Speaker 1: Hey, Orge, do you think stuff in science is typically
Speaker 1: well named or sometimes misleading?
Speaker 2: Depends on which science. Do you mean physics? Then no?
Speaker 1: I mean sometimes we do a good job, right, like
Speaker 1: black hole?
Speaker 2: Yeah? Yeah, those are black and their holes in space.
Speaker 2: So I guess you did good on that one.
Speaker 1: But in other cases you're saying we didn't do as well.
Speaker 2: Well, let's see, how about quark flavors which don't have flavor,
Speaker 2: how about quantum colors which don't have colored?
Speaker 1: All right, that's fair, So let's do a little experiment.
Speaker 1: I'm going to give you a physics name, and you're
Speaker 1: gonna guess what it means. All right, go for it.
Speaker 1: It's called the dark flow.
Speaker 2: That sounds like maybe a plumbing product to get your
Speaker 2: toilet moving again?
Speaker 1: Nice?
Speaker 2: Did I get that right? Stay tuned and find out
Speaker 2: you have to go with the flow. Is that what
Speaker 2: you're saying?
Speaker 1: Hi?
Speaker 2: I'm Horia mccartoonist and the author of Oliver's Great Big Universe.
Speaker 1: Hi, I'm Daniel. I'm a particle physicist and a professor
Speaker 1: at U see erline, and I hate plumbing projects.
Speaker 2: Who loves plumbing projects? I guess plumbers?
Speaker 1: Yeah? Maybe, I mean I hope they love plumbing projects.
Speaker 1: Otherwise it's kind of sad.
Speaker 2: It's probably interesting to them. Sure, But how many plumbing
Speaker 2: projects have you worked on recently?
Speaker 1: Oh? Too many? You know, anytime you have a plumbing project,
Speaker 1: that means bad news. Something's leaking somewhere, something's broken somewhere.
Speaker 1: You know, the joys of home ownership.
Speaker 2: What did you call a plumber?
Speaker 1: Yeah, that's like paying somebody to deliver the bad news
Speaker 1: to you.
Speaker 2: Hey, yeah, and then then hopefully they fix it for you.
Speaker 2: What kind of plumbers are you calling?
Speaker 1: Yeah, they fix it. We had a leak in our
Speaker 1: upstairs bathroom recently and had to tear out and rehab
Speaker 1: all the walls in our garage. It was a big
Speaker 1: pain in the butt.
Speaker 2: Oh boy, what happened? The tub overfluid or something?
Speaker 1: It was a dark flow.
Speaker 2: You took too long of a bath there thinking about physics.
Speaker 1: I was dunking stuff in the bathtub hoping for a
Speaker 1: Eureka moment, and it just didn't happen.
Speaker 2: It just didn't happen. You didn't figure out how bullyiancy works.
Speaker 1: No, it's too complicated for a particle physicists.
Speaker 2: Too many particles in a tub of water. But anyways,
Speaker 2: welcome to our podcast Daniel and Jorge Explain the Universe,
Speaker 2: a production of iHeartRadio.
Speaker 1: In which we teach you to think like a physicist
Speaker 1: instead of a plumber. Don't dunk stuff in a tub
Speaker 1: of water, but immerse your brain in the mysteries of
Speaker 1: the universe and try to learn how the universe works.
Speaker 1: By Oz most is hanging out with us as we
Speaker 1: talk and joke about everything that's out there in the
Speaker 1: universe and try to explain all of it to you.
Speaker 2: That's right. We try to fill the tub in your
Speaker 2: mind of knowledge and wonder until it overflows and hopefully
Speaker 2: maybe leaks into your garage a little bit.
Speaker 1: You know, in that scenario, maybe a plumbing project is
Speaker 1: good news because you got to rip everything out and
Speaker 1: build up a new understanding of the universe. That's what
Speaker 1: I'm always saying I want to do. Wait.
Speaker 2: Wait, you're saying the universe is plumbing.
Speaker 1: I'm saying, our mental understanding of how the universe works
Speaker 1: has plumbing. When you got to tear all that out
Speaker 1: and rebuild it, that means you're coming up with a
Speaker 1: new idea about how the universe works, and that's a revolution.
Speaker 2: Oh wait, what's the plumbing for ideas? Also, what do
Speaker 2: you flush down the toilet of physics?
Speaker 1: Imagine the inside of a physicist mind and model it
Speaker 1: as a bunch of pipes with ideas flushing and flowing
Speaker 1: and swirling.
Speaker 2: I see. I always love physicists sticky pipes themselves in
Speaker 2: their brains. But aren't physicists sort of the plumbers of
Speaker 2: the universe? Kind of you're trying to figure out how
Speaker 2: the plumbing of the universe works.
Speaker 1: Yeah, everything in the universe is out there, sloshing around,
Speaker 1: banging it into itself. It's not like a stable situation.
Speaker 1: You look up at the night sky and you might think, Hi,
Speaker 1: everything's just sort of hanging it out. But that's just
Speaker 1: because we live for such brief moments. On cosmic scales,
Speaker 1: if you look at the universe over millions or billions
Speaker 1: of years, you would see things expanding and exploding and
Speaker 1: smashing and flowing.
Speaker 2: Well, usually pipes expanding is not a good sign. Like,
Speaker 2: if your pipe expands, it's probably going to burst.
Speaker 1: M Yes, that's true. Dark energy would be bad news
Speaker 1: for your plumbing.
Speaker 2: Yeah, better flush it down the toilet or call a
Speaker 2: real professional.
Speaker 1: Someone who has actually useful skills you mean.
Speaker 2: Yeah, someone who can fix a leak, someone who knows
Speaker 2: how to use a wrench.
Speaker 1: You mean not a physicist and not a cartoonist.
Speaker 2: No, but an engineer. Maybe I'm also an engineer. I
Speaker 2: have that skill.
Speaker 1: So you tackle the plumbing projects in your house to
Speaker 1: do you call it professional?
Speaker 2: Sometimes I do fix it. Yeah, I fix my washing
Speaker 2: machine the other day. I was pretty proud of myself.
Speaker 1: My kicking it and swearing at it.
Speaker 2: No, but looking it up on YouTube and looking at
Speaker 2: a video of how to replace the little valve there inside.
Speaker 2: We're all YouTube engineers.
Speaker 1: Did you type how to replace that little valve there
Speaker 1: into Google and just follow the instructions?
Speaker 3: No?
Speaker 2: I think I just typed the model of my dishwasher,
Speaker 2: and then, you know, helpfully, a lot of people out
Speaker 2: there have posted videos about it. I had to order apart,
Speaker 2: pulled it apart. Surprisingly, nothing exploded.
Speaker 1: So far.
Speaker 2: That's right.
Speaker 1: So far the Jorge has mastered his washing machine and
Speaker 1: the plumbing of his house. Physicists are still trying to
Speaker 1: figure out how the universe out there works.
Speaker 2: Yeah, because it is an amazing universe full of what
Speaker 2: seems like internal plumbing where forces and things like particles
Speaker 2: and quantum fields all flow around and flush together and
Speaker 2: get mixed up and flow from one side to the other.
Speaker 1: There's all sorts of stuff involved. There's normal matter, there's
Speaker 1: dark matter, there's dark energy. There's the expansion of the universe.
Speaker 1: There's the shape of the universe, in the size of
Speaker 1: the universe, all of which play a role in what's
Speaker 1: going on out there.
Speaker 2: Yeah, we've learned a lot about the universe, but there's
Speaker 2: still a lot we don't know, big giant concepts that
Speaker 2: are still a big mystery. So today on the podcast,
Speaker 2: we'll be tackling the question what is the dark flow? Now,
Speaker 2: this is not related to on flow, is it.
Speaker 1: Well, there are a lot of bodily functions that do flow,
Speaker 1: but we are strictly a physics podcast asked not biology.
Speaker 2: Although putting the word dark in front of it it
Speaker 2: does make me think of toilet functions.
Speaker 1: There are a lot of dark matters that get flushed
Speaker 1: down the toilet, that's true.
Speaker 2: Yeah, a lot of dark matter that requires a lot
Speaker 2: of dark energy. Especially if you haven't been eating fiber.
Speaker 1: When your plumber is telling you you need a new diet,
Speaker 1: then you definitely have a problem.
Speaker 2: Yeah, aren't doctors sort of like body plumbers?
Speaker 1: Hmmm, yeah, exactly. Yeah, the digestive system sometimes needs a
Speaker 1: new valves.
Speaker 2: But this is an intriguing name for a concept, that
Speaker 2: dark flow. And so as usual, we were wondering how
Speaker 2: many people out there had heard of this concept or
Speaker 2: have any idea what it could meet.
Speaker 1: Thanks to all the volunteers who answered this question. If
Speaker 1: it sounds like fun to you to get a dose
Speaker 1: of random physics questions in your inbox every week, right
Speaker 1: to me too, questions at Danielandhorghey dot com.
Speaker 2: So think about it for a second. What do you
Speaker 2: think is the dark flow? Here's what people have to say.
Speaker 3: That sounds like it has to do with dark energy
Speaker 3: and or dark matter and it's relative distribution through the
Speaker 3: cosmos over time.
Speaker 1: I want to say, has something to do with like
Speaker 1: dark matter flowing between stars or galaxies, or like how
Speaker 1: we have a solar wind, so like a dark matter wind.
Speaker 2: All right, I'm going to go with Batman as well.
Speaker 2: It's like when Batman's in a state where he's totally
Speaker 2: concentrated in his crime fighting, it's just you know, having
Speaker 2: the artistic flow of fighting crime.
Speaker 1: At least it would be something new in the Batman story.
Speaker 1: I feel like Batman is just the same story over
Speaker 1: and over again.
Speaker 2: Oh boy, are we're going to get into Batman in
Speaker 2: this episode? How long is this episode? It sounds like
Speaker 2: you've thought about Batman a lot.
Speaker 1: I just saw like Batman Year one rebooted. It's like,
Speaker 1: how many times can you go back and tell the
Speaker 1: same story about his parents getting shot and he's a
Speaker 1: dark soul? Dot dot dot, Like, come up with something new, people, Well,
Speaker 1: do you mean like change his origin story?
Speaker 2: Then he wouldn't be Batman.
Speaker 1: Tell us a story about somebody else. Don't just keep
Speaker 1: rehashing the same.
Speaker 2: Ipedi tell us about possum Man.
Speaker 1: Yeah, exactly, And squirrel Man squirrel girl is a thing, right.
Speaker 1: Ryan North writes that it's great.
Speaker 2: Yeah, yeah, But anyways, without getting too dark here about
Speaker 2: comic books, this is apparently a real physics concept, dark flow.
Speaker 1: It is a real physics concept, and it's kind of
Speaker 1: connected to dark matter and dark energy, but it's neither
Speaker 1: of those things. It's more related to the overall expansion
Speaker 1: and flow of galaxies in the universe.
Speaker 2: I wonder if maybe you've overused the word dark in
Speaker 2: physics that's a dark thought, yeah, because people think it's
Speaker 2: just all connected. If you use the same word for
Speaker 2: the set for different things.
Speaker 1: You're absolutely right. People confuse dark matter and dark energy
Speaker 1: all the time because they're both called dark. Really, what
Speaker 1: we mean by dark is we don't understand this. It's
Speaker 1: mysterious and unknown.
Speaker 2: And invisible, right that you can't see it in space.
Speaker 2: M hmm, yeah, exactly, all right, Well, Daniel step us
Speaker 2: through this. So what is the dark flow? What's exactly
Speaker 2: flowing in the universe and where is it flowing too
Speaker 2: or down too?
Speaker 1: That's part of the question. How is the universe flowing
Speaker 1: and where is it flowing to? So to understand the
Speaker 1: dark flow, we have to think about the expansion of
Speaker 1: the universe, like where all the galaxies are going, because
Speaker 1: the dark flow is a mystery that sort of sits
Speaker 1: on top of our current understanding of the expansion of
Speaker 1: the universe, how all the galaxies are moving away from
Speaker 1: each other.
Speaker 2: And like expansion, you mean, like the expansion of space itself,
Speaker 2: Like the space where all the galaxies sit in is expanding,
Speaker 2: It's getting bigger and bigger.
Speaker 1: Exactly, space is being created between galaxies, which effectively increases
Speaker 1: the distance between those galaxies, and that's the expansion of
Speaker 1: the universe, which is a really interesting and fascinating history. Right.
Speaker 1: We know that the universe expanded very quickly early.
Speaker 2: On, that's the Big Bang, right, and inflation.
Speaker 1: Yeah, loosely speaking, though we don't understand what caused that
Speaker 1: initial expansion. And then after that period things were still expanding,
Speaker 1: but the signs sort of flow. The universe took its
Speaker 1: foot off the gas and started hitting the brake. So
Speaker 1: still expanding, but now it was decelerating. There's all this
Speaker 1: mass in the universe that was starting to slow down
Speaker 1: the expansion. So universe still getting bigger, space still getting
Speaker 1: created between the galaxies, but at a lower.
Speaker 2: Rate now, meaning like the mass of the stuff in
Speaker 2: the universe was somehow slowing down the expansion. Why was that?
Speaker 2: Was it like the gravity of the stuff or just
Speaker 2: the having stuff in space makes it not expand.
Speaker 1: That's just the gravity, right. What determines whether space is
Speaker 1: expanding or contracting, and whether it's accelerating or decelerating is
Speaker 1: the amount of stuff in space, like the matter density,
Speaker 1: and that sends to pull stuff together. And then also
Speaker 1: the shape of the universe, is it flat, is it negative?
Speaker 1: Is it open? And the amount of dark energy which
Speaker 1: is pushing things out and accelerating things. Early on in
Speaker 1: the universe, there really wasn't a whole lot of dark energy,
Speaker 1: and so matter dominated and it was slowing down that expansion.
Speaker 1: But because it was still expanding, and as the universe expands,
Speaker 1: it makes more dark energy because dark energy doesn't get diluted,
Speaker 1: and then dark energy makes the universe expand more. That
Speaker 1: continuing expansion, even though it was decelerating, made more dark energy,
Speaker 1: which turned things around again back to acceleration. So we
Speaker 1: had constant expansion, but we had like initial acceleration, then deceleration,
Speaker 1: and now for the last five or six billion years
Speaker 1: we've had acceleration. Again that's dark energy.
Speaker 2: So it's been sort of a roller coaster ride for
Speaker 2: the universe, right, Like it stressed out really fast and
Speaker 2: then its slowed down and now it's picking up speed
Speaker 2: again and potentially it's going to keep picking up speed
Speaker 2: until it grows it and super duper fast rate.
Speaker 1: Yeah, we don't know what the future holds because we
Speaker 1: don't really understand dark energy like at all. If this
Speaker 1: simple model of dark energy is just like some energy
Speaker 1: that permeates space and causes the expansion of the universe
Speaker 1: to accelerate. Is valid, then yeah, I'll just keep going
Speaker 1: forever and things will get more and more distant. Space
Speaker 1: will continue to expand between galaxies, creating vaster and vaster
Speaker 1: distances between superclusters which will collapse into supermassive black holes.
Speaker 1: And that's sort of the far future of the universe.
Speaker 1: And we're talking about this today because this expansion of
Speaker 1: the universe, this creation of space between galaxies, is sometimes
Speaker 1: called by astronomers the Hubble flow.
Speaker 2: Now, first of all, I guess this expansion of space
Speaker 2: is happening everywhere, all at once, the same everywhere, or
Speaker 2: does it happen more in certain spots.
Speaker 1: We think it happens the same everywhere.
Speaker 2: Even the spots with lots of stuff in it.
Speaker 1: Yeah, that's the current idea, and it's sort of a
Speaker 1: theory and an observation. It's sort of the simplest idea
Speaker 1: you could have if you don't know what dark energy is.
Speaker 1: Possibly it's something like a cosmological constant, some energy in
Speaker 1: space that causes this to happen. The simplest thing to
Speaker 1: do is to say, oh, that's just a number it's
Speaker 1: the same everywhere. We don't see any evidence for it
Speaker 1: being different in different parts of the universe. But you know,
Speaker 1: that's a tricky because we can't see it in different
Speaker 1: parts of the universe. At the same time, we can
Speaker 1: see the expansion happening locally now ish we can see
Speaker 1: the expansion happening far away, like a billion years ago.
Speaker 1: It's hard to compare point to point.
Speaker 2: But didn't you say stuff slows down the expansion of
Speaker 2: the universe. So I wonder if in this bus, where
Speaker 2: there's a lot of stuff in it, maybe it's expanding
Speaker 2: less fast.
Speaker 1: Stuff does slow down the expansion of the universe. That
Speaker 1: sort of contributes overall, but everything is actually balanced so
Speaker 1: that overall the universe is flat. But yeah, we don't
Speaker 1: know how stuff and dark energy interact with each other.
Speaker 1: Dark energy is so dilute and so actually weak that
Speaker 1: anywhere there is stuff, you can basically just ignore the
Speaker 1: dark energy and just treat it as if there was
Speaker 1: gravity there, just like from its stuff.
Speaker 2: But I guess as we look around, we see the
Speaker 2: whole of the universe expanding evenly in all.
Speaker 1: Directions exactly, And the idea of the Hubble flow, I
Speaker 1: think It's kind of helpful visually for you to think
Speaker 1: about this because we tend to think about this in
Speaker 1: terms of velocities, Like those galaxies are moving away from
Speaker 1: us at some speed, and we measure those velocities in
Speaker 1: terms of red shift. How the light from those galaxies
Speaker 1: is getting stretched out because those galaxies are moving away
Speaker 1: from us. This is how Hubble sort of initially described it.
Speaker 1: You know, the velocity of these galaxies. It makes more sense,
Speaker 1: it's sort of more natural to think about it in
Speaker 1: terms of general relativity. Has expansion of space itself, That
Speaker 1: those galaxies aren't accelerating away from us, but space is
Speaker 1: being created between us, and so like, there is an
Speaker 1: increased distance between us and those galaxies, but you're not
Speaker 1: feeling any acceleration.
Speaker 2: And you said, this motion is called the Hubble flow.
Speaker 2: What do you mean motion? And why is it called
Speaker 2: a flow?
Speaker 1: So motion there is a bit misleading. The distance between
Speaker 1: the galaxies is increasing, and so for nearby galaxies, you
Speaker 1: can like measure their velocity and say that galaxy has
Speaker 1: a certain velocity in our reference frame. But across really
Speaker 1: really big distances, it doesn't make sense to talk about
Speaker 1: relative velocity for objects that are super far away from
Speaker 1: each other and have different non inertial frames. So astronomers
Speaker 1: sometimes use velocities as a sort of way to talk
Speaker 1: about those objects. But it doesn't actually make any sense
Speaker 1: in terms of general relativity. And why it's called the
Speaker 1: hubble flow. Yeah, it's a good question. I think it
Speaker 1: just helps you visualize how like space is being created
Speaker 1: everywhere and things are being carried along by that stream.
Speaker 2: Okay, so the hubble flow. And then I guess maybe
Speaker 2: it's sort of like the flow of space being created
Speaker 2: pushing everything apart in the universe.
Speaker 1: Yeah, exactly, And so you can think about those galaxies
Speaker 1: as sort of like sitting in that hubble flow. Space
Speaker 1: is being created, everything is moving further and further apart.
Speaker 1: But there's a wrinkle on top of that motion. Right,
Speaker 1: the hubble flow is not the only thing that's happening
Speaker 1: in the universe. Each of these galaxies is also moving
Speaker 1: relative to the hubble flow. Like people often write in
Speaker 1: and ask, hey, if galaxies are being pushed apart because
Speaker 1: of dark energy, why is the Milky Way going to
Speaker 1: slam into Andromeda? And the answer is that these things
Speaker 1: are moving because of gravity. Right, we talked earlier about
Speaker 1: how dark energy sort of wins over really large distances,
Speaker 1: but for short distances it's too weak. Well, gravity is
Speaker 1: the opposite. Gravity gets really weak at large distances, so
Speaker 1: like between superclusters you hardly feel anything. But short distances,
Speaker 1: like just between two galaxies or between the Sun and
Speaker 1: the Earth, gravity totally dominates dark energy right like.
Speaker 2: Right now, even where I'm sitting, space is expanding, right like,
Speaker 2: if I look at my hand in front of me,
Speaker 2: the space that my hand is in is technically expanding,
Speaker 2: just like the rest of the universe is just expanding
Speaker 2: so little that the force is keeping my hand together
Speaker 2: when over the expansion of the space.
Speaker 1: Yeah, that's exactly right. And in the case of your
Speaker 1: body and your hand, the forces are electromagnetic, those chemical
Speaker 1: bonds are tying everything together, So the hubble flow is
Speaker 1: like the gentless little current that your body can overpower.
Speaker 1: Even on the scale of the Solar System, gravity winds
Speaker 1: and the motion of the Earth around the Sun is
Speaker 1: totally dominated just by gravity. Like if it wasn't, we
Speaker 1: might have discovered dark energy much much earlier, because we
Speaker 1: wouldn't have understood the orbits of the planets without it,
Speaker 1: and we have very precise measurements of things happening in
Speaker 1: the Solar System. Any little deviation from gravity we would
Speaker 1: have noticed if it was at all measurable. But dark
Speaker 1: energy is not even measurable on the scale of our
Speaker 1: Solar System. You have to go out to between clusters
Speaker 1: of galaxy in order to see it. So on the
Speaker 1: local scale, dark energy is basically irrelevant, and it's gravity
Speaker 1: that winds over and so these galaxies are moving in
Speaker 1: random directions and feeling tugs from gravity and having all
Speaker 1: sorts of dynamics. We talked about how galaxies merge and
Speaker 1: form together and make superclusters. So this motion relative to
Speaker 1: the hubble flow, that's called the peculiar motion of a galaxy.
Speaker 1: It just means how it's moving relative to the flow
Speaker 1: I see.
Speaker 2: So like, for example, in our Solar System, the space
Speaker 2: is expanding, but our planet is tied to the Sun,
Speaker 2: just like we're tied to the Earth. So like me
Speaker 2: sitting here on Earth and not moving relative to the Earth,
Speaker 2: there is sort of almost like space flowing through me
Speaker 2: as it expands through.
Speaker 1: Me exactly, So your peculiar motion is towards the Earth,
Speaker 1: and the Earth's peculiar motion is towards you. And then
Speaker 1: on the scale of galaxies, like the Milky Way's peculiar
Speaker 1: motion is relative to Andromeda, and Andromeda's peculiar motion is
Speaker 1: relative to the Milky Way. If we only were following
Speaker 1: the Hubble flow and had no peculiar motion, then the
Speaker 1: space between us would be expanding, right, the distance would
Speaker 1: be growing. But it's not. Milky Way Andromeda are coming
Speaker 1: towards each other. And that's the case for lots of galaxies,
Speaker 1: even the ones that aren't going to smash into each other.
Speaker 1: They're all basically pointing in random directions. Very few are
Speaker 1: like at rest relative to the Hubble flow.
Speaker 2: So like our galaxy and Andromena galaxy are moving relative
Speaker 2: to each other. And there's two components to that. You're saying, right, like,
Speaker 2: there's how much we're moving because the space is expanding
Speaker 2: between us, and there's also how much we're moving if
Speaker 2: the space wasn't expanding between us.
Speaker 1: Yeah, that's exactly right. You can think about it either
Speaker 1: just from the reference frame of the Milky Way and say, look,
Speaker 1: everything is moving away from us, and then just break
Speaker 1: that motion into two components. Say one of that is
Speaker 1: due to dark energy and the other is everything else
Speaker 1: relative to that, And then a more natural way to
Speaker 1: think about it is, instead of just having one reference
Speaker 1: frame centered at our heads, think about dark energy is
Speaker 1: just creating this like expanding frame, and then think about
Speaker 1: the motion relative to those frames. The hubble flow is
Speaker 1: sort of the most natural way to think about the
Speaker 1: expanding universe. And in that frame they're called co moving frames.
Speaker 1: The only motion is the peculiar motion, because you just
Speaker 1: sort of set the expansion of the universe as like
Speaker 1: the baseline thing.
Speaker 2: Right, It's sort of like maybe like the river analogies
Speaker 2: you just brought up earlier, like we're all moving in
Speaker 2: a river, and the flow of the river is the
Speaker 2: hubble flow, or like the expansion of the universe. But
Speaker 2: you know, within my little raft, I can have something
Speaker 2: moving relative to each other, or I can try like
Speaker 2: rowing towards your raft, and then there would be some
Speaker 2: peculiar motion between our rafts.
Speaker 1: Yeah, exactly right. Maybe we have engines and we're trying
Speaker 1: to push towards each other against the current, or we're
Speaker 1: trying to get away from each other or whatever, so
Speaker 1: that's our peculiar motion.
Speaker 2: All right, Well, these are peculiar concepts. And so let's
Speaker 2: dig into Now, what is the dark flow of the universe?
Speaker 2: Why is it dark? Where is it flowing? And is
Speaker 2: it something we want to touch or take a bath in.
Speaker 2: So let's dig into that. But first let's take a
Speaker 2: qu break. All right, we're talking about the dark flow
Speaker 2: of the universe, which sounds a little bit sinister. I
Speaker 2: feel like there's something going on under the surface.
Speaker 1: That is not light, sort of like a Batman tinged
Speaker 1: story the Dark Knights of Gotham.
Speaker 2: I can underplot you like, there's something going on in
Speaker 2: the streets.
Speaker 1: Of Gotham exactly.
Speaker 2: There's a flow of villainy in the sewers.
Speaker 1: Yeah, or maybe it's financial plumbing, right, it's a flow
Speaker 1: of dark money through politics.
Speaker 2: Oh there you go. Well again, maybe you should have
Speaker 2: called it something different and not dark and confuse everybody.
Speaker 1: You there means the entire physics community, right, because I
Speaker 1: certainly didn't call this dark flow.
Speaker 2: But you keep using it though, so you're complicit.
Speaker 1: Yes, I am complicit. Admit to that. Toss me in
Speaker 1: physical naming jail and throw away the key.
Speaker 2: Yes, we'll lock you up an archem asylum with the
Speaker 2: rest of the Batman villains. All right, we're talking about
Speaker 2: the dark flow of the universe, and we talked about
Speaker 2: how the universe is expanding, and it's because space is expanding.
Speaker 2: All of space in the universe is expanding. And that flow,
Speaker 2: that flow of new space flowing everywhere, being created everywhere,
Speaker 2: stretching things out they're in space. We call that the
Speaker 2: hubble flow. Now is that the same as the dark flow?
Speaker 1: So that's the Hubble flow. The dark flow is a
Speaker 1: question about this peculiar motion. Are there any patterns in
Speaker 1: the peculiar motion? Like we expect that the universe is
Speaker 1: the same everywhere and it's all just random, there's nothing special,
Speaker 1: and so that if you look at the directions and
Speaker 1: the magnitudes of these peculiar motions, like where's every galaxy going,
Speaker 1: and you add them all up, they should basically add
Speaker 1: up to zero, right, there should be no preferred direction
Speaker 1: in the universe.
Speaker 2: Meaning like like let's say we ignore the expansion of
Speaker 2: the universe, or like take that into account, or you
Speaker 2: substract the expansion of the universe from the motion of everything.
Speaker 2: How is everything moving? How are all those superclusters of
Speaker 2: galaxies on those galaxies and stars? How's it all moving?
Speaker 2: If the universe wasn't expanding, that's what you mean?
Speaker 1: Yeah, essentially relative to the expansion. How is everything moving?
Speaker 1: Does it point in some particular direction? Does it average
Speaker 1: out to zero? We expect, based on like very simple,
Speaker 1: kind of naive but powerful arguments, that it should average
Speaker 1: out to zero. We think the universe is the same everywhere,
Speaker 1: and the Big Bang happened at every point in the
Speaker 1: universe the same time, and there was no global over
Speaker 1: density anywhere, and so we expect it to basically average
Speaker 1: out to zero.
Speaker 2: Now, what do you mean by average out to zero?
Speaker 2: Like why should that be the case? Like the motion
Speaker 2: of our solar system does it add up to zero?
Speaker 1: So our solar system reflects the spin of the initial
Speaker 1: blob of gas that formed it. Right, that blob of
Speaker 1: gas had a bunch of particles heading and render directions,
Speaker 1: and it was a regularly shape. Then it was spinning,
Speaker 1: and so the spinning, the motion of our solar system,
Speaker 1: including the motion of the Solar system around the center
Speaker 1: of the galaxy, does reflect the motion of that initial blob.
Speaker 1: And we expect all those blobs were basically created equal,
Speaker 1: and so you'll get some spinning one way and some
Speaker 1: spinning another way, and some moving this way and some
Speaker 1: moving that way, and you do see a big variation
Speaker 1: of like directions of stars, et cetera.
Speaker 2: I see, it's sort of like maybe if you had
Speaker 2: like a gas canister full of gas molecules, you sort
Speaker 2: of expect all the gas molecules, all the motions of
Speaker 2: all those particles and molecules to average out to zero, right,
Speaker 2: because it's all sort of random. The canister's not going anywhere,
Speaker 2: and it's had some time to like diffuse and even.
Speaker 1: Out exactly unless there was something outside that cannister pulling
Speaker 1: on them, or something pushing on the whole canister, or
Speaker 1: something else acting that you weren't aware of, the motion
Speaker 1: of all the molecules should add up to zero. So
Speaker 1: that's the question.
Speaker 2: And so that's kind of what you were saying we
Speaker 2: should expect from the universe. Like if you subtract the
Speaker 2: expansion of the unverse from the motion of everything, then
Speaker 2: basically all the stars and galaxies out there shots sort
Speaker 2: of look like a canister of gas where everything's just
Speaker 2: moving in random directions, but it should all add up to.
Speaker 1: Zero, exactly. And there's one more little bit of trickiness
Speaker 1: that we need to think about before we're ready to
Speaker 1: actually look at those galaxies and answer the question. And
Speaker 1: that's the frame of reference when we talked about subtracting
Speaker 1: out the expansion of the universe, and that's helpful for
Speaker 1: like removing something we already understand that's causing everything to
Speaker 1: get away from each other. But we still need to
Speaker 1: pick a frame of reference in order to calculate the
Speaker 1: velocity of a galaxy, because galaxies don't have velocity relative
Speaker 1: to space, they only have velocity in some reference frame.
Speaker 1: Like put a reference frame on one galaxy, measure the
Speaker 1: velocity of another one relative to that or your spaceship
Speaker 1: or something. Right, velocities are not a property of objects.
Speaker 1: There are properties of pairs of objects. So basically have
Speaker 1: to pick a reference frame in which you're going to
Speaker 1: measure the velocity of all these objects.
Speaker 2: And is it going to change if you changed the
Speaker 2: point of reference, like it shouldn't it add up to
Speaker 2: zero no matter what?
Speaker 1: Yeah, this is a really interesting and subtle point, like
Speaker 1: space itself has no frame of reference, doesn't prefer any
Speaker 1: frame of reference, and so it's often said like the
Speaker 1: universe has no preferred frame, and that's mostly true, But
Speaker 1: the stuff in the universe definitely does have a frame
Speaker 1: of reference. Like if you add up the velocities of
Speaker 1: all the stuff in the universe, that has a frame
Speaker 1: of reference, right, that has a location that has an
Speaker 1: average velocity, and so the stuff in the universe has
Speaker 1: a frame and you can sort of pick that frame
Speaker 1: of reference just by looking at the cosmic microwave background radiation.
Speaker 2: But wait, I thought every other motion of the other
Speaker 2: stuff out there should add up to zero. Are you
Speaker 2: saying it doesn't or it may not. Like if you
Speaker 2: calculate the average velocity of all the galaxies in the
Speaker 2: universe observable universe, shouldn't that be zero?
Speaker 1: It should be if you pick the right reference frame, right,
Speaker 1: and you understand everything that's happening. So that's why the
Speaker 1: answer depends a little bit on the reference frame. So
Speaker 1: we go back to the very early universe and say, well,
Speaker 1: what was the reference frame of all the stuff in
Speaker 1: the early universe, like when things were still hot and
Speaker 1: dense and a big plasma. We can actually measure that
Speaker 1: because we can look at the cosmic microwave background radiation
Speaker 1: the life from that plasma, and we can look at
Speaker 1: like whether it's bluer or redder in one direction. So
Speaker 1: we can measure our motion through this cosmic microwave background
Speaker 1: radiation and that tells us what the frame of the
Speaker 1: universe was like fourteen billion years ago, and so it
Speaker 1: gives us a frame. Then we can ask, like, are
Speaker 1: the galaxies now moving relative to the cosmic microwave background radiation?
Speaker 1: We expect that to be no, because we think the
Speaker 1: galaxies came from the same stuff that formed the CMB,
Speaker 1: so it'd be weird if that stuff was moving relative
Speaker 1: to the CMB.
Speaker 2: Wait wait, wait, wait, are you saying the cosmic microwave
Speaker 2: background radiation is not the same in all directions? You're saying, like,
Speaker 2: it's redder to the right than to our left.
Speaker 1: Kind of yeah, because we're moving through the CMB, we
Speaker 1: have a non zero velocity relative to the CMB. If
Speaker 1: you just measure the CMB overall is very obviously redder
Speaker 1: in one direction and bluer in the other. And then
Speaker 1: typically you see these maps of the CNB, but they've
Speaker 1: already subtracted that out. They've already measured our overall motion
Speaker 1: relative to the CMB and subtracted that out, and then
Speaker 1: they're looking for like tiny little wiggles in the CMB
Speaker 1: on top of that.
Speaker 2: Oh, I see what it's saying. So like, if you
Speaker 2: look at the cosmic microwrate background, it has a certain
Speaker 2: velocity or motion to it, and so you have to
Speaker 2: sort of assume that's like the motion of the universe
Speaker 2: kind of, that's like the home base of the universe.
Speaker 1: Yes, the zero of the universe. That's really the only
Speaker 1: thing we can compare two. Right, You can't just measure
Speaker 1: velocities relative to space. That doesn't have any meaning. You
Speaker 1: have to measure relative to something. So you need like
Speaker 1: a baseline. So we go back to the early universe
Speaker 1: and take the frame of the CMB and say, our
Speaker 1: galaxies overall moving in some direction relative to the motion
Speaker 1: of the CNB. That would be weird. It's not weird
Speaker 1: for one galaxy to be moving relative to the CNB
Speaker 1: or like stuff happens, it gets pulled in some direction.
Speaker 1: The Earth is moving relative to the CNB. No big deal.
Speaker 1: But if you add up everything relative to the CMB
Speaker 1: after subtract the expansion also, then the question is where
Speaker 1: is everything going?
Speaker 2: Okay, I see, so we've measured the velocities of all
Speaker 2: the galaxies and superclusters and stars out there in space,
Speaker 2: and we've measured how they move relative to the cosmic
Speaker 2: microrerate background radiation. And you're saying it should be zero
Speaker 2: if you think about it, because it should average out,
Speaker 2: but is it.
Speaker 1: It does not average out to zero. And that's the
Speaker 1: dark flow. The dark flow is this extra unexplained velocity
Speaker 1: of all of these galaxies relative to the CMB. After
Speaker 1: subtracting out the expansion, turns out the galaxies are pointing
Speaker 1: in a certain direction.
Speaker 2: Wait, what where are they going?
Speaker 1: So they did this really cool study where they measured
Speaker 1: the velocities of a bunch of different galaxies.
Speaker 2: And this is you've measured like everything like the whole
Speaker 2: observable universe that we can see.
Speaker 1: No, definitely not. That would take way too long. What
Speaker 1: they did is they found like seven hundred clusters out
Speaker 1: there in the universe and they measure the velocities of those.
Speaker 1: So again, not even individual galaxies like galaxy cluster. They
Speaker 1: wanted to like scan as far as they could across
Speaker 1: the observable universe. But you know there's zillions and zillions
Speaker 1: of galaxies. If you did the whole project, it would
Speaker 1: take forever.
Speaker 2: So then isn't that a really tiny sample of the
Speaker 2: whole universe.
Speaker 1: Absolutely, it's a tiny sample, but they measured it across
Speaker 1: the universe, so they hope there's no bias. And they
Speaker 1: know how much data they have, so they can measure
Speaker 1: the statistical uncertainty there. So if they measure a velocity
Speaker 1: that's smaller than their uncertainty, then they say, okay, it's
Speaker 1: consistent with zero. If they measure something much bigger than
Speaker 1: their uncertainty, they can still tell that it's happening. Like
Speaker 1: if you only measure the speed of two cars zooming
Speaker 1: biber they're going one hundred miles an hour, then that's
Speaker 1: very likely the average speed of your road is not zero,
Speaker 1: it's closer to one hundred.
Speaker 2: Okay, so that's a big mystery. Like you've measured the
Speaker 2: velocity of while the superclusters out there, or at least
Speaker 2: a sample of them, and they don't seem to be
Speaker 2: standing still relative to the background of the universe.
Speaker 1: And they use this very cool technique to measure the
Speaker 1: velocities of these galaxies because these clusters can be hard
Speaker 1: to see. And so what they did is they looked
Speaker 1: at CMB photons passing through these clouds of hot gas.
Speaker 1: As the CMB photons passed through them, they get a
Speaker 1: little bit of boost of energy from interacting with these clouds.
Speaker 1: You can use this to see distant galaxies just by
Speaker 1: like their effect on the CMB, Like if those photons
Speaker 1: have passed through the blob of gas, then you can
Speaker 1: tell and you can tell the velocity of that blob
Speaker 1: of gas. It's like an extra little Doppler shift there.
Speaker 2: M Okay, So everything seems to be moving relative to
Speaker 2: the background of the universe by a lot or is
Speaker 2: it like a little tiny drift.
Speaker 1: Well, it depends on your scale of reference. But like
Speaker 1: it's moving an eight hundred to one thousand kilometers per second.
Speaker 1: You add it all up, it's really a non zero number.
Speaker 1: It's pointing sort of in the direction of the Centaurus
Speaker 1: and Hydra constellations, which listeners might remember is sort of
Speaker 1: in the same direction as what we call the great attractor,
Speaker 1: which is behind the zone of avoidance in our galaxy.
Speaker 1: Like our galaxy is a big disc, right, and so
Speaker 1: you can look up above the disc, or down below
Speaker 1: the disc, or sort of out away from the center
Speaker 1: of the galaxy. Those directions are pretty easy to see
Speaker 1: because you're not looking through a lot of dense galaxy.
Speaker 1: But if you try to look through the center of
Speaker 1: the disc itself, or even through the center of the galaxy,
Speaker 1: there's a lot of stuff there, a lot of gas
Speaker 1: and dust and other stars and black holes that block
Speaker 1: our view. And in that direction there also tends to
Speaker 1: be like a lot of gravitational motion within the observable universe.
Speaker 1: So it's called the Great Attractor. We don't know what's
Speaker 1: there exactly, because we can't see in that direction very well,
Speaker 1: but they're already We thought there was a lot of
Speaker 1: local gravitational motion in that direction towards sort of the
Speaker 1: center of the Linikia supercluster, and now it looks like
Speaker 1: the whole universe is also moving in that direction. Like
Speaker 1: you add up all the galaxies in the universe, their
Speaker 1: velocity on average points in the same direction as our
Speaker 1: motion towards the Great Attractor.
Speaker 2: And by all the galaxies in the universe, you mean
Speaker 2: all the galaxies we can see.
Speaker 1: I mean only these seven hundred clusters that were measured
Speaker 1: by this one study.
Speaker 2: Right right, Well, a zero sample of all of them.
Speaker 1: Yeah, exactly, a tiny, tiny sample, but yes, statistically significant
Speaker 1: sample of all of them, and they measured a big velocity.
Speaker 1: This is a really surprising result.
Speaker 2: So they've measured how things are moving, and you're saying
Speaker 2: they're moving in a particular direction, which seems to be
Speaker 2: in the direction of something really really extra big compared
Speaker 2: to the size of the observable universe.
Speaker 1: Yeah, And I don't want to confuse people, because there's
Speaker 1: two different motions we're talking about here. Like one is
Speaker 1: our particular peculiar motion, the motion of Andromeda and the
Speaker 1: milky way in our little cluster is being pulled towards
Speaker 1: the center of the Lina Kia Clue supercluster by some
Speaker 1: big mass that's within our universe. Right, that's our sort
Speaker 1: of local peculiar motion. But then you add up all
Speaker 1: the peculiar motion of the whole universe, and that also
Speaker 1: seems to be headed coincidentally, maybe I don't know, in
Speaker 1: the same direction. So like, the whole universe is also
Speaker 1: moving and its arrow is in the same direction as
Speaker 1: our local motion towards the great attractor. So sort of
Speaker 1: like back to the analogy of boats in the river,
Speaker 1: we discover that, oh, our boat is moving in some
Speaker 1: direction relative to the river, and then we add of
Speaker 1: all the boats and like, oh my gosh, everybody's moving
Speaker 1: in that direction. What's going on? It's this as something
Speaker 1: much bigger than just the Great Attractor.
Speaker 2: The Great Attractor is within our supercluster of galaxies, or
Speaker 2: it's outside of it.
Speaker 1: We don't know what the Great Attractor is. We think
Speaker 1: it might be something super massive at the core of
Speaker 1: our supercluster. So yeah, probably it's within our supercluster. But
Speaker 1: the dark flow is the motion of everything in the universe,
Speaker 1: coincidentally or not, I don't know, in the same direction
Speaker 1: as our motion towards our supercluster.
Speaker 2: Well, like, we're next to this great attractor and we're
Speaker 2: moving towards it. Are you saying then that when you
Speaker 2: measure out the whole universe, even the stuff behind the
Speaker 2: Great Attractor is moving towards the Great Attractor, meaning towards us.
Speaker 1: No, the stuff behind the Great Attractor is moving in
Speaker 1: the same direction as us, meaning away from the Great Attractor.
Speaker 1: So the overall motion is in the same direction, not
Speaker 1: towards the Great Attractor, but in the same direct is
Speaker 1: our motion towards as a great attractor.
Speaker 2: Oh so the Great Attractor is just maybe a red herring,
Speaker 2: it's not really doing anything.
Speaker 1: Maybe, I don't know. It's sort of weird that it
Speaker 1: would be a coincidence, but it might just be Yes.
Speaker 2: All right, well, let's dig into what that could mean,
Speaker 2: what could be causing the dark flow of the universe,
Speaker 2: and whether or not we do have to call the
Speaker 2: plumbers to fix it or not. So let's dig into that.
Speaker 2: But first let's take another quick break. All right, we're
Speaker 2: talking about the dark flow of the universe, which is
Speaker 2: this idea that it seems like everything in the universe
Speaker 2: is moving in a particular direction and we don't know why.
Speaker 1: We definitely don't know why, which is why we slapped
Speaker 1: our familiar dark label on this thing.
Speaker 2: Or I guess it's moving relative to the cosmic microwave background.
Speaker 2: Could it be a the microwave background is the one
Speaker 2: that's moving or has a bias.
Speaker 1: Well, the motion is relative, so I'm not sure what
Speaker 1: it means to say the CNB is the one that's
Speaker 1: moving like relative to what relative to all the galaxies? Yes,
Speaker 1: but the motion is relative, so you can't just like
Speaker 1: ascribe it to one and not the other. But it's
Speaker 1: the relative motion that's curious, right, because we think that
Speaker 1: the stuff that made the galaxies, it's the same stuff
Speaker 1: that made that CMB light. So that plasma that made
Speaker 1: the CMB light, its overall motion should be the same
Speaker 1: as the galaxies, because the plasma is what turned into
Speaker 1: the galaxies, unless there was something else involved, unless there's
Speaker 1: something else pulling on these galaxies that didn't pull on
Speaker 1: the CMB.
Speaker 2: I mean, like, if you zoom out of where we
Speaker 2: are our supercluster, our galaxies are superclusters, maybe we're orbiting
Speaker 2: around something bigger then, maybe even the bigger than the
Speaker 2: observable universe exactly.
Speaker 1: And we often talk about the observable universe as our
Speaker 1: limit of the things that we can see, But there's
Speaker 1: an important subtlety there. The observable universe is what we
Speaker 1: can see now, what we can interact with now. The
Speaker 1: universe is expanding, right, and things used to be closer
Speaker 1: to us. So there are things that used to be
Speaker 1: in our observable universe that we used to be able
Speaker 1: to interact with us, that used to be able to
Speaker 1: affect us, which no longer can. So it might be
Speaker 1: that by looking deep into our past and understanding how
Speaker 1: the universe has been affected, we can see hints of
Speaker 1: things that affected us which have now left our observable universe.
Speaker 1: It's sort of like a way to sneak into seeing
Speaker 1: things that are now outside the observable universe.
Speaker 2: Wait, what because of this motion, like, maybe it tells
Speaker 2: us where it's been.
Speaker 1: Yeah, exactly. What if there was something really massive, something
Speaker 1: crazy massive, in our universe early on, and it created
Speaker 1: this gravitational tug towards it, and now it's left our
Speaker 1: observable universe, so it's not in our universe anymore. If
Speaker 1: you then looked at the motion of just the stuff
Speaker 1: in our observable universe, you would see it all moving
Speaker 1: towards this mysterious source of gravity and attraction, even though
Speaker 1: you wouldn't see that thing itself because it's now left
Speaker 1: our observable universe. So something that's now outside of our
Speaker 1: observable universe was once inside of it, right then it
Speaker 1: might have still left an imprint on the motion of galaxies.
Speaker 2: Meaning like maybe we could deduce where the Sun is
Speaker 2: and what the sun is just by looking at the
Speaker 2: orbit of the Earth without having to actually look at
Speaker 2: the Sun.
Speaker 1: Yeah, exactly. You don't need to see the Sun in
Speaker 1: order to know that the Sun is there, and so
Speaker 1: even though this thing is outside our observable universe, it's
Speaker 1: within our past light cone. Our past light cone is
Speaker 1: all the stuff that we have interacted with in the
Speaker 1: past that could have affected us in the past. And
Speaker 1: the expansion of the universe is pulling things outside of
Speaker 1: our light cone, making it impossible to interact with that
Speaker 1: stuff because space is expanding faster than the speed of light.
Speaker 1: But it is possible that there was once something very dense,
Speaker 1: some intense mass, some source of incredible gravity that affected
Speaker 1: the formation of the whole universe that we now can't see.
Speaker 2: I see it like you know, how we found out
Speaker 2: that the Earth is just orbiting around the Sun, and
Speaker 2: then eventually we just find out that the Solar system,
Speaker 2: the Sun is just kind of in a corner of
Speaker 2: the Milky Way galaxy, orbiting around the center of the
Speaker 2: Milky Way galaxy, not even close to the center of it.
Speaker 2: And now maybe we're finding out that our whole observable universe,
Speaker 2: everything that we can see, all those bazillions of stars,
Speaker 2: maybe we're just like at the corner or at the
Speaker 2: edge of some sort of a bigger massive of stuff
Speaker 2: in the universe.
Speaker 1: Exactly. And we like to imagine that the whole universe,
Speaker 1: if it's infinite, is filled with the same kind of stuff,
Speaker 1: and that our chunk and the observable universe we happen
Speaker 1: to live in is probably an average chunk, and any
Speaker 1: chunk you would all average out to zero. But it
Speaker 1: might not be the case, right. It might be that
Speaker 1: there is some larger structure. There are like things that
Speaker 1: are denser and heavier and chance so that if you
Speaker 1: take a random slice of the universe, you don't on
Speaker 1: average get zero. It gives you a picture as to like,
Speaker 1: what is that nearby larger structure. I think it's super
Speaker 1: cool that we could like make measturements in our observable universe.
Speaker 1: It get glimpses for what's beyond, right, which sometimes felt
Speaker 1: like an impenetrable wall, like a dark wall beyond which
Speaker 1: we couldn't see. But we can dig out clues from
Speaker 1: the history of the universe to figure out what has
Speaker 1: happened that we can no longer see.
Speaker 2: Well, it's a little bit sort of like the picture
Speaker 2: we have of the Milky Way galaxy where we live,
Speaker 2: like we really can't see it. We're in the middle
Speaker 2: of it. But we can sort of reconstruct what it
Speaker 2: might be or what it is, just by looking at
Speaker 2: the immediate things around us.
Speaker 1: Yeah, that's true. The things that are obscuring our vision
Speaker 1: are different. In the case of the Milky Way galaxy,
Speaker 1: it's like the gas and the dust and other sort
Speaker 1: of practical stuff, and in the case of the observable universe,
Speaker 1: it's the speed of light. But yeah, it's a good analogy. However,
Speaker 1: of course there's controversy about this.
Speaker 2: Is it about the name beyond it? A dark flow
Speaker 2: was maybe a terrible idea because it's not really dark.
Speaker 1: It does kind of flow, though, the controversy is about
Speaker 1: whether it exists. So this measurement was made originally with
Speaker 1: like CMB data from the w MAP satellite, sort of
Speaker 1: an intermediate satellite, not the most precise data we have
Speaker 1: about the cosmic microwave background radiation.
Speaker 2: Wait, wait, what is the WMAP.
Speaker 1: I think it's named after Wilkinson. It's a satellite out
Speaker 1: in space that picks up these CMB photons. They're very
Speaker 1: very low energy photons, super long wavelengths. You need a
Speaker 1: very sort of specialized equipment to pick them up. The
Speaker 1: w MAP satellite is part of a long history of
Speaker 1: these satellites. It was Kobe was the first one, then
Speaker 1: w MAP and then PLANK these more and more precise
Speaker 1: based telescope. Yeah, it's a space telescope. It's an instrument
Speaker 1: out in space that picks up these photons.
Speaker 2: And this one who specialize on the CMB and then
Speaker 2: what we also used to measure the expansion and the
Speaker 2: motion of these galaxies.
Speaker 1: Yeah, you can learn so much from the CMB. Absolutely,
Speaker 1: very very general, very powerful. That's why WMAP is such
Speaker 1: an important thing, and why Kobe won, why the Kobe
Speaker 1: satellite folks won a Nobel prize, and why PLANK was
Speaker 1: such an important thing. Plank is the follow up to WMAP. Anyway,
Speaker 1: these results came from an analysis of seven hundred clusters
Speaker 1: with the w MAP data, and then they reanalyze this
Speaker 1: using the PLANK data, so more precise, more recent data,
Speaker 1: larger data set. And there's a disagreement about the results there.
Speaker 1: Like one group says, yes, we see the dark flow
Speaker 1: in the PLANK data. Another group says, no, we analyze
Speaker 1: that same data, we don't see anything.
Speaker 2: What cann't They just crosscheck and figure out why they're different.
Speaker 1: They're working on that but it's complicated because when you
Speaker 1: do these analysis, there's so many assumptions, and two different
Speaker 1: groups are going to make different assumptions, and those assumptions
Speaker 1: are sometimes hard.
Speaker 2: To spot, like what kind of assumptions.
Speaker 1: Well, there's all sorts of details you need to understand
Speaker 1: about how the CMB photons are boosted as they pass
Speaker 1: through this hot gas, so people have a model of that.
Speaker 1: Nobody's modeling all of the details of every individual photon
Speaker 1: down to all the microphysics. It's always a simplification, and
Speaker 1: how those simplifications are made, and whether they're valid, and
Speaker 1: whether the simplifications introduce errors, and whether those errors are important.
Speaker 1: There's a long series of decisions people make when they
Speaker 1: analyze these data. That's why it's important to have cross
Speaker 1: checks because helps you reveal where those decisions could be
Speaker 1: biasing your results. So what that tells us is like,
Speaker 1: there's something funky in one of these analyzes, and you're right,
Speaker 1: they need to cross check and drill down. But it's
Speaker 1: not trivial. It's not like they're doing the same calculation
Speaker 1: and expect to get exactly the same number. They're probing
Speaker 1: the same physical thing, but they're doing the calculation in
Speaker 1: very different ways.
Speaker 2: M I mean, couldn't they just like go to a
Speaker 2: meeting together and figure it out, like I'm doing this,
Speaker 2: what are you doing? Or are you doing that?
Speaker 1: Oh?
Speaker 2: Wait, that doesn't make any sense. What if you tried that?
Speaker 1: Yeah? I think they are working on that. But I
Speaker 1: think there's also a little bit of acrimony between these
Speaker 1: two groups. I'm not sure it's always a friendly disagreement. Yes,
Speaker 1: exactly is it dark drama? I mean, nobody's like murdering
Speaker 1: other people's parents in the alleyway and leading them to
Speaker 1: become the dark Knight of Justice or anything. It's not that.
Speaker 2: Dark, but not yet apparently, stay tuned. Maybe there's a
Speaker 2: Nobel prize at stake. Things will get ugly.
Speaker 1: But what lies in the future is a deeper analysis
Speaker 1: of this same data, and we hope maybe even more
Speaker 1: refined data. Future measurements of the CNB can give us
Speaker 1: an even clearer picture of what's out there, the motion
Speaker 1: of the CMB and the motion of all these blobs
Speaker 1: of gas and galaxies relative to the CMB.
Speaker 2: So these folks are talking, and what are they saying
Speaker 2: is maybe the probable cause if if there is a
Speaker 2: relative motion to all these stars and galaxies around us.
Speaker 2: What could be the cause of it?
Speaker 1: So the only idea that's out there, if this thing
Speaker 1: is real, is what we talked about earlier, some big
Speaker 1: blob of stuff that's out there beyond our observable universe,
Speaker 1: past our horizon, something very dense that once pulled on
Speaker 1: all of us. So we all have this overall bias
Speaker 1: in our peculiar motion. What that could be, nobody knows.
Speaker 2: Wait, are you saying that maybe there was something bigger
Speaker 2: than the observable universe that was pulling on all of
Speaker 2: our galaxies that we can see, but now that it's
Speaker 2: so far away from us, it doesn't affect us anymore,
Speaker 2: or can't affect us.
Speaker 1: Yeah, that's exactly right. Because the expansion of the universe
Speaker 1: is faster than light. This expansion increases as distance grows,
Speaker 1: and so things fall off the edge of our horizon.
Speaker 1: There are some things which could interact with us, could
Speaker 1: send us photons or pull on us, which now can
Speaker 1: no longer. Like things near the edge of the observable universe.
Speaker 1: We see photons from them, but some of them are
Speaker 1: now past the edge of the observable universe, and if
Speaker 1: they send us photons, those photons will never arrive because
Speaker 1: space is expanding between us and those galaxies faster than
Speaker 1: the speed of light. Sort of mind boggling to think about, right.
Speaker 2: This thin gravity can only go as fast as the
Speaker 2: speed of light. Right, so if something's moving or being
Speaker 2: expanded away from most faster than the speed of light,
Speaker 2: then even where we'll never feel it's gravity.
Speaker 1: Exactly. Like if they shine a flashlight at you, you'll
Speaker 1: never see it no matter how long you wait, which
Speaker 1: also means you'll never feel its gravity. You might have
Speaker 1: seen that flashlight earlier on, you might have felt their
Speaker 1: gravity early on, but no longer. So again, this is
Speaker 1: like a way to probe things that we might have
Speaker 1: interacted with earlier in the history the universe that we
Speaker 1: can no longer see.
Speaker 2: You're saying, we'll see it like the spin it gave
Speaker 2: us or the orbit it gave us.
Speaker 1: Yeah, exactly. It's sort of like what happens if you
Speaker 1: walk into a party five seconds after some celebrity does right,
Speaker 1: and everybody's looking in the same direction, and you're like,
Speaker 1: what's going on? Who was just here?
Speaker 2: Who was that? Was that badman?
Speaker 1: Or was that Kim Kardashian or is Kim Kardashian actually batman?
Speaker 1: And you see the effect on the conversations. In everybody's
Speaker 1: head is pointing in some direction. You don't see the person,
Speaker 1: but you see the effect they left on.
Speaker 2: The room, or so we think, right, we still have
Speaker 2: to confirm these measurements.
Speaker 1: Yeah, exactly. These are very difficult measurements to make. And
Speaker 1: as you hear, there's not a consensus about whether the
Speaker 1: dark flow is even a thing.
Speaker 2: Well, boy, maybe should give it a different name. Would
Speaker 2: you call it?
Speaker 1: What would you call it? The dark Kardashian?
Speaker 2: Well, I feel like the word dark didn't that originate
Speaker 2: from the being invisible? Like dark matter. You can't see
Speaker 2: it because it doesn't interact with the electromagnetic light. Because
Speaker 2: if you're going to call anything that's mysterious dark, then
Speaker 2: that's everything, isn't it.
Speaker 1: The whole universe is kind of mysterious. I think that
Speaker 1: dark means mysterious. It doesn't mean invisible because dark it
Speaker 1: doesn't actually mean invisible, right, You can have invisible things
Speaker 1: in bright light. Also, dark corners mean dark corners are
Speaker 1: things that are like obscured, right, right.
Speaker 2: But I guess what I mean is if you start
Speaker 2: calling everything that's mysterious dark, then that's just going to
Speaker 2: confuse everybody.
Speaker 1: Yeah, that's probably true. Dark.
Speaker 2: There's dark particles, there's you know, dark things in my fridge.
Speaker 2: They're going to think people are thinking they're related to
Speaker 2: dark matter and dark energy in the dark night.
Speaker 1: Well, they're all mysterious, so they are all related in
Speaker 1: that sense, the dark in the dark minds of physicists.
Speaker 2: Maybe not in the physical in the physics sense, right.
Speaker 1: Yeah, no, absolutely, they could have completely different physical explanations.
Speaker 1: They're just currently not understood.
Speaker 2: All right, Well, maybe you should just call it the
Speaker 2: hubble flow and then just follow it up with we
Speaker 2: don't know what the hubble flow is or what's causing.
Speaker 1: It, but this is an addition to the hubble flow, Like,
Speaker 1: what on top of the hubble flow is happening in
Speaker 1: the universe? Is really the question?
Speaker 2: All right? Well, it sounds like the answer once again
Speaker 2: is stay tuned. There are mysterious workings in the universe,
Speaker 2: mysterious flows plumbing that we can't yet see out there
Speaker 2: in the universe that is hopefully leaking a little bit
Speaker 2: so that we know and we can to study.
Speaker 1: It, and we'd love to understand the universe, and so
Speaker 1: we're looking out there into the nice sky and trying
Speaker 1: to squeeze every tiny little drop of information out of
Speaker 1: the photons that do arrive to us. And it's incredible
Speaker 1: that we can even figure out that the universe is
Speaker 1: out there and what it's doing, and maybe even get
Speaker 1: a glimpse of what's past the edge of the observable universe.
Speaker 2: I wonder if physicists are like Batman. You know, you're
Speaker 2: looking up at the sky, you see a bad signal
Speaker 2: in the motion of the stars, and you're like duty calls.
Speaker 1: We're all totally ripped, just like Batman. Ripped. That wasn't
Speaker 1: a joke, man, Why are you laughing?
Speaker 2: Ripped in your minds and the abs of your minds
Speaker 2: and in the biceps of your typing fingers. Well, in
Speaker 2: any case, we hope you enjoyed that. Thanks for joining
Speaker 2: us us see you next time.
Speaker 1: For more science and curiosity, come find us on social
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Speaker 1: and Jorge Explain the Universe is a production of iHeartRadio.
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