Is dark matter hot?

Scientists don't know what dark matter is, but they know if it's hot (or not!)

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Episode Transcript

Speaker 1 (00:08):
Hey, Daniel, I have a question about dark matter. Oh, man,
don't we all? I mean, I know that we don't
know what it is, right, but what is it like?
I mean, is it quishy? We don't know. What does
it taste like? Well, you know, our tongues can't taste it,
so again we don't really know. But is it fuzzy
maybe we don't know, or scratchy? Probably not. But again

(00:29):
we just don't know. You know, for such a hot topic,
you would think you guys would know more about it. Well,
that's one thing we do know, whether dark matter is
hot or not. Hi am more Hammon, cartoonists and the

(00:57):
creator of PhD comments. I'm Daniel. I'm a particle physicist,
and I have no opinion about the attractiveness of dark matter. Well,
it's definitely attractive, right, gravitationally speaking on a cosmological level.
That's right. It is the great attractor from that point
of view. But welcome to our podcast, Daniel and Jorge
Explain the Universe, a production of I Heart Radio in

(01:19):
which we talk about all the amazing and crazy things
in our universe, the things that scientists have understood and
the things that scientists are now working to understand. We
break down all the crazy for you and explain it
in a way that hopefully makes you smile. It's right,
all the things that are hot in this universe and
all the things that are not hot or cold or
super cold, because the universe has a broad range. Right,

(01:41):
things can be as hot as a million degrees or
as cold as zero degree. That's right. Everything has a temperature,
even black holes, we all have a rating. That's right.
Most of the universe out there is at a very
cold two point seven three degrees kelvin. But there are
a few hot spots a place like Earth where hot
little bits of temperature clue to make life an interesting podcast.

(02:03):
And so we like to talk about in this podcast
about dark matter a lot. And I feel like we
talked about it a lot because it's such a huge mystery.
I mean, it's of the universe and we don't know
what it's made out of. I think it's one of
the biggest open questions in science. You know, the person
or the group that figures out, like what is dark matter? Anyway,
that will be a historic moment, that will be a

(02:26):
an understanding and achievement, a breakthrough that will go down
in history for sure. Do you think a Nobel Prize
would be enough for that discovery or they need to
like stack him up or something, or maybe make up
like a special Nobel Prize, the dark Nobel Prize. You know,
they should have already given a Nobel Prize to Vera
Reuben for the discovery that dark matter was out there.
Even if we don't know what it is, we know

(02:46):
it's there, we know it's matter, and Nobel Prize committee
overlooked very Reuben. Some say because she's a woman, that's terrible.
That's the dark history of the Nobel Prize. It's the
dark history of dark matter. But we know something. There's
a little bit about dark matter. Let it's there, and
that it's affecting things gravitationally and keeping galaxies together. But

(03:06):
the question is how much more do we know about it?
What else do we know about this mysterious thing, if
it even is a thing. That's right, We would love
to know what dark matter is made out of. And
particle physicists like me scratch their heads all day wondering
what kind of particle is it made out of? For
many particles or is it a particle at all? But
along the way while we're looking for its particle nature,

(03:28):
we have other ways to try to get clues as
to what it might be. By looking at how it
moves and how it clumps, and how it squishes and
how it buzzes, we can try to get a handle
on what it is or isn't. Yeah, and so to
be on the program, we'll be asking the question is
dark matter hot or not? Well, for those of you

(03:49):
who are a little bit older, you might remember a
popular website a few decades ago called hot or Not,
which was probably inappropriate these days, totally inappropriate exact Yeah,
rated people based on their hotness. And I'm guessing it
was not the temperature. No, it was not the temperature,
although maybe we should revive it in a physics version,
like is the top cork hot or not? Our neutrinos

(04:11):
hot or not? That might be or cold maybe based
on how much fun thing you can get for it.
That's right, And it's a weird combination of ideas, you know,
dark matter, mysterious blobs of stuff out there in the universe,
and temperature, But it turns out to be very important.
It's one of the most powerful handles we have on
the nature of dark matter and one of the most

(04:32):
valuable clues we have that tells us what it is
and what it can't be. Yeah, So, as usual, we
were wondering how many people out there had thought about
this question of whether dark matter is hot or cold,
and so as usual Daniel went out there into the
wilds of the Internet to ask people this question. That's right.
So thank you to everybody who was willing to participate
in our random person on the internet questions. And if

(04:54):
you'd like to answer random questions from me in preparation
for a future podcast, please is right to us two
questions at Daniel and Jorge dot com. To think about
it for a second, what would you answer if someone
asked you, is dark matter hot or cold? Here's what
people have to say. Yes, it seems most natural to
me that dark matter would interact with itself, so it's

(05:16):
doing so. It's reasonable think that it could have a
temperature um it's relative to other dark matter, so I
guess it will be hot. What I think it's that
are parts that of the dark matter that can be
hot and parts that are gonna be colder. I think

(05:37):
dark matter is cold, or at least cooler than normal matter.
On average average temperature of the universe is a few
codings about zero. And since we have no idea, what
what is what other constituent particles of dark mastra? I
think the answer is we have no idea. I don't
know a lot about dark matter, but I don't usually

(06:00):
ink of matter having a specific temperature. I'd say we
don't know, because we don't even know what it is.
I would say that it's probably not hot. Well, hot
and cold are relative terms, So if what you mean
is does dark matter have a temperature, then I would
say probably not, because everything with the temperature gives off
infrared radiation. I had to consult my eleven year old,

(06:21):
who is the cosmologist in our family. So we think
that dark matter is cold. The only reason we know
it exists is because it reacts with gravity, and I
don't think it will react with anything on the electromagnetic spectrum,
so it wouldn't be hot or cold. Both knowing scientists,

(06:43):
they fancim intrinsic property of dark matter and named it
hot and cold, even though it doesn't mean anything like
hot or cold. All right, I like how people evaded
the question very expertise. You're impressed by that are you disappointed?
I'm impressed. They're like, Oh, they're thinking, like physics avoid
answering the question. It's like what is hot and cold?

(07:05):
Let's divert into that discussion. Well, we do this a
lot in physics. We apply weird sounding characteristics to things,
you know, like when we're talking about particles, we're talking
about their spin that's not really spin, and we're talking
about their mass, but they don't have any stuff to them.
And so I understand why people are a little wary
of interpreting like the temperature of dark matter, Like what
does that actually mean? What are we really talking about? Yeah, Like,

(07:28):
we don't even know if it's a thing, So how
can I not thing have temperature? That's right? It feels
like a detail, Like are you worried about what color
it is? You don't even know if it exists. Why
do you care if it's purple or brown? Right? Yeah? Yeah?
What color is dark matter? Then you it's dark? Alright,
So let's break it down for folks. And first of all,
I guess the question is, how can dark matter even
have a temperature if we don't know what it is? Right? Well,

(07:50):
let's remember what temperature really means. For us, temperature is
a macroscopic quantity. Right, you touch something, it feels hot
or it feels cold, and that's really actually about the
heat difference, Like if something has more energy in it
than you do, then the heat flows from it into
your finger, like when you touch a hot burner, and
that's what you're feeling. So you don't actually measure temperature

(08:11):
with your finger. You measure like a relative heat. But
when we think about temperature, like microscopically, we try to
understand how that experience of feeling things being hot or
cold translates to like the motion of the particles inside it.
And so most loosely, we think about temperatures relating to
how fast those particles inside something are moving. Like a gas.

(08:33):
If it's a hot gas, then the particles in it
are moving really fast, that's right, and that's in fact
what's happening. But also for liquids and for solids, and
in fact that's why liquids and solids are more solid
than gases, right, because their particles are not moving as much,
they're more easily trapped by all the bonds, and solid
has various temperatures because the atoms and it can wiggle

(08:54):
more or less they can shake and vibrate in that
kind of stuff. So it's all about the energy stored
in those parts, like the motion of the particles inside,
like the speed almost. Yeah, if you're talking about a gas,
then it's mostly about the speed. And I think this
is really interesting stuff to like take something that's macroscopic
and kind of qualitative, you know, this feeling of temperature,
and try to understand it on the microscopic scale, and

(09:16):
it sometimes works and it doesn't always work. And we
had a whole podcast where we talked about like the
hottest things in the universe, and some of these things
are counterintuitive. Like some of the hottest stuff in the
universe is the interstellar plasma, which is like some crazy
high temperature like three thousand degrees kelvin. But if we
dropped you in it, you would freeze to death immediately.

(09:38):
And that feels counterintuitive because there isn't much of it
out there that's right of this plasma. It's very hot,
but it's very dilute, so it doesn't contain a lot
of heat, and so you're much denser blob of heat.
If we dropped doing it, most of your heat would
leak out but the particles of that plasma individually are
moving super duper fast, and so you can still call
it hot, right, So it's related to the speed and

(10:00):
the get or the vibration or like the kinetic energy
of the molecules and particles in something. But how does
that apply to dark matter, because we don't really know
if dark matter is made out of particles or not.
We don't really know. Well, we know that something is
out there creating gravity. We know there's a kind of matter,
and that's really about it. We know sort of where

(10:20):
it is in the universe, but you're right, we don't
know that it's a particle that could turn out to
be something else. And you know, all the matter that
we've ever seen in the universe so far has been
made out of particles. So it seems tempting to say, well,
then the dark matter must also be made out of particles.
But you know, remember that dark matter is most of
the stuff in the universe. We've only seen a little slice.

(10:40):
We've seen five percent of the universe, so it's dangerous
to extrapolate to like a full and say the rest
of it must also be made out of particles. But
we don't really have better ideas, and so we typically
just assume dark matters made out of particles. So that's
kind of like the working hypothesis. Yeah, it's like, let's
try this, let's see if it works. If it breaks,

(11:01):
then we'll go back and examine all the assumptions we made.
But when you're exploring the unknown, you've got to make
some assumptions just to like have something to do, because
you can't just sit at home and go like, I
don't know what dark matter is. You know, it sort
of ends there. So we say, maybe dark matters of
particle and then we can ask if dark matter is
made of particles, are those particles moving fast or are

(11:22):
they moving slow? Right? Are they hot or not? Are
they hot or not? That's exactly what that really means.
It means is dark matter made out of super fast,
zippie particles moving relativistic speeds or is it made of
like heavier, slower moving particles that just sort of like
float around its slower speeds. I guess it's kind of
weird to think of something being hot but not being

(11:42):
able to touch it, you know what I mean? Like,
that's weird, right, I was just thinking, do neutrinos have
a temperature, like in a neutrino who we can't interact
with through electromagnetism, Can that have a temperature? Yes, absolutely,
neutrinos are very hot, and the reason is that neutrinos
have almost no mass, and so these through the universe
at very very high speeds, and so they contain a

(12:03):
lot of energy. You would say they have a high temperature,
but you're right that you can't feel them, and the
reason is that you have no interaction in common with them,
or almost none, because all they feel is the weak force.
So they have all this energy, but they have no
way to transmit it to you. So it's like you
pass right through each other. And so they can have
that high temperature, they can have that high energy, but
if there's no common interaction, no way to communicate, there's

(12:27):
no way for that energy to flow to you, and
so you won't feel them being hot. What about like
if what if dark matter is not a particle? Can
it still have a temperature and something that's not a
particle still be hot? WHOA? You just blew my mind.
Could something that's not made of particles have a temperature?
We've never seen anything that's not made of particles, So

(12:48):
that's quite a reach. But I guess macroscopically you could
like see if it emits light, and everything in the
universe that does emit light has a temperature, it's black
body radiation. But I don't know. That would be an
amazing thing to explore if we discover the dark matter
wasn't made of particles, because we do know something about
its temperature, which is what we're going to talk about today.

(13:10):
I see, so that it's made out of particles is
not just a working hypothesis. It's it's like your only hypothesis.
It's all we got at this point. It's like the
one idea we've been using for a hundred years, or
you know, empty box for crazy new ideas somebody should
come up with. Really, like, could it be something that's
not a particle? It certainly could be. I mean, we
have no concrete evidence that it is a particle other

(13:32):
than all matter so far discovered is made of particles,
but it certainly could be. We're open to surprises. I mean,
dark matter itself is a surprise. Its existence was a surprise,
and there have been some ideas about un particles matter
made out of things that are not quite particles that
you know, don't have a definitive size, but it's a
bit fuzzy, and nobody's really worked out the math for

(13:55):
how it could be dark matter, So they're just sort
of like the beginnings of ideas, I guess. I mean,
you know, like energy, is energy also particle based, because
you know, energy can have gravity or exert gravity or
effect gravity. Energy density certainly has gravity, and some energy
is particle based, like photons, right, Photons are basically just energy.

(14:15):
They have no mass to them, and photons contribute to
the energy density of the universe, and therefore it's curvature.
So certainly, all right, Well, I guess there's no maybe
room and your equations so far to account for something
that's not a particle, Is that kind of what you're saying.
That's right, yeah, But I would love to blow up
those equations. I would love if we found something about

(14:36):
dark matter that proved that it wasn't the particle and
then we have to go back to the drawing board
and think from scratch. That would be a tremendous breakthrough,
an intellectual crack in the very foundations of physics, which
is the kind of thing we're all hoping will happen,
you know, because those are the moment you get, like
the real insights. It pulled back the curtain and discover
something surprising and fascinating about the universe. So yeah, this

(14:57):
is all we got so far, and I would love
to see it break into pieces. You'd love to prove
that they're not so hot, all these theories precisely. All right, Well,
let's get into how we could tell whether or not
dark matter has a temperature besides like I guess, feeling
its forehead. Daniel, that's right, These days were very sensitive

(15:18):
to high temperatures. But let's get into how we could
tell and what it tells us about dark matter. But first,
let's take a quick break, all right, Daniel, we're talking
about whether dark matter is hot or cold, and so

(15:40):
we talked about how we have to kind of assume
that it's a particle because that's the only idea that
we have. And so if it's a particle, then you
can talk about whether those dark matter particles are moving
a lot or vibrating a lot, which case wouldn't make
them technically hot even though we can't feel it. That's right,
and we're really interested in whether dark matter is fast

(16:01):
moving or slow moving, because it tells us also whether
the particle is heavy, in which case it is more
likely slow moving and cold, or very very low mass,
in which case is probably faster moving and hot. So
we're we're using this as a way to sort of
get a clue as to the nature of dark matter.
It's but it could dark matter be both, like I mean,
it could it be like regular matter that some of

(16:21):
it is hot and some of it is cold. Totally absolutely,
dark matter could be lots of different particles, some of
which are very heavy and some of which are very light.
But we know that dark matter sticks around for a
very very long time. It's like cosmologically stable. It's been
here since the beginning. It's affected the structure of the universe.
We've seen it put its imprint on the whole history
of the universe. And so that suggests that it's probably stable,

(16:43):
that it's not changing a lot from one kind of
mass to another. But you know, we really just don't know.
All right, Well, let's get into now how we could
tell whether dark matter has the temperature or not, Like,
how do you how would you even measure the temperature?
Of dark matter. If a particle of dark matter is
moving a lot or vibrating a lot, or could we
even tell the different We can actually tell the difference,
and I think this is really clever. It's one of

(17:04):
the most elegant pieces of science that I've seen recently.
We can tell whether dark matter is moving fast or
slow because of the way it makes an imprint on
the growth of the universe. You know, the universe started
from like the Big Bang, and back then things were
hot and dense and mostly uniform, and then you've got
little quantum fluctuations, little pockets of density here and less

(17:28):
density there, and those pockets are critical because that's what
seeds the whole structure of the universe. Like the reason
we have a galaxy here and not over there is
because some initial fluctuation made things a little dense, and
then gravity clumped them together and clumped them together even further.
So you've got these little fluctuations in the early universe,
which see the structure of the universe, right, because gravity

(17:49):
takes over from these little wrinkles. But dark matter plays
a really big role in that because dark matter basically
is gravity, right, It's the biggest source of gravity in
the universe, and so where dark matter is and how
it's distributed determines the shape and the structure of the
whole universe. And so we can tell from like pictures
of the Big Bang until the temperature of dark matter

(18:11):
at the beginning of time or right now. Well, we
can tell the temperature of dark matter over the history
of the universe. Everything is cooling down, but we can
tell whether dark matter was made hot or made cold.
Everything is getting colder over time, but we can tell
whether dark matter started out hotter or colder. And we
can do that by seeing whether or not it's moved
around a lot, whether or not it's been wiggling around

(18:34):
and that's affecting the structure universe, or whether it's been
mostly staying in the places it was made. I see
because I guess you assume that it's kind of like
a gas, right Like you don't assume it's a solid.
You assume that it's you know, kind of moving around freely.
It's not tied together to itself except with gravity. That's right,
only held together with gravity. And so we think of
it like a diffuse gas, like a pressureless gas that

(18:56):
doesn't even bounce against itself, and so basically it just
has gravity tational effects. And so we can sort of
walk through the history of the universe with a cold
version of dark matter, a version where dark matter is
mostly staying where it was, and then we can walk
through a version of the universe where dark matter is hot,
where it's zipping around really fast, and we see that
those two things predict different shapes of the universe that

(19:17):
we see today and also different histories of the universe,
and then we can compare those histories to what we
actually see because like, if the dark matter at the
beginning of time was super cold, then I guess it
particles themselves don't have enough speed to like go off
and spread out. They would sort of stay clumped together.

(19:37):
That's exactly right. So if dark matter is very cold,
then the structure of the universe forms sort of bottom up.
Everything is where it was and it's not zipping around
very much, and so you get these little clumps of
density from those initial wrinkles, and that's what seeds like
the formation of stars, and then stars get together and
they form galaxies, and galaxies pull themselves together to form

(19:58):
galaxy clusters. You get this structure formation that's sort of
bottom up. Everything starts clumping where it was and then
pulls together, so you get, for example, galaxies forming before
galaxy clusters. You get stars forming, then galaxies, then galaxy
clusters in that order. And we can look back through
the history of time because remember as we look out

(20:20):
through space, we're looking backwards in times, so we can
see where their galaxies a billion years after the universe started,
where their stars. Which order did things get made? We
can tell by looking deep into the history of the
universe just by looking far out into space. Right, And
I guess you're using relative terms right, like cold and
hot here. You're not thinking about a specific temperature because

(20:41):
that could maybe also depend on how heavy these particles are.
That's right. We're mostly talking about whether or not they're relativistic,
like are they moving it close to the speed of
light or are they not relativistic? You know, they're moving
a much less than when you say hot, you mean
like super duper hot light speed hot. Yeah, exactly. And
when we think about what hot dark matter would look like, well,

(21:03):
you have the early universe, and you know dark matter
is made just with everything else, and you get these
initial little clumps of density from quantum fluctuations. But if
dark matters most of the stuff and it's moving really
really fast, then those initial little blobs of density don't
really matter because dark matter sort of washes them all out,
Like the dark matter is flying everywhere super duper fast,

(21:23):
and so those initial little clumps get evened out, they
get smoothed out, so you don't get stars forming first.
Instead you get these like these really big supermassive blobs
of stuff because only the really big over densities, only
the really big clumps from the beginning stick around and
survived the dark matter spreading everything out to form some structure.

(21:44):
What do you mean? So if the dark matter is hot,
it means that the it's particles are moving a lot.
And so are you saying that dark matter is more
diffuse or like the blobs are moving around fast. Both,
they're moving around faster and so they spread out and
so it gets more even and so it's harder for
gravity to get a handle and start forming stars, for example,

(22:05):
because things get smooth. For gravity to form a star,
you need like a little blob that's denser than the
stuff around it that it can gather stuff together using gravity.
But if dark matter, which is most of the stuff,
is moving fast, then it's spread everything out, its smoothed
everything over. There's nothing for gravity get a handle on,
except for the really really big stuff because that's the

(22:25):
stuff that dark matter can't smooth out. And so instead
of getting stars and then galaxies and the galaxy clusters
and then superclusters, you start out with supercluster sized blobs
of stuff and then it breaks up into galaxy cluster
sized blobs of stuff, and those break up into galaxy
size blobs of stuff, and then you get stars forming.

(22:46):
So it's sort of like top down instead of bottom up.
Interesting just based off of the temperature of dark matter. Yeah,
so the temperature of dark matter totally determines the entire
history of the universe. Like the universe would be very
different if we had no dark matter because it wouldn't
have been around to clump the normal matter together into
stars and galaxies. And also the universe would be different

(23:08):
if we had hot or cold dark matter, just it's
such a dominant force. It's most of the gravity. So
it affects how the universe came together. And we can
actually tell the history of the universe whether things foreign,
buttom up or top down. Yeah, because we can look
back in time and we can say, well, we're there
galaxies in the first billion or two years after the

(23:29):
Big Bang, or did it take a while for galaxies
to form? And so we can look back in time
and we can ask whether these things were made, in
what order were they made. And also it affects the
way things look today, because things would be smoother today
if dark matter was hot, and things would be sort
of clumpier today if dark matter was cold, like for example,

(23:51):
our galaxy is the Milky Way, and if dark matter
was cold, then we expect that the Milky Way has
a bunch of like little galaxies orbiting it, the way
the Earth has the Moon. We expect that the Milky
Way has its own little like many galaxies that orbit
our galaxy. If dark matter was super cold, if dark
matter was cold, then there should have been these blobs

(24:12):
of stuff formed outside of our galaxy, these dwarf galaxies,
which would now be orbiting the Milky Way, and that
we should see that today, So that would be a
sign that dark matter is cold. Affects not just the
history of the universe, but also affects the shape of
the way things look today. Yeah, I guess it's. I mean,
it's such a huge part of the universe that you
know whether it's hot or not. It should be no

(24:34):
surprise that the term is the fate of the universe
because it's such a huge chunk of it. Yeah, exactly.
It's not a little detail. It's not like a tiny
bit of salt that you add to your recipe. Right,
it's most of the stuff in the universe, and so
of course it's going to have big consequences for how
the universe looks and how it comes together. All right,
it could be hot or cold, and we could probably

(24:55):
tell by looking at the structure and the history. I
guess the history is also important of the universe. The
history kind of tells us a clue about whether it's
hot or not. That's right. Did the structure form top
down big stuff first and then small stuff or to
the form bottom up like small stuff first, which then
came together to make the bigger stuff. And it also
affects the way things look in our universe today. Right,

(25:17):
all right, let's now answer the question whether dark matter
is hot or not and what that tells us about it.
The first, let's take another quick break. All right, Daniel,

(25:39):
is dark matter hot or not? Is it a swipe
laughter or a swipe right for you? Well, I love
dark matter. I'm very excited about dark matter. I'm very
attracted to dark matter. But I have to say that
the universe tells us that dark matter is quite cool.
It's not hot, It's definitely not hot. I mean, it's
still be beautiful. It's just you know, a little chilly.

(26:00):
That's right. It's got its own standards of beauty, and
it's pretty cool, you know, dark matter. And we know
that because we look at the history of the universe
and we see that stars formed first, and that then
galaxies formed, and that then galaxy structure is formed. Because
we look back in the very early universe and we
see galaxies forming before there were clusters, and we see

(26:21):
stars forming before there was galaxies. Can we tell that?
Can we? How can we tell? I felt like we
can only see really far out and tell the distance
and the age of things by looking at like supernovas,
And so how can we tell how things formed if
our only way of knowing is through stars? That's right? Well,
the supernovas tell us sort of like the distance ladder

(26:42):
and so we can tell how far away something is
and therefore when it happened. And you're right that we
need stars to happen to give us that distance ladder.
But we can go back and look at the early universe,
right that tells us like, okay, this is really really
far away. And for example, you would expect that there
would be galaxy clusters formed in the very early universe

(27:04):
if dark matter was hot. And so we look out
past the most distant supernovas into the deep early universe,
you know, and we can tell that these things happened,
you know, thirteen billion years ago, for example. We don't
see galaxy clusters forming out there in the very edges
of the things that we can observe. That's the very
earliest universe. And you're right. We we we can't get
as precise an estimate for those distances because we don't

(27:27):
have the supernovas, but we can extrapple it a little bit.
And also we know it's super duper old. So like
the the oldest stars that we can see tell us
that things were not as formed as they are closer
to us or closer to the present, that's right. They
tell us that the structure formed bottom up that things
came together in small clumps first, and then those small

(27:48):
clumps organized themselves into bigger stuff. So you get stars,
and then galaxies, and then galaxy clusters, and then superclusters
of galaxies, which is the latest structure to form. And
that's why they're the biggest gravitationally bound objects in the
universe because they have most recently come together. It takes
a while for gravity to do this, and galaxy superclusters

(28:11):
are the last thing to have formed. It's all that
we've had time to form so far in the universe.
All right, Well, I guess so then that tells us
that dork matter is cold, and I guess it. Do
we have a sense of how cold it is, like,
you know, not going at the speed of light. I
know that's how you define cold. But is it like chili?
Or is it like warm? Or is are we talking

(28:32):
like the temperature of the sun? What are we talking about?
It's definitely not the temperature of the sun. I mean,
if it's out there and it's a particle, it's going
to be very very cold. You know, it's going to
be a few degrees kelvin. Really, we think dark matter
is only a few degrees kelvin probably, yeah, And you know,
it's not interacting in the same way that like hydrogen
does in the core of the Sun to produce a

(28:53):
huge amount of energy. But there's still a lot we
don't know about dark matter that could have self interactions
that contain energy that we are not aware of. And
so everything we say here should be taken with a
big grain of salt because it's all pretty speculative. But
you know, also, the cold dark matter picture is pretty good.
It works pretty well, but it's not perfect, Like it
doesn't perfectly explain everything that we see, right, Like you're saying,

(29:15):
cold dark matter predicts that we would have baby galaxies
floating around us. That's right. We expect to see a
bunch of these dwarf galaxies orbiting the Milky Way, and
we see some, but we don't see nearly as many
as we expect, and we don't know yet. Is that
because dark matter isn't as cold as we thought, or
is it because those dwarf galaxies are harder to see
than we thought they would be. And recently people have

(29:36):
developed extra good techniques to find dwarf galaxies and they
found a few more, and that sort of closes the
gap a little bit, but there's still some tension there.
It's still something that we don't quite understand. And you know,
we like those details. We like getting those things right
because those are the things that tell us that our
theory is really working. And so there's still some question
marks about it. But it's definitely not hot. It's some

(29:58):
version of cold. I guess we can't make any version
in our simulations work out to be just like the
universe we have now, like you tweaking further, you don't
get the right proportion of dwarf or baby galaxies, not yet.
But you know, these simulations are very very hard to
do because you're simulating an enormous number of particles. And
when they do these simulations, they usually just like leave

(30:21):
out all the normal matter because the normal matter is
a small fraction and it's much harder to model because
normal matter has complicated interactions, right, you know, stars and
gas and all that stuff. It has pressure and complicated
flows because of the electromagnetic interactions and the strong interactions
and all that stuff. So until recently, these simulations have
mostly just removed all the bionic matter. But you know,

(30:43):
baryons are important. I'm a baryon, you're a baryon. Stars
or buryons. The whole visible part of the galaxy has
made a bury on. So what does it mean, Like,
that's the particles that we're made out of, regular matter
like quarks and electrons. And so when they do these
simulations to describe the structure the universe, they don't have
the computational power to describe all the barry on its

(31:04):
all the things that make me and you corks and
all that stuff, So they mostly just remove it as
a as a simplification because that's the most complicated stuff
to describe, and so our simulations are really approximate right now.
So people are working on ways to include normal matter
in these simulations and try to get more precise estimates,
more precise predictions for how many dwarf galaxies we should see. Yeah,

(31:26):
I guess people are complicated. They're hard to predict, for sure,
they are. They are hard to describe. So we know
we think dark matter is made out of particles, and
if it is, we think it's cold, because that's what
the universe is telling is So what does that tell
us about dark matter? Like, does it give us a
clue about what it is or what kind of particle
it is, or you know, is the fact that it's cold.

(31:49):
Does that tell you something about how it interacts with
other forces? Yeah, it tells us a lot. And what
it can do is remove candidate particles from the list,
and most specifically, it is the neutrino as a candidate
for dark matter. For a long time, people thought, oh,
there's a lot of invisible matter out there, a matter
that almost never or never interacts with us except for gravitationally.

(32:11):
Maybe it's just neutrinos. And it's a very tempting candidate
because we already know about neutrinos. We know neutrinos are
these whispy particles that can pass through a light year
of lead without interacting. The air around us is filled
with neutrinos, but we can't feel them or taste them.
They have a lot of energy, but they don't deposit
it on us. And so it's tempting to assign these

(32:32):
two mysteries together, right, the weirdness of neutrinos and the
mystery of the missing matter. Maybe one plus one just
equals too, And so for a long time people suspected
maybe the missing matter was just like a ridiculous number
of neutrinos. And remember neutrinos are very very light, that
hardly any mass per particle. It's not zero, but it's

(32:52):
a small number. So if you're gonna explain most of
the stuff in the universe with neutrinos, it would have
to be an ungodly number of neutrinos. Could it be
like a heavy neutrino, Like I know, neutrinos they can
have different masses, right, The neutrinos that we're aware of,
the three, the electron, mu and town netrinos all have
very very very small masses. And so what we can
do is we can rule out those We can say

(33:14):
it's not one of the neutrinos that we know one
of the neutrino lights. Yeah, exactly, because those neutrinos have
such small mass that they're always moving basically at the
speed of light, are very close to the speed of light.
For example, when neutrinos come from a supernova, they arrive,
you know, very close to the same time as the
photons arrive because they're traveling basically at the speed of light. Actually,

(33:34):
the neutrinos get here first because the photons get slowed
down by interacting with the star. But it's basically a race.
The neutrinos fly almost the speed of light. You're saying
they're faster than light, Thaniel, They're not faster than light.
They leave sooner. The photons spend more time packing, but
they do travel a little faster. But you're exactly right
that there's the possibility that there could be some weird

(33:56):
heavy neutrinos, so not the neutrinos that we're familiar with,
but if they're is another kind of neutrino, fourth neutrino,
or many other kinds of neutrinos that are very heavy,
then those are still valid candidates for the dark matter.
And those go by the terms like sterile neutrinos because
called sterile because maybe they interact with our kind of
matter even less. Wow, it's like a neutral neutrino. Yeah,

(34:19):
that's right. It's like an even more standoffish and snobbish
particle than the neutrino. And that's a hard standard to meet.
I was just thinking, like shy or you know, loth
to interact with other particles. They know the introvert neutrinos,
but you just assume that you know, it's just not
my apologies sterile neutrinos, I take it back, right, So
that tells that they can't be neutrinos because neutrinos you

(34:41):
usually go really fast, but they could be. Basically, that
doesn't leave you much, does It just tells you that
it's another kind of part of it. Yeah, and that
we don't know about that. That's an important clue because
that means that there's no particle on our current list
that fits the requirements. There's no particle out there that
doesn't have electromagnetic or strong interactions and is heavy. Right

(35:01):
there just isn't one. The only particle in our current
list that had any chance of being the dark matter
or new trinos, and this piece of evidence rules that out.
It says it can't be one of the new trinos
we know. So it has to be a new particle.
And that's exciting. A new heavy particle, a new heavy
particle exactly. It means that there's something new to discover.
It's not just oh, there are more of this particle

(35:22):
than we thought. It means there's a new particle. And
a new particle is interesting because you wonder, like why
does it exist? How many new particles are there, Where
did it come from? Why is it different from these
other particles? You know, it gives you a whole new
set of questions to ask, a whole new way to
look at the universe. And you guys are looking for
these in the particle colliders, right, you're smashing particles hoping
that a new kind of particle will pop out. And

(35:44):
you might say, hey, that's dark matter. That's right. And
we have specific ideas for what this new particle could be.
We have ideas like the whimp particle weakly interacting massive particle.
It's just a generic name meaning some big, heavy particle
that doesn't interact much. And it has to not interact
very much in order to be the dark matter. And
it has to be massive in order to be cold

(36:06):
because of the structure of the universe. And another idea
is the axion. The axon could be the dark matter,
and we have specific experiments to look for whimps and
for axons. We just did a podcast episode about axons.
They're not the same thing. They are not the same thing.
There are two very different kinds of particles. The axion
is like a heavier version of the photon, and the

(36:27):
whimp is like it's like a heavier version of the neutrino,
but maybe interacts even less. And we have experiments underground
to look for WIMPs, these big tanks of liquid argon,
for example, or liquid zenon that look for one whimp
coming through and knocking into a bunch of particles and
then giving us a signal. We're using space telescopes to

(36:48):
look to see if occasionally whimps bounce into each other
and give off a little flash of light that we
could see, which would be really really rare because dark
matter is dark. But you know, we look at places
where there is a lot of dark matter and try
to see the occasional blip, and then we try to
make dark matter in the collider to see if we
can create it and play with it there. So far,

(37:09):
none of these experiments have turned up any evidence for
dark matter that anybody believes, and so we're still in
the hunt. But you know, even though we don't know
what dark matter is, we're able to say some things
about what it isn't right, is it weird that you
haven't found dark matter in these colliders. I mean, like
in the universe there's five times more dark matter than
regular matter, which might make you think that it's like

(37:32):
it's more likely to happen, but in our colliders you
can't seem to make even a little bit of it.
That's right. It is a little weird. Now. On one hand,
it may be the dark matters everywhere, but we can't
make it because we're playing with our kind of matter,
like our kind of matter might not interact with dark matter,
which means that we can't use our matter to look
for dark matter, and we can't use our matter to

(37:54):
make dark matter like for that to work for any
of the experiments I just described to work to this
govern the particle nature of dark matter means there has
to be some way for our particles to talk to
the dark matter particles, to share some sort of new
dark photon, or some new force has to exist that
works on both particles. And it could be that it

(38:15):
just doesn't. It could be the dark matters out there.
It's a particle and it just feels nothing except for gravity,
in which case it's basically hopeless for us to discover
its particle nature because gravity is so weak that we
can only detect dark matter when you have enormous, like
galaxy sized blobs of it. Which makes it pretty hard
to do particle experiments. But I thought when you smash particles,

(38:37):
it turns into like pure energy, and then anything can
come out of it. You're saying that maybe it's possible
that not even dark matter can come out of that.
That's right. When you smash particles together, it's not exactly
pure energy. It turns into one of the bosons of
the forces that can interact with those particles. So, for example,
when you smash a quark and an antiquark together, you
can get a glue on, or you can get a photon,

(38:58):
or you can get a w boson. But if those forces,
the weak and the strong force and electromagnetism don't interact
with dark matter, then those bosons which represent that energy
can't then turn into dark matter. And so that is
one limitation I know that I like to say in
this podcast that we can use colliders to explore the
universe because anything that can be made will be made.

(39:18):
But there is an important caveat there that whatever can
be made has to somehow interact with the particles that
were smashing. If there's no way to interact, then you
just can't make it. You need a dark matter collider, Daniel. Obviously,
to discover dark matter, you have to build a dark
matter collider. All right, Well, um, it sounds like we

(39:38):
don't know what dark matter is still, but we know
that it's pretty cool. It's a pretty cool thing in
the universe. It's cool, that's right. Dark matter is pretty chill.
You know. It wants to come over and watch Netflix
with you, even if you don't think it's hot. Yeah,
all right, Well again, just makes me think about all
the crazy things we don't know, you know, and all

(39:59):
the sort of fun and clever ways we can tell
about things we don't know even though we don't know
anything about it. Yeah, And this is what science does,
is we probe things from every direction. We're trying to
uncover a real truth about the universe, and that has
lots of facets. And so if we get stumped in
one direction, like we can't seem to find it in
our detectors, then we go another route and say, well,

(40:20):
can we say anything about it from this perspective or
from that perspective? And we're trying to be clever in
the field of particle physics, and science in general is
filled with clever people having new ideas about ways to
answer these questions, and so to me, this is one
of the most elegant ways to put a really important,
really insightful constraint on what dark matter is and isn't.
All Right, Well, I think we answered that question pretty good,

(40:43):
and I think we can all learn a little bit
from dark matter to just be cool. Don't get too excited.
Thanks for joining us, See you next time. Thanks for listening,
and remember that Daniel and Jorge Explain the Universe is
a production of I Heart Radio. For More podcast. For

(41:06):
my heart Radio, visit the I heart Radio app, Apple Podcasts,
or wherever you listen to your favorite shows. H