Speaker 1: Hey, Daniel, remember a few weeks ago when you insulted
Speaker 1: America's favorite chocolate bar hershees.
Speaker 2: How could I forget? I got some strong emails about
Speaker 2: that comment.
Speaker 1: Oh yeah, did you get us barred from the airwaves?
Speaker 2: No? But I think I might have to raise the
Speaker 2: bar in terms of my comments.
Speaker 1: You mean we're not already at the bottom.
Speaker 2: We're scraping the bottom of the bar roll.
Speaker 1: Yeah, it sounds like you need to spend less time
Speaker 1: in bars drinking away your chocolate sorrows.
Speaker 2: I'll have a chocolate TEENI please.
Speaker 1: Sure give it a shot. Hi am Hori, my cartoonist
Speaker 1: and the author of Oliver's Great Big Universe. Hi.
Speaker 2: I'm Daniel. I'm a particle physicist and a professor at
Speaker 2: U SEE Irvine, and I do have strong opinions about chocolate.
Speaker 1: Do you have opinions about strong chocolate? Or do you
Speaker 1: have strong opinions about chocolate?
Speaker 2: Yes? And yes, it's.
Speaker 1: Because you're really passionate about chocolate. It gives you a
Speaker 1: lot of comfort.
Speaker 2: I feel like, if you're gonna eat chocolate, then you
Speaker 2: should eat chocolate that you like. But you know, people
Speaker 2: prefer lots of different kinds of chocolate. I don't mean
Speaker 2: to judge people who like Hershey's. You know, all the
Speaker 2: power to them if that's what they want. It's everywhere.
Speaker 1: I feel like you're maybe being a bit of a
Speaker 1: chocolate snob.
Speaker 2: I mean, are you being a snob to know what
Speaker 2: you like and know what you don't like. I don't
Speaker 2: want to insult anybody else's preferences to anybody who is
Speaker 2: offended by me calling Hershey's chocolate garbage, I do apologize.
Speaker 1: That sounds like a non apology, Daniel.
Speaker 2: It did sound like an apology. I agree, But it
Speaker 2: sounded like I was trying, didn't it.
Speaker 1: I don't think that counts these days. I think you
Speaker 1: could raise your bar in terms of apologies to the masses.
Speaker 2: Well, I'll say this, I enjoy chocolate. I hope everybody
Speaker 2: else out there enjoys chocolate, whatever flavor or variety they prefer. Yeah,
Speaker 2: there you go.
Speaker 1: But anyways, welcome to our podcast Daniel and Jorge Explain
Speaker 1: the Universe, a production of iHeartRadio.
Speaker 2: In which we whip up a delicious concoction of everything
Speaker 2: that's out there in the universe, all the dark energy,
Speaker 2: all the dark matter, all the dark ideas that describe
Speaker 2: how the universe actually works, what it's made out of,
Speaker 2: and how it came to be. We melt it all down,
Speaker 2: form it into squares, and ship it to you over
Speaker 2: the airwaves.
Speaker 1: That's right. We try to lower the bar of science
Speaker 1: and make it accessible to everyone out there. We try
Speaker 1: to make it silky and smooth and delicious and easy
Speaker 1: to go down.
Speaker 2: Because wondering about how the universe works is something everybody does,
Speaker 2: and everybody should have access to what we know and
Speaker 2: what we don't know about the universe. Thinking like a
Speaker 2: physicist is not something only professional physicists can do. It's
Speaker 2: something everybody can do and should do about how the
Speaker 2: world works. And we're here to walk you through that.
Speaker 1: It's right because we all live in the universe. We're
Speaker 1: all members of the universe, and we can all wonder
Speaker 1: about how it all works, how it's all put together,
Speaker 1: and why things are the way they are.
Speaker 2: Do you have like a membership card for the universe?
Speaker 2: I don't have one.
Speaker 1: Oh oh uh, awkward?
Speaker 2: Awkward.
Speaker 1: Maybe you just got lost in the mail, Daniel. I'm
Speaker 1: sure that's what happened. Sure you are invited.
Speaker 2: Maybe it's part of the Hershey's Cabal. That's secretly organizing
Speaker 2: the whole universe.
Speaker 1: Oh you've been you've been blacklisted or dark chocolate listed.
Speaker 2: Yeah, milk chocolated.
Speaker 1: That's what you get for insulting hours that be accessible
Speaker 1: chocolate to the masses. Yeah, but you can still come
Speaker 1: to the universe.
Speaker 2: I guess, all right, thank you, you give.
Speaker 1: Be somebody's plus one. Just don't invite physicist, maybe because
Speaker 1: there they'll prefer to be on the outside of the
Speaker 1: universe looking in.
Speaker 2: No. I love being the part of the universe that's
Speaker 2: looking at itself, right. That's what makes the questions so important.
Speaker 2: That we're not outside the universe studying it like some
Speaker 2: weird objects. It's part of our context, our lives. It's
Speaker 2: our existence that we're trying to understand. Physics is sometimes
Speaker 2: sold is like bigger than human But to me, it's
Speaker 2: the fundamental question of humanity, answering the deepest questions about
Speaker 2: our own existence.
Speaker 1: Is it sold as bigger as humanities? Where is that listed?
Speaker 2: That's how it was sold to me. I guess, so
Speaker 2: maybe I shouldn't generalize from any equals one. But one
Speaker 2: reason I got into physics originally was that it seemed
Speaker 2: like the questions were universal, They weren't limited to things
Speaker 2: happening on Earth. You know, you study biology, and you
Speaker 2: know it's fascinating and important, but it might be very
Speaker 2: different on other planets.
Speaker 1: Maybe that's why you're not invited, Daniel. You think you're
Speaker 1: bigger than the rest of us.
Speaker 2: I think some of the pelophysics is that it does
Speaker 2: seem to be universal, though of course we don't know,
Speaker 2: and that's a question we can only answer in the
Speaker 2: future when we meet aliens.
Speaker 1: I guess it does sort of tap into that sense
Speaker 1: of the cosmos, the grand pictures that we all kind
Speaker 1: of have inside of us. And it is pretty amazing
Speaker 1: what we've been able to understand and study and come
Speaker 1: to terms with in terms of where we are in
Speaker 1: the universe, within our Solar System, within our galaxy, within
Speaker 1: our supercluster of galaxies, within the observable universe. It's pretty
Speaker 1: amazing that we can see so much structure out there
Speaker 1: just by looking out to the night sky.
Speaker 2: Yeah, it's amazing what we know about the universe, never
Speaker 2: having left our tiny little neighborhood. Everything we know about
Speaker 2: the structure of galaxies and superclusters and all that stuff
Speaker 2: is based just on the few photons that happen to
Speaker 2: hit Earth from those distant locations. It would be incredible
Speaker 2: to get in a warp ship and like actually visit
Speaker 2: some of these places and learn more about what's actually
Speaker 2: going on there right now. But we can do a
Speaker 2: pretty impressive job even from here. But of course, there
Speaker 2: are still lots of unanswered questions about how solar systems form,
Speaker 2: what planets there are in our Solar system, what planets
Speaker 2: there have been in our Solar system, and larger questions
Speaker 2: about galaxies, why they exist, how they form, and why
Speaker 2: they take their weird shapes.
Speaker 1: That's right, because we're orbiting around our Sun, and the
Speaker 1: Sun is orbiting in a galaxy, which is a huge
Speaker 1: cluster of stars. There are one hundred billion stars in
Speaker 1: the Milky Way galaxy. But how these galaxies get form
Speaker 1: is kind of a still up in the air, right.
Speaker 2: It is, And it's an important question because galaxies are
Speaker 2: kind of the basic building block of the universe. They're
Speaker 2: like the atoms of the universe from which you can
Speaker 2: put together structures and superstructures and all sorts of other stuff.
Speaker 2: It's incredible that like galaxies actually exist and they tend
Speaker 2: to be a certain size so it's an important thing
Speaker 2: to understand, like why are universe features galaxies? How they
Speaker 2: come to be, how long they will continue to be
Speaker 2: the basic building block of the universe?
Speaker 1: Yeah, and why do they have some strange features? And
Speaker 1: so today on the podcast we'll be asking the question
Speaker 1: why do some galaxies have bars? Are these like drinking bars?
Speaker 1: Like our galaxy has a bar, can go and get
Speaker 1: a drink.
Speaker 2: There's definitely bars in our galaxy, right, I've been to
Speaker 2: some of them. So that's a question we can definitively
Speaker 2: answer today on the podcast.
Speaker 1: They seem to be closer around Earth at least all
Speaker 1: the bars we know are on Earth. Is that kind
Speaker 1: of a physics question or is a biology question?
Speaker 2: Well? I think there's some history of drinking alcohol in space, right.
Speaker 2: I think Zach and Kelly's new book has some stories
Speaker 2: about astronauts who have smuggled things up to the ISS,
Speaker 2: So perhaps there is an unofficial bar off Earth as well.
Speaker 1: I wonder if anyone has tried a like brew beer
Speaker 1: on the space station.
Speaker 2: Space moonshine, either purpose or by accident. The Moon does
Speaker 2: shine more brightly in space, so maybe it's easier to
Speaker 2: make moonshine m there you go. Yeah, And so of
Speaker 2: course we don't know if aliens enjoy relaxing in bars
Speaker 2: or whether they like Hershey's chocolate in their chocolate tenies.
Speaker 2: But today we're not talking about places to drink. We're
Speaker 2: talking about the structures at the centers of galaxies.
Speaker 1: I see. So when you say a galaxy bar, it's
Speaker 1: not like a like a buffet bar or a beer bar.
Speaker 1: It's more like a destructor. Is a structure in a
Speaker 1: galaxy that looks like a bar or a structures multiple ones.
Speaker 2: Yeah, there's a kind of galaxy out there called a
Speaker 2: barred galaxy, but not every galaxy has this kind of
Speaker 2: bar at the center, and so it's an open question
Speaker 2: about why some galaxies have bars.
Speaker 1: Well, as usually, we were wondering how many people out
Speaker 1: there had thought about galaxy bars and why some galaxies
Speaker 1: have them, why some don't? Are they over twenty one?
Speaker 1: Can you get your favorite drinks in them? And so
Speaker 1: Daniel went out there into the internet to find out
Speaker 1: what people think about this question.
Speaker 2: And whether you like salad bars or chocolate bars or
Speaker 2: any other kind of bar, You are welcome to participate
Speaker 2: in this segment of the podcast. Just write to meet
Speaker 2: you questions at Danielandjorge dot com and I'll set you up.
Speaker 1: So think about it for a second. Why do you
Speaker 1: think some galaxies have bars? Here's what peop glad to say.
Speaker 3: So I didn't know galaxies had bars in the center.
Speaker 3: So I'm going to say maybe it has it because
Speaker 3: gravity is aliding them in a special way, but can't
Speaker 3: be made into sphere because of the black holes gravity.
Speaker 4: I think planets don't share orbits because the center of
Speaker 4: gravity would have to be absolutely stationary or it would
Speaker 4: oscillate and shake itself out of sync. I think that
Speaker 4: the bars in the center of galaxies are there to
Speaker 4: give people somewhere to go. But like if women's clothes
Speaker 4: shops have bars, they have somewhere for men to go
Speaker 4: instead of trying to look interested.
Speaker 3: I expect that galaxies have bars at their centers because
Speaker 3: you're going to need a place to kill some time
Speaker 3: and have have a beer if your interstellar flight is delayed.
Speaker 3: I have no idea what does a bar in the
Speaker 3: center of abilitum?
Speaker 1: All right, some fun ideas here, some of them a
Speaker 1: little bit inappropriate.
Speaker 2: Perhaps I think this might be the highest fraction of
Speaker 2: joke answers we've ever gotten, which means people really just
Speaker 2: don't know anything about bars and galaxies.
Speaker 1: Maybe they're just a barring and not knowing anything about them.
Speaker 2: Or maybe they'd already spent too much time at a
Speaker 2: bar when they got these questions and they were in
Speaker 2: silly mood.
Speaker 1: Oh there you go. Yeah, maybe you shouldn't send these
Speaker 1: questions up during happy.
Speaker 2: Hours Monday morning nine am. Is that what happy hour is?
Speaker 1: Yeah? Depends on how much you led your job, I guess.
Speaker 2: And where you are in the world. Right, we have
Speaker 2: listeners all over the world.
Speaker 1: Yeah, we are a global podcast. All right, Well, Daniel,
Speaker 1: maybe step us through this. What exactly do you mean
Speaker 1: by a galaxy bar?
Speaker 2: So, galaxy bars are features of one particular kind of galaxy,
Speaker 2: and galaxies, it turns out, can have all sorts of
Speaker 2: different shapes in them. Galaxies, of course, are clusters of
Speaker 2: stars and they can be very very small, from dwarf
Speaker 2: galaxies that have thousands of stars to super mega galaxies
Speaker 2: that have billions and billions of stars. But we tend
Speaker 2: to notice some features in them. And you know, the
Speaker 2: beginning of any science is basically just bought it, just
Speaker 2: like look at it. Categorize it, sort it, see what
Speaker 2: patterns emerge. And very early on when people were studying galaxies,
Speaker 2: they noticed that they could have different structures, even things
Speaker 2: we could see here from Earth with limited telescopes.
Speaker 1: Right, And we've talked a little bit about before the
Speaker 1: different shapes that galaxies can have, right, And they can
Speaker 1: be sort of like a blob like a football, or
Speaker 1: maybe like a spiral, or maybe just like a pancake.
Speaker 2: Right, yeah, exactly. And there are regular galaxies that are
Speaker 2: probably in the process of merging between two galaxies. There
Speaker 2: is a galaxy out there that looks like a question mark,
Speaker 2: for example. But the most common galaxy out there is
Speaker 2: a spiral galaxy.
Speaker 1: How common is it and is it like fifty?
Speaker 2: It's somewhere around two thirds of all the galaxies that
Speaker 2: we have studied, our spiral galaxies. And there's a reason
Speaker 2: for that. And we went into all of that in
Speaker 2: our recent podcast about the various shapes that galaxies can have.
Speaker 2: But basically, galaxies form from huge blobs of gas which
Speaker 2: spin and collapse and then form a disc. And that
Speaker 2: dense disc gas tends to form those stars, and the
Speaker 2: spinning of the galaxy turns that disc into a spiral.
Speaker 1: That's kind of like the default shape of a galaxy, right,
Speaker 1: But then you can get other shapes, as we talked about,
Speaker 1: when they merge with each other, other galaxies or they
Speaker 1: crash into other galaxies.
Speaker 2: Right exactly. So a spiral galaxy is like the basic
Speaker 2: building block of the universe in that sense, it's the
Speaker 2: most important one to understand because the other ones, ellipticals
Speaker 2: and irregulars are all made out of spiral galaxies. And
Speaker 2: also our galaxy is a spiral galaxy. So if we're
Speaker 2: going to understand like the context of our lives, let's
Speaker 2: begin at.
Speaker 1: Home, right, And I guess maybe just to paint the
Speaker 1: picture for folks, by a spiral, it sort of looks
Speaker 1: like a like a toilet flushing, right, Like, It's not
Speaker 1: like one like they're tracing out a spiral with a
Speaker 1: pen and a paper. It's more like a swirl that's
Speaker 1: kind of converging in the middle.
Speaker 2: Yeah, And the spiral galaxy usually has several arms. They
Speaker 2: can have two, sometimes they have three. There are galaxies
Speaker 2: out there with four arms. Each one is like a
Speaker 2: spiral that usually goes like one time around. The galaxy
Speaker 2: starts from the center and then comes out, So it's
Speaker 2: sort of more like a pinwheel.
Speaker 1: Right, So that gives you the image of the spiral galaxy.
Speaker 1: And so that's the basic shape of most galaxies out there.
Speaker 1: But you say, you're saying some of them have a
Speaker 1: bar through them or on them.
Speaker 2: At the center. Some galaxies don't have bars. The spiral
Speaker 2: arms just start at the very center. But a good
Speaker 2: fraction galaxy is the center of them. Isn't part of
Speaker 2: the spiral arm. It's a separate thing. It's like a bar.
Speaker 2: Like the Milky Way galaxy has this big blob of
Speaker 2: stars and gas and dust at the center of it,
Speaker 2: which forms this big bar and the spiral arm that
Speaker 2: comes out of each end of the bar. So the
Speaker 2: spirals don't go to the very center of the galaxy.
Speaker 2: They start at the edges of this bar.
Speaker 1: WHOA, wait, wait, wait too, you're saying our Milky Way
Speaker 1: galaxy is a bar.
Speaker 2: Yes, we live in a bar galaxy.
Speaker 1: Whoa is it a milk chocolate bar? Since we're the
Speaker 1: Milky Way.
Speaker 2: Now I'm afraid to answer because I don't want to
Speaker 2: anger the Hershey universe cabal that's in charge of everything.
Speaker 1: Yeah, what if the Milky Way is a hercheese product.
Speaker 2: This is why it's so important for science to be
Speaker 2: free of this kind of oppression. People should feel free
Speaker 2: to speak their minds on chocolate, right.
Speaker 1: Well, people shouldn't disparage other people if theyread chocolate. I
Speaker 1: think that is maybe the most important part of your yeah,
Speaker 1: se point take. But our Milky Way is a bar?
Speaker 1: Are you saying so? Like our spirals, the arms of
Speaker 1: our galaxy don't spiral into a single blob in the middle.
Speaker 1: It's they spiral into a bar.
Speaker 2: That's right. Our galaxy is two major arms, and then
Speaker 2: there's some spurs that come off of them, and each
Speaker 2: arm comes from a different side of the bar. So
Speaker 2: the two major spirals that make the Milky Way galaxy,
Speaker 2: they don't actually meet at the center. They come to
Speaker 2: different sides of this bar that's at the core of
Speaker 2: the galaxy.
Speaker 1: Interesting, the picture I'm getting is sort of like I
Speaker 1: don't know if you've seen those like Hawaiian fire dancers
Speaker 1: where they have a stick and at each end of
Speaker 1: the stick it's on fire and so then they spin it.
Speaker 1: That's kind of sort of what's the picture that we
Speaker 1: get of the Milky Way galaxy? Isn't it like there's
Speaker 1: a bar and from the each end of the bar
Speaker 1: you get these swirls that spiral out.
Speaker 2: Well, I am not fancy enough to take vacations to Hawaii.
Speaker 2: Tend to spend my money on chocolate instead. But it
Speaker 2: sounds accurate.
Speaker 1: You know that you can get chocolate in Hawaii. In fact,
Speaker 1: it's kind of known for they're kind of known for
Speaker 1: having good chocolate.
Speaker 2: Macademian that chocolate here, I come, there you go. But yeah,
Speaker 2: that sounds accurate. Wow. Or if you imagine like a
Speaker 2: baton twirler at a football game and ribbons at the
Speaker 2: end of the baton. If he or she spins the baton,
Speaker 2: then the ribbons spiral around it. But the ribbons, of course,
Speaker 2: don't meet at the very center. They come from the
Speaker 2: edges of the bed.
Speaker 1: Now, this is an interesting idea just because to me
Speaker 1: and I wonder if to a lot of our listeners
Speaker 1: this is kind of news. Right, Like most of the
Speaker 1: time that you see a picture or a drawing of
Speaker 1: the milk Away Galaxy, they don't it doesn't have the
Speaker 1: bar in the middle, right like the famous isn't the
Speaker 1: famous image of like you are here? Or it points
Speaker 1: to a point in the Milk Away. Of a drawing
Speaker 1: of the milk Away that one doesn't have a bar,
Speaker 1: I don't think.
Speaker 2: Yeah, that's a good point. I'm not sure if that
Speaker 2: image is scientifically vetted. It also really doesn't have big
Speaker 2: distinct arms the way you might expect, But a more
Speaker 2: scientifically accurate picture of the Milky Way shows the specific
Speaker 2: arms and the bar. The bar is not like a
Speaker 2: very rigid rod. It's a bit of a blob also to.
Speaker 1: Me now of stars, like a just an elongated cluster
Speaker 1: of stars.
Speaker 2: Yeah, it's stars, and it's gas and it's dust like
Speaker 2: the rest of the galaxy. But you know, this is
Speaker 2: us imposing order on a huge swirling mass of stuff.
Speaker 2: So part of this, of course, is just the impression
Speaker 2: we make, how our brains filter the full details of
Speaker 2: the buzzing chaos of reality.
Speaker 1: All right, Well, so then the Milkway has a bar.
Speaker 1: Lots of galaxies maybe have bars in the middle of them,
Speaker 1: and so the question is why do they have bars,
Speaker 1: What's going on there in the middle of the galaxy?
Speaker 1: And can you play galactic limbo with that bar? And
Speaker 1: so let's get to these deep and profound questions. But
Speaker 1: first let's take a quick all right, we're talking about
Speaker 1: the Milky Way galaxy and the fact that it has
Speaker 1: a bar of stars in the middle. The swirls of
Speaker 1: the galaxy don't converge to a round blob in the middle.
Speaker 1: They converse to kind of like a rod of stars. Right.
Speaker 2: Yeah, it might be more accurate to call it like
Speaker 2: a football because it's a little wider in the center
Speaker 2: than it is at the edges. But yeah, there's sort
Speaker 2: of a long.
Speaker 1: But it's not a bar. Why don't we just call
Speaker 1: them football galaxy?
Speaker 2: Football galaxy? That sounds great. I am done defending the
Speaker 2: naming schemes of astronomy.
Speaker 1: Because it's indefensible, because you just can't do it.
Speaker 2: Sorry, astronomers, you have no champion in me anymore.
Speaker 1: Oh boy, you're throwing the astronomers under the bus.
Speaker 2: No, I'm just throwing.
Speaker 1: Naming peculiarities.
Speaker 2: Oh, it certainly does. And nobody's ever asked me my
Speaker 2: opinion when they make up these names.
Speaker 1: Okay, So the Milky Way has a bar or football
Speaker 1: kind of in the middle, which is weird, right, because
Speaker 1: you would expect a galaxy to be sort of, you know,
Speaker 1: round and symmetric, right.
Speaker 2: You would kind of expect it. And that's exactly why
Speaker 2: we are really interested in these questions like what structures
Speaker 2: form in the universe. The answer to that question reveals
Speaker 2: the forces at play. Like wire galaxies created in the
Speaker 2: first place, Well, there's gravity, why don't they just collapse
Speaker 2: into a central ball, while there's angular momentum that turns
Speaker 2: them into disks? Why do the arms form at all?
Speaker 1: Well, maybe a question we can ask here is like
Speaker 1: what percentage of galaxies are barred galaxies? Like, is the
Speaker 1: Milky Way super weird that it has a bar? Or
Speaker 1: is it like most spiral galaxies have bars or some
Speaker 1: of them or fifty to fifty.
Speaker 2: That's a really interesting question, and the answer depends on
Speaker 2: the mass of the galaxy in a super weird way
Speaker 2: for galaxies about the mass of the Milky Way, like
Speaker 2: billions and billions of solar masses. It's around two thirds
Speaker 2: of spiral galaxies that have bars. The more massive the galaxy,
Speaker 2: the smaller the fraction of the spirals have bars. And
Speaker 2: also the less massive the galaxy, the smaller fraction have bars.
Speaker 2: So there is this like sweet spot in the middle
Speaker 2: where galaxies are more likely to have bars. And if
Speaker 2: you move away from that lower mass or higher mass,
Speaker 2: then the fraction that have bars decreases.
Speaker 1: M interesting, So it depends on the size and mass
Speaker 1: of the galaxy. But maybe let's take a back to
Speaker 1: the basics, because it sounds like there's a lot of
Speaker 1: physics lying on here, as you might expect from our podcasts,
Speaker 1: But so what's going on at the center? Like how
Speaker 1: can this bar still be there after all these billions
Speaker 1: of years?
Speaker 2: It all comes from asymmetries like little over densities. If
Speaker 2: you start from a galaxy that was perfectly smooth, like
Speaker 2: all the matter was evenly distributed, or like you had
Speaker 2: a cluster of stars that were perfectly distributed in circles
Speaker 2: on perfectly circular orbits, then it wouldn't form a bar.
Speaker 2: You tweak it a little bit, like one little spot
Speaker 2: has more density than another spot, then that spot's going
Speaker 2: to have more gravity and it's going to attract more
Speaker 2: stars to it, and so you get these density waves
Speaker 2: forming inside the galaxy. So the same process that forms
Speaker 2: the arms, theives density waves of the arms that we
Speaker 2: describe as like traffic passing through a freeway, can also
Speaker 2: form density structures in the center of the galaxy and
Speaker 2: in this case, they form bars.
Speaker 1: Well, yeah, I remember we spent a whole episode talking
Speaker 1: about this idea of density waves in a galaxy. It's
Speaker 1: not like the arm of a galaxy what you see
Speaker 1: as a soirl like, it doesn't rotate around the galaxy.
Speaker 1: It's more like a ripple through a whole bunch of stars.
Speaker 2: Exactly. It's more like a ripple. It's like people doing
Speaker 2: the wave in a football stadium. They're not moving sidewaves,
Speaker 2: but the wave itself is moving, right, And so the
Speaker 2: galaxy arms are waves in the density of stars, and
Speaker 2: the bar is the same way. The bar is like
Speaker 2: a density wave. So it's a structure that forms, but
Speaker 2: it moves through the stars. It doesn't move at the
Speaker 2: same speed the stars are moving around the galaxy. It
Speaker 2: can move faster, it can move slower.
Speaker 1: Wait, what do you mean the bar? Does a bar
Speaker 1: rotate and bar devastates moving like, which direction is the
Speaker 1: bar moving?
Speaker 2: The bar rotates the same direction as the galaxy and
Speaker 2: it definitely rotates.
Speaker 1: Oh, kind of like the arms and a clock.
Speaker 2: Kind of like the arms and a clock, and also
Speaker 2: like the arms of the galaxy. Right, because the arms
Speaker 2: are density waves, they move relative to the stars, So
Speaker 2: one star might be in the density wave and then
Speaker 2: they'll get passed by the density wave which moves on.
Speaker 1: WHOA are there like videos online about this? That would
Speaker 1: be cool to see.
Speaker 2: You mean, like actual footage of bars rotating, not.
Speaker 1: Like a simulation or something.
Speaker 2: Yeah, I would love to see that, but the time
Speaker 2: scales are ridiculous, Like the Milky Way takes two hundred
Speaker 2: and fifty million years to rotate, So there's no chance
Speaker 2: for like, watch a galaxy rotate unless you're gonna set
Speaker 2: it your camera for tens of millions of years.
Speaker 1: So wait, how do we know it's rotating.
Speaker 2: Well, we build up a model of how galaxy works
Speaker 2: that includes angular momentum, and you need those components that
Speaker 2: rotation to explain what we see. If they weren't rotating,
Speaker 2: there's no anglar momentum, and these things would collapse a
Speaker 2: lot faster. A lot of galaxies and a lot of these
Speaker 2: structures are supported by angular momentum. Like, where else would
Speaker 2: the spiral arms come from if it wasn't spinning.
Speaker 1: I don't know.
Speaker 2: Yeah, So we do a lot of simulations to try
Speaker 2: to explain the galaxies we see. We said, well, we
Speaker 2: think we understand all the forces at play. We put
Speaker 2: it in the computer, we press the button, it goes,
Speaker 2: and we see what comes out of the simulation. Then
Speaker 2: we compare that to what we see in the sky. Interesting,
Speaker 2: that's what we do to try to explain what's happening
Speaker 2: or try to gain some understanding of the forces at work.
Speaker 1: Wow, so nobody's ever actually seen a galaxy swirling. We
Speaker 1: just think it does. I mean, it's the most likely
Speaker 1: or obvious explanation, but still it's a fact that we
Speaker 1: haven't seen a galaxy swirl. Like, we've only seen a
Speaker 1: photograph of it. It's like you've only seen a photograph
Speaker 1: where Merry go round. Ever, like you've never seen a
Speaker 1: mirror go around move.
Speaker 2: Well, that's true when we've only even known about other
Speaker 2: galaxies for like one hundred years. It was Hubble who
Speaker 2: saw these little smudges in the sky and deduced their
Speaker 2: distance from analyzing special stars in them, the sephids, whose brightness,
Speaker 2: of course is famously connected to their pulsing frequency. And
Speaker 2: he's the one that understood that these things were outside
Speaker 2: our galaxy. So we only known that other galaxies exist
Speaker 2: for like one hundred years and had good enough telescopes
Speaker 2: to like resolve their structure for a few decades, which
Speaker 2: is basically no time at all relative to their actual emotion.
Speaker 2: So yeah, we're studying galaxies basically frozen in time.
Speaker 1: Well that's interesting. Now you're saying that about two thirds
Speaker 1: of galaxies have a bar in the middle of them,
Speaker 1: and this bar is rotating and rippling through the center
Speaker 1: of the galaxy. Like what's the mechanism for making these bars?
Speaker 1: Like why not? Why a bar shape? Why not a
Speaker 1: star shape or a banana shape.
Speaker 2: I would love to see a banana shape at the
Speaker 2: center of galaxy and probably some galaxy out there that
Speaker 2: has that, because there's so many galaxies out there and
Speaker 2: so many of them are doing weird things.
Speaker 1: I guess if you think about a galaxy, is banana shape,
Speaker 1: Like each arm is like a banana. So it's like
Speaker 1: if you take if you take several bananas and arrange
Speaker 1: them in a star shape, you would get a spiral galaxy.
Speaker 2: Right, Yeah, that's true. That's a message from the universe
Speaker 2: that bananas are important, and you.
Speaker 1: Start with them in chocolate this they're galactic.
Speaker 2: And that's why we sell frozen chocolate covered bananas here
Speaker 2: in Orange County.
Speaker 1: That is the purpose of the universe, isn't it? Is
Speaker 1: it deep fried too? Why not fried chocolate covered bananas?
Speaker 1: I think I've had that at a restaurant, actually.
Speaker 2: But you asked a great question that wasn't about dessert,
Speaker 2: which is why do some galaxies have bars? Why do
Speaker 2: they form bar shapes and not other things? And the
Speaker 2: answer is that we're not sure, Like we don't understand
Speaker 2: why some galaxies form bars and some don't. We don't
Speaker 2: totally understand the mechanism for the formation itself. Like we've
Speaker 2: seen this happen in simulation sometimes and it comes out
Speaker 2: of instabilities. You put a bunch of stars a little
Speaker 2: bit closer to each other, they tend to form these
Speaker 2: density waves. And what's happening here at the center is
Speaker 2: that the stars are no longer totally in a circular orbit.
Speaker 2: The bar like twists the orbits so that some stars are
Speaker 2: moving more on radial orbits passing close through the center
Speaker 2: of the galaxy.
Speaker 1: What do you mean, radar like more oval shape orbits?
Speaker 2: Yeah, more oval shape, So they're not equally distributed around
Speaker 2: the center of the galaxy. They're like passing back and forth.
Speaker 1: Oh interesting, I guess maybe it's sort of rare or
Speaker 1: sort of a coincid is that you would have circular
Speaker 1: orbit orbits, right Like, if I throw a rock at
Speaker 1: the Sun right now, it would not necessarily go in
Speaker 1: a round orbit. It might maybe more likely go in
Speaker 1: an oval shape orbit.
Speaker 2: Yeah, there's lots of possible solutions, and a circle is
Speaker 2: like a special case of it. And also to maintain
Speaker 2: a circular orbit requires resisting the tugs and the pulls
Speaker 2: of everything else around you. Remember that you're not orbiting
Speaker 2: by yourself. The Earth is being on by Jupiter, and
Speaker 2: Jupiter is being pulled on by everything else. That's one
Speaker 2: of the reasons why our orbits are eccentric. So the
Speaker 2: more complicated dynamics you have, the more lack of symmetry,
Speaker 2: the more these things are getting pulled by other stuff,
Speaker 2: the more they're going to fall out of perfectly circular
Speaker 2: orbits and clump together in other kinds of orbits.
Speaker 1: So that you're saying, the stuff at the center of
Speaker 1: our milk away galaxy is not necessarily going around in
Speaker 1: a circle, it's maybe going in an oval shape around
Speaker 1: the center. But then how does that explain the ripple
Speaker 1: that is the bar or all of these things going
Speaker 1: in an oval in sync.
Speaker 2: Yes, the bar itself is like a denser region which
Speaker 2: passes through the center of the galaxy, and it influences
Speaker 2: other stuff. So gas near the center of the galaxy,
Speaker 2: for example, moves at faster speeds than gas further out,
Speaker 2: So the gas can catch up to the bar and
Speaker 2: pass through it. But then the bar is stronger gravity
Speaker 2: slows it down, so the material tends to lose some energy.
Speaker 2: This actually ends up funneling a bunch more gas towards
Speaker 2: the center of the galaxy and can create new stars
Speaker 2: right there in the bar.
Speaker 1: But then, I guess, like making the bar is so common,
Speaker 1: you're seeing about two thirds of galaxies, spiral galaxies have
Speaker 1: a bar in them. Why don't we see that effect
Speaker 1: like in other places, like when I flushed the toilet,
Speaker 1: or you know, in our Solar system. Even we don't
Speaker 1: have a bar in our solar system. I guess we
Speaker 1: don't have spiral arms in our Solar system either, Like
Speaker 1: you know, things coalesced into the planets, although we do
Speaker 1: have an asteroid built.
Speaker 2: I think it comes down to the balance of all
Speaker 2: the things at play. You know, we have gravity in
Speaker 2: the Solar System, and of course in the galaxy. But
Speaker 2: the galaxy is a lot more friction. You know, there's
Speaker 2: gas and there's dust. This stuff is interacting much more
Speaker 2: than things in the Solar system. So there's a lot
Speaker 2: more transfer of energy between the stuff in the galaxy
Speaker 2: than there is in the Solar system. Solar systems like
Speaker 2: a cleaned up little galaxy. There's not nearly as much
Speaker 2: stuff between the planets as there is between the stars.
Speaker 2: In the galaxy. There's a lot more of this exchange
Speaker 2: of energy and friction and stuff bumping into other stuff
Speaker 2: in the galaxy, which can give you more interesting complex
Speaker 2: basically turbulent structure. And we don't know how long these
Speaker 2: bars last. There's some theories that say these bars will
Speaker 2: last forever, and we see some galaxies that have had
Speaker 2: their bars for like ten billion years. There are other
Speaker 2: theories that the bars could be oscillatory, like maybe the
Speaker 2: bar forms, but then the formation of the bar tends
Speaker 2: to destroy the bar, and then another bar forms a
Speaker 2: billion years later. So it could be that every galaxy
Speaker 2: goes through phases where it has a bar and doesn't
Speaker 2: have a bar. These are really open questions in the
Speaker 2: study of galaxy formation.
Speaker 1: Mmmm, it's doing like pull ups or something on a bar,
Speaker 1: going back and forth. But this is an interesting idea
Speaker 1: that it maybe depends on time. Like you're saying, they
Speaker 1: can come in and out. What do you see when
Speaker 1: you look at the galaxies out there in the universe, Like,
Speaker 1: don't can't we see them across different ages and times? Yeah?
Speaker 1: Do we see that the older galaxies have more bars
Speaker 1: or younger galaxies have more bars? What do we see?
Speaker 2: Yeah, so we're not sure. We have a lot of
Speaker 2: different clues that sort of point us in different directions. Like,
Speaker 2: on one hand, we see some galaxies with really really
Speaker 2: old bars. I was reading a paper about a galaxy
Speaker 2: we're looking at ten billion years in the past, and
Speaker 2: we can see the formation of a bar back then.
Speaker 2: So that's just like some bars formed really early in
Speaker 2: the universe and may have lasted a really long time.
Speaker 2: Of course, you can't see the same galaxy over time.
Speaker 1: Yeah, how do we know?
Speaker 2: We don't know necessarily how long a bar lasts when
Speaker 2: it's formed. But in this one particular galaxy, they could
Speaker 2: see evidence of the bar over about ten billion years
Speaker 2: because they could see how the bar had disrupted the
Speaker 2: structure of the galaxy. So it's a little speculative, but
Speaker 2: they think that that bar lasted for about ten billion
Speaker 2: years in that galaxy because of how it like shepherded
Speaker 2: the stars and influence the gas to create new stars.
Speaker 2: So it's not everybody that believes that paper, but that
Speaker 2: one paper argues that there are galaxies with really long
Speaker 2: lived bars.
Speaker 1: And then I guess, how do you know that they
Speaker 1: can go away?
Speaker 2: Yeah, another great question. That's again from simulation. We see
Speaker 2: in some simulations that a bar can like drive gas
Speaker 2: inwards towards the center of the galaxy. It's an over
Speaker 2: density and so it pulls stuff in and that can
Speaker 2: disrupt the motion of stars through the galactic core. So,
Speaker 2: like the formation of the bar could create gravity which
Speaker 2: pulls in more gas, which ends up disrupting the bar itself.
Speaker 2: So we don't know. And this is the kind of
Speaker 2: thing we study in simulations, and these simulations are never perfect, right,
Speaker 2: You can't include everything in the galaxy in your simulation,
Speaker 2: every tiny particle. It would take forever. So whenever they
Speaker 2: do these simulations, they're always making approximations. Well, let's leave
Speaker 2: out the dark matter, or let's ignore this effect. Or
Speaker 2: something to make it tractable. And so you've got lots
Speaker 2: of different simulations with sort of different answers, and people
Speaker 2: are still exploring this, like right now, nobody really knows
Speaker 2: the answers to these questions.
Speaker 1: So when you simulate it, you add a little asymmetry
Speaker 1: or like an offset to the distribution of the stars,
Speaker 1: and then you get a bar. And then what you
Speaker 1: see in the simulation is that sometimes the bar disappears
Speaker 1: after a while exactly, and then maybe it comes back
Speaker 1: later mm hmm.
Speaker 2: And there's some other hints from the simulations. Some suggest
Speaker 2: that it depends on the thickness of the disk. Like
Speaker 2: our galaxies mostly flat, but it's not a totally thin disk.
Speaker 2: It's not like it's a paper thin, right, There's a
Speaker 2: width to the galaxy, and some galaxies have thinner disks
Speaker 2: and some galaxies have thicker discs, And some of these
Speaker 2: stimulations suggest that galaxies with thinner discs are more likely
Speaker 2: to have bars than galaxies with thicker discs in them.
Speaker 1: WHOA why would they be?
Speaker 2: I think it has to do with basically the gravitational
Speaker 2: environment in which a disruption forms, Like if there's a
Speaker 2: lot of other gravity happening anyway, then the small over
Speaker 2: density is less likely to cause a big pile up
Speaker 2: in the galactic spin. Like, for example, you had a
Speaker 2: huge amount of dark matter in your galaxy and then
Speaker 2: the gravity of the galaxy is mostly dominated by that
Speaker 2: dark matter. In the same way, if you have lots
Speaker 2: of other stars like above and below where the over
Speaker 2: density forms, then those stars are going to tend to
Speaker 2: smooth stuff out because of their gravity. A thicker disc
Speaker 2: might prevent a little over density from turning into a
Speaker 2: big bar.
Speaker 1: Interesting, it's like a two D effect, like it only
Speaker 1: happens in thin sheets.
Speaker 2: Yeah, it could be. Another study suggested that mergers can
Speaker 2: destroy bars. Remember, spiral galaxies merge and form sometimes new
Speaker 2: spiral galaxies, sometimes elliptical galaxies, And it could be that
Speaker 2: bars are destroyed in these mergers, and then the new
Speaker 2: galaxy doesn't necessarily have a bar. It depends on the
Speaker 2: instability and the formation of the stars in that new galaxy. WHOA,
Speaker 2: but there's another really interesting clue. And to answer a
Speaker 2: question you asked a few minutes ago about whether bars
Speaker 2: form more recently or not, we find that it also
Speaker 2: depends on the mass of the galaxy. So a lot
Speaker 2: of the bars tend to have formed more recently in
Speaker 2: lower mass galaxies. So it might be that lower mass
Speaker 2: galaxies take longer to form stars and then take longer
Speaker 2: to form bars. So most of the bars that are
Speaker 2: forming in the universe today we think are in the
Speaker 2: lower mass galaxies. They might not just have had time
Speaker 2: to make a bar until now.
Speaker 1: So you need like time and mass and flatness to
Speaker 1: make a bar.
Speaker 2: And some sort of seed, right, some gravitational over density.
Speaker 2: I think it's really fascinating that basically all the structure
Speaker 2: in the universe comes from one place being heavier than another.
Speaker 2: When the universe began, if it was totally smooth, nothing
Speaker 2: would have happened because all the gravity would have been balanced.
Speaker 2: Basically everything that happened in the universe is because of
Speaker 2: gravitational imbalances.
Speaker 1: M yeah, I definitely happen, and I definitely have a
Speaker 1: mass imbalance in certain parts of my body. Gravity. Yeah,
Speaker 1: it's all good. I'm blaeing the chocolate bars. So then,
Speaker 1: but then, how does the mass of the galaxy affect
Speaker 1: these bars? Like why would having more mass or less
Speaker 1: mass affect the bar formation?
Speaker 2: Because structure formation takes time, right, Gravity is very very weak,
Speaker 2: and it takes time for gravity to overwhelm these other things.
Speaker 2: It takes time for gravity to pull stars together, it
Speaker 2: takes time for those stars to then influence each other.
Speaker 2: So all these things take time, and lower mass galaxies
Speaker 2: have less gravity, so it just takes them longer to
Speaker 2: sort of come together and form these things. And structure
Speaker 2: is sort of hierarchical, right, it's bottom up. You build
Speaker 2: the stars, the stars build a galaxy, You build a
Speaker 2: structure on that, and then you build structures on those structures.
Speaker 2: The barred spiral galaxy is sort of like the pinnacle
Speaker 2: of universal structure. It's like the most complex organized thing
Speaker 2: the universe has made so far.
Speaker 1: Well, it definitely sounds like we've raised the bar here
Speaker 1: in terms of galactic mysteries. It seems like this is
Speaker 1: still a question for many scientists, and in fact, it
Speaker 1: might even have interesting consequences for our conception of dark
Speaker 1: matter and how they influence the formation of these galaxies.
Speaker 1: So let's dig into those dark matters. But first, let's
Speaker 1: take another quick break. Right, we're talking about galactic bars now, Daniels,
Speaker 1: this is where single stars go to meet other single stars.
Speaker 2: This is where molecular clouds hang out hoping to one
Speaker 2: day make stars.
Speaker 1: They're like celebrity bars. You see a lot of stars
Speaker 1: in them.
Speaker 2: This is where stars are born. Baby.
Speaker 1: All right, Well, we talked about how we're not quite
Speaker 1: one hundred percent sure why some galaxies are bars. It
Speaker 1: seems like it's just a matter of the physics, like
Speaker 1: the weirdness of how stars and the gravity interact and
Speaker 1: form ripples in a cluster of stars. But there is
Speaker 1: sort of another component to it, which is dark matter. Right.
Speaker 1: Dark matter plays a big role in how galaxies form,
Speaker 1: how fast they form, what shapes they have, and so
Speaker 1: what do these bars tell us about dark matter?
Speaker 2: Yeah, you're exactly right. And remember that the stars themselves
Speaker 2: are just like tracers that tell you where most of
Speaker 2: the mass is in the universe. So most of the dynamics,
Speaker 2: the gravitational forces, are hidden from us. We live in
Speaker 2: a dark universe where we only see a tiny, little
Speaker 2: fraction of what's going on out there. So by studying
Speaker 2: the motion of the gas and the dust and the stars,
Speaker 2: we could try to get a glimpse for what's going
Speaker 2: on sort of behind the curtain. What the fuller picture
Speaker 2: of the universe is, and you're absolutely right that dark
Speaker 2: matter controls the formation of the galaxies. We wouldn't have
Speaker 2: galaxies this early in the universe if it wasn't for
Speaker 2: dark matter. Eighty percent of the matter in the universe
Speaker 2: is dark matter, but more like ninety percent of the
Speaker 2: matter in galaxies is dark matter. So galaxies are basically
Speaker 2: mostly dark matter. And because dark matter feels gravity, it
Speaker 2: has a huge influence on gravitational turbulence, like the formation
Speaker 2: of bars.
Speaker 1: Absolutely so, all this time that we've been talking about
Speaker 1: the mass of a galaxy, I thought we were talking
Speaker 1: about the stars, But are we also talking about the
Speaker 1: mass due to dark matter.
Speaker 2: Yeah, we're talking about the total mass, not just the
Speaker 2: visible mass of the galaxy.
Speaker 1: Or are they pretty correlated, like the more stars you
Speaker 1: see in a galaxy, the more dark matter it has.
Speaker 2: Yeah, great question. We did a podcast recently about the
Speaker 2: variation of dark man matter in galaxies. Most galaxies have
Speaker 2: around ninety percent of their mass is dark matter, but
Speaker 2: there is a variation. There's some that are like ninety
Speaker 2: nine percent dark matter and some that have almost none
Speaker 2: or maybe zero dark matter. It tells a really fun
Speaker 2: story about the history of galaxies and how they form
Speaker 2: and merge and might even strip each other of dark
Speaker 2: matter on the whole. There's a reasonable correlation, and like
Speaker 2: ninety percent of a galaxy's mass is invisible ninety percent
Speaker 2: ninety percent.
Speaker 1: Yeah, but isn't like the proportion of the universe something
Speaker 1: like five to one dark matter to normal matter.
Speaker 2: Yeah, so galaxies tend to have more dark matter than
Speaker 2: most of the universe.
Speaker 1: Why is that? Is it just because there's a lot
Speaker 1: of like stars and does kind of in between galaxies
Speaker 1: out there.
Speaker 2: It's because dark matter is really responsible for the formation
Speaker 2: of galaxies, and once a galaxy forms, it's hard for
Speaker 2: it to lose its dark matter, but it's possible for
Speaker 2: it to lose its gas and its dust and even
Speaker 2: its stars. There's all sorts of processes that tend to
Speaker 2: blow out gas and dust from a galaxy, you know,
Speaker 2: supernova or radiation from the central black hole. All these
Speaker 2: things can push normal matter out of a galaxy, but
Speaker 2: they don't affect its dark matter. So the dark matter
Speaker 2: is sort of there in the background, forming the gravitational
Speaker 2: structure on which the rest of the universe lives, and
Speaker 2: it's much less influenced by everything else that happens to
Speaker 2: the normal matter.
Speaker 1: Okay, so if our galaxy is ninety percent dark matter,
Speaker 1: I would think that was like the most powerful thing.
Speaker 1: That term is the shape with a galaxy. So does
Speaker 1: that mean that the dark matter in our galaxy is
Speaker 1: also in a spiral shape and maybe also has a
Speaker 1: bar of dark matter in the middle.
Speaker 2: Yeah, great question. We don't know the answer to that
Speaker 2: because we don't know the distribution of dark matter in
Speaker 2: the galaxy with a lot of precision. We know roughly
Speaker 2: that it's denser in the core and then it's spread
Speaker 2: out like a big halo. But we also know something
Speaker 2: about the physics of dark matter, like we know that
Speaker 2: dark matter doesn't feel friction with other dark matter the
Speaker 2: way normal matter does. Like two clouds of gas that
Speaker 2: pass through each other will exchange energy them will tend
Speaker 2: to fall towards the center for example. These kinds of
Speaker 2: energy exchanges just don't happen for dark matter because dark
Speaker 2: matter passes right through itself. It only feels gravity, and
Speaker 2: gravity is not strong enough for those kinds of exchanges.
Speaker 2: So the complex structures you see forming in normal matter
Speaker 2: don't always happen in dark matter. That's why we think
Speaker 2: dark matter is sort of a bigger, fluffier halo than
Speaker 2: the more compact visible matter in the galaxy.
Speaker 1: The shape of the regular stars is mostly due to gravity, right,
Speaker 1: and dark matter feels gravity with itself, doesn't it.
Speaker 2: It's mostly due to gravity, but also crucially relies on
Speaker 2: losing energy. Like if stuff is just swirling around gravitationally,
Speaker 2: it won't collapse nearly as easily if it's not sticky,
Speaker 2: if it just passes right through itself. When things are
Speaker 2: orbiting a black hole, for example, they could orbit there forever,
Speaker 2: but they're much more likely to fall in if they
Speaker 2: exchange some energy, so one of them gives up energy
Speaker 2: to another one rather than passing right through.
Speaker 1: You mean, like if the stuff swirling around crashes with
Speaker 1: itself or like bumps into itself and it's going to
Speaker 1: lose some energy.
Speaker 2: Yeah, So things are just much more stable if there's
Speaker 2: no interactions other than gravity. The other interactions, they tend
Speaker 2: to exchange energy and then stuff can fall into the
Speaker 2: center to make stars or to make galaxies, or to
Speaker 2: make arms, this kind of stuff. But the truth is
Speaker 2: we don't really know what structure there is to the
Speaker 2: dark matter. We don't have a lot of great probes
Speaker 2: for exactly where the dark matter is throughout the galaxy.
Speaker 1: It could be a dark chocolate bar in the middle.
Speaker 1: It could be just like a moose spread out throughout
Speaker 1: the whole galaxy.
Speaker 2: It could be We're getting better and better techniques to
Speaker 2: map where the dark matter is in the galaxy, but
Speaker 2: we don't have a very accurate picture. But you know,
Speaker 2: simple models of the physics of dark matter tell us
Speaker 2: how it should be distributed, though we don't know if
Speaker 2: those models are accurate, of course.
Speaker 1: But then in all these simulations you're talking about where
Speaker 1: we are finding out all these things about the bars,
Speaker 1: do they include dark matter in those simulations? And kauds
Speaker 1: of assumptions do you have to make there?
Speaker 2: They do include dark matter, and they treat dark matter
Speaker 2: as like a collisionless fluid, like they're in the backdrop
Speaker 2: of providing more gravity. And the dark matter does definitely
Speaker 2: influence the formation of bars in these simulations. The more
Speaker 2: dark matter you have, the less often you get a bar,
Speaker 2: even when you have some sort of like gravitational instability
Speaker 2: or over density in the visible matter.
Speaker 1: So does dark matter help the bars or prevent the bars?
Speaker 2: It prevents the bars from forming. So the more dark
Speaker 2: matter you have, the more it tends to smooth out
Speaker 2: the gravity of the disc, just like having a thicker
Speaker 2: disc right, as you were saying earlier, sort of like
Speaker 2: a two D effect. The more three d R galaxy becomes,
Speaker 2: the less susceptible it is to these effects. In the
Speaker 2: same way dark matter basically reduces the effect of gravity
Speaker 2: of the visible matter provides this like background gravity pulling
Speaker 2: on everything.
Speaker 1: I see, like it evens out the strength of gravity
Speaker 1: in the galaxy. Right, because you think dark matter is
Speaker 1: distributed spread all over the place, the more significant that is,
Speaker 1: the more generally speaking, gravity is in that galaxy exactly.
Speaker 1: And so if you have no dark matter, do you
Speaker 1: always get a bar?
Speaker 2: Nothing happens every time in simulation, but the proportion definitely
Speaker 2: goes up if you have no dark matter. But we
Speaker 2: don't expect that to happen in the universe very often,
Speaker 2: as we talked about recently on the podcast. We don't
Speaker 2: think galaxies can form without dark matter, though they might
Speaker 2: later be stripped of their dark matter in some collisions,
Speaker 2: But then collisions are also going to destroy a bar.
Speaker 1: So is the bar kind of a sign of how
Speaker 1: much dark matter there is in a galaxy, then.
Speaker 2: Yes, exactly. And people have used bars to try to
Speaker 2: measure the dark matter fraction in galaxies. Like if you
Speaker 2: look up at the night sky and you measure the
Speaker 2: fraction of galaxies that have bars in them, that tells
Speaker 2: you something about how much dark matter there can be
Speaker 2: in those galaxies, because if there's a lot of dark
Speaker 2: matter in all those galaxies, they wouldn't have bars, And
Speaker 2: if there was less dark matter in those galaxies, more
Speaker 2: of them would have bars. So we can actually use
Speaker 2: the bars as a way to like indirectly estimate the
Speaker 2: dark matter fraction of the universe in a way that's
Speaker 2: separate from the galactic rotation speed or any of the
Speaker 2: other probes.
Speaker 1: Like the bar sets the bar for dark matter.
Speaker 2: Yeah, it raises the bar on our understanding.
Speaker 1: Our precision to measure dark matter.
Speaker 2: Yeah, this is not a very precise measurement because we
Speaker 2: don't have that many galaxies where we can measure this
Speaker 2: kind of thing. But it's consistent with everything else we
Speaker 2: know about dark matter. So the fraction of galaxies with
Speaker 2: bars is consistent with our understanding that eighty percent of
Speaker 2: the matter in the universe is dark matter, and something
Speaker 2: like ninety percent of the matter in galaxies is dark matter.
Speaker 1: And then what percentage is Hershey's chocolate versus snob chocolate.
Speaker 2: I see, now you're disparaging people's choice chocolate. It goes
Speaker 2: both ways. I'm all right, I can take it. If
Speaker 2: I can dish it out, I can take it. That's fair.
Speaker 2: I'm sure that Hershey's dominates the chocolate market, but I'm
Speaker 2: glad that there are now a variety of chocolate offerings
Speaker 2: for people to enjoy.
Speaker 1: All right. Well, another interesting feature of our galaxy and
Speaker 1: of our u universe the way physics works to create
Speaker 1: these weird characteristics of the things we see out there,
Speaker 1: which then kind of reveal what things are made of
Speaker 1: and how things work in the grand cosmic scale things.
Speaker 2: Absolutely, and it's just part of the game we're playing,
Speaker 2: looking out at the night sky and out deep into
Speaker 2: the universe to try to get a handle on what's
Speaker 2: going on out there, what structures form, why do they form,
Speaker 2: and what do they reveal about the flash of titanic
Speaker 2: forces that's happening underneath it all.
Speaker 1: It sounds like these bars are giving us a good
Speaker 1: handle on galaxy formation and dark matter content of the Universe.
Speaker 2: I'm going to go hit the lunch bar after this.
Speaker 1: Yeah, just don't hit the dessert bar too hard. It
Speaker 1: depends on the kind of chocolate ban Yeah, that's right.
Speaker 1: The brand apparently.
Speaker 2: Love all you and love all your chocolate choices. Keep
Speaker 2: chomping on bars. That sounds really credible, Daniel. I'm working
Speaker 2: on it, man, I'm working on it all right.
Speaker 1: Well, we hope you enjoyed that. Thanks for joining us.
Speaker 1: See you next time.
Speaker 2: For more science and curiosity, come find us on social media,
Speaker 2: where we answer questions and post videos. We're on Twitter, Discorg, Insta,
Speaker 2: and now TikTok. Thanks for listening, and remember that Daniel
Speaker 2: and Jorge Explain the Universe is a production of iHeartRadio.
Speaker 2: For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts,
Speaker 2: or wherever you listen to your favorite shows.