark minds of physicists. Maybe not in the physical, in the physics sense, right? Yeah. No, absolutely. They could have completely different physical explanations. They're just currently not understood. All right. Well, maybe you should just call it the Hubble flow and then just follow it up with,

we don't know what the hobble flow is or what's causing it. But this is in addition to the Hubble flow, like, what on top of the Hubble flow is happening in the universe is really the question. All right. Well, it sounds like the answer. Once again, is stay tuned. There are mysterious workings in the universe, mysterious flows, plumbing that we can't yet see out there in the universe that is hopefully leaking a little bit so that we know

and we can study it. And we'd love to understand the universe. And so we're looking out there into the nice sky and trying to squeeze every tiny little drop of information out of the photons that do arrive to us. And it's incredible that we can even figure out that the universe is out there and what it's doing and maybe even get a glimpse of what's past the edge of the observable universe. I wonder if physicists are like Batman, you know, you're looking up at the sky, you see a bat signal in the motion of the stars, and you're like, duty calls.

We're all totally ripped just like Batman. Ripped. That wasn't a joke, man. Why are you laughing? Ripped in your minds, in the abs of your minds, and in the biceps of your... Typing fingers. Well, in any case, we hope you enjoyed that. Thanks for joining us. See you next time.

For more science and curiosity, come find us on social media where we answer questions and post videos. We're on Twitter, Discord, Insta, and now TikTok. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcast from IHeartRadio,

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December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then, everything changed. There's been a bombing at the TWA terminal. Just a chaotic, chaotic, chaotic. seen in its wake a new kind of enemy emerged terrorism listen to the new season of law and order criminal justice system on the iHeart radio app apple podcasts or wherever you get your podcasts

my boyfriend's professor is way too friendly and now i'm seriously suspicious wait a minute sam maybe her boyfriend's just looking for extra credit well dakota luckily it's back to school week on the okay story time podcast so we'll find out soon this person writes my boyfriend's been hanging out young professor a lot. He doesn't think it's a problem, but I don't trust her. Now he's insisting we get to know each other, but I just want her gone. Hold up. Isn't that against school policy?

That seems inappropriate. Maybe find out how it ends by listening to the OK Storytime podcast and the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. This is an IHeart podcast.