0:02

The cosmic web is kind of like the

0:04

skeleton of the universe. The

0:06

clusteriest of clusters and the voidiest of

0:08

voids is the things that come to

0:10

mind, right? If you get things that challenge

0:12

that, well then we just rethink the physics. Hello

0:19

and welcome to the Supermassive podcast

0:21

from the Royal Astronomical Society with

0:23

me, science journalist Izzy

0:25

Clark and astrophysicist Dr Becky Smithers.

0:28

This month we're untangling the cosmic web,

0:30

not the James Webb Space Telescope, but

0:32

the large scale structure of the universe.

0:34

What do we know about it and

0:36

what can it tell us? Yeah, I

0:38

just thought it was about time we

0:40

got into something really complicated because we'd

0:43

just be taking a bit too easy

0:45

with all this planetary science and our

0:47

tour of the solar system. What, is

0:49

your head not hurt very recently Izzy when you've

0:51

been recording? No, I'm due another headache so here

0:53

we go. As always,

0:55

Dr Robert Massey, the deputy director of the

0:58

Royal Astronomical Society is here. So Robert,

1:01

how would you describe the

1:03

cosmic web? Because it

1:05

might not be something that everyone is

1:07

familiar with. So let's begin this brain

1:10

stretch right now. Exactly. Brain stretch

1:12

headache questions that I struggle to answer.

1:14

You know, everything that makes a good

1:16

Supermassive episode. The good question. Exactly. The

1:18

good questions. The good questions. Yeah.

1:20

So look, I mean, if you take a kind

1:22

of casual look at the sky, a very quick

1:24

line of sight, you think, okay, randomly distributed stars,

1:26

but even in our own galaxy, you look a

1:29

bit longer, you see the band of the Milky

1:31

Way. So you can quite easily deduce there's some

1:33

structure in the way the stars are distributed. And

1:35

if you look on a much bigger scale, if

1:38

you map galaxies on a huge scale and you

1:40

do that in 3D, then it turns out they

1:42

grouped into these huge clusters and those clusters are

1:44

on this filaments of this giant web. And there

1:46

are these filaments and voids and

1:48

that's the sort of bubbly structure of the

1:50

universe on the bigger scale. And we didn't

1:52

know about it until the 1980s because that

1:54

was when we got better telescopes and we

1:56

were able to measure those distances more reliably

1:58

and push them out further. than before. And

2:00

those galaxies, the galaxies like the one we live

2:03

in, they're concentrated into these huge clusters. They tend

2:05

to be concentrated along the nodes or the knots

2:07

and strands of the web. And they're around voids

2:09

where there are far fewer of them. And these

2:12

are really big. These are billions of light years

2:14

across. So they're certainly the biggest

2:16

structures in the whole universe. So understandably, there's

2:18

a lot of interesting understanding things on the

2:20

biggest scales. Yeah. I like to joke that,

2:22

you know, instead of like turtles all the way down, it's just filaments

2:24

all the way. It's exactly filaments, bubbles,

2:27

soap, what do you describe it as?

2:29

I don't know, soap or spongy or

2:31

something like that. Yeah. I've always looked at it as

2:33

like a big sponge. Right. And I like how you were

2:35

saying, you know, we didn't discover this to the eighties because

2:37

we almost couldn't zoom out far enough to see it. Right.

2:40

Cheers Robert. We'll catch up with you later in

2:42

the show for some more questions. And this month's

2:44

stargazing tips. So buckle up everyone.

2:47

The mind stretching continues because we're going to

2:49

dive into the world of cosmology. So far,

2:51

we've sort of talked about the structure of

2:53

the cosmic web, but you're about to hear

2:55

a little bit more on that. Why

2:58

is it important and what is it made

3:00

of and where the heck has it come

3:02

from? These are all questions that I

3:04

put to Dr. Kiara Mingarelli from Yale

3:07

University. The cosmic web is the name

3:09

that we give to this structure

3:12

that we can see on very large

3:14

scales, on the largest scales. So

3:16

let's just start where we are right now

3:18

and then zoom out. Okay. So we're

3:20

on the earth. We're in the Milky

3:22

Way galaxy. Next door is the Andromeda galaxy,

3:25

but we're part of a galaxy

3:27

cluster. And then if you zoom out of

3:30

our galaxy cluster, far away,

3:32

there's another galaxy cluster. And there's like

3:34

these filamentary structures that can connect these

3:36

galaxies. And then if you zoom out

3:38

again, you can see even

3:40

more galaxy clusters with more of these filamentary

3:43

structures that connect them. So they kind of

3:45

looks like a brain with all

3:47

of these bundles of neurons that are

3:49

sort of talking to each other. Now, I'm

3:51

not saying that we can talk to each

3:53

other through the cosmic web, but we are

3:55

all connected. And so it

3:58

begs the question, like, why is that? there and

4:00

how did that form? Okay, so we've got

4:02

this big connection of galaxies, galaxy

4:04

class A. I can just picture this like we

4:07

start as a little spot and we move out

4:09

and we've got this, I don't

4:11

want to say a tangle, but it's

4:13

kind of like a tangle. Some people

4:15

might want to think of it as

4:17

like a net kind of structure. That's

4:19

exactly right. So to picture this, is

4:22

it a physical structure? What is it made

4:25

of? Do we know? Right.

4:28

So it's kind of like the skeleton

4:30

of the universe, right? If you were

4:32

to take this kind of skeleton and

4:34

then paint on hydrogen gas, then you

4:36

would see it. And so

4:38

it's mostly made up of gas, like

4:41

the filamentary structures are really largely made

4:43

up of gas. But then

4:45

the big nodes, you know, that are the things

4:47

that are being connected, there's a

4:49

lot of hydrogen, but then you have

4:52

other heavier elements that help to make

4:54

up galaxies. And that's all formed by

4:56

the early stars. So the earliest stars

4:58

were only made up of hydrogen, because

5:00

that's like a primordial element that was

5:02

there in the beginning, hydrogen and helium.

5:04

And then as stars burn, they can create

5:07

these heavier elements. And so everything that we

5:09

have was created in these

5:11

early burning stars, and then in supernova

5:13

explosions. And then recently, we also know

5:15

that some of the heaviest elements like

5:18

gold and platinum were created

5:20

by merging dead

5:22

cores of stars called neutron stars.

5:24

That's also really exciting. Where

5:27

has this come from? Where does this begin?

5:29

How do we even begin to unpack this?

5:32

Yeah, that's a great question. So

5:34

it all started about 380,000 years

5:37

after the Big Bang. There was

5:39

this cosmic soup, and

5:41

the universe was so hot, that

5:44

light and particles that make

5:46

up you and I like protons and neutrons

5:48

and you know, things that we call baryonic

5:50

material, which is just stuff that you can

5:53

touch with your hands, right, your desk, your

5:55

watch, your headphones. All of this

5:57

was in this primordial goop, right, which includes

6:00

It was so hot that even light couldn't escape

6:02

from this really hot soup. And

6:04

inside of this soup, there were

6:06

quantum fluctuations from the big bang that

6:09

had been amplified by inflation in the

6:11

early universe. Right after the big bang,

6:14

about 10 seconds, the universe inflated to

6:16

be an enormous size compared to what

6:18

it was. And so the

6:20

small fluctuations that had happened right at the

6:22

quantum level after the big bang

6:25

then became huge, right? Because everything expanded

6:27

in all directions. And so those

6:29

tiny little fluctuations are now really big. And

6:32

so some of them are

6:35

troughs and dark matter can

6:37

go into those troughs. And

6:40

it forms these places that then regular matter

6:42

wants to go later. So when

6:44

the universe can cool down and then

6:46

light can finally escape from this

6:48

cosmic soup, we get the

6:51

cosmic microwave background that maybe some of

6:53

the listeners are familiar with. It's at 2.7 degrees Kelvin. But

6:57

where I was going with this is that

7:01

those fluctuations created these little nests

7:03

for regular matter to fall into

7:05

and to cool off. And that's

7:08

how we formed galaxies

7:10

and galaxy clusters. And

7:12

so these little cosmic potholes that were

7:14

really tiny at the beginning of the

7:17

universe and then grew to enormous sizes

7:19

now are the homes of lots

7:21

of regular baryonic matter, including and

7:23

also dark matter that surrounds them.

7:26

So it's like the dark matter is their

7:28

house. Yeah. Right. And then all of the

7:30

baryonic matter goes inside and then

7:32

it forms all of these structures as it cools

7:34

down, but it has to cool down because when

7:36

it's hot, stuff is moving everywhere, but as it

7:38

cools down, then it can like go to its

7:41

neighbor's house, see what's going on,

7:43

find out what's happening. It can get

7:45

together in little groups and then you

7:47

can form galaxies and then galaxies can

7:49

eventually merge and they get bigger and

7:51

that's how we think the universe works. A

7:54

nutshell. Yeah. Okay. Thank you. Just blow

7:56

my mind. Thanks very much. I

8:00

think this is a really interesting idea.

8:02

So we're saying we've got these pockets

8:04

of matter essentially, and that's where we

8:06

have our galaxies forming and clustering together.

8:08

And so are we saying like

8:11

matter can travel between these webs

8:13

from say one little cluster of

8:15

galaxies, say over here in the

8:17

left to say another one

8:19

over here on the right, you know, obviously

8:22

we're talking about astronomically massive scales, but they

8:24

are connected. They can, what

8:27

matter flows through them, is it correct to

8:29

say that? Well, it's

8:31

hard to say if it's flowing or not,

8:34

because in order to see any kind of

8:36

flow, we would have to observe it moving.

8:38

Okay. And these distances are

8:40

so vast that it's kind

8:43

of like trying to watch

8:45

a turtle go 100 kilometers and

8:48

you're wondering like, can the turtle actually travel that far?

8:50

And then if you just watch the turtle for

8:53

a second, you're like, this turtle doesn't move.

8:55

But if you give it enough time, it's

8:57

gonna move and it can go a hundred

8:59

kilometers, right? I'm Canadian. And so I tend

9:01

to use the metric system. Hey,

9:04

we're all fine with that. That's right.

9:06

Okay, good. So it's

9:08

really a time scale issue. We certainly

9:10

know that we can see the filaments,

9:13

which means that there's at least hydrogen

9:15

gas in those filaments, but

9:17

you know, which way it's flowing and if

9:19

it's flowing or if it's just there, it's

9:22

kind of hard to say. Are universes expanding?

9:24

So what does that mean for the cosmic

9:26

web? And how do we

9:28

know how that interaction plays out or

9:30

how does that interaction play out if

9:32

you've got galaxies merging as well? Right,

9:35

that's a great question. So each

9:37

one of these nodes has

9:39

galaxies in it and they're all

9:42

gravitationally bound, these galactic super clusters.

9:45

And so the force of gravity inside

9:47

of those is stronger than the acceleration

9:49

of the universe. But what

9:51

that means is that eventually we're gonna

9:53

get really far away from the other

9:55

super clusters. And so like these filaments

9:57

are gonna get thinner and thinner and...

9:59

and thinner as we move further away.

10:02

So imagine a piece of toffee

10:04

getting thinner and thinner and thinner as

10:07

it goes. And eventually, there won't be

10:09

anything left. We can already

10:11

see some of these superclusters accelerating away from

10:13

us. And so in

10:15

the future, if we as a species

10:18

can survive another 100 million billion years,

10:21

what we see today is going to be

10:23

completely different from what our descendants will see. We

10:26

might be very much alone in a little

10:29

island in our little supercluster, because the other

10:31

ones will have accelerated away. So

10:33

does this mean that as our universe

10:36

expands, the formations

10:38

of galaxies, does that slow down? They're

10:40

not going to have that, I

10:43

want to say, swap of hydrogen

10:45

to help continue to build

10:47

them. Is that what we're talking about here?

10:50

Well, so what we're talking about is

10:53

competing forces. So we know, for

10:55

example, the universe is expanding right

10:57

now. But we're humans,

10:59

and we're here, because the forces

11:02

that keeps ourselves together is stronger

11:04

than the force of gravity that's

11:06

expanding everywhere. And so

11:08

similarly, in our supercluster right now,

11:11

the force of gravity and all of the

11:13

dark matter that's holding us together is stronger

11:15

than the expansion of the universe. It's making

11:17

everything else fly out. But

11:20

eventually, what will happen is something called

11:22

the heat death of the

11:24

universe. And that means

11:26

that eventually, the galaxies will all

11:28

merge, they're supermassive, black holes will

11:30

merge, all of the stars will

11:33

eventually burn out, and we'll have

11:35

just a black hole universe, where everything is

11:37

in some sort of black hole. And

11:39

then the black holes start to evaporate. And

11:41

that takes a really long time. They

11:44

emit something called Hawking radiation. But

11:47

all that means is that they eventually lose all

11:49

of their mass to radiation. And

11:51

the universe just becomes this cold, dead

11:53

place. OK.

11:56

I mean, it sounds lovely. Doesn't it?

11:58

Yes. Yes. Exactly. How

12:02

much can we actually know

12:04

about, because obviously

12:07

a big part of this is dark

12:09

matter. So can

12:11

we study it or does that depend

12:14

on being able to detect dark matter,

12:17

which in itself is a whole other

12:19

podcast episode? Yeah,

12:21

we can study it in different ways.

12:23

We can study the cosmic web by

12:26

looking at large surveys of

12:28

galaxies. We have really big

12:30

telescopes that look at the sky and

12:33

then we can map the sky and

12:35

we can see these filamentary structures emerging

12:37

so we can actually see the cosmic

12:39

web. We can also

12:42

make large computer simulations like

12:44

Illustris. And if you

12:46

look up Illustris on the internet, you can

12:48

find lots of beautiful animations that show you

12:50

where the dark matter has to be, where

12:52

the regular matter is. And

12:54

then what's really amazing is that you

12:57

can match the two, that

12:59

these simulations have to match what we

13:01

actually see with our telescopes in order

13:03

to be credible. So

13:06

one of the fun things you can do with the simulations

13:08

is that you can turn off dark matter. And

13:11

then what you see is that you can't make a universe.

13:13

There's no scaffolding. There's no skeleton for

13:16

the matter to clump on and cool

13:18

down on. There's no little house that

13:20

all the baryons can go into and

13:22

chill out. Right? Like they're just kind

13:24

of floating everywhere. And

13:26

it's really difficult to create any kind of

13:28

large scale structure, as we call it, which

13:31

is this cosmic web. Oh,

13:35

I really enjoyed speaking with Kiara. Yeah, she's great.

13:37

Yeah, yeah. It's so good. We've got part two

13:39

of an interview with her coming up in just

13:41

a moment because there was literally too much to

13:43

cover. Turns

13:45

out covering the biggest thing in the

13:47

entire universe. It's quite difficult. It's quite

13:49

difficult. Yeah. So

13:52

Becky, some follow up questions to that.

13:54

How important is dark matter for

13:57

this web light structure? I

14:00

mean, it literally wouldn't exist without it.

14:02

So dark matter, I think it's often

14:04

referred to as like the scaffold of

14:06

the cosmic web, right? It holds galaxies

14:08

together and it holds together clusters of

14:10

galaxies as well. And therefore,

14:12

if you keep going on that, it holds

14:14

together the whole structure of the web. So

14:16

without it, in just a

14:18

few billion years, those structures would completely disperse,

14:20

right? And it just wouldn't be held together

14:23

anymore. And that's because gravity just wouldn't be

14:25

strong enough to hold it together against all

14:27

the random motions that these galaxies have in

14:29

different directions, right? Like we talk about sort

14:31

of like redshift when we look out into

14:33

the universe, right? And all the galaxies appear

14:36

to be moving away from us, but that's

14:38

like an overall global thing that's going on

14:40

or universal thing that's going on. But like

14:42

think about how Andromeda is actually coming towards

14:44

us, towards the Milky Way, because in our

14:46

little local group, everything's got random motions

14:49

with respect to each other. Yeah. And

14:51

so it's the dark matter that sort

14:54

of pervades this entire structure and sort

14:56

of connects galaxies along these

14:58

filaments that holds everything together. And it's just

15:00

without it, it'd be like a whole house

15:02

of cards that falls apart. This is one

15:05

of the actual like big

15:07

pieces of evidence we have for dark matter

15:09

is that we can't get the

15:11

universe to exist and to look like

15:13

it does without it. Yeah, absolutely. And

15:15

so is there a repeatable pattern within

15:17

the web itself? Like, can we see

15:19

that or is it just random? We

15:22

don't think it repeats, no. So

15:24

there are recurring like similar structures,

15:27

like filaments, walls, voids, that Robert

15:29

was talking about before, they show

15:31

up everywhere and in every direction

15:33

we look, but we don't

15:35

see like the same patterns of those

15:37

structures repeating. No, it's not like some

15:39

sort of weird fractal or anything like

15:41

that. So when we go out to

15:43

large enough scales beyond around 300 million

15:45

light years across, the universe starts to

15:47

look what we call homogeneous. So it

15:49

looks the same in all directions on

15:51

a large scale. But like, if

15:53

you zoom into those smaller scales, it's still very

15:56

different in terms of like a

15:58

pattern. Yeah, okay. covered

28:00

and I think we can all agree that the

28:02

answer to that part is yes. So the second

28:04

half of David's question is, if that

28:06

is the case, is it possible

28:09

to calculate how long the process will

28:11

take, very best wishes to you all

28:13

and keep up the excellent podcast. Hi

28:16

David, first of all, great, great question.

28:18

Now, as we know, and as Kiara

28:20

said, yes, okay, this is going to

28:22

happen. The cosmic web will gradually thin

28:24

out and disappear due to the expansion

28:26

of the universe. Now we know the

28:29

rate of expansion of the universe. We

28:31

know the rate is accelerating at, but

28:33

extrapolating forward is a little bit difficult

28:35

because, you know, there are different models

28:37

for what the expansion is going to

28:39

do. So that does affect things slightly.

28:41

However, the timescales involved

28:44

are really, really quite long.

28:46

So I think in the grand scheme of

28:48

things, I can give you like an earmark

28:50

figure for what would happen. We

28:52

think in around about a hundred billion

28:54

years because of the accelerated expansion and bear in

28:57

mind, you know, the universe currently has 13.7 billion

28:59

years. This is far in the future in terms of its

29:01

sort of history. That any

29:03

galaxy that's not bound to our local group,

29:05

so in the way and Andromeda, they'll

29:08

have moved so far away that we actually

29:10

won't be able to see them beyond the

29:12

observable universe. So the structure will still

29:14

exist, but it'll be so thin that we won't even be

29:16

able to see it. It's only

29:18

in a trillion years time

29:20

that structures like within clusters and

29:22

the web itself will actually begin

29:25

to be affected, right? So it

29:27

will be stretched out.

29:30

And then over trillions to tens of

29:33

trillions, hundreds of trillions of years, we

29:35

think is when eventually that gravitationally bound

29:37

sort of nature of clusters will

29:40

actually be overtaken by the universe's expansion.

29:42

And I think that just puts it

29:44

into perspective, like, okay, yes, the universe

29:46

is expanding at an incredible rate, but

29:49

you know, gravity does its job pretty well in

29:51

holding things together. And thanks to, you know, dark

29:54

matter, as we talked about before, you know,

29:56

and this is why, you know, when people ask why

29:59

isn't the space between stars. expanding in the

30:01

Milky Way, it's because well gravity is stronger

30:03

on those smaller scales to hold everything together.

30:05

So like, you know, the space between stars

30:07

isn't getting bigger in the Milky Way because

30:10

everything's bound by gravity. And so

30:12

over a much larger scale is when

30:15

the acceleration of the universe starts to

30:17

take hold, but still gravity locally is

30:19

the strongest thing until trillions to tens

30:21

of trillions of years time, when eventually

30:23

that is overcome, at least, we

30:26

think, based on our current models of what's happening

30:28

in terms of the expansion rate of the universe.

30:31

Okay, thanks Becky. I hope that answers

30:33

your question, David. And Robert

30:35

Adrian111 asks, can

30:38

Euclid help with mapping the cosmic

30:40

web? Will it make a dark

30:42

matter and dark energy map? So

30:44

Euclid is this telescope that we've

30:46

all been talking about recently. I'm

30:48

so excited. Yeah, it's great. So

30:50

stay released next year. So excited.

30:52

Okay, good stuff. But back to

30:54

Adrian's question. Yeah, it's good stuff.

30:56

Adrian111, it's a good question to

30:58

ask. The answer is yes, because Euclid is designed

31:00

to map galaxies out 10 billion light

31:02

years away across a third of the sky. So looking back

31:04

a long way into the past in the universe as well.

31:06

So it'll definitely help us make a map of the web

31:09

because the whole objective is to make a 3D map of

31:11

a chunk of the universe. So not the whole universe that

31:13

we can see by any means, but quite a big bit

31:15

of it. And when you look

31:17

at things like phenomena like gravitational lensing and

31:19

the bending of light by gravity, that's a

31:21

way of mapping dark matter and understanding exactly

31:24

where it is because you see this lensing.

31:26

And you know, if you don't associate it

31:28

with visible matter or even implied visible matter,

31:30

normal matter as it's called, then you know

31:32

there's dark matter there. Now, dark

31:34

energy is sort of more pervasive and

31:36

uniform and we have really,

31:39

it's fair to say, not a good idea of what

31:41

it is still, even compared with dark matter where they

31:43

release more candidates. So mapping it is a bit

31:46

of a challenge. But by

31:48

getting the spectra of the galaxy, so looking when

31:50

I talk about a spectrum, you think about a

31:52

rainbow, think about the light being dispersed across colors

31:54

and then think about that happening in radio and

31:56

x-ray as well. But for this, in

31:58

this case optical. And infrared. And

32:01

infrared. So I'm sorry, infrared. We can

32:03

use that to make... Don't get infrared, please. Yeah,

32:05

no, no, I do need to be corrected. Mostly

32:07

infrared, the infrared. But we can work out how

32:09

fast galaxies are moving through redshift, basically by seeing

32:11

how the lines in the spectrum are shifted, and

32:14

then understand the expansion of the universe, and

32:16

then we can deduce how much of an

32:18

effect dark energy is having. We have a

32:20

good idea overall, but just verifying that, thinking

32:22

about all that stuff. So Euclid is making

32:24

a big contribution to that too. So the

32:26

answer to your question is, yeah, we'll get

32:28

better maps as a result, including dark matter.

32:30

Yeah, the dark energy one's so exciting as

32:32

well. The idea that we can trace the

32:34

expansion rate of the universe by looking back

32:36

in further weather distances is just

32:39

so cool. I'm so excited for it. And someday we'll

32:41

know what it is, right? Well, fingers crossed, you know?

32:43

Come on, guys, I'm out of this. Come on, Becky, you

32:46

need to change field. You need

32:48

a Nobel Prize, surely. No

32:50

pressure. Yeah, no pressure. Okay,

32:52

and Becky, there's not a name on

32:54

this one. But they've asked, how sure

32:57

are we about the structure of the

32:59

cosmic web? How precise can we be,

33:01

especially in far areas? Yeah, that's

33:03

a pretty good question. So I mean, we're

33:06

pretty sure about the structure because many surveys

33:08

have looked at this and all seen the

33:10

same thing. And obviously with newer telescopes, new

33:12

observatories, we're pushing to higher redshifts or greater

33:14

distances away from us all the time, like

33:16

with Euclid, as we just talked about. But

33:19

as we do that, obviously we're only seeing

33:21

the brightest of galaxies. We're not seeing the

33:23

faintest of galaxies when we go to those

33:25

huge distances. Even with something

33:27

like James Webb, right? You're still

33:29

not going to see the faintest of

33:31

things, even though it's got this incredible

33:33

sensitivity and light-collecting power. We know there's

33:36

some that we're still missing. However,

33:38

the brightest galaxies still do trace that

33:40

overall structure, right? And as I said

33:43

before, 300 million light

33:45

years is when things start to look homogeneous

33:47

and the same in all directions and have

33:49

that overarching structure of the filaments and the

33:51

clusters and the voids and the knots. So

33:54

we know that it's sort of the same everywhere

33:56

else we look. Now, to put context on that

33:58

number of 300 million.