What can we learn from dwarf galaxies?

Daniel 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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Episode Transcript

Speaker 1 (00:08):
Daniel, what's your favorite thing about looking up at the stars?
You mean, other than the hot coco? Did you always
drink hot coco when you look at the stars? Do
you have like a peplold in response? There? Can you
see stars if you're not holding hot coco? I've never tried.
It might steam up your glasses. No, But I mean
like about the actual stars, not not what you're drinking
when you look at them. I don't know. I love

(00:29):
how big everything is up there, the stars, the galaxies,
all of us, 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

(00:52):
way I like my hot coco. Big and dark him
or handy cartoonists and the creator of PhD comics. Hi,

(01:14):
I'm Daniel. I'm a particle businessist and a professor at
uc Herbhine, 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 cocoa. You know you can do all
those three things independently. What that's not true. You can't

(01:38):
just make hot coco at home in your living room
or look at the stars. Next, you're telling me you
can like make s'mores over your range. You could even
do it on the microwave. What you have under those
super smart microwaves? Did you got for free? I think
that would cause offense to the fundamental nature of space
and time making smores in the microwave. Yes, you'd be

(01:58):
blacklisted by the Boy and Girls Couts. 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

(02:20):
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 twoing and frowing that's happening up there in
the night sky, and to explain all of it to you,
that's right. We'd give you some more of this amazing universe,
all the hot stuff, all the cold stuff, all of

(02:41):
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 is 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 black page. The

(03:01):
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 flavors 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

(03:24):
little gas clouds all the way up to supermassive galaxies.
That's right, All of the big stuff in the universe
usually gets all the big headlines. People mostly pay attention
to supermassive 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

(03:46):
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

(04:07):
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 the
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

(04:27):
the universe then 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 they 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

(04:51):
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

(05:12):
and have been listening to podcast for a while and
would like to share your voice for everybody else, please
don't be shy right to us two questions at daniel
an Horgay dot com. So think about it for a second.
What do you think our 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

(05:32):
up by a big galaxy. I would imagine at 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 a tiny little galaxies, not that big.

(05:55):
I don't know. Maybe a dwarf galaxy is a galaxy
with not enough mass to be considered a galaxy, like
what happened to Pluto. I don't know. I'm guessing land
or 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

(06:19):
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

(06:43):
different shape, maybe 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, straightforward answers here, everyone said they're just a galaxies,
but smaller. It's like a dwarf serving of ice cream

(07:07):
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 oldest diet. Well that's a thing
on the internet, right, there are all these videos of
people making like little, tiny, like lego sized foods. Really
like instead of having a saying which you just have
like a tiny little sandwich. Yeah, there's this whole genre

(07:27):
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 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

(07:47):
a dwarf galaxy. So everybody was basically right, Dwarf galaxies
are a little cute galaxies. Because it turns out that
galaxies come in all sorts of sizes. We tend to
think about galaxies in term of ones like our own
the Milky Way that has hundreds of billions of stars,
Like galaxies that are much much bigger than the Milky Way,

(08:08):
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, but like, how big
is our galaxy? What are the size ranges that qualify
a galaxy is a dwarf galaxy? So our galaxy has
somewhere around two hundred to four hundred billion stars. That's

(08:29):
a really difficult number to wrap your mind around. Two
hundred 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 pointier soul stars just for themselves in the Milky Way.
It's really an incredible number of stars out there, hold

(08:49):
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

(09:09):
podcast and I will give you a star in return. Now,
do you offer free home delivery for 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
gonna cost you ten trillion dollars to deliver it to
your house and also the life of very human on Earth.
M Yeah, And in this case, it's not just the shipping,
it's the handling, right, because that's particularly tricky when you're

(09:31):
dealing with something several thousand degrees kelvin. 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?

(09:53):
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 of is about a billion light years away.

(10:14):
It's called I See one one oh 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 a 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,

(10:35):
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

(10:57):
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, Well, I'm not
sure trees are collecting stars these days, but you're welcome

(11:19):
to assign a star for every tree in the galaxy.
Any tree that donates to the podcast, I will email
them a certificate of ownership. They technic 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

(11:42):
a question is like, is there an upper limit to
the size of a galaxy or 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 fundamental the history of the universe is that galaxy

(12:02):
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,

(12:24):
and that expansion is accelerating, so it's increasing the distances
between galaxies, so it sort of like keeps the galaxy
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 universes started
accelerating right in terms of its expansion, So maybe we

(12:46):
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

(13:08):
we have superclusters which are sort of on the edge
of whether gravity can hold them together or dark energy
will rip 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

(13:29):
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 or 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

(13:50):
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 contract
and then we'll 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

(14:13):
the way that it has that's 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.

(14:35):
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,

(14:56):
as you say, into one big superstructure, one megaga. 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,

(15:16):
Like at the beginning of the universe, everything was spread out,
but then these things started to clump together, and so
everything started with 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

(15:37):
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,

(16:00):
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, and
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

(16:22):
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 kind of
things expand from an early characteristic quantum fluctuation size lown
up into something macroscopic, which, as you say, then determines

(16:46):
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 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

(17:08):
characteristic size where galaxies start, and so that's on the
little end, and so these dwarf galaxies are basically on
that smaller end of little gravitational clumps that formed little
stellar neighborhoods. And 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

(17:30):
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

(17:51):
enormous spectrum of size there, from really just a handful
of stars all the way up to billions of stars. Mm. 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 form. But first let's take
a quick break. All right, we're talking about dwarf galaxies,

(18:22):
and we talked a little bit about how basically dwarf
galaxies were the og galaxies in the universe, right like,
at the beginning of time, every galaxy was a dwarf back. 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

(18:44):
maybe this nice spiral shape or form that the Milky
Way has. It's actually 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 disc 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

(19:06):
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 disc. Now, when dwarf galaxies
merge together to make bigger galaxies, then you have like
disc 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

(19:29):
why like the Milky Way is mostly a disc. All right. Well,
then like how many. I guess that question is like
how many dwarf galaxies do you need to come together
to make a galaxy like the Milkyway, Because you're saying
the Milky Way 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,

(19:50):
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 smooth together into one big galaxy.
But you said the range is between like a dwarf
galaxy is between a thousand and several billion. Yeah, Well,
you know, this is one of those sort of artificial
distinctions in astronomy, like what do you call a galaxy

(20:12):
and what do you call a dwarf galaxy. There's this
threshold of a few Above a few billion or a
few tens of billions of stars, it's called a galaxy,
and below that it's called a dwarf galaxy. For example,
the large Magellanic 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,

(20:32):
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. I feel like, if
you have a thousand stars, maybe you shouldn't be called
a galaxy, you know, that's such a that's more like

(20:56):
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

(21:18):
see and then discovering, oh, that wasn't representative. It turns
out we need to revise our whole picture of how
things work. And so we've been seeing the biggest, brightest,
most exciting galaxies, but there 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,

(21:38):
there could be aliens out there in one of these
megan galaxies looking at us and saying, that's not a
real galaxy. It only has four hundred billion stars. That's
nothing exactly. Remember if we demote other galaxies that they
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

(21:59):
the universe started with. Hundreds or thousands or maybe even
millions of dwarf galaxies have been smooshed 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

(22:20):
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 you're saying? But then
within that giant cloud, little galaxies form that then eventually
clumped together. Well, I mean they clumped together eventually, the

(22:40):
same way that for example, Andromeda and the Milky Way
will eventually merge in a few billion years. Gravity is
inexorably 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 and drama in tribute to the

(23:00):
greatest flavor. Right. Sounds good? And then you can ask like, well,
you know, did the Milky waan 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.

(23:21):
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 gas they 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

(23:42):
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

(24:02):
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 are 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 slurped up by the

(24:25):
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 gets slurped up. WHOA, how many
satellite galaxies do we have? That's an interesting and complicated question.

(24:45):
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 ga taxies 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

(25:06):
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 interesting. So you're saying that, like, we
run a simulation of the universe based on what we

(25:27):
know and then explains the big stuff out there, like
the galaxy superclusters and the bubbles 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 formations of a bunch of little ones,
but not all the little ones should get slurped up

(25:49):
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, try
to spot them. We've seen some of them, but we
don't see as many as we expect. It sounds like
there's something wrong with the simulation, not necessarily with the universe.

(26:09):
You know, 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 done something wrong and
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

(26:31):
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 hundred thousands or
maybe millions of stars in them, so they are much
fainter than other galaxies, which make them more challenging to spot.

(26:54):
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

(27:15):
it harder to see out of the Milky Way because
there's so many stars and 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

(27:37):
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

(27:58):
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 Andromeda galaxy, do you also see less
or fewer dwarf galaxies than you think you would? Yeah,
it's a problem everywhere. It's harder to study for more
distant galaxies because they aren't more distant, and these galaxies

(28:21):
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

(28:42):
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 far 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 distant Oh, I see that's the real mystery.

(29:02):
I just change how you call them, then done. You
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

(29:24):
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

(29:44):
you saying, like a bunch of dark matter with a
few stars in it, that's a galaxy. Stick, 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 stuff out there in space that's maybe separate
from other clubs of stuff. Yeah, But then you get

(30:05):
in the question 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 That's why
I'm confused. Now, 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,

(30:26):
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.

(30:50):
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 every thing 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.

(31:10):
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 usually change your name from physicists to stuffists,
stuff is this, 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,

(31:32):
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 is
just waiting to be discovered. Interesting. All right, Well, let's

(31:52):
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. All right, we're

(32:18):
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

(32:40):
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 a blob of
dark matter and a blob of normal matter you know, gas, etc.

(33:01):
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

(33:22):
sprinkles of stars, like by mass, where the Milky Ways
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

(33:43):
river of dark deliciousness with a few sprinkles of marshmallows,
like the stars are the marshmallows on top of the
dark matter hot coco give means more. 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 matter. Right, All dwarf galaxies
are just these last few that we found. Most dwarf

(34:04):
galaxies are overwhelmingly dark matter. There's are 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 if you
have a smaller clump of stuff, you have less gravity
sort of holding those initial stars together because stars are

(34:28):
sort of poison to other stars. Like, what happens when
you form stars is you get a bunch of radiations
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

(34:49):
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 supernova, it basically blows
out all the gas from a dwarf galaxy. But if
you have a big enough clump, then they can retain
that gas anyway. Right, So as you're sort of serving
of gravity, you get smaller, you get too small to

(35:09):
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 a star will explode, but
the dark messines it doesn't interact with dark matter, and
the dark matter doesn't care, but it will blow out
all the other star stuff that's in that Meni galaxy.

(35:31):
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, 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

(35:51):
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
the much earlier part of the universe. But also they
are these very cool blobs of dark matter. And you know,

(36:12):
dark matter a continuing source of mystery and consternation for
a 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 spending more than

(36:35):
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
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 discover dark matter. We saw that stars are

(36:55):
moving really really fast, but that there's not enough stuff
in the galaxy to hold them together 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

(37:16):
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 milkway, like what's

(37:38):
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

(38:00):
enter the galaxy. Everything between us and the center is
about fifty 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 arout and at some point it's only
dark matter. I guess anytime you're in between stars, you're

(38:22):
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. Were actually 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,

(38:44):
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 Milky Way galaxy
has kind of like a higher concentration of dark matter

(39:04):
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
board dark matter on average and Andromeda. Andromeda 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

(39:25):
the dwarf ones, they're really fun ways to study dark
matter because what 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
of like pointing their telescopes at these dwarf galaxies to
try to see signals from the dark matter. Oh, I

(39:48):
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. Point your telescope too, and say, okay,
I know for sure there's a lot of dark matter
in this one spot. It's the dark matter doing anything
interesting that might tell us a little bit about what

(40:10):
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

(40:30):
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 raise. 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 raise. But you

(40:52):
like swamped in gamma rays from other stuff. Dwarf galaxies
have very little other stuff. They're mostly dark matter. So
if you point 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

(41:14):
they've given us some really powerful limits telling us what
dark matter doesn't do. Wait, are you saying that dark
matter it might be actually shining and might a 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

(41:35):
dark matter. You know, people complain to me sometimes like
you guys are still looking for dark matter. You haven'
found it? When are you 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

(41:56):
be called dark matter? I look forward to having that
argument with you when we collect our Will Prize for
discovering dark matter. Well, if dark matter doesn't midlight, it's
gonna be kind of dim and it's gonna be a
dark ish and it's gonna be sort of red shifted,
right because these galaxies are probably moving away from us,
which means that you could technically call it chocolate. And

(42:16):
if it's red shifted, it should be like rose chocolate matter, right,
Is that a thing? Is rose chocolate a 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.
M Well, there you go. Physicists are inventing new things
all the time. That was definitely not a physics invention.
I think it was Nestlie that came out with it.

(42:38):
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

(42:58):
really clumped, is it smooth 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.
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

(43:18):
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

(43:38):
we see weird stuff that we don't understand. What I 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 get as
peaky towards the core. The density of dark matter at

(43:59):
the very part of these 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

(44:20):
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. Yeah,
so yeah, over time it will tend to clump more
and more and more. But it hasn't clumped as much
as we expected. 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

(44:41):
dark matter and explaining the large structure of the universe.
And some people think it's a good argument for Mond
one 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
the different accelerations works differently than we expected, and the
whole dark matter thing is mistake. And it's true that

(45:01):
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 Mond 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 MOND, But this is

(45:23):
one place where Mond 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

(45:44):
also these cores 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
baryonic physics, you know, with photons and protons and electromaticism.
It's very complicated. Dark matter should create simpler structures. And

(46:04):
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?

(46:27):
It could be boring like rose chocolate bars for all
you know, right, huge cups of rose chocolate cosmic hot
coco 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 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

(46:49):
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 but maybe you can get
away with trademarking. 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

(47:11):
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

(47:31):
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

(47:53):
off for him. Do you prefer a cup of hot
cocoa or hot vanilla? Hot vanilla? You mean like pure
vanilla extract? I don't know. You said nila is your
favorite flavor, better than chocolate. So what's a delicious vanilla
beverage you enjoy on 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 milkshake. I'll

(48:17):
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. Or more podcast

(48:37):
from my heart Radio. Visit the iHeartRadio app, Apple Podcasts,
or wherever you listen to your favorite shows.