Could dark matter be a superfluid?

Daniel and Jorge talk about whether a new idea for dark matter might overcome some discrepancies in galaxy behavior.

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Episode Transcript

Speaker 1 (00:08):
Jorge, I have an idea for our Daniel and Jorge
explain the universe food truck. Oh nice, wait, are reactually
doing that? I thought it was a joke. Well, it
was a joke, but then our listener Tim Lazarov road
in to ask when our food truck is going to
be in his neighborhood. May should be more like a
food spaceship it would be more appropriate. Well, that goes
perfectly well with my idea. You see, normal food trucks

(00:31):
sort of drinks right fluids to wash down their taste treats.
But our food spaceship should serve super fluids like zero
viscosity beverages nice, so they go down easier. Or do
they have Bose Einstein condensation on the outside. I'm thinking
we should test it on some undergrads, you know, make
sure it's safe before we sell it to the public.

(00:51):
I'm sure the FDA is all over that. I don't
think they have a physics food division yet. That's the
p d A Physics and Drugs Administration. Hi, I'm or

(01:14):
hand me a a cartoonists and the co author of Frequently
Asked Questions about the Universe. Hi I'm Daniel. I'm a
physics professor and a particle physicist who does research at CERN.
And every time I'm in a lab, I'm always tempted
to taste everything that sounds like a terrible Daniel, especially
if you visit a virology lab. Yeah, but you know
they got like weird glowing goo. You're like, m I

(01:37):
wonder what flavor that is. I'm never gonna do it,
of course, I hope, but you know, the curious mind wonders, right,
Remind me never to invite you to my lab, otherwise
you'll be licking everything like a dog or a two
year old, like that kid in the movie that licks
the flagpole and wonders if his tongue is really going
to stick to it. We should get into the physics

(01:57):
of a Christmas story will shoot your eyes out. But
welcome to our podcast Daniel and Jorge Explain the Universe,
a production of I Heart Radio in which we invite
you to taste the entire universe to enjoy the flavor
of knowledge as well as ignorance, to take a bite
out of everything that we do and do not know
about the universe. We don't shy away from the big mysteries.

(02:20):
On this podcast. We talk about the smallest things, the
medium sized things, and even the biggest things, and we
attack all of them and try to explain them to you. Yeah,
it is a delicious looking universe, like a giant cosmic
bouffet of amazing ideas and incredible phenomena that are there
for us to dig into and fill our bellies with
amazing knowledge. Yeah. And if you're not a curious person,

(02:43):
then you wouldn't take a bite to these weird things.
And I think that's why when I'm in some kind
of laboratory and I see something weird bubbling in a
flask and I wonder, I wonder if that would make
a good soda for our food truck, It's that same
curiosity that inspires me to try to take a bite
out of the whole universe because I want to know.
I want to understand. It's not enough to just say
I bet that green bubbling thing tastes something like lemon

(03:05):
lime soda. I want to actually know the truth. I
think there are easier ways to find out what something is, Daniel,
rather than bringing it in your mouth and in your body,
Have you tried asking what it is? It seems like
the polite thing to do. But then how would they know,
right if they haven't tasted it. Maybe some questions are
not meant to be answered, What like, are you curious
about how sanide tastes? I am curious? Actually, you know,

(03:27):
I like almond pastries and so hey, maybe you know
a nice cyanide after flavor is not the worst thing
in the world. But yeah, I'm curious about all this stuff,
even the stuff that might kill you to find out
the answer. It's just this deep desire to know these truths. Well,
speaking of dark matters like tasting poison, there are amazing
mysteries out there, including one that is maybe one of

(03:47):
the biggest mysteries in the universe. At least it's the
second biggest mystery in the universe by percentage. Boys, right,
that's right. If the universe was laid out as a buffet,
most of it would be dark energy, but a huge,
heaping pile of it would be dark arc matter. Most
of the actual stuff in the universe, the matter, the things,
the bits and pieces that move around in our universe,

(04:08):
are not the things that make up me and you
and weird beakers of bubbling green goo in chemistry laboratories,
it's something else, something different, something we do not yet understand,
but we have a cool name for it. That's right,
we have a cool name for it. It's dark matter.
And that is an interesting analogy, Daniel. I guess the
universe is like a giant buffet, and five percent of

(04:29):
it is like regular food, right, chicken and bread and
pasta slad, And about twenty seven percent of that buffet
is a big giant mystery, some kind of weird dark stuff.
And I guess you would be right in there tiling
it on on your plate. Yeah, I don't know if
I'll go back for seconds. You know, I got to
try the first serving, but yes, serve me up some

(04:50):
dark matter. I want to know what it is. Does
it in the end just taste like chicken? Would it taste?
I guess it would just go through your tongue, wouldn't it.
That's right. Dark matter sounds like something black and heavy,
but actually it's invisible and intangible. Dark matter would pass
right through you like a cloud of neutrinos, because we
don't think that it interacts with normal matter in any

(05:12):
way other than with gravity, and gravity is a very
very weak force, so you couldn't even like pick up
a spoon of dark matter. You had like a blob
of dark matter, and you dropped it, it would fall
to the center of the Earth, right, And we don't
even know if it is matter. We don't even know
if it is stuff. All we know about it is
its effect on the rest of the universe and the

(05:33):
rest of the stars and galaxies out there that we
can see. That's right. We have never confirmed the particle
nature of dark matter. We don't even really know a
hundred percent that it's stuff. And we've talked about it
on the podcast a lot of times and we've said
that we are pretty sure it's matter, but there are
some questions that remain. There are some things that we
see out there in the universe that the idea that
dark matter is stuff is some kind of invisible new

(05:55):
matter in the universe doesn't quite explain. And there are
a few other our ideas, different hypotheses to try to
explain it. And I know that it's one of the
favorite pastime of our listeners because they're always writing the
emails about maybe dark matter is actually this other thing,
or what if dark matter it could be something else
totally different. It is one of the biggest most accessible
mysteries in the universe. That's right. Maybe it is a

(06:18):
giant tree of some kind of cosmic buffet for some
giant beings. Perhaps. Yeah, maybe it's just like weird pasta
instead of squidding, because they put something else into it
to make it like really dark or invisible. But I
guess what you mean, right, Yeah, if there's some kind
of animal, let's bray something that makes it invisible, maybe
they've just inserted that into the dark matter pasta. Well,

(06:38):
hopefully it's more like the dessert of the universe, because
that would be pretty neat, right. A third of the
universe is just desserts. Does that tell us about your diet?
Or Hey, are you like desserts? Sure? Why not? I
guess it all depends on your perspective. Vegetables can't be dessert, right, sure, yeah,
I guess so I like a nice zucchini bread. Yeah, which,

(06:58):
it's that's technically what whread you eat at the end? Right,
sounds good. Well, I'm hoping that we can gobble of
the mystery of dark matter one day, but until then,
we have to think carefully about what we have seen,
what we know, what we can explain, what we can't
explain and what new ideas we might need to tell
a complete and true story about everything that's happening out
there in the universe. Yeah, it is an ongoing debate

(07:20):
about what dark matter is, and it's an ongoing exploration
of what it could be. So today we'll be talking
about one possible idea for dark matter. So today on
the program, we'll be tackling the question could dark matter
be a super fluid? And if so, is it's sparkling?

(07:41):
And will it kill you? I guess if you drink
it question number two? Right, First, I want to know
if it's carbonated, and then question number two is can
I survive drinking it? Well, it dissolve all the carbon
in your body is the second question. I guess you
would want to know before you drink anything, unless you're
Daniel whites It. Yeah, and it's a really interesting question.
Other dark matter is a super fluid, and you might

(08:02):
wonder like, why do we care? Why do we think
it might be a super fluid. And for all the
successes that dark matter has had in explaining the large
scale structure of the universe and gravitational lensing and the
wiggles we see in the cosmic microwave background radiation. We'll
talk about it in a minute, But there are some
things that dark matter as a theory of some weird
invisible particle really struggles to explain in our universe. It

(08:25):
needs a little bit of help. Yeah, and I feel
like we maye skipped the question here, Like I feel
like we never even tackled the question could dark matter
be a fluid? Like do we even know if it
could be a fluid or a solid or gas? You will, Actually,
dark matter already we think is kind of a collisionless fluid,
sort of like an ideal gas. You know, we think
about it as these particles flying around in the universe,
not interacting with each other at all, because again, the

(08:48):
only interactions we think it has is gravity, and gravity
between particles is basically zero. So already we think of
dark matter sort of like a collisionless fluid or an
ideal gas. So here we're talking about it having like
special produced from being a super fluid like a superhero.
But shouldn't be like a super gas then? Yeah, exactly,
Maybe it gets bitten by a radioactive spider and then

(09:09):
turns into a super fluid or super gas. Yea radioactive
fluid spider, I guess it would have to be the
marvel theory of the universe. Well, as usually, we were
wondering how many people out there had considered this question
whether dark matter could be a super fluid. So Daniel
went out there into the wilds of the internet, or
maybe the campus of UC Irvine, which one this time? Daniel,
these are Internet answers, So thanks very much to everybody

(09:32):
who participates in these and waits patiently for us to
get to the episode. If you'd like to participate for
future episodes, please don't be shy right to us. Two
questions at Daniel and Jorge dot com. So think about
it for a second. Do you think dark matter could
be a super fluid? Here's what people had to say. Well,
I wish I knew what a super fluid was, because

(09:52):
that sounds like a really awesome thing to get to know.
The dark matter could be a lot of different things
still at this point, um, so sure it could be
a super fluid. It could be a great soft drink
that sadly just passes right through your body. I don't
really think that's true because the other superfluids that we've
created are all made out of hawks, they're just in

(10:12):
a different arrangement, and I don't think quarks can, you know,
display the behaviors that dark matter does. So no, I
don't think that dark matter is a super fluid, but
who knows, well, what's a super fluid. I believe that's
material that doesn't lose energy when it's moving, So it
sounds like document that would really hit that point because
it doesn't even inter fear with itself. On the other hand,

(10:34):
we know that doc meto is impacted by gravity, so
that would speak against it. So I'm not really sure.
But if I would have to make a bed, I
would say, yeah, it's a super fluid. I don't see
how can anything out in space be considered a fluid
given how low density it is, and if it isn't
a fluid, it could be a super fluid. No idea.

(10:55):
That's fascinating. I would not have thought dark matter was
dense enough to be a fluid in the sense that
we understand it. Um my understanding that dart matter is
going to be as diffuse as regular matter and its
distribution through a nearly infinite universe, so that it's going
to behave like a guess. I would think if I
remember correctly, a super fluid doesn't as a fluid fluid

(11:17):
without viscosity. I think it was viscosity. And well, we
know dark matter as supposed to interact with matter only
through gravity. But I suppose if it interacts any through gravity,
that implies some form of attraction, which means, if we
regarded as a fluid, it must have some sort of viscosity, which,
if I remember the definition of a superfluid currictly means

(11:38):
that can't be a super fluid. Sure, I guess it
could be anything. We really don't know much about dark
matter other than it exists and it has gravity. Um,
so why not? Could be, could be super fluid, could be,
could be anything? Really m M interesting answers some skepticism.
Some people were like, I don't know, don't think so

(12:01):
this one sounds weird to people. I think the idea
of having like a fluid out in space sounds weird.
People think of spaces like cold and mostly empty, maybe
filled with tiny little crystals or particles flying around, But
like a fluid is a weird thing to think about
having in space, right, And it seems like a lot
of people are like, maybe it could be a fluid,
but a super fluid, I don't know if I would

(12:23):
give it, you know, supernatural powers. Yeah, well, at least
one guy is ready to taste it, though, you know
he's thinking about super soft drinks. Hmm, interesting, super soda,
super dark soda. Exactly. It's very massive. It's like Coca
cola dark coke. Give us a call. We got ideas,
special dark recipe. But a lot of people didn't seem
to know what even a super fluid is. I guess

(12:46):
that's not a common word even I'm not sure what
it quite means. Yeah, super fluids are not the kind
of thing you have experience with that you don't see
them in your everyday life. The river that runs through
the park in the middle of your town doesn't ever
become a super fluid. You don't make super fluids in
your kitchen. They're like a weird quantum state of matter
that we only recently even we're sure could exist. And

(13:09):
so it's a sort of a new theoretical idea, and
whenever that happens, people have fun applying into like, oh,
this is new and cool. Maybe this works also over
here in this different part of physics where there's something
we don't understand we have a new hammer. So let's
see what else could be a nail. Yeah, it seems
like maybe dark matter being a super fluid could maybe
explain some of the things we can't quite figure out

(13:29):
about it. And so Dania, let's step people through this.
First of all, I guess let's recap what dark matter
is and what is it that we don't understand about
it that is making us consider this idea. So we
think that the universe has a bunch of invisible matter
in it because we see a lot of gravity out
there in the universe that we can't explain from visible matter.
You know, we know that stars are huge balls of

(13:52):
hydrogen and they have a lot of mass, and so
they have a lot of gravity, and the Earth spins
around the Sun, for example, because of the mass of
all that grab But if you look at a galaxy
and you add up all of the mass from all
the stars that you can see, you can't explain all
the gravity that's happening in the galaxy. Like the galaxy
is spinning like all galaxies do, and that spinning would

(14:12):
tend to toss stars into outer space the way like
ping pong balls on a Merry Go Round. If you spin,
it would toss those ping pong balls out past the
Merry Go Round, But the gravity of the galaxy keeps
those stars in place. That's why this Milky Way is
not just like throwing all of our stars away. But
the galaxies are spinning really really fast, and in order
to hold them in place, they need more gravity than

(14:34):
we can account for. The gravity from the stars we
can see doesn't give us enough gravity to hold the
galaxy together. And on a bigger scale, we also need
to hold galaxy clusters together. Galaxy clusters are big groups
of galaxies, and we don't think those galaxies seem to
have enough mass to hold themselves together. Galaxy clusters are
also spinning. But if you add a bunch of invisible

(14:57):
mass to the galaxies, then it all works because it's invisible,
explains why you can't see it, and it's new matter,
so it adds more gravity, and so it solves those
problems if you add this weird new invisible stuff to
the universe. The weirdest thing about it, though, is that
you need much more dark matter than visible stuff. It's
not like you just add a little sprinkling of dark matter.

(15:19):
You need to take every star and add five stars
worth of dark matter to explain all the missing gravity. Yeah,
it's it's almost like dark matter is kind of like
the missing piece. And what we see of the universe, right,
and the way the galaxies stick together and the galaxy
clusters stick together, they stick together more than they should
given what we can see in them. And so one

(15:40):
solution is that maybe there's invisible stuff out there that's
holding it together. Yeah, and this is the kind of
thing we're always trying to do, is reconcile everything we see.
We think we understand how gravity works. Let's check it.
Let's make sure that our explanation makes sense, that it
works for this scenario. And that's how we discovered oh
my gosh, it doesn't. And that was the clue that
maybe it was something else going on, or there was

(16:02):
something else new out there. But before you believe that,
just like crazy new idea that the universe is filled
with a huge amount of invisible stuff you just happened
and never noticed before, you want other pieces of evidence.
And so we have other clues that dark matter might
be real. We see it affecting the way light moves
through the universe because if it has mass, it changes

(16:22):
the curvature of space, and so it can lends light.
We see that it existed in the very early universe
because it affected the wiggles in the cosmic microwave background radiation,
these photons from the plasma that filled the universe very
very early on. And we also know that affected the
whole way that the universe form, the large scale structure
of the universe. Galaxies we think wouldn't even exist if

(16:45):
dark matter hadn't created little gravitational wells to pull stars
into mostly all hangs together into a very nice story, right,
But it's kind of interesting. I guess that it's a
story we made up assuming that what we know of
the rest of the universe is true. Right, Like, if
we assume that the laws of fixed work the way
we think they do, then you sort of need this
invisible matter to just make what we see make sense.

(17:07):
But that's only assuming that we're right about the laws
of physics. Yeah, that's saying gravity works a certain way,
and so in order to explain this missing gravity, we
need more mass. But you're right, there are other ideas.
People have also thought, well, maybe gravity doesn't work the
way that we thought maybe there isn't any missing gravity.
It's just signs that our theory of gravity is wrong.

(17:28):
And people have tried to modify the theory of gravity
to explain what we see, and this is called Mond
modified Newtonian dynamics. The theory says that instead of gravity
going like one over distance square the way Newton said,
there's another factor there that when things have very low accelerations,
gravity is a little bit different. It gets a little stronger.

(17:48):
So if you tweak gravity in just this way, you
can also explain how galaxies rotate without using dark matter.
That's really interesting. So it could be that dark matter
is not dark matter at all, Like maybe we just
have the laws of physics a little bit wrong. But
does that explain everything about the way the galaxies stick
together and even the gravitational lensing. No, So Mond, this

(18:09):
alternative theory gravity does in fact a better job at
explaining how galaxies rotate than dark matter does. There's some
things about galaxy rotations that dark matter just can't seem
to get right, but it doesn't explain everything else that
dark matter does. Like Mond does not do a good
job of describing how galaxy clusters rotate and spin. Around themselves,
and it's much more difficult for it to explain, like

(18:31):
the cosmic microwave background and lensing and all sorts of
other very strong evidence for dark matter. So mond is
sort of a nice idea you can explain one thing
actually better than dark matter can. But dark matter is
sort of like a stronger idea across the board. But
neither of them, I guess, is perfect. And let's maybe
dig into that a little bit, like what is it
about the idea that dark matter is invisible stuff that

(18:53):
doesn't explain what we see out there? So by now
we've seen a lot of galaxies, and what we try
to do is understand like how am my dark matter
is in a galaxy versus how much normal matter? And
is that common across galaxies? Like do all galaxies have
the same amount of dark matter and the same amount
of normal matter? And we also try to understand how
that could have happened. You know, if dark matter is

(19:13):
this weird particle, this new heavy, invisible thing, then it
would have clustered together and we can make models for
how that would have happened informed galaxies. We look out
into the universe, the galaxies we see don't really line
up with what we expect for dark matter. So one
thing specifically we look at is a relationship between how
bright the galaxies are and how fast they are spinning.
How bright they are is really interesting because it tells

(19:35):
us like how much normal matter is there, how many stars,
how fast they're spinning should tell us something about how
much dark matter there is in the galaxy. And if
you do a bunch of simulations, then you expect like
a loose relationship there. You expect, like some galaxies to
have a lot of dark matter and something to have
a little bit, but there to be a lot of variation.
What we see when we look at these galaxies, though,
is that there's a very very tight relationship. It's like

(19:58):
almost no variation, like the amount of stars in a
galaxy and amount of dark matter and the galaxy has
a very very close relationship, which we think is weird
and we can't explain with our models. You mean, like
when you look at the galaxies out there in the universe,
they almost have the same proportion of regular matter and
dark matter. I think that's what you're saying, right, Like,
there aren't galaxies out there with a lot of dark matter,

(20:18):
and there aren't a lot of galaxies with a little
bit of dark matter, which is weird. But I guess
to me, it's weird that you would think it's weird.
Why wouldn't they all it's sort of the same. But
they were all made in the Big Bang, you know,
because there's a random element here, right, Like, how do
galaxies form? Anyway? It comes from a quantum fluctuation in
the initial seeds of the universe that gave you a
slight over density in the dark matter that pulled together

(20:42):
a little well and then grab some stuff. And you know,
we do expect some relationship. We expect there to be
a relationship between the amount of matter and the amount
of dark matter, because in the beginning we think this
stuff is mostly evenly spread out. But we also expect
some variation. And when they do simulations to try to
predict what kind of variation we see and we run
all of our laws of physics, we see a much

(21:03):
wider variation in our simulations than we see out there
in the actual universe. That tells us like, maybe there's
something wrong with this theory. That's something that we're putting
into our simulations that isn't doing a good job of
describing what we're actually seeing. I think you're saying that
the ratio between dark matter and regular matter is too constant,
like it's too consistent across the board of the universe,

(21:25):
which means that maybe the problem is that what we
expect to see in the universe is wrong or something. Yeah,
because for that to happen, you might expect some sort
of interaction between them, for them to like turn back
and forth into each other, or interact with each other,
some process that's keeping them so tightly coupled. But we
think that dark matter and atomic matter don't interact except
through gravity, so we don't have a process for making

(21:46):
this happen. The alternative theory MOND actually predicts this perfectly,
like Mon says, there is no dark matter, there's just
normal matter, and gravity changes how things spin, and the
apparent rotation of velocity of these galaxies should be very
tightly connected to their brightness because the rotation velocity just
comes from stars. So the Mond prediction is like bang

(22:09):
on exactly what we see, whereas the dark matter prediction
is sort of like scattered all over the place and
doesn't do a good job of describing what we see.
For astronomy nerds out there. This is called the Tully
Fisher relation. M I think you're saying that maybe this
idea of Mond, that maybe our laws of physics are wrong,
does a better job of explaining the consistency of what

(22:29):
we see out there in the galaxies. Like, it makes
more sense that gravity works differently than we think it does,
and then it would be for other to be a
bunch of invisible mass. Yeah, it does a better job
of explaining the relationship between galaxy brightness and rotation that
we see in the universe. We see them much more
tightly coupled and connected in the universe than we would
expect if it was due to dark matter. If you've

(22:50):
got like a random sampling of how much dark matter
and how much normal matter, you expect there to be
more of a spread. But Mond predicts a very tight
relationship because there is no dark matter, So it does
a better job of predicting what we actually see out
there in the universe. This is like kind of a
headache for dark matter as a theory. Interesting, like it
has its failings. The idea that it's invisible matter, and

(23:11):
this idea that maybe the laws of physics are wrong
comes in and says, hey, I can fix that, but
maybe it doesn't fix everything, which is why it's still
not the prevailing theory. Yeah, exactly, But it's been like
a real thorn in the side of dark matter for
a long time and keeps a lot of people, I think,
from accepting this idea that dark matter might be real. Well,
there is a new theory, a new idea that maybe
would make dark matter make more sense, and that's the

(23:33):
idea that it maybe it's a super fluid. So let's
get into what a super fluid is and whether dark
matter could be one of these superpowered fluids. But first
let's take a quick break. All right, we're asking the question,

(23:57):
could dark matter be a super fluid? It? And we're
asking the question because there are some things about dark
matter that we can't quite explain out there. Even if
we assume it's some kind of stuff, some kind of particles,
some kind of fluid, it doesn't quite explain the ratio
of regular matter and dark matter we see out there
in the universe. Yeah, that's right, And so people are
trying to be creative. They're saying, like, dark matter by

(24:17):
itself doesn't quite work as a theory. Mond by itself
has lots more problems than dark matter by itself, like,
neither of them are perfect. Is there some way we
could take dark matter and make it a little Mond
dear right, to try to capture some of the things
that Mond has. Remember, the key feature of minds that
it changes the effect of gravity over some distances. So

(24:38):
people were like, well, is there any way we could
change dark matter or tweak dark matter so that it
basically has the same effect as Mond, but only in
these scenarios the hearts of galaxies where dark matter seems
to be having a problem all right, And so the
idea is that maybe dark matter is a super fluid,
which is like a superpowered fluid that was I don't know,
born in krypton or something inherited power ring from an am.

(25:01):
The superfluid is not from cartoons and not just from
science fiction. It's a real thing. A couple of Nobel
Prizes have been won already because of superfluids, and they're
called superfluids sort of analogy to superconductors, right. A superconductor
is something that conducts electricity, so you can send energy
down a wire if it's superconducting and lose none of
it right, you don't turn any of it into heat,

(25:23):
and so super fluid is similar. It's a liquid that flows,
but without any internal resistance, so the bits just sort
of slide by each other. It doesn't heat up at
all as it flows, doesn't lose energy. Like if you
take a bucket of water and you put your finger
in it and spin it, you get a little vortex
that's forming. But eventually that vortex will sort of peter
out right, the energy will diffuse and the water will

(25:45):
stop moving. In a superfluid, that doesn't happen. You start
a vortex and it just spins forever. Wow, it's super interesting.
I guess that's the idea of zero viscosity, which means
no friction between the molecules of the fluid. But I
guess maybe let's dig in a little. This might be interesting.
What exactly is viscosity or what exactly is friction? Like?
Where does that come from? So it comes from the

(26:06):
interaction of the bits inside of it. Right, when we
talk about like an ideal gas, we're talking about particles
flying through space, but we ignore the possibility that they
can bump off each other and exchange energy. In real life,
the particles inside of gas can bump off each other,
can exchange energy, and the same thing with a liquid.
In the case of a zero viscosity liquid a super fluid,

(26:27):
then the particles don't really bump off each other and
they can like change places without losing any of their energy.
What do you mean they don't bump into each other.
They do bump, but they don't lose energy, or they
don't bump at all, Like the things inside of the
liquid don't interact with it with the self. When you're
thinking about a liquid, it's sort of like an emergent object,
and it's easiest to think about it like layers of liquid.

(26:47):
Imagine like two layers of a liquid passing by each other,
and think about whether there's like friction between those layers. Say,
for example, say for example, you have a tube and
you're pushing some liquid through it, right, then if it's
a very vis gets liquid, it's going to flow more
rapidly near the center than near the walls, because I
get the fluid particles near the walls of the tube

(27:08):
are hitting the particles of the tube, right, and so
they lose energy. They're like bumping against the wall, and
it's not just that they're bumping against the wall. Think
about like, as the layers are passing by each other,
are the particles grabbing at each other? Like what is
friction anyway? Like if you run your finger along the surface,
why is kinetic energy getting turned into heat? Because the
particles in your finger are grabbing at the particles on

(27:30):
the table. There's little deformities and there's bonds between them
that are getting broken and reformed. And the same thing
happens inside liquid. When you have like layers of liquid
passing by each other, they have an interaction, then they
can grab at each other and sort of like slow
the next layer down and it's super fluid, that doesn't happen,
and the layers can sort of like pass by each
other without any friction at all. And why is it?

(27:53):
I guess it just depends on the interaction between the particles. Right.
It's not something you can do with the normal liquid
very easily. It's a quantum property, right, It's not something
you can really understand at an intuitive level just thinking
about little balls. Instead, you need to think about these
objects as quantum objects, which means you think about their
wave functions and when these things get really really cold,
then you have very little uncertainty on their temperature that

(28:14):
their wave functions grow really really wide, because the Heisenberg
and certainty principle tells you can't know something's momentum and
its location very very well. So when you cool something down,
its wave function grows very very large. So now instead
of having just like a bunch of little particles bouncing
around that you can sort of think of a particles,
you have these overlapping wave functions between these objects, and

(28:34):
they form like one big quantum state, and they tend
to move like all together instead of interacting with each other,
so they're like more tightly coupled to each other weirdly,
which gives this super fluid state. I see, I guess
that you're saying. I think the cold or something gets
the bigger the wave function of the particles gets, which
means that their things get fuzzier almost in a way,

(28:55):
right like instead of a little tiny ball, suddenly it's
more like a hazy blob. And it's kind of hard
maybe for too hazy blobs to really drag on each other.
Is that kind of what you're saying. Yeah, And it
acts more coherently instead of individual particles which you can
grab at each other. Now it's like a huge train
of particles that tend to move together rather than bumping
against each other. So it's like it's all much more coordinated. Now.

(29:17):
It's not like a random crowd of people bumping into
each other. Now it's like a tightly packed formation of
a marching band walking down the street. But they don't
bump into each other at all. They just sort of
like flu and so this happens. For example, here on Earth,
of you cool helium a lot, you get a Bose
Einstein concert which is super fluid. The first demonstration of
this was in super fluid helium. We get the Nobel

(29:38):
Prize and Physics at Stanford for that. That wasn't actually
a Bose Einstein condensate, but it is. A super fluid
can flow without losing any energy. Another example of a
super fluid is a Bose Einstein condensate. Another special state
of matter rape a whole podcast about that again, where
you're cooling atoms down in a trap to make them
very very cold and overlapping. So they have other weird

(29:58):
quantum properties as well, which include being a super fluid.
We also think we have seen super fluid sort of
indirectly inside the large Hadron collider. When we smash big
atoms together, like lead nuclei and gold nuclei, we can
make this state of matter called a cork gluon plasma.
And one of the features of it, we think, is
that there's a little bit of super fluidity, very very

(30:20):
briefly at the heart of that thing while it exists.
I mean, when you smash particles together, you get so
much density of energy, and these particles really packed together
that they behave like a super fluid for a tiny
little bit. Yeah, for a tiny little bit, even though
it's super duper hot. It's also so dense that these
particles undergo this new phase change into a cork gluon plasma,

(30:41):
which can also be a super fluid. So a super
fluid isn't like one unique state of matter. It's like
a description of a phase of matter, the way like
some phases of matter conduct electricity and some don't. Some
phases of matter are fluid and some are super fluid
and others aren't. So these are just examples of places
where we have seen super fluidity happening, Like really is
a thing in the universe. We're sure about that. That's

(31:02):
not a hypothetical thing, and we think it might even
be what's happening inside of neutron stars, right, Yeah, the
inside of neutron stars is a lot like a cork
glow on plasma. It's very very dense, and the particles
get squeezed together and their wave functions start to overlap.
And we don't really know what happens inside a neutron
star because it's some state of matter that we can't
really access anywhere else, and all the forces come into play,

(31:25):
including gravity and the strong force, and it really tests
our ability to even do calculations or predicular what might happen.
We think that weird states of matter like nuclear pasta
might occur, and there might also be super fluid states
inside the heart of a neutron star. But is there
generally like a recipe for making super fluids? Like, what's
the thing that makes all of these different examples? What

(31:46):
do they have in common? They all have in common
high density. So you squeeze these particles together basically so
that their wave functions are overlapping, and that's how you
achieve it. It's easier to do that if they're very
very low temperature because they're way from are larger. But
if you get high enough density, you can also achieve
it even at high temperatures, like the inside of a
neutron star. But the crucial thing is density. Wonever. It's like,

(32:09):
you know, taking a bunch of water balloons. When you
squeeze them together, they almost become super fluid. Haven't you
been to a birthday party recently? He saw this happen. Well, like,
if you have a bunch of water balloons out and
they're really far spread, far apart, they sort of behaved
like maybe like little particles, but if you sort of
packed them together in a bucket, they kind of act
like a fluid. Right, Yeah, that's true. I never thought

(32:31):
about what it's like for a bunch of water balloons
to slide out of a bucket. Have you like dumped
a bucket of water balloons on somebody's head before? Yeah? Yeah,
super super flat water balloons. Actually that sounds like super fun.
But you're exactly right. When simple things come together, they
can do new, weird things. And that's the whole amazing
size of chemistry. Right. We have these phases of matter

(32:52):
that come out of the way, these particles interact with
each other or don't interact with each other, and generate
these emergent properties electrical activity or shiny nous or liquid
phases or other weird phases of matter. It's incredible what
matter in the universe can do. The variety of things
that come out just of the basic laws from the
interactions at the microphysical level. It always amazes me. M Okay,

(33:13):
So that's what a super fluid is. And now the
idea is that maybe dark matter could be a super fluid.
So it's the idea that dark matter is made out
of little particles and then when you somehow get them
really close to each other, they behave like a super fluid. Yeah.
The idea is that dark matter is still dark matter.
It's still some particle that's invisible and intangible and doesn't

(33:33):
interact with us except through gravity. But you get enough
dark matter together under the densest conditions, maybe it's forming
a super fluid. And now we give it new properties
like those water balloons in the bucket. It can do
things when it's all together in those conditions that it
couldn't do otherwise. And the idea is that this new
super fluid state of dark matter might explain what's happening

(33:57):
inside galaxies that currently dark matter as a theory can't explain.
Interesting But I thought maybe dark matter didn't interact with itself.
So isn't it already a super fluid that doesn't have
any internal friction. Yeah, so currently we think that dark
matter doesn't have any any interactions. So you might think, oh, yeah,
dark matter out there in space is a collisionless fluid.
Isn't that also a super fluid, right, Not technically, because

(34:19):
they don't have overlapping wave functions. Like if you just
have a really dilute gas of dark matter, like we
think exists out there at the edge of the galaxy
and beyond, that's not really a super fluid because the
particles are just really far away from each other. To
have a super fluid have these new phenomena emerge, you
really have to have them close enough to each other,
so the wave functions overlap. So it's more about the

(34:40):
quantum overlap of the individual particles and less about the
kind of frictionless flow of the fluid for dark matter. Yeah,
And in this case, it's the overlap of those particles
that generates new phenomena like frictionless flow, and in the
case of dark matter super fluid, they think it can
effectively create a new force. What emerges from a dark

(35:01):
matter super fluid. It's sort of like a new force
which effectively can change the way that gravity works to
give you exactly the same behavior that we see in
the mond theory of dark matter. To me, the question
should be is dark matter a super duper fluid? All right, Well,
let's get into how dark matter being a super duper
fluid at the heart of galaxies could explain some of

(35:21):
the things we can understand or explain about the current
model of dark matter. But first let's take another quick break.
All right, we're asking the question could dark matter be

(35:43):
a super fluid? And we're asking the question because there
are things about dark matter that we can't quite explain.
I mean, we don't know what it is or what
it could be. But even our idea of it as
a particle, as a bit of matter, as bits of matter,
doesn't quite explain some of the things we see out
there in the universe. And so the idea is that
maybe dark matter is a super fluid, which might explain
these things. Yeah, and I think the kernel of the idea.

(36:05):
What generated it is noticing that dark matter as a
theory seems to have trouble in the densest situations. It
works really well between galaxy clusters, it works out there
in space for gravitational lensing of lots of galaxies. It
works in the early universe, but inside galaxy is currently
the densest places in the universe is where it struggles.

(36:26):
So people thought, well, maybe when dark matter gets denser,
it forms this new state, this super fluid, and then
we can figure out how to give this superfluid new properties.
Maybe you can solve the problems of dark matter inside
galaxies without breaking with dark matter is already so good
at explaining everywhere else, right, So the idea is that
it only forms a super fluid inside the dense environment

(36:47):
of a galaxy. Everywhere else it's just this normal, boring,
old dark matter, normal boring, but it's still quite mysterious
and elusive. They're saying that at the center of galaxies,
where things are pretty dense anyways, right, there are sometimes
black holes in the middle of galaxies. There are a
lot of stars clustered together. The ideas that maybe dark
matter is super duper compact at the center of galaxies

(37:08):
more so than like at the edges of galaxies, and
not just at the center, but yes, definitely denser at
the center. But essentially we're thinking about galaxies as like
a dense place in the universe. And as we talked
about earlier, how do you make a super fluid. You
need to get the particles close enough so their wave
functions overlapped and make the super fluid thing happen. And
so that can happen in a galaxy because there's a

(37:29):
lot of gravity there, it gathers together a lot of
dark matter. Another trick they pull is to make dark
matter very very very very low mass. We know like
how much mass the dark matter we need, but we
don't know how much mass each particle has. So if
you make dark matter out of really massive particles, you
have fewer of them. If you make dark matter out

(37:49):
of really low mass particles, you need more of them.
So the folks who are working on this theory say
that dark matter is really really low mass, then there's
a huge number of them, right, And so they're imagining
the centers of galaxies being swarmed with zillions and zillions
of very very low mass dark matter particles. That come
together into a super fluid, and when they do that,

(38:10):
they get all sorts of new weird behaviors. Cool, well,
it's getting too a little bit of what those behaviors are, Like,
what do you think happens when dark matter is that
close together that the wave funtionings overlap. So it's really
hard to do these calculations because you're talking about like
overlapping wave functions of lots and lots and lots and
lots of particles. So when physics needs to do that,
they try to describe these new behaviors in terms of

(38:30):
something they're already familiar with. So the way they usually
talk about it is in terms of sound waves propagating
through this super fluid. I think about like shock waves
moving through it. How if you pull on one part
of it that would affect the other parts of it.
And dark matter is this super fluid, so it's like
weirdly tightly couple that acts like a big coherent blob

(38:50):
instead of individual pieces. And so they talk about phonons,
which are like sound waves moving through this super fluid,
and they build up this whole theory which comes out
looking a new force sort of like this dark matter
when it enters this super fluid as a new way
to interact with itself that it didn't have before. Well,
I guess maybe one thing that's confusing me is that
I thought dark matter didn't interact with itself, right like

(39:12):
it The particles of dark matter don't usually or can't
bump into themselves. That's what was part of the idea
of dark matter. So why would bring them together really
close together make them interact in a different way. But
if they can't interact with each other, you're right, at
a particle level, they don't have that kind of interaction.
We're only talking about gravity, but now we're adding other
weird quantum effects, and quantum effects when they all work together,

(39:35):
can make it feel like there's a force. Another example
is the poly exclusion principle. As someone that tells you that,
like two fermions can't be in the same location. That's
the thing that keeps some kinds of stars from collapsing.
It allows you to resist the force of gravity is
trying to push it in. It's not technically a force
at the particle level, there's no force there, but this

(39:55):
quantum behavior of the objects basically acts as a block
to g so in the same way, this weird quantum
behavior of dark matter when it's a super fluid acts
sort of in a way to change gravity. It's sort
of like there's a new force. It's not an individual
new force on the particles. It's a way to describe
what happens to all these particles when they do this

(40:17):
new quantum thing. You can tell a story about it
as if it was a new force. Okay, I think
maybe I'm starting to get it. Like if I have
two particles of dark matter, and I have them really
far apart, then they do interact with each other, and
not like they can't bump into each other, but they
can attract each other gravitationally, like there's a gravitational force
between the two particles of dark matter. I think what

(40:39):
you're saying is when you bring them really really close
to each other, so that wave function of these two
dark matter particles starts overlap, then there are other effects
that start to kick in, other quantum effects then maybe
affect the gravity between them. Yeah, that's precisely it. And
you can talk about those new quantum effects as a
new force and introduce even like new particles for that force.

(41:00):
Call these things phone ons, or you could just say,
maybe that changes the overall effective gravity, right, it changes
the impact of gravity because now you have to factor
in this new, weird quantum effect. And the amazing thing
that comes out of the math is that the change
it makes to gravity is to make it look exactly
like Mond predicts. Remember, Mind, is this change in Newtonian

(41:21):
gravity that would beautifully describe everything we see at the
hearts of galaxies, but fails everywhere else. It turns out
if you make dark matter of super fluid, it changes
the gravity within this dark matter to make it look
just like Mind. But wait, I thought that you know
the gravity The gravity is just between the two particles, right,
Like the gravity between these two dark matter particles maybe

(41:42):
changes when you bring them closer together, so that maybe
they feel or not feel more or less gravity. But
to someone standing far away from these two particles, why
would they why would the gravity change for them? No,
it doesn't. You're absolutely right. And so if you're outside
of a galaxy, it doesn't matter whether the dark matter
is fluid or not. But we're talking about inside of galaxy.
We're talking about what's happening internally, how fast things are

(42:04):
spinning the gravity that like one blob of dark matter
is feeling on another blob of dark matter inside the
same galaxy. So this affects how dark matter inside the
Milky Way, for example, is pulling on other dark matter
inside the Milky Way, which is exactly what keeps the
whole galaxy together as it spins. So these quantum effects
make the gravity stronger of the superfluid or weaker. It

(42:27):
makes the gravity stronger, right, enhances their gravity. What do
you mean, like, do you know what the quantum effect
is or are just kind of postulating that there's maybe
some quantum effect that would make the gravity stronger. The
quantum effect comes from these overlapping wave functions, and when
you put it together and you do the math, and
you squeeze it theoretically sort of into the box of
a force and say, how do I interpret this as

(42:49):
a force. Then the calculations come out to predict a
change in the force of gravity that looks just like
the math you get from Mond. It's not just speculation.
You can go directly from these quantum effects to calculating
the new effective force of gravity, and it looks just
like Mon's prediction, which is as we know something that
works very very well well. But I think what you're

(43:09):
saying is that the super compact dark matter forms a
super fluid which has stronger gravity between the dark matter
particles that are in the super fluid. But with something
outside of the super fluid, would it feel this extra
gravity or not? Well, something inside the galaxy would, but
something outside the galaxy wouldn't. Right, So something outside far

(43:30):
far away other galaxies in the cluster wouldn't feel any
change in the effective gravity. And that's key because we
don't want to change the predictions for dark matter in
the cluster that already works really really well. The way
we see the galaxies rotate around each other, and big
clusters of galaxies rotate around other clusters of galaxies. That's
very well described by the dark matter theory. So we
don't want to change that. So the super fluid thing

(43:52):
only changes what happens inside galaxies, not between galaxies. And
so how would that explain what we talked about earlier
was one of the shortcomings of dark matter, which is
that the proportion of dark matter and regular matter is
too consistent between galaxies. How would this explain it? Well? Conceptually,
you can imagine that it gives us away for like
dark matter and normal matters sort of talk to each
other more intimately, so effectively it solves a problem by saying,

(44:15):
you do still have some variation in how much dark
matter and normal matter you have, but dark matter itself
acts a little bit differently, so it changes how galaxies rotated,
changes the effective gravity of that dark matter, and that's
what determines how fast a galaxy can rotate without tearing
itself apart. And remember the discrepancy we saw it was
not actually directly in the dark matter density of these galaxies,

(44:36):
but the rotation speed of the galaxies versus their brightness.
So now we have a new way for dark matter
and normal matter to side interact a little bit more
strongly because this new force that's inside the dark matter
super fluid, because we think that's sort of like couples
the stars and the dark matter a little bit more tightly,
makes it possible for them to have like more feedback
mechanisms to potentially explain what we're seeing out there in

(44:59):
the universe. I think what you're saying is that maybe
there's like an extra effect here that comes from the
super fluidity of dark matter that maybe makes it not random, right,
because before the problem was that we expected the ratio
of dark matter and normal matter to be is a
little bit more random, more variation. But maybe this special
effects kind of acts in a way that gives you

(45:19):
less variation. Like if you have more dark matter, it
acts in a way so that you have more regular
matter as well. And if you have less dark matter,
then maybe acts in a way to give you less
regular matter. That's the kind of feedback effect we're looking for.
We see out there in the universe, this is strangely
tight relationship between the dark matter and the normal matter.
We didn't understand that if the only relationship between the

(45:40):
two was this fairly weak gravity. But if gravity gets
a little bit stronger, it helps solve those problems. And
more specifically, we see that the effective gravity inside these
galaxies now follows exactly the prediction of mind, which, as
we said before, predicts very precisely the ratio of these
dark matter to normal matter inside the galaxy. So it

(46:00):
all clicks very nicely into place. M M. It seems
like a pretty super idea. This is a super fluid,
but it also sort of constrains dark matter in a
bit right like it it depends also, like it can
only be a super fluid if dark matter is made
out of really light small particles, right, And there are
ideas out there for dark matter to be very very

(46:22):
light particles. The most common idea is a whimp, a
weekly interacting massive particle where the mass of the particle
would be like a hundred times the mass of the proton.
But there are other ideas where dark matter could be
very very light. We've talked about it before on the podcast,
the idea of an axion sort of like a photon
with a little bit of mass to it, And so
this idea is a little bit more like an axion

(46:43):
than a whimp, which would also maybe make dark matter
harder to eventually detect and study right directly, Yeah, a
lot of our searches for dark batters, these big underground
tanks that are looking for a dark matter particle to
come and bounce off a zenon atom for example, are
not will of sensing dark matter and very very low mass.
But we have other experiments that are looking for very

(47:05):
very low mass dark matter particles. But there are also
other ways to test this theory. People think that if
this is true, it will also affect like how galaxies merge.
I mean, if you have two merging galaxies and they
each have their own halo of dark matter, then what
happens when they emerge. You will see the super fluidity
effect because the halos won't merge as fast as they

(47:27):
would otherwise. Like two halos made of a normal fluid,
you'll expect a little bit of friction. Two halos made
of a super fluid, they'll basically pass through each other
and it'll be gravity that pulls them back, so they
would like oscillate more times as they merge. If it's
a super fluid, then if it's just a fluid, So
if we can like study merging galaxies as a chance,

(47:48):
we can see whether dark matter is a super fluid
or a normal fluid. I think we're saying is that
the dark matter at the outsides and the edges of
the galaxy, which is not as compact or super fluid,
would sort of become a superfluid once it crashes into
another galaxy. I'm saying that we can test the super
fluidity of dark matter by slamming into another blob of

(48:09):
super fluid. Dark matter, if it really is super fluid,
should basically pass right through. If it isn't super fluid,
then we should see some friction between the two blobs
of dark matter. And we can't do this very easily,
but sometimes galaxies collide, right, huge galaxies slam into other galaxies,
basically testing this hypothesis, doing this experiment of slamming one
blob of dark matter into another. So if we can

(48:32):
study those collisions, we might be able to tell the
difference between super fluid collisions and normal fluid collisions because
they should look a little bit different. Pretty interesting to
think that dark matter, which we can see or touch,
could be doing things that we can maybe imagine and
even the dunk right and figure out, yeah, because we
can't see the dark matter directly, but we can see

(48:52):
it indirectly because of gravitational lensing and because of its
impact on the other stars. So doing a lot of
statistics and very careful measurements, we can get a sense
for where the dark matter is and what's happened to it.
And then we can check that against our calculations and
see does it look like it's being a superfluid or
a normal fluid? Right, And if you see that it
has a cape on it, then you know, like Hey,
it definitely has superpowers, right, that's right, And then it

(49:14):
needs to stop by our food truck so we can
promote our super fluid beverages, which we has not quite
passed the p d A approval. Right, that's right. Don't
go at drinking any super fluid yet, please people. Yeah,
and don't invite Daniel to your lab because he will
definitely put his own super fluids on everything. Hey, I'm
a curious person. What can I say? All right, Well,

(49:37):
another interesting idea about dark matter, the one that could
explain what is going on out there, and another example
of how this is still work in progress. We don't
know what this thing is. We're trying to figure it out,
and there are still new ideas coming up that could
explain what's going on. Yeah, there are whole categories of ideas.
Some of them even try to combine dark matter with
mond and say, maybe dark matter is real but also

(49:59):
gravity and used to be modified. Lots of people out
there trying to make the best of both worlds, and
this is like a cool alternative to try to capture
all of the best bits of all of those theories.
You could make a dark Mond to the theory, right, Yeah,
and we could have a dark mond flavored coke. There
you go, and you can make a dark Mont Sunday
ice cream Sunday as well. All right, well, we hope

(50:20):
you enjoyed that. Thanks for joining us, see you next time.
Thanks for listening, and remember that Daniel and Jorge explained.
The Universe is a production of I heart Radio. For
more podcast from my heart Radio, visit the i heart
Radio app, Apple Podcasts, or wherever you listen to your

(50:44):
favorite shows. Ye