How the Universe Works

The night sky.

Countless stars and the majestic sweep

of the Milky Way, but beyond our local neighborhood,

across the cosmos, there are over

two trillion more galaxies.

When we first began to observe galaxies,

we collected them like butterflies.

Little by little, we realized that

they formed a web.

The cosmic web is the infrastructure that connects

every corner of the universe.

You don't know anything about our universe

if you don't understand the cosmic web.

It feeds galaxies. It forms galaxies.

It is made of galaxies.

It's the architect of everything,

and our cosmic future depends on it.

The cosmic web is one of the most important parts

of our universe... It plays a key role

in the evolution of the cosmos.

Without the cosmic web, there would be no stars,

no planets, nowhere in the universe where

the conditions of life could exist.

How did the universe go from a hot soup of gas

to a cosmic web, sprinkled with galaxies,

planets, and us?

The universe may appear random.

Two trillion galaxies, spread across the cosmos.

But in this cosmic chaos,

scientists detect water.

When we first saw that the universe was full

of galaxies, it seemed like overwhelming chaos,

but it's not... They're all connected.

Galaxies link up in a gigantic cosmic network

spanning the entire universe.

How this pattern emerged may be cosmology's

biggest puzzle.

In some senses, you don't understand something

unless you understand how it comes into existence

and how it's formed.

And galaxies are the basic building block

of our universe.

To solve this mystery, scientists need to go deep,

to the very edge of the observable universe,

and study light from the first galaxies.

Chile, 2021.

Scientists point the VLT, or Very Large Telescope,

towards the Hubble Ultra Deep Field.

It's a patch of sky famously photographed

by the Hubble Space Telescope in 1995.

The VLT's power allows astronomers to see

much deeper into space.

Imagine you take a grain of sand,

and you put it on your fingertip,

and you hold your arm out like this,

and you block a part of the sky looking

at that grain of sand... That's the size

of the Hubble Ultra Deep Field, and yet it contains

thousands of galaxies in it.

The telescope stares at those galaxies

for 155 hours and picks up the faintest of glows...

ancient hydrogen gas concentrated along a strand

of space 15 million lightyears long.

The filaments are just one tiny section

of the cosmic web, the largest known

structure in the universe.

The scale of the cosmic web is enormous.

It is, by definition, the largest thing

that we can see in our universe.

Today, the cosmic web is a lattice of filaments,

linked streams of hydrogen gas

that form an intergalactic network spanning

the entire universe.

Inside the nodes of the cosmic web,

you'll find galaxies and stars and black holes.

Along the filaments, you'll find gas

that connects these nodes, and the gas will connect

to the other galaxies and clusters of galaxies.

It's this beautiful superhighway of

large cities that are connected through these filaments.

We can see the cosmic web

about as far back as we can look,

and really, galaxies are forming along that web

all the way back.

This cosmic infrastructure dates back

to the earliest days of the universe.

13.8 billion years ago,

the universe ignites in a tiny ball

of super hot energy.

It expands and begins to cool.

Energy transforms into primitive,

subatomic particles of matter.

The heat from the Big Bang is so intense,

gravity is effectively powerless.

The very early universe was super hot,

super energetic, and regular particles

of matter were zipping around so fast

that not even gravity could hold them together.

But regular matter wasn't the only thing

in the early universe.

In the background, gravity is working

on something else...

Regular matter's ghostly cousin,

the invisible substance known today as dark matter.

It makes up about 85 percent of all the matter

created in the early universe.

Normal matter and dark matter both existed

around the time of the Big Bang, but they way they played out

was very different.

Just ten seconds after the Big Bang,

the infant universe is billions of degrees Fahrenheit,

still far too hot for regular matter particles

to clump together, but dark matter plays by different rules.

Dark matter isn't affected by the Big Bang's intense

radiation in the same way that regular matter is,

and so because it's able to cool,

it clumps together in a way that regular matter doesn't.

As dark matter clumps grow, they exert

a gravitational pull and begin to form shadowy structures.

As soon as the dark matter gets a foothold,

we have a place where there's a bit more stuff,

then that attracts more and more dark matter.

380,000 years after the Big Bang,

the intense heat drops to a few thousand degrees.

Normal particles of matter move around more slowly.

Protons and electrons bind together and form

atoms of hydrogen and helium gas.

Then gravity from dark matter starts to work

on regular matter.

Before you know it, you have this very clumpy

universe with these huge dark matter halos

that can now start to draw in also ordinary matter

in the form of gas.

A billion-year building project begins.

The dark matter clumps pulled in clouds of gas...

the foundations of the cosmic web and the galaxies.

Just as when you build a building, you know,

there's a lot of work that happens before

the building goes up, our universe spent

a lot of time laying the groundwork for

this cosmic web before it switched on the lights.

The foundations are complete,

but the job isn't finished.

How did those clouds of gas transform into the greatest

structure in the universe?

The secretive dark matter that brought the gas together

is also on site, managing the build.

It was really the dark matter that called the shots

in cosmic clustering, because it outweighed

the ordinary stuff by a big factor.

In essence, the cosmic web is made of dark matter.

Tendrils of material are stretched out across the cosmos.

As the sprawling structure builds, its gravitational

pull strengthens, pulling in more dark matter.

The clumps begin to collapse and shrink down

into filaments... these meet at even more tightly

packed clusters, creating a huge, dark scaffold

that drags in more hydrogen gas.

Imagine drops of dew on a spider web.

That's like hydrogen blobs being pulled in

to dark matter's cosmic web.

After tens of millions of years of construction,

strands of gas stretch across the cosmos.

Fast forward to now... The web appears

in all its star-spangled glory, lit up with galaxies.

We know at some point, stars and galaxies formed.

The big question is when... What were the first

galaxies like? That's a big mystery.

So how then did the lights of the cosmos switch on?

Evidence suggests that as the universe assembled its web

of dark matter and hydrogen gas,

the biggest stars that have ever lived

set the cosmos ablaze.

2018, scientists study an ancient galaxy,

the catchily named MACS1149-JD1.

There, they find some of the oldest stars

ever detected.

This particular galaxy is exciting, because it's

forming stars just a very short time after the Big Bang.

Those stars could hold clues as to how

the cosmic web that supports the universe

first lit up, but as astronomers study

starlight from when the universe was just

250 million years old, they get a shock.

The stars are not just made up of hydrogen

and helium produced in the Big Bang.

They also contain what astronomers call metals.

Metals in astronomy is everything heavier

than hydrogen and helium.

No matter where it is on the periodic table,

if you're not hydrogen or helium, you are a metal,

even though that makes no sense.

If I were king of astronomy, metals is right out.

The Big Bang only made hydrogen and helium.

Anything heavier than that was churned up in

the cores of dying stars.

The bright stars of this ancient galaxy

dating back to just 250 million years after

the Big Bang contain chemicals that were created

in even earlier stars.