Daniel and Jorge Explain the UniverseDaniel and Jorge talk about whether the whole Universe is in motion, and what could be pulling on it
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I'm David Eagleman from the podcast Inner Cosmos, which recently hit the number one science podcast in America. I'm a neuroscientists at Stanford and I've spent my career exploring the three pound universe in our heads.
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Guess what, Well, that's that Mago.
I've been trying to write a promo for our podcast part Time Genius. But even though we've done over two hundred and fifty episodes, we don't really talk about murders or cults.
I mean, we did just cover the Illuminati of cheese, so I feel like that makes us pretty edgy. We also solve mysteries like how Chinese is your Chinese food? And how do dollar stores make money? And then of course can you game a dog show?
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Hey Org. 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 which don't have colored?
All right, that's fair, So let's do a little experiment. I'm going to give you a physics name, and you're gonna 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? Didn't find out?
You have to go with the flow? Is that what you're saying?
Hi?
I'm Horia mcgurtoonez and the author of Oliver's Great Big Universe.
Hi, I'm Daniel. I'm a particle physicist and a professor at U see Erine, and I hate plumbing projects.
Who loves plumbing projects? I guess plumbersumbers?
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 means bad news. Something's leaking somewhere, something's broken somewhere. You know, the joys of home ownership.
What don't you call a plumber?
Yeah, that's like paying somebody to deliver the bad news to you.
Hey. Yeah, 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 that had to tear out and rehab all the walls in our garage. It was a big pain in the butt.
Oh boy, what happened? The tub overfluid 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 bulliancy works.
Now 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 Osmos is 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 is plumbing.
I'm saying, our mental understanding of how the universe works has plumbing. When you 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, what do you flush down the toilet of physics?
Imagine the inside of a physicist mind and model it as a bunch of pipes with ideas flushing and flowing and swirling.
I see, I always love physicists thicky pipes themselves in their brains. But aren't physicists sort of the plumbers of the universe kinda? 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, Hi, everything's just sort of hanging it out. But that's just because we live for such brief moments. On cosmic scales, if you look 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.
M 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 you tackle the plumbing projects in your house or do you call it professional?
Sometimes I do fix it. Yeah, I fix my washing machine the other day. I was pretty proud of myself.
My kicking it and swearing at it.
No, but looking at 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 a part, pulled it apart. Surprisingly, nothing exploded so far. That's right.
So far the Jorge has mastered his washing machine and 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, in 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 on flow, is it.
Well, there are a lot of bodily functions that do flow, but we are strictly a physics pot cast, not biology.
Although putting the word dark in front of it 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. Oh, aren't doctors sort of like body plumbers?
Hmmm, yeah, exactly. Yeah, the digestive system sometimes needs a new valve.
But this is an intriguing name for a concept that dark flow, and so, as usual, we were wondering how many people out there had heard of this concept or have 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, right to me too, questions at Daniel Landhorhey dot com.
So think about it for a second. What do you think is the dark flow? Here's what people have to say.
That sounds like it has to do with dark energy and or dark matter and it's relative distribution through the cosmos over time.
I want to say, 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 gonna go with Batman as well. It's like when Batman's in a state where he's totally concentrated in his crime fighting, it's 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 one 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, Like, come up with something new, people, Well.
Do you mean to change his origin story? Then he wouldn't be Batman.
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 this stept 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 unknown and invisible, right, like you can't see it in space. Mm hmm, yeah, exactly.
All right, Well, Daniel, step us through this. What is the dark flow? What's exactly flowing in the universe and where is it flowing too or down too?
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 like 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. And that's the expansion of the universe, which is a really interesting and fascinating history. Right. We know that the universe expanded very quickly early.
On, that's the Big Bang, right, and 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 lipped. The universe took its foot off the gas and started hitting the brake. So still expanding, but now it was decelerating. There's 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 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 five or six billion years we've had acceleration again that's dark energy.
So it's been sort of a roller coaster ride for the universe, right, Like it's stressed out really fast and then it slowed down, and now it's picking up speed again, and potentially it's going to keep picking up speed until it grows it in 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 supermassive 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 if you don't know what dark energy is. Possibly it's 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 and happening locally now ish we can see the expansion happening far away, like a billion years ago. It's hard to compare point to point.
But then, 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 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. This is how Hubble sort of initially described it. You know, the velocity of these galaxies. It makes more sense, it's sort of more natural to think about it in terms of general relativity. Has 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 increasing, and so for nearby galaxies you can like 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, it's a good question. I 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 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 superclusters 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 is 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 gentless little current that your body can overpower. Even on the scale of the Solar System, gravity winds 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 superclusters. 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 is sort of almost like space flowing through me as it expands through.
Me 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 Andromeda, and Andromeda's peculiar motion is releigt 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 Way. 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. They're all basically pointing in random directions. Very few are 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 the 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 analogies 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 we're 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 break.
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Hi.
I'm David Eagleman from the podcast Inner Cosmos, which recently hit the number one science podcast in America. I'm a neuroscientists at Stanford, and I've spent my career exploring the three pound universe.
In our heads. We're looking at a whole new.
Series of episodes this season to understand why and how our lives look the way they do. Why does your memory drift so much? Why is it so hard to keep a secret? When should you not trust your intuition? Why do brains so easily fall for magic tricks, and why do they love conspiracy theories. I'm hitting these questions and hundreds more because the more we know about what's running under the hood, the better we can steer our lives. Join me weekly to explore the relationship between your brain and your life by digging into unexpected questions. Listen to Inner Cosmos with David Eagleman on the iHeartRadio app, Apple Podcasts or wherever you get your podcasts.
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 Tinge story The Dark Knights have Gotham.
I can underplot you, like, there's something going on in the streets.
Of Gotham exactly.
There is 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 are complicit.
Yes, I am complicit. I will admit to that. Toss me in physical naming jail and throw away the key.
M 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 in the university 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. 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. Right, there should be no preferred direction in the universe.
Meaning like like let's say we ignore the expansion of the universe, or like take that into account, or you substract the expansion of the universe from the motion of everything. How is everything moving? How are all those superclusters of galaxies on those galaxies and stars? How's 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 arounder 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, et cetera.
I see, it's sort of like maybe if we 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. The canister is not going anywhere, and it's had some time to diffuse and even.
Out exactly unless there was something outside that cannister 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. Shoots 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 it 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. Right, velocities are not a property of objects. There are 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 if you change the point of reference, like shooting it at up to zero no matter what.
Yeah, this is a really interesting and subtle point, Like space itself has no frame of reference, 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 and you can sort of pick that frame of reference just by looking at the cosmic microwave background radiation.
But wait, I thought every other motion of the other 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 the life 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 fourteen 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 galaxy came from the same stuff that formed the CMB, so it'd be weird if that stuff was moving relative to the CMB.
Wait wait, wait, wait, so are you saying the cosmic microwave background radiation is not the same in all directions? You're saying, like, it's red er 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 is very obviously redder in one direction and bluer in the other and then typically you see these maps of the CNB, 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, the zero of the universe. That's really the only thing we can compare two. 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, are 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, where 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 micro rate 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 CMB. 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 seven hundred 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 of velocity that's smaller than their uncertainty, then they say, okay, it's consisting 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 biber they're going one hundred miles an hour, then that's very likely the average speed of your road is not zero, it's closer to one hundred.
Okay, so that's a big mystery. Like you've measured the velocity of while the superclusters out there, or at least a sample of them, and they don't seem 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 CMB photons passing through these clouds of hot gas. As the CMB photons passed 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. It'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 an eight hundred to one thousand 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 disc, right, and so you can look up above the disc, or down below the disk, 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 disk itself, or even through the center of the galaxy, there'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 Lini Kia supercluster. 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.
By all the galaxies in the universe. You mean all the galaxies we can see.
I mean only these seven hundred clusters that were measured by this one study, right.
Right, Well, just a zero sample of all of them.
Maybe 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 Linikia Clue supercluster 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 this is something much bigger than just the Great Attractor.
The Great Attractor is within our supercluster of galaxies, or it's outside of it.
We don't know what the Great Attractor is. We think it might be something super massive 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 super cluster.
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 Great Attractor, meaning towards us.
No, the stuff behind the Great Attractor 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. Hi.
I'm David Eagleman from the podcast Inner Cosmos, which recently hit the number one science podcast in America. I'm a neuroscientists at Stanford, and I've spent my career exploring the three pound universe.
In our heads. We're looking at a whole new.
Series of episodes this season to understand why and how our lives look the way they do. Why does your memory drift so much? Why is it so hard to keep a secret? When should you not trust your intuition? Why do brains so easily fall for magic tricks? And why do they love conspiracy theories? I'm hitting these questions and hundreds more because the more we know about what's running under the hood, the better we can steer our lives. Join me weekly to explore the relationship between your brain and your life by digging into unexpected questions. Listen to Inner Cosmos with David Eagleman on the iHeartRadio app, Apple Podcasts, or wherever you get your podcast us.
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We think of Franklin as the doddling dude flying a kite and no rain, but those experiments are the most important science discoveries of the time.
I'm Evan Ratliffe.
Last season, we tackled the ingenuity of Elon Musk with biographer Walter Isaacson. This time we're diving into the story of Benjamin Franklin, another genius who's desperate to be dusted off from history.
His media empire makes him the most successful self made business person in America. I mean he was never early to bed, an early to rise type person. He's enormously famous. Women start wearing their hair and what was called the cliffor a la Franklin.
And who's more relevant now than ever.
The only other person who could have possibly been the first president would have been Benjamin Franklin, but he's too old and wants Washington.
To do it.
Listen to On Benjamin Franklin with Walter Isaacson on the iHeartRadio app, Apple podcasts or wherever you get your podcasts.
Hey, it's Jake Alburn. We have a new limited series of my podcast Deep Cover out now, all about George Santos, the Republican congress from New York who told a lot of stories about his life and his credentials, many of which turns out were not true.
That's like, you know, mister Ripley needs catch me if you can.
I mean, the guy who'd wing to everyone.
He was very ambiguous and sketchy, quite quite honestly about what the company did and how it made so much money overnight.
What prosecutors a legend indictment is that most of that twelve thousand dollars goes directly to Santos's personal bank account.
I would go down these rabbit holes and start thinking about, like what is the nature of truth? You know, like what can I what can I actually like tell the reader is real about this guy's story.
My phone is literally blowing up inquiries about saying, is George going to jail? What's going on? And I though, like why are you doing this?
Like why listen to Deep Covered George Santos on the iHeartRadio app, Apple podcasts or wherever you listen to podcasts.
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 a 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 CNB 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 ascribe 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, it's 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.
I mean, like, if you zoom out of where we are our supercluster, our galaxies, our superclusters, maybe we're orbiting around something bigger then, 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, which 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. It's 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 it tells us words 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 of 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.
Yeah, exactly, you don't need to see the Sun in order to know that the Sun is there. And so even though the 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 a 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, the observable universe we happen to live in it 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 chance 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. 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 dark flow was maybe a terrible idea because it's not really dark.
It does kind of flow, though, the controversy is about whether it exists. So this measurement was made originally with like CMB data from the w MAP satellite, sort of an intermediate satellite, not the most precise data we have about the cosmic microwave background radiation.
Wa 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 w MAP satellite is part of a long history of these satellites. It was Kobe was the first one, then w MAP and then Pluck these more and more precise based telescope. Yeah, it's a space telescope. It's an instrument out in space that picks up these photons, and.
This one to specialize on the CMB and then but we also used to measure the expansion and the motion of these galaxies.
Yeah, you can learn so much from the CMB. Absolutely, very very general, very powerful. That's why WMAP is such an important thing, and why Kobe won, why the Kobe satellite folks won a Nobel Prize, and why PLANK was such an important thing. Plank is the follow up to WMAP. Anyway, these results came from an analysis of seven hundred clusters with a w MAP data, and then they reanalyze this using the PLANK 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 PLANK data. Another group says, no, we analyze that same data, we don't see anything.
What cann't They just crosscheck 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 differentsumptions, 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 CMB 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 cross checks, 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 analyzes, 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? Or are you doing that? Oh that doesn't make any sense? What if 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 a 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 Knight of Justice or anything. It's not that dark, but not yet apparently, stay tuned. Maybe there's a Nobel prize at stake. Things will get ugly. But what lies in the future is a deeper analysis of the same data, 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 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. There 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 life. 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 Ghax, he's faster than the speed of light. Sort of mind boggling to think about.
Right, the spin 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 it's 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 badman?
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 to in some direction. You don't see the person, but you see the effect they left on the room.
Or 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.
Well, boy, maybe you should give it a different name.
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.
It's 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 dark. There's dark particles, there's you know, dark things in my fridge are going to think people are I 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, or.
Maybe not in the physical in the physical 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 hubble 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 night 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 bad 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 and the apps 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, Instant, and now TikTok. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. How is us dairy tackling greenhouse gases? Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as Dairy dot COM's Last Sustainability to learn more.
Guess what Will?
What's that Mago?
I've been trying to write a promo for our podcast, Part Time Genius, but even though we've done over two hundred and fifty episodes, we don't really talk about murders or cults.
I mean, we did just cover the Illuminati of cheese, so I feel like that makes us pretty edgy. We also solve mysteries like how Chinese is your Chinese food? And how do dollar stores make money? And then, of course can you game a dog show?
I'm saying is everyone should be listening.
Listen to Part Time Genius on the iHeartRadio app or wherever you get your podcasts.
Hi.
I'm David Eagleman from the podcast Inner Cosmos, which recently hit the number one science podcast in America. I mean neuroscientists at Stanford and I've spent my career exploring the three pound universe in our heads.
Join me weekly to explore the relationship.
Between your brain and your life.
Because the more we know about what's running under the hood, better we can steer our lives. Listen to Inner Cosmos with David Eagleman on the iHeartRadio app, Apple Podcasts, or wherever you get your podcasts.
