Edge Of Knowledge
science

What Is Dark Matter? An Astrophysicist Explains

We see evidence for dark matter everywhere we look but proving hypotheses around it has been exceptionally difficult. Astrophysicist Paul Sutter explains everything about this mystery in a way almost anyone can understand - what we know about dark matter, what don’t, and how we’re trying to find out more.

Find more with Dr. Paul Sutter here:
Website: https://pmsutter.com
YouTube: https://youtube.com/paulmsutter
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Video courtesy of:
ESA and the Planck Collaboration
https://www.esa.int/ESA_Multimedia/Images/2013/03/Planck_CMB
CC BY-SA IGO 3.0ESO/L. Calçada
https://www.eso.org/public/unitedkingdom/videos/eso1709c/

Released on 1/19/2022

Transcript

00:00
Dark matter is kind of frustrating.
00:03
We see evidence for it everywhere we look,
00:05
from the rotation curves of galaxies
00:07
to the cosmic microwave background.
00:09
In all attempts to explain it away
00:12
as some quirk of gravity fail.
00:15
Whatever dark matter is,
00:17
it must be cold, collision-less and abundant.
00:21
We'd like to know what dark matter is made of
00:23
because it can help us understand the dynamics of galaxies
00:26
and the evolution of the universe.
00:28
Oh, and by the way,
00:30
it is by far the most common particle in the universe.
00:34
So, it'd be nice to know what it's actually made of.
00:37
[bright anticipatory music]
00:53
Let's say you're on a small boat
00:55
in the middle of the ocean.
00:56
Or it could be a big boat,
00:58
it doesn't really matter for this metaphor.
00:59
Anyway, it's the middle of the night
01:02
and you see lights on a distant shore.
01:05
Now, those lights tell you that the shore exists,
01:08
but it doesn't tell you anything about it.
01:10
There could be mountains or jungles,
01:12
you just don't know.
01:13
This is the situation with dark matter.
01:15
The luminous material in our universe,
01:18
the stars and galaxies tell us that dark matter exists,
01:22
but it doesn't tell us what it's made of.
01:24
Now taking together,
01:26
there's so much evidence for dark matter.
01:28
I could spend the rest of this episode
01:30
going over all the evidence.
01:31
I mean, I wrote a book about it.
01:35
And check this out, there's the rotation curves of galaxies.
01:38
There's the temperatures of galaxy clusters.
01:41
There's the bending of light around massive structures.
01:44
There's the large-scale structure of the universe itself.
01:46
There's the cosmic microwave background.
01:48
It goes on and on and on and on.
01:50
We know that dark matter exists,
01:53
but we don't know what it is.
01:55
But we do know what it isn't.
01:59
It's not just normal matter
02:01
that happens to be dim and hard to see
02:04
like rocks or planets or black holes.
02:06
We know this because we've made measurements
02:09
of the very early universe
02:10
that tell us how much normal matter is in the universe.
02:13
And there simply isn't enough
02:15
to account for all the gravitational effects.
02:18
Instead, dark matter has to be cold and collision-less.
02:24
Cold means that the dark matter moves slowly
02:27
compared to the speed of light.
02:28
And collision-less means that it doesn't interact
02:31
with itself or normal matter.
02:33
That means dark matter
02:35
is
02:37
everywhere.
02:38
Dark matter particles are streaming through
02:40
this room right now,
02:42
we are swimming in an ocean of dark matter particles,
02:45
but because the dark matter doesn't interact
02:48
with normal matter,
02:49
we can't directly detect it.
02:51
We can only learn about it through indirect methods.
02:55
There are some things we do know about dark matter,
02:58
but to tell you, I need to go to the chalkboard.
03:03
Let me show you one of the pieces of evidence
03:06
that we have for dark matter
03:08
and I wanna show you this
03:09
'cause it's more than just using this evidence
03:11
to infer that dark matter exists.
03:14
We can also figure out how it acts in our universe.
03:17
And to do that,
03:18
we're gonna look at rotation curves.
03:21
So, rotation curves are a connection
03:23
between the velocity of stars in orbit around a galaxy
03:27
and the amount of stuff the luminous matter
03:30
that we can see in that galaxy.
03:32
This is basic Newtonian mechanics.
03:35
It's just simple gravity.
03:36
Yap, thanks, Isaac.
03:38
So, check this out.
03:39
When we look at a galaxy like this,
03:41
most of the material is actually compressed into the core.
03:45
And then the further away you get from the center,
03:47
it thins out.
03:49
And so, if we make a plot of the velocity of stars
03:53
versus the distance,
03:54
it should look something like this.
03:57
There's more stuff in the center.
03:59
So, the stars orbit faster and faster,
04:01
but then it starts to thin out
04:03
and you get further away.
04:06
And eventually, the most distant stars
04:09
should not be orbiting very quickly at all.
04:12
This is not what we observe at all.
04:15
Instead, we see something completely different
04:17
in galaxies across the universe.
04:20
It goes up like this
04:23
and then it stays up.
04:26
The stars in galaxies are orbiting the center
04:29
much faster than they should be
04:31
if we just accounted for all the luminous matter.
04:35
There is something else going on,
04:36
there is some invisible kind of matter,
04:39
there is dark matter.
04:42
Let me show you something cool.
04:44
Shoot, I'm out of room.
04:45
Is this one of those fancy chalkboards that spin up?
04:49
It is.
04:51
All right.
04:52
What we discovered about dark matter
04:56
is that every single galaxy in the universe
04:59
is surrounded by a ball of it,
05:00
something we call a halo,
05:02
and that every halo in the universe
05:05
shares a common structure, a common shape,
05:08
something we call a universal density profile.
05:11
This one that I'm about to show you, by the way,
05:13
it's called the NFW profile for Navarro, Frenk and White,
05:17
the three astronomers who figured it out.
05:19
And the equation looks something like this.
05:21
It tells us that all dark matter,
05:24
it's density as a function of radius
05:28
looks like a scale density
05:31
divided by the radius over a scale radius
05:35
times one plus radius over scale radius squared.
05:43
This scale density and scale radius are different numbers
05:47
for every single halo,
05:48
but no matter what,
05:50
they all share this common shape
05:52
and the shape looks like this,
05:55
density as a function of radius,
05:57
starts like this
05:59
and then goes down.
06:01
And the scale radius tells us where this breaking point is.
06:05
It tells us something interesting
06:08
about the evolution of dark matter halos.
06:12
It tells us that they first form
06:14
with a central ball of density
06:17
and then slowly over time
06:19
accumulate more dark matter particles.
06:22
This is amazing.
06:23
This is telling us about the history of structure
06:27
in our own universe.
06:29
But this is all theory,
06:30
let's see what experiments
06:32
could possibly tell us about dark matter.
06:36
That's how we know dark matter behaves theoretically.
06:39
But what we really care about
06:41
is directly detecting it.
06:43
We can see its gravitational influence
06:45
everywhere in the universe,
06:46
but we wanna know, we wanna feel it,
06:48
we wanna taste it, we wanna smell it.
06:50
I mean, this aquarium has normal matter in it.
06:54
Not too much, but you know enough,
06:57
but really it's full to the brim of dark matter.
07:00
We just can't see it, but let's pretend that we can.
07:03
Oh, perfect, perfect.
07:06
Thank you.
07:08
All right.
07:09
Look at all that dark matter.
07:11
Dark matter makes up 80 to 90% of all the mass
07:15
in every single galaxy.
07:17
We just can't directly see it.
07:19
We wanna know how dark matter interacts with itself
07:22
and with the normal or baryonic world.
07:26
That's where every single theorist with time to kill
07:29
makes up their own pet theory
07:31
of how dark matter might work
07:32
and we have so many candidates.
07:34
We have, let's see,
07:35
weakly interacting massive particles,
07:37
we have self-interacting dark matter,
07:39
we have axions and axion-like particles,
07:43
we have primordial black holes
07:44
and sterile neutrinos and on and on and on.
07:47
But what really matters
07:48
is that all these different ideas,
07:50
all these different theories
07:52
predict how dark matter might behave in our universe.
07:55
And then we can go out and try to detect it, observe it,
07:59
somehow catch a glimpse of dark matter
08:02
and prove one of these hypotheses right.
08:05
And there's all sorts of possibilities
08:07
of how dark matter might interact.
08:09
So for example,
08:10
two dark matter particles may occasionally collide
08:14
and annihilate each other
08:15
in a flash of gamma ray energy that we can see.
08:18
Or it could be that dark matter buries itself
08:22
deep in the heart of a star
08:23
and raises the temperature higher
08:25
than what you might normally expect.
08:27
We might even detect it here on the earth.
08:29
We could set up, say, a cryogenic ultra cold detector
08:33
and wait a really long time.
08:35
And then occasionally, a dark matter particle
08:37
will hit the detector and heat it up just a little bit
08:41
and we can detect that heat.
08:43
Or we can set up really pure xenon or argon
08:46
and again, wait a really long time
08:48
and dark matter comes in
08:51
and releases a flash of light that we can see.
08:53
We have dozens of detectors and instruments
08:56
and observatories around the world hunting for dark matter
08:59
every single second of every single day.
09:02
We just haven't seen any yet.
09:04
Ah!
09:08
Sorry about that, it can get a little bit frustrating.
09:11
I mean, does dark matter even exists?
09:14
Luckily, I know an expert.
09:17
I'm Janna Levin.
09:18
I'm a professor of physics and astronomy
09:22
at Barnard College of Columbia University.
09:25
In our episode on dark matter,
09:26
we're covering some of the possibilities,
09:29
some of the candidates
09:30
of what the dark matter particle could be.
09:33
What is the difference between, say, a WIMP
09:36
and like an axion or one of the ultralight bosons?
09:40
How are these particles different
09:41
and where in our theories do they come from?
09:44
Well, the first thing that I wanna say
09:45
is that we know that dark matter exists,
09:48
even if we can't explain the bulk of it,
09:51
The problem isn't whether or not dark matter exists,
09:55
we see neutrinos and they are dark matter.
09:57
We know that there are particles
09:59
that do not interact with light
10:01
and they have mass
10:02
and they contribute to the weight of the universe.
10:06
But they're not sufficient to explain the bulk of it.
10:10
The surprising part isn't that they are particles
10:12
that don't interact with light,
10:14
the surprising part is that it's so hefty.
10:19
We account for some kind of 5% residual ashy residue
10:25
left over from the Big Bang
10:26
and the dark matter is more like 27%.
10:30
And that's the bizarre part that we don't understand.
10:33
So, we have definitely seen dark particles.
10:36
It's kind of thrilling for a theoretical physicist
10:39
to think that it's a hint to something
10:42
beyond what we already know
10:45
and that's what dark matter is giving us almost as a gift,
10:48
what we don't know, it's giving us a clue.
10:51
It's a gift.
10:52
Why can't the neutrinos be the dark matter?
10:55
Why is that ruled out?
10:56
It's a great question
10:57
'cause neutrinos are absolutely
11:00
a physical undeniable verifiable example of dark matter.
11:04
They do not interact with light.
11:06
They have all the properties of dark matter,
11:09
but they're not heavy enough
11:11
or abundant enough to explain
11:15
the extreme dominance in the energy pie.
11:19
So, if you think of the energy pie of the universe,
11:22
dark matter is taking up like some twenty-five percent,
11:25
let's just say roughly.
11:27
The neutrinos that we know about are not hefty enough
11:31
to make up for that pie,
11:32
but they're definitely an undeniable example of dark better.
11:36
So, I think the question is really,
11:38
are there really heavy neutrinos?
11:40
And that's basically a lot of people are looking for that.
11:42
They're looking for WIMPs,
11:44
weakly interacting massive particles, WIMPs,
11:48
which is what neutrinos are.
11:50
They're weakly interactive massive particles
11:52
and they're looking for WIMPs
11:54
that are much, much heavier than neutrinos
11:56
and that don't fit into our standard understanding
12:01
of particle physics.
12:03
Does dark matter do more
12:05
than just sit there and gravitate?
12:08
Did it potentially play a role in the very early universe?
12:12
I'm thinking like baryogenesis
12:14
and matter/anti-matter asymmetry
12:16
and all the crazy physics happening
12:18
in the first few seconds of the Big Bang.
12:21
Could it be that dark matter played a role back then too?
12:23
I mean, that's a really great question.
12:26
I think when we're searching for dark matter,
12:29
we're cross correlating with explanations
12:33
of the baryon asymmetry.
12:35
When the universe was created
12:36
in principle of all the symmetries exists,
12:38
there should be an equal amount of matter and anti-matter
12:41
and they should annihilate and there'd be nothing.
12:43
And so, we know that there's a violation of that symmetry.
12:48
We know that for some reason,
12:49
matter is preferred over anti-matter.
12:52
And so there's a tiny, tiny excess.
12:54
Should dark matter play a role in that?
12:57
Probably, one would hope so by the economy of explanations,
13:02
but we don't really know.
13:03
So, if we find the dark matter,
13:06
for sure the hope is that we're going to be like,
13:08
Whoa, does it explain baryogenesis
13:11
and where does it fit into the bigger scheme?
13:13
And all of these things are like clues nudging us
13:18
towards the right explanation.
13:20
It's almost too ambitious
13:21
to try to grope for it all at once.
13:25
We're lucky if we find one thing
13:27
and that thing will definitely redirect other searches.
13:31
Janna, thank you so much
13:33
for your time- Thank you.
13:34
And joining us on this episode.
13:37
As you can see, when it comes to dark matter,
13:40
there are more questions than answers.
13:42
Is dark matter simple
13:44
consisting of just a single kind of particle
13:46
or is it complex
13:47
with lots of different kinds of particles participating?
13:50
Is there still some undiscovered theory of gravity
13:53
beyond Einstein's relativity
13:55
that could just explain a way all of these results?
13:59
Not now, Albert.
14:01
Are there new forces of nature involved?
14:03
What role did dark matter play
14:05
in the earliest moments of the universe?
14:07
Dark matter is more than a hypothesis.
14:10
It's a framework for understanding vast swaths of phenomena
14:14
across the universe,
14:16
but it's like a house that isn't finished.
14:18
We have the foundation,
14:19
we just can't live in it yet.
14:21
And that's why dark matter is on the edge of knowledge.
14:25
[gentle anticipatory music]