Daniel and Jorge Explain the UniverseDaniel and Jorge talk about how the smaller galaxies might hold the secrets to understanding the structure of the Universe and the truth about dark matter.
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Hey, Daniel, what's your favorite thing about looking up at the stars?
You mean other than the hot coco?
Drink hot cocoa? When you look at the stars, you have like a Peplodian response.
There can you see stars if you're not holding hot cocoa? I've never tried.
You might steam up your glasses. No, But I mean, like I about the actual stars, not just not what you're drinking when you look at them.
I don't know. I love how big everything is up there, the stars, the galaxies, all of it's just so overwhelmingly huge.
But aren't you a little bit biased about that?
What do you mean?
I mean, you're only seeing the big stuff when you look up at the sky. There's plenty of little cute stuff on those stars or on those planets.
Oh yeah, that's true, I suppose, But it doesn't change my mind. You still like big stuff. I like my universe the way I like my hot cocoa.
Big and dark.
Or Hamming cartoonists and the creator of PhD comics.
Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I don't really drink that much hot coco. But does that mean you don't look at the stars very much? It does, unfortunately, mean I don't look at the stars very much. Basically, it's because I don't do as much camping as I used to, and camping was my number one way to see the stars and also to drink hot coco.
You know you can do all those three things independently.
What that's not true? You can't just make hot coco at home in your living room or look at the stars next to you telling me you can like make s'mores over your range.
You could even do it on the microwave. What didn't you have one of those super smart microwaves that you got for free.
I think that would cause offense to the fundamental nature of space and time making s'mores in the microwave.
Yes, you'd be blacklisted by the Boy and Girl Scouts. But anyways, welcome to our podcast Daniel and Jorge Explain the Universe, a production of iHeartRadio in which we take a.
Long, deep sip of this sweet, sweet universe, trying to appreciate all of its incredible flavors and colors and mysteries. We look out into the cosmos and we wonder why things are the way they are, why they look the way they do, and if it's possible to explain all of it, to understand the swirling and the dancing and the frothing and all the tuing and frowing that's happening up there in the night sky, and to explain all of it to you.
That's right, We give you some more of this amazing universe, all the hot stuff, all the cold stuff, all of the chocolate stuff, and even all of the vanilla stuff. That's also pretty interesting.
Did you say even all of the vanilla stuff, like vanilla's an afterthought. I thought you were a great defender of vanilla as an actual flavor.
I am a big defender. It's my favorite flavor.
That's like saying your favorite color is white.
Dude, it is. Actually I love nothing more than a blank page. The more white I can put into my drawings, the less work I have to do.
A white page is usually the enemy of creative types, but I'm glad that it inspires you. But it's true that there's a lot of stuff out there to enjoy and to experience, of all different flavor and all different sizes. Sometimes people focus on the biggest, craziest, most extreme stuff in the universe, but there's a whole scale of things happening out there, tiny little gas clouds all the way up to super massive galaxies.
That's right, All of the big stuff in the universe usually gets all the big headlines. People mostly pay attention to super massive black holes or giant superstars that are millions of times bigger than our sun. But sometimes it's the little stuff that can tell you a lot about the big ideas in the universe.
Because remember that we don't get to control what happens in the universe. If we want to learn the way the universe works, we just got to sit back and watch the experiments that nature has arranged for us. We don't get to say what happens if you shoot two black holes together. We just have to look to see if somebody has already smashed them together. And so, because we are beggars, we don't get to be choosers, and that means that we need to make the most of everything that's out there. We need to think about what we can learn from the big stuff and also what we can learn from little stuff, because everything out there in the universe has something to teach.
Us, and there is a lot to be taught out there in the universe, a lot of amazing things big and small. And so today we're going to focus on one type of thing out there in the universe that maybe doesn't get as much attention as some of the big stuff. So to be on the program, we'll be asking the question, what are dwarf galaxies?
I love that we're going to get to talk about dwarf galaxies today because they are some of the most fascinating and interesting and reviewing aspects of the universe. They have so much to teach us about what's going on and where everything came from.
Yeah, they're pretty exciting and pretty awesome, and so as usual, we were wondering how many people out there had thought about dwarf galaxies or know what they are.
So thank you very much to everybody who answers these questions for the podcast. We love hearing your thoughts, as does everybody else. And if you are out there and have been listening to the podcasts for a while and like to share your voice for everybody else, please don't be shy. Write to us two questions at Danielanjorge dot com. So think about it for a second. What do you think are dwarf galaxies? Here's what people had to say.
My guess is they're just smaller galaxies, smaller collections of stars that have not yet been swallowed up by a big galaxy.
I would imagine that all galaxies kind of start out that way and grow and merge and until they become big, beautiful spirals like the Milky Way.
Dwarf galaxies are like those little mini galaxies, and they can be like satellite galaxies to galaxies like the Milky Way, just tiny little galaxies, not that big. I don't know.
Maybe a dwarf galaxy is a galaxy with not enough MOUs to be.
Considered a galaxy, like what happened to Pluto.
I don't know.
I'm guessing the dwarf galaxies, as the name suggests, are smaller galaxies.
I guess that by the name, dwarf galaxies have a lot less stars and planets and other stuff. But I don't know how smaller it has to be to be considered a dwarf galaxy.
The term dwarf galaxies kind of reminds me of the galaxies that are like globular clusters. So or it might just be as it says in the name, they're just smaller galaxies, maybe much less stars, maybe a different shape, maybe less dark matter keeping them together.
I don't believe.
There probably won't be any black hole in the center, but there might be. I don't know.
All right, some pretty straightforward answers here, everyone said they're just galaxies but smaller.
It's like a dwarf serving of ice cream or a dwarf cup of cocoa.
Oh yeah, that is that a new diet, perhaps.
Little servings of everything. I think that's maybe the old this diet.
Well, that's a thing on the internet, right, there are all these videos that people making like little tiny, like lego sized foods.
Really like instead of having a sandwich, you just have like a tiny little sandwich.
Yeah, there's this whole genre of YouTube videos they make like tiny food.
Oh but that's not for eating, right, that's just for like being silly. Nobody's sitting down to tuck into like a tiny roast chicken, are they.
Well maybe they could, they could, I don't know. They usually cut the videos after they'd make the food.
I do like those tiny kitchen videos. Those are really fun.
All right, Well let's dig into it, Daniel. What is a dwarf galaxy?
So everybody was basically right, dwarf galaxies are little cute galaxies, because it turns out that galaxies come in all sorts of sizes. We tend to think about galaxies in terms of ones like our own the Milky Way that has hundreds of billions of stars, but galaxies that are much much bigger than the Milky Way all the way down to galaxies that are very very small things that you probably wouldn't even call a galaxy.
Maybe let's put things into perspective, like how big is our galaxy? What are the size range you said qualify a galaxy as a dwarf galaxy.
So our galaxy has somewhere around two hundred to four hundred billion stars. That's a really difficult number to wrap your mind around.
Two hundred billion, like a billion times two hundred exactly.
There are more stars in the galaxy than people on Earth, right, It's incredible, like every single person on Earth could have like point to or so stars just for themselves in the Milky Way. It's really an incredible number of stars out there, all them firing and burning with planets around them, lots of Earth like planets. It's really hard to sort of like get the whole scope of the galaxy in your mind. But that's the size of our galaxy. A few hundred billion stars.
You'd be like Oprah, you'd be giving up stars to everyone. You get a star, and you get a star, You're all stars.
That's right. Donate to the podcast and I will give you a star in return.
Now, do you offer free home delivery that or do you have to pay for shipping? See that's how they get you the shipping. It's the free star, but it's going to cost you ten trillion dollars to deliver it to your house and also the life of very human on Earth.
Mm hmm.
Yeah. And in this case, it's not just the shipping, it's the handling, right, because that's particularly tricky when you're dealing with something several thousand degrees Calvin, But no, I will email you a plaque of ownership of your star if you donate to the podcast.
Oh boy, I feel like you just made a serious offer.
Let's see if we get any takers. But our galaxy, as big as it is, is not even the biggest galaxy out there.
How big do galaxies get? Like Andromeda? How big is Andromeda?
Andromeda has more than a trillion stars in it. It's about five times as big as the Milky Way, like totally dwarfs us in terms of galaxies. And there are other galaxies out there that are even bigger.
Whoa, what's the biggest galaxy that we know of? Or what's the biggest galaxy that Google knows of?
So the biggest galaxy that we know is about a billion light years of a way. It's called I See one one one, and there's a lot of uncertainty, but the current estimate is that it has the mass of about one hundred trillion stars, so like one hundred times more stars than Andromeda.
Whoa, which is already five times bigger than us, So it's like five hundred times bigger than us in terms of mass.
Yeah, there's some nuances there because there's a big variation in the masses of stars. Actually, more stars are smaller than the mass of our sun. Remember, the most common kind of star out there is a red dwarf, which is smaller than the kind of star that we have. So if you're just measuring the mass in terms of like our solar masses, that's going to underestimate the number of stars that are out there in that galaxy. So it may even be more. This is just like a really shocking number, hundreds of trillions of stars. You know. For comparison, there's like a few trillion trees on Earth, so that means that like every tree on Earth could have like twenty stars in that mega galaxy.
WHOA.
Sure trees are collecting stars these days, but you're welcome to assign a star for every tree in that galaxy.
Any tree that donates the podcast, I will email them a certificate of ownership.
They technical they kind of do already because I print out the outline every single time on paper.
Wow, which means you're sacrificing trees for the podcast.
Yeah. I like to think they donated for the good of the of knowledge, But I guess maybe a question is like, is there an upper limit to the size of a galaxy? Where can galaxies just be infinitely big? And if there's a limit, what causes that limit? Is it something about the conditions at the beginning of the universe.
There's no technical limit to the size of a galaxy. Galaxies just form and get bigger and bigger. That's fundamentally the history of the universe is that galaxy started out basically a small clumps of stars, which then merge with other clumps of stars, and so you get this like hierarchical formation, this merging of mergers of mergers, and so there's no reason why you can't just like keep clumping galaxies together, and they are going to keep clumping together. Really, the only thing that limits the size of the galaxy is the fact that the universe is expanding, and that expansion is accelerating, so it's increasing the distances between galaxies. So it sort of like keeps the galaxies separated a little bit and prevents them from colliding all into one huge mega galaxy. So it's a bit of a race against time, right because recently the universe has started accelerating right in terms of its expansion, So maybe we have seen the biggest galaxies that will ever form. Some cosmologists think that we live at the time of the biggest structures in the universe because of the accelerating expansion of the universe, then size of structures cannot grow anymore because so much space is being created between existing galaxies. And so, like we have galaxies, and we have clusters of galaxies that are mostly held together by gravity. Then we have superclusters which are sort of on the edge of whether gravity can hold them together or dark energy will re them apart. And so it might be that, like our cluster of galaxies eventually collapses into one big galaxy, maybe even our supercluster collapses into a super galaxy. But the stuff and other superclusters. Probably dark energy will keep us from ever merging with them, So we might get future bigger galaxies, but we won't ever get like bigger blobs of stuff. We might have reached sort of like peak size of blob.
We're like at the end of purity. It's all downhill after that.
Stuff just starts falling apart after that.
Yeah, hopefully let's get to party as much as we can right now. But I guess it also depends on what dark energy is going to do in the future, right Isn't it a possibility that dark energy will reverse and it will cause everything to start contracting and then will very basically at the entire universe is going to collapse into a clump and then it'll be like one giant galaxy.
Basically, it certainly does depend on that. The scenario we just outlined assumes that dark energy continues the way that it has that is constant in space, and that as space gets bigger, you add more dark energy. So dark energy is an increasing fraction of the energy density of the universe, which just further accelerates the expansion. If you just extrapolate that out naively, then yeah, you get the scenario we just outlined. But as you say, we don't really understand dark energy where it comes from. What is this source of potential energy that's accelerating the expansion of the universe. Could it change? And in fact it might, right because we don't know the underlying mechanism that creates it. It could be that there's some complicated dynamics there that change with time and give us a different future. Like it could just all turn off suddenly and then we have a big crunch where everything collapses down, as you say, into one big superstructure, one megagalaxy. But we're not here today to talk about the super big galaxies. They get enough attention. Let's turn our mental eyes down to the other end of the spectrum.
Yep, yep, we're talking about dwarf galaxies. And like you said, it's the case that all galaxies started out as dwarf galaxies, right Like at the beginning of the universe, everything was spread out, but then these things started to clump together, and so everything started with the small galaxies.
Yeah, everything started with these little fluctuations due to quantum mechanics. A little bit that was more dense over here, a little bit that was less dense over there, and then gravity did its work and pulled that stuff together and made little clumps of gas which then turned into stars, and that's how you got the first galaxies. So there was like a size of those clumps that formed the first galaxies. And you know, some of those have merged into bigger galaxies, and some of them have not, and some of them are more recent and haven't yet merged into other galaxies. So at the small end of the scale are those little mini galaxies that have not merged or not merged as many times.
I wonder if there's like an average size galaxy at the beginning of the universe, do you know what I mean? Like the universe presumably was kind of the same everywhere when there's a certain density of stuff, which means that on average there was probably like like every galaxy was almost the same size, right, some small size.
Yeah, And we can actually see this in the cosmic microwave background radiation. We can see this pattern of overdensity and under density, and we can use that size actually to measure like the expansion rate of the universe. We have a whole podcast episode about like measuring the curvature of space and the history of it, and you can see those kinds of things expand from an early characteristic quantum fluctuation size blown up into something macroscopic, which, as you say, then determines basically the size of these initial clumps.
Yeah, but again it's quantum base, right, so it's totally random. So there could have been maybe a spot in the universe, but that had a big fluctuation which maybe would have made a big galaxy out there at the beginning of time.
Yeah, it is random, you're right, and so it's less likely, but it's possible to get a larger gravitational collapse an early galaxy that started out big. But there's also a typical characteristic size where galaxies start, and so that's on the little end. So these dwarf galaxies are basically on that smaller end of little gravitational clumps that formed little stellar neighborhoods.
So right now we have these galaxies and giant structures of galaxies. But it used to be the case, maybe at the beginning of the universe, where like the entire universe was just kind of evenly distributed with tiny little galaxies.
Yeah, and these galaxies get to be pretty small, like remember the milky ways hundreds of billions of stars. Dwarf galaxies can go all the way down to like hundreds or thousands of stars, all the way up to like several billion stars. So there's an enormous spectrum of size there, from really just a handful of stars all the way up to billions of stars.
Hmmm. Interesting. All right, let's get more into actual dwarf galaxies and what they can tell us about dark matter and the rest of how the universe formed. But first, let's take a quick break.
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All right, we're talking about dwarf galaxies, and we talked a little bit about how Basically, dwarf galaxies were the og galaxies in the universe, right, Like the beginning of time, every galaxy was a dwarf galls.
Yeah, there may have been some larger galaxies formed randomly as you said, but the original galaxies, yeah, we're all dwarf galaxies. That's how it all began.
And they usually kind of have a fuzzy shape to them, right, They don't have maybe this nice spar shape or form that the Milky Way has.
It's actually an interesting and really subtle point there, because if you have an initial clump of stuff that collapses under gravity, it tends to form a disk. And it forms a disk because it's spinning, and it's spinning like around some particular axis. Gravity can squeeze it down sort of along that axis, but on the plane perpendicular to it, it can't squeeze it down as much because it's still spinning and it retains that angular momentum. So if you have just like an initial spinning blob of stuff, it tends to form a disk. Now, when dwarf galaxies merge together to make bigger galaxies, then you have like disks spinning in lots of different directions. And you end up with like more ellipsoid galaxies which eventually later then also collapse into like some big overall disc, which is why, like the Milky Way is mostly a disc.
All right, Well, then like how many I guess the question is, like how many dwarf galaxies do you need to come together to make a galaxy like the Milky Way, Because you're saying the Milky Wave probably formed out of dwarf galaxies coming together, right, I'm just wondering how many it takes.
Yeah, I mean if dwarf galaxies start out as a few thousand stars, right, and the Milky Way has a few hundred billion stars, then that means that the Milky Way might be like a million dwarf galaxies all smoohed together into one big galaxy.
But you said the range is between like a dwarf galaxy is between one thousand and several billions.
Yeah, Well, you know, this is one of those sort of artificial distinctions in astronomy, like what do you call it galaxy and what do you call a dwarf galaxy. There's this threshold of like above a few billion or a few tens of billions of stars, It's called a galaxy, and below that it is called a dwarf galaxy. For example, the large Magallanic Cloud is orbiting the Milky Way and it has like thirty billion stars in it. Some people call it a dwarf galaxy. Some people say no, no, it's its own galaxy. And so that's a bit of an artificial distinction. But if you want to go like all the way back to the og galaxies out of which everything was built, then those are all going to start out pretty small. So if those are like a few thousand stars, then it's going to take millions of those to make a Milky Way.
Mmmm.
I feel like if you have a thousand stars, maybe you shouldn't be called the galaxy, you know. That's such a sad that's more like a I don't know, like a star neighborhood or something star clump associated stars.
That's the bias, right, that's us looking at our neighborhood and observing other galaxies. But something we learn as we develop better tools is to see fainter stuff, is to discover the stuff that is not as easy to spot. And the whole history of science is us drawing big conclusions from the stuff we first see and then discovering, oh, that wasn't representative in terms of we need to revise our whole picture of how things work. And so we've been seeing the biggest, brightest, most exciting galaxies, but the whole spectrum of other kind of stuff out there, whether you want to call it a galaxy or mini galaxy or galaxy no or dwarf galaxy. You know that's just the name.
You mean, Like, there could be aliens out there in one of these mega galaxies looking at us and saying that's not a real galaxy.
It only has you know, four hundred billion stars. That's nothing exactly. Remember if we demote other galaxies, that day might come for us.
Yeah, but you're saying so, you're saying that our galaxy, the Milky Way, it probably is made up of hundreds of these original dwarf galaxies that the universe started out with.
Hundreds or thousands or maybe even millions of dwarf galaxies have been smoothed together. Like the stars that are in the Milky Way did not all start in the same part of the universe. They all came together after they already formed clumps of stars. So all the stars in the Milky Way did not form out of the same big gas cloud. You had like millions of different gas clouds that made millions of little pockets of stars which then formed together later into a bigger galaxy.
You mean they were all sort of clumped together initially maybe, is that what we're saying? But then within that giant cloud little galaxies form that then eventually clumped together.
Well, I mean they clumped together eventually, the same way that for example, Andromeda and the Milky Way will eventually merge in a few million years. Gravity is inextually pulling us together and we will form some big combined galaxy. I don't know what you call it, like the Andromeda Way or Milky Andromeda or something.
Vanilla Andromeda. I vote for Vanilla Andromeda. Contribute to the greatest flavor.
Right, sounds good. And then you can ask like, well, you know, did the Milky Way Andromeda did they form together out of the same big clump. Well, their gravitational future is secure that they will eventually be together, But really they formed separately and then came together. And that same idea applies to all the dwarf galaxies that formed the Milky Way and the Dwarf galaxies that formed Andromeda. They sort of formed separately and then later came together to make bigger galaxies.
So then has enough time gone by to explain how our galaxy form out of maybe millions of little galaxies, like I know, that's a big mystery with black holes, right.
Yeah, that's a great question. And that's actually one of the great triumphs of dark matter is that when we do simulations of our universe and we say, here's so much dark matter there was, and here's some quantum fluctuations, and then we just like run the clock forward, we can actually reproduce the large scale structure of the universe, formation of the big galaxies that we see. And actually the bigger galaxies we see do appear in our simulations. But as we'll talk about later in the podcast, the dwarf galaxies, we don't really understand why there aren't more of them. So we do understand some of it, but not all of it.
Interesting, are there still dwarf galaxies merging into our Milky Way galaxy or within our Milky Way galaxy? Or are we pretty much like just one big unified family right now?
There's so many dwarf galaxies still out there right now. A bunch of them are orbiting the Milky Way, right, which means eventually they might get slurred up by the Milky Way.
Wait, what we have like a galaxy system?
Oh?
Yeah, like we have our galaxy and we have little galaxies orbiting around this.
Yeah, we have satellite galaxies, right, little dwarf galaxies orbiting the Milky Way, trapped by our gravity, and eventually they'll get slurped up.
WHOA, How many satellite galaxies do we have?
That's an interesting and complicated question. We have something around a couple dozen satellite galaxies that we've discovered, and one of the big questions about the research right now is why don't we have more. So if you run these simulations, they suggest that you should get galaxies about the size of the Milky Way, and we do, and that all makes sense, but they also suggest that the Milky Way should have a lot more dwarf galaxy satellites. There should be like five hundred of them orbiting the Milky Way, but we only see a couple dozen. And that's one of the things people are still confused about. And that's why dwarf galaxies are so interesting, because they're one of the things that remain not well understood. M interesting.
So you're saying that we run a simulation of the universe based on what we know, and it explains the big stuff out there, like the galaxy superclusters and the bowls and the walls of superclusters, but it doesn't match what we see kind of at the local level around us, around our galaxy.
Yeah, exactly. It suggests that if you're going to have big galaxies that comes out of formation of a bunch of little ones, but not all the little ones should get slurped up into the big ones, that you should have lots and lots of little galaxies still left over orbiting the bigger galaxies. But when we look out into the night sky, we just don't see them like we look for them. We try to spot them, We see some of them, but we don't see as many as we expect.
Sounds like there's something wrong with the simulation, not necessarily with the universe.
No, the universe has to follow our program, man.
Yeah, that's what I'm saying. I don't think the universe cares.
No, it's not that the universe has like done something wrong. It needs to be chastised or something. This is just the process we have. We think we understand the rules that control how things happen, and so then we do a bunch of simulations to say what do the rules predict, And if they predict something we don't see, that means obviously something is wrong with the simulation. But the question is what Or the other thing is maybe something is wrong with what we're seeing, like maybe we're just not seeing everything that's out there. Dwarf galaxies are tricky to spot because they're small. They only have hundreds, thousands, or maybe millions of stars in them, so they are much fainter than other galaxies, which make them more challenging too spot.
So we were expecting from the simulations to see about five hundred dwarf galaxies orbiting the Milky Way, but we've only seen about twelve. And you're saying, like, I wonder if you could maybe even see them from our point of view, right, like they're so small, maybe to us they just look like a little cluster of stars, not necessarily a whole other galaxy.
It is complicated by the fact that we are inside the Milky Way, which makes it harder to see out of the Milky Way because there's so many stars and a's gas and dust in between. They have taken that into account, Like how many galaxies should we have seen from our point of view that they have factored in. Something that they're not sure about has to do with the dark matter in these dwarf galaxies. Like we also suspect, and I want to dig into this in a minute, that these dwarf galaxies are much heavier in dark matter than in normal matter. A typical galaxy is about eighty five percent dark matter fifteen percent normal matter. The Milky Way is a bit above that, like ninety percent dark matter ten percent normal matter. But these dwarf galaxies might be overwhelmingly dark matter. They might have a lot more dark matter in them than normal matter, which of course make them harder to spot.
Now, this mystery about not seeing enough dwarf galaxies around the Milky Way, is that also true for other galaxies? Like if you look at the Androma galaxy, do you also see less or fewer dwarf galaxies than you'd think you would.
Yeah, it's a problem everywhere. It's harder to study for more distant galaxies because they are more distant, and these galaxies are small. So you know, our observations of dwarf galaxies around Andromeda are not even as good as our observations of dwarf galaxies around the Milky Way, which are already very challenging to see. So the Milky Way is sort of like the best laboratory for studying this. But yeah, we see similar stuff in Andromeda. Beyond that, it's just too difficult to study.
I guess it's kind of like trying to detect the wisps of smoke around, like a giant cloud of smoke, right, that's kind of what these galaxies look like from.
Afar exactly, and it's not always easy to tell, like where is the edge of a galaxy and is that the count is a dwarf galaxy or is it already falling into the main galaxy right against all sorts of distinctions.
Oh, I see, that's the real mystery. I just change how you call them, then done, we can check off that box.
No, it doesn't matter what you call them, because there's just a disagreement about the distribution of stars in our simulations and what we see out there in the universe. But a lot of astronomers think that probably this will be resolved if we improve our abilities to discover these dwarf galaxies. For example, recently they found eight new Milky Way dwarf galaxies that they hadn't spotted before because they are ultra faint, because they are more than ninety nine point nine percent dark matter. They're basically dark matter galaxies with a little sprinkling of stars in them.
WHOA wait, I feel like now you're getting into the definition of a galaxy itself. Like are you saying, like a bunch of dark matter with a few stars in it, that's a galaxy.
Still, that's a galaxy according to astronomers. Yeah, it has the mass, right, it has stars in it, so yeah, they call that a galaxy.
I guess maybe the definition then is just like a clump of stuff out there in space that's maybe separate from other clumps of stuff.
Yeah, But then you get in the question of like, what do you call a globular cluster? Why is that not a dwarf galaxy? Why is it a cluster?
Exactly right, That's what I would ask. Why I'm confused?
Yeah, well, welcome the club. Astronomy is a disaster when it comes to naming things, and that comes from a particle physicist, and I know that we have no high ground when it comes to naming things.
Yeah, I guess it's hard to name things in general, right, It's hard to name your kids. I can only imagine naming the entire universe.
Well, the real challenge here is that a lot of this is historical. You know, we didn't always understand the connections between things. We saw stuff in the sky, we gave it different names. Later we realized, oh, this is really another kind of that. You know, even if you just look in our solar system, you know, we have like comets and asteroids, and then we have like centaurs, which are sort of like between comets and asteroids. We have planets, and we have moons, and like, you know, the distinctions between these things are fuzzy. What's really going on is that you have a whole spectrum of stuff out there, from big to small and everything in between. So the distinctions between things are sort of artificial labels that we are just putting on stuff because what we historically saw first, what we sort of originally called things. The truth is that there's a smooth spectrum of all sorts of stuff out there.
Sounds like you just need to call everything stuff. Like a lot of galaxy. It's just stuff that's not a black hole. It's just stuff. If I should change your name from.
Physicists, stuff is is yeah exactly.
I'm just you can be stuffy. Stuff is this.
Yeah, I'm just trying to stuff as much knowledge in my mind about stuff. Basically, you know, it's different from like biology. Cats and dogs really are different things. There's not an entire spectrum of every creature between a cat and a dog that doesn't exist. But I think out there in the universe there really is like every kind of thing between every other kind of thing. So there's a whole spectrum of stuff out there. It's just waiting to be discovered.
Mm.
Interesting. All right, Well, let's get a little bit deeper into this connection between dark matter and dwarf galaxies and how maybe dwarf galaxies can help us understand or finally figure out what dark matter is. But first, let's take another quick break.
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All right, we're talking about dwarf galaxies, and it's pretty interesting that what you said earlier that like a clump of dark matter, which is a few sprinkles of stars, you would still call that a galaxy.
I would still call that a galaxy. I mean, think about where that came from. Originally, you had a clump of stuff in the very early universe a tiny little bit denser than everything else. Mostly that means the dark matter, because it was more dark matter than everything else. That dark matter makes like a little we call it a gravitational well. Everything likes to roll downhill towards lower gravitational potential. Gravity gathers stuff together, and so every little gravitational well gathered together blob of dark matter and a blob of normal matter gas, et cetera. And that led to star formation. And so it's because of dark matter that gas clumped together and made the first stars in the early universe. And so every sort of like original og clump there. I guess we call a dwarf galaxy.
I see, okay, I guess it's kind of also like our galaxies mostly dark matter too, Like the Milky Way is mostly dark matter with a few sprinkles of stars, like by mass, where the Milky Way's with fifteen percent.
Yeah, exactly. On average, the universe is about eighty percent dark matter in terms of mass, and so everything out there is mostly dark matter with a sprinkling of stars.
Wait, are you saying the Milky Way should actually be called the milk chocolate the way.
It's more like the hot cocoa Way, right, It's really a river of dark deliciousness with a few sprinkles of marshmallows, like the stars are the marshmallows on top of the dark matter. Hot cocoa give mesmore. But what's fascinating is that these dwarf galaxies have much more dark matter than typical Like they can be up to ninety nine point ninety nine percent dark.
All dwarf galaxies are just these last few that we found.
Most dwarf galaxies are overwhelmingly dark matter. There's a few that are like satellites of the Milky Way that have had their dark matter stripped out of them, but the great majority of them are overwhelmingly dark matter. It's just sort of like the size of that clump of stuff tends to have fewer stars. Really, why is that, Well, if you have a smaller clump of stuff, you have less gravity sort of holding those initial stars together because stars are sort of poisoned to other stars. Like what happens when you form stars is you get a bunch of radiation shooting out from that star, and that tends to heat up and blow out all the gas that you need to make stars. So remember to make a star, you need a blob of cold gas. The gas can't be like moving around too fast, or gravity which is super weak won't have a chance to suck it together. So as soon as you start forming stars, then those stars like push out all the other gas. And then as soon as you have the first super nova, it basically blows out all the gas from a dwarf galaxy. But if you have a big enough clump, then it can retain that gas anyway. Right, So as you're sort of serving of gravity, you get smaller, you get too small to sort of overcome these supernova and these other effects that are killing your star formation.
Oh, I see, because I guess when a star explodes in a supernova, the dark matter doesn't care, right, Like, star will explode, but the dark mess is it doesn't interact with dark matter. The dark matter doesn't care, but it will blow out all the other star stuff that's in that Meni galaxy. And that's why if you're small, then you'll most likely blow out all of your star stuff, but you'll keep your dark matter stuff. So it's like you're super concentrating the dark matter. You're purifying it, distilling it. There you go, Yeah, distilling.
So the dwarf galaxies that are still around, they're the ones with overwhelmingly dark matter and very very few stars in them. They had like one initial round of star formation and then they basically poison the well. So they're also super fascinating from that point of view because they're like fossils of star formation they didn't have like many many cycles like that, sort of like a window into a much earlier part of the universe. But also there are these very cool blobs of dark matter, and you know, dark matter a continuing source of mystery and consternation for physicists, and these are really awesome laboratories to study dark matter.
But I guess you can't really see this dark matter, right, you're just inferring that it's there, or that this clump of stars has ninety nine point whatever amount of dark matter. You're just seeing a little bit of stars that are clumped together and spinning more than they should, and so you're inferring that there's a bunch of dark matter there.
Yeah, we're not seeing this dark matter like directly using gravitational lensing for example. I mean, in a few cases we can, but mostly we're inferring that these clumps of stars have a lot of dark matter based on the motion of the stars, which is originally how we discovered dark matter. We saw that stars are moving really really fast, but that there's not enough stuff in the galaxy to hold them together, or if they're moving that fast, and which you can do if you look at the velocity of stars, how fast they're moving around the center of a galaxy, is you can tell how much gravity does there have to be to keep that star at that distance from the center of that galaxy. And that gives you like a map of the gravity of that galaxy, which you can turn into a map of the mass of the galaxy. And so you can say, based on the spinning stars that I see, where is the mass in that galaxy? And that tells you where the dark matter is in that galaxy. So like a few tracers in a galaxy will tell you basically where the invisible mass is.
What about our Milky Way, Like what's our percentage of dark matter to regular stars?
So the Milky Way is a little bit more dark matter than the rest of the universe. We're like ninety percent dark matter and ten percent other stuff, whereas the rest of the universe is about eighty percent dark matter. But it also varies with distance from the center of the galaxy, like where we are where the Sun is relative to the center of the gas galaxy. Everything between us and the center is about fifty to fifty dark matter and other kinds of matter, whereas if you go further out then it starts to be overwhelmingly dark matter. And remember that the dark matter halo for the Milky Way is much bigger than the distribution of stars that goes out much much further, So the stars peter out and at some point it's only dark matter.
I guess anytime you're in between stars, you're basically sitting in dark matter.
Right, Yeah, Well, we don't really know the sort of fine scale structure of dark matter. We have these very coarse probes from like how stars move, and we have stellar streams. We actually do the whole podcast episode about like trying to see the fine scale structure of dark matter within the galaxy. It's really hard, and the bottom line reason is that gravity is just super weak, and so in order to measure where the dark matter is, you need really big blobs of it, which means we can't see small blobs of it. But we can look at these dwarf galaxies and trace the motion of their stars and use that to figure out where the dark matter is in those galaxies and how much of it there is.
Well, it's interesting that our milk Kay galaxy has kind of like a higher concentration of dark matter than the rest of the other the universe in general and other other galaxies as well. Are we a higher concentration of dark matter than like Andromeda.
We do have bored dark matter on average and Andromeda. And Dromeda is a bigger galaxy, and so it's a smaller chance to like fluctuate up to have more dark matter than a smaller galaxy like the Milky Way. But these smaller galaxies, like the dwarf ones, they're really fun ways to study dark matter because one thing we can do, for example, is we can look to see whether the dark matter in these dwarf galaxies is banging into itself and giving off some sort of like telltale signature. Particle physicists particular are like pointing their telescopes at these dwarf galaxies to try to see signals from the dark matter.
Oh, I see what you're saying. Like, we can use dwarf galaxies as kind of like a way to know where there's a lot of dark matter out there in the emptiness of space. Like if you see a dwarf galaxy, then that gives you a target to point your telescope to and say, okay, I know for sure there's a lot of dark matter in this one spot. Is the dark matter doing anything interesting that might tell us a little bit about what it is exactly?
And one particularly interesting thing that people hope dark matter will do is that two dark matter particles, whatever they are, we don't know what they are, might smash into each other and they might annihilate, might turn into something else, and occasionally that will involve turning into photons. So normally we think of dark matter as dark not creating any photons, but there are some theories where it has some kind of interaction which eventually can turn into photons. And so you see this like characteristic flash of gamma rays. Problem is the universe filled with gamma rays. All sorts of other stuff generates gamma rays. So one thing you can do is point your telescope at the center of the galaxy where you expect there to be a lot of dark matter and look for gamma rays. But you're like swamped in gamma rays from other stuff. Dwarf galaxies have very little other stuff. They're mostly dark matter. Points your telescope at the heart of these dark matter galaxies, these dwarf galaxies, and you see gamma rays there, then you can be more certain that it comes from dark matter. We haven't seen any there's nothing unusual emanating from the hearts of these dwarf galaxies. But they've given us some really powerful limits telling us what dark matter doesn't do.
Wait, are you saying that dark matter might be actually shining and might e mid light? Would you have to change the name then, from dark matter to like dim matter darkish matter.
There's so many theories of dark matter that you can't even really describe all of them, and so many ways to look for dark matter. You know, people complain to me sometimes like you guys are still looking for dark matter, you haven't found it when you're going to give up. The problem is that there's so many ways that dark matter could be discovered, and so many different ideas for what it could look like because we know so little about it that we've got to try lots of different ways. And in some of those theories, yeah, dark matter can annihilate and turn into photons, so yeah, what is still be called dark matter. I look forward to having that argument with you when we collect our Nobel Pride for discovering dark matter.
Well, if dark matter doesn't midlight, it's going to be kind of dim and it's going to be a dark ish and it's going to be sort of red shifted, right, because these galaxies are probably moving away from us, which means that you could technically call it chocolate matter.
And if it's red shifted, it should be like rose chocolate matter.
Right, Is that a thing?
Is rose chocolate thing? Yeah? Absolutely, they invented it recently. He had dark chocolate, milk, chocolate, white chocolate, and now rose chocolate. It's a whole new process.
Hmmmm, Well, there you go. Physicists are inventing new things all the time.
That was definitely not a physics invention. I think it was Nestle that came up with it. But we can do more than just look for dark matter annihilating with itself. We can also study in detail the distribution of dark matter, like where in these dwarf galaxies did did dark matter end up? And does it agree with our simulations and our calculations, because we can tell not just how much dark matter there is, but also like is it mostly at the core. Is it really clumped? Is it smoothly spread out? This kind of stuff, and what we see is that it does not agree with what we predict. That our simulations get it wrong.
Wait, how can we tell how it's distributed. If it's invisible, it's invisible, but it affects the motion of the stars. And so if, for example, you have all the dark matter at the very very center, then the stars closer to the center will be going really really fast. If the dark matter is more spread out, then the stars closer to the center are not as affected by all that dark matter. So by looking at how the velocity of the stars changes as you get further from the center, we can make a map of where in the galaxy that dark matter is. Is it all clumped in the center, is it more spread out? And when we do that, we see weird stuff that we don't understand. What do you mean weird stuff?
So our simulations predict that you should have like a really hard core of dark matter that yeah, you have a big fluffy halo, but the density should rise really rapidly as you get towards the center. And what we see in our telescopes is not the same thing. We see like a flatter distribution. It doesn't like get as peaky towards the core. The density of dark matter at the very heart of the dwarf galaxies is lower than what we expect. In astronomy. This is called as the core versus cusp problem. Simulations predict a sharp cusp in the density, but what we see is more like a flat core.
They're fuzzier than you expected. But isn't that just kind of a matter of time, Like over time it should clump together towards the center, right, because that's what dark matter does.
It is and we factor that time into our simulations, and the predictions just disagree with what we expect to see at a universe of this age. Yes, so yeah, over time it will tend to clump more and more and more. But it hasn't clumped as much as we expected.
Mmm, what could it be? What could be the explanation?
Well, there's lots of really fun ideas. This is a big crack and sort of the success of dark matter and explaining the large structure of the universe, And some people think it's a good argument for moond or of these alternative theories. It says, you know, dark matter doesn't even exist at all. It's just that we've misunderstood gravity, and the gravity at different distance scales and at different accelerations works differently than we expected. And the whole dark matter thing is a big mistake. And it's true that dark matter does not do a good job of explaining what we see in these dwarf galaxies. It's like a big open problem for dark matter, and mind does a good job of explaining what we see in these galaxies, and so that's a bit of a puzzle, right. Mind also fails to explain lots of other stuff in the universe. Lots of reasons why we think dark matter is a better sort of overall picture than mind. But this is one place where mind does better than dark matter.
What are some of these ideas then that maybe dark matter does have some strange interaction with itself, or maybe there's like a dark matter sun in the middle of that galaxy blowing out some of the dark matter stuff like that.
Yeah, one really interesting clue is that there's actually a lot of variation, Like they're not as cuspies you expect, but also these cores, there is a lot of diversity of these cores, so you see lots of different sort of shapes, and people wonder like, why would you get so many different shape if the only thing that's happening here is gravity. Gravity is pretty simple. It's not as complicated as like buryonic physics, you know, with photons and protons and electromagnetism, very complicated. Dark matter should create simpler structures. And so one idea is just what you suggested, that maybe dark matter has some complicated self interaction that we don't know about that's creating interesting sorts of structures in the hearts of these galaxies that we just can't see because it's all made out of dark matter. So it's sort of like the cutting edge of current research is to try to understand what's going on at the hearts of these dwarf galaxies. What is dark matter doing right?
Right? It could be fororing like rose chocolate bars, for.
All you know, right, huge cups of rose chocolate cosmic hot cocoa could just be out there waiting for us to sip them.
But then if you're out there sipping them, you're also looking at the stars, but you're inside of the stars.
Kind of oh my gosh, I don't even know what to do, I'd have to be camping at the same time.
Yeah, camping in space, space camping.
That's absolutely the title of my new science fiction TV series that I'm pitching to Netflix.
There you go. Well, I think they already have space camp, of course, but maybe you can get away with trademarking camp.
This is why we have lawyers. They'll figure it out.
Yeah, they'll figure it out, all right. Well, another interesting journey into a corner of the universe that maybe a lot of people don't pay attention to, but that could actually reveal a lot about how things work. Dwarf galaxies.
And remember that as we develop better and more powerful technological eyeballs to look out into the universe, we see fainter stuff and smaller stuff, which might hold some of the answers to some of the enduring mysteries we've been puzzling over for a long time.
So the next time you're out there camping or not, or looking at the stars or not, or drinking hot chocolate or not, you can do those three things independently. Think about the little structures of the universe out there and how they maybe have special properties that can really kind of reveal some of the more interesting inner workings of the universe.
I have one last question for you before we sign off for here. Do you prefer a couple of hot cocoa or hot vanilla?
Hot vanilla? You mean like pure vanilla extract?
I don't know. You said vanilla's your favor flavor, better than chocolate. So what's a delicious vanilla beverage to enjoy in a camping trip?
Oh? Boy, yeah, just warm milk. I think it's just called warm milk.
Somebody invented that too.
Man, Yeah, yeah, vanilla milk. Check. I'll take that camping any day. All right. Well, we hope you enjoyed that. Thanks for joining us, See you next time.
Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact, but the people and the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions house US dairy tackling greenhouse gases. Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
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