What Is The Universe Expanding Into?

“You must not attempt this approach to parallels. I have traversed this

bottomless night, which extinguished all light and joy in my life. I entreat you,

leave the science of parallels alone…” Two trains hurtle along tracks side by

Two trains hurtle along tracks side by side, straining every rivet and bolt.

It’s neck and neck, one locomotive inching in front of the other, before conceding ground.

Crowds line the sidings, waving flags and cheering for their favourite engine in this unusual race. Then the unthinkable happens.

Gasps ring out as the trains smash into each other, metal folding like

paper as they burst into flames.

But how could this have happened?

After all, the tracks the trains were racing along were parallel. The very definition of

parallel lines is that they will never meet, no matter how far you extend them.

How could they have come together? Mathematicians have studied parallel lines

for millennia. Among the earliest to juggle with these ideas was the Greek polymath, Euclid - often

referred to as the Father of Geometry. Euclid penned one of the most influential books ever

written, Elements - containing many of the rules that underpin mathematics to this day.

And the fifth of these rules is called the parallel postulate.

This effectively states that two trains travelling along parallel tracks should never, ever meet. The

other four postulates were quickly proven, but the parallel postulate remained evasive,

unproven for almost two thousand years. Until finally, in the 19th century,

mathematicians dropped an existential bombshell. The postulate hadn't been proven

because it couldn't be. Two parallel lines could meet after all.

Suddenly, Euclidean geometry was no longer the only game in town.

It became possible to bend and contort space in ways that completely upend the usual

rules. Indeed, among those who broke Euclid’s parallel postulate was Hungarian mathematician

night that extinguished all joy in his life. But what does this mean? And why does

this matter outside of mathematics? The answer, as we will see, is truly bizarre.

For today, non-Euclidean geometry lies at the heart of one of the most fundamental questions

in the universe. A cosmic question close to the top of the list of those asked to astronomers.

Our journey towards answering this question will take us to bizarre, twisted universes

where light loops round and we can see the same galaxies multiple times in the sky. It will guide

us through universes folded back on themselves, universes where if you look hard enough you may

see yourself staring back - and universes where parallel lines meet again and again and again.

It is a trip that will defy common sense, but is guaranteed to leave you with a much deeper

understanding of the cosmos in which we live - and possibly even which cosmos in which we live.

And the question we will be answering?

If the universe is expanding, just what is it expanding into...?

On the 27th of December 2024, a telescope in Chile discovered something that caused the

UN to activate a planetary defence protocol for the very first time.

The telescope had discovered 2024 YR4, an asteroid the size of a football field, that

if it hits Earth in 2032 will unleash hundreds of times more energy than the Hiroshima bomb.

But the question is - will it? With breaking news like this,

especially science breaking news, which is very susceptible to hyperbole, it is hugely

important to know where your information is coming from, which is why I use Ground News

as an indispensable resource when researching, and they've kindly helped make this video possible.

Ground News gathers the world’s news in one place so you can compare coverage and verify

your information. For the 2032 asteroid, it lists 224 news sources all on one handy page,

and rates each publication for bias and factuality, as well as providing

information about the publication's ownership. For example, one source listed as 'mixed

factuality' originally ran with the headline 'Graphic shows asteroid the size of a football

pitch on course to hit Earth' whereas most of the sources listed as high or very high

factuality were more up front that the chances of impact with earth are only between 1 and 2%.

And so I encourage you to visit ground.news/HOTU or scan my QR code if you're looking for a quick

and easy way to stay fully informed, on any topic, Make sure you use my

link to save 40% off unlimited access to their Vantage plan – the same one I use.

In Medieval Naples, Pope Innocent IV lies on his sickbed. The Pontiff's advisors have

just delivered the crushing news that his Papal forces have been overrun by Manfred,

the King of Sicily. This devastating development is widely credited as the reason for his

death just days later at the age of 59. And yet, in some circles at least, there are

growing whispers that the Pope's early demise came from an entirely different source. That he was,

in fact, murdered. The proposed culprit? The ghost of a little known English bishop

with whom he'd clashed time and time again. A clergyman by the name of Robert Grosseteste.

This supposed spiritual assassin was born in the 13th century and rose to become the bishop of the

English cathedral city of Lincoln. Quarrelsome and restless, he sought reforms to the Catholic Church

that would bring him into direct conflict with Pope Innocent IV as well as King Henry III.

Yet it is Grossteste's contributions to the fledgling field of modern science

that are far more noteworthy. For he was a particularly early

advocate of the scientific method - and the crucial role of experiment in revealing the

hidden laws that invisibly govern our world. To begin with, Grossteste was the first person

in history to correctly explain rainbows as the result of the refraction of light. And

light became somewhat of an obsession and played a central role in his version of something bishops

and cosmologists both fixate on: the creation of the universe. According to Grossteste, the

universe began when light expanded outwards from a central point, before condensing into matter.

This was a full seven centuries before modern astronomers would hit upon a similar notion.

And so today, Grosseteste is known in some circles as the “Big Bang bishop”.

Grossteste died in 1253 and is buried in Lincoln cathedral. Miracles were reported at his shrine

and he was widely considered a saint in England as a result. Although Grosseteste’s sainthood was

never ratified by the Vatican, in large part due to the rumour that his ghost murdered the Pope.

But as we know Grosseteste´s idea for an expanding universe would not be developed

in the following years - it would take more than half a millenia for it to raise its head again.

And interestingly, it was another Catholic man of the cloth that would ultimately

rekindle Grossteste's idea in the early 20th century: the Belgian priest Georges Lemaître.

Lemaître was lucky to even be alive at this point. As an artillery officer at

Ypres during the First World War, he narrowly escaped the horrors of a cloud of chlorine gas

when the wind changed direction and blew it away from him. Then, in the Second World War,

the Americans accidentally bombed his home. In 1927, Lemaître published his solutions to

the equations of Einstein's General Theory of Relativity, our best and most complete theory

of gravity. He wasn´t the first to do this - people had been doing it for years - indeed

Karl Schwarzschild had been one of the first more than a decade earlier, using

his solution to propose the idea of a black hole. Lemaître's solutions however were different - they

implied that the entire universe was expanding. But most ignored his findings. Einstein was among

those who were brutally dismissive: “Your calculations are correct,

but your physics is abominable,” he said. Einstein famously would go on to tweak his

own equations to maintain a static universe. However, the seeds of the proof that Lemaître

was right – and Einstein wrong - had already been sown - for in 1915, the American astronomer Vesto

Slipher had announced his discovery that galaxies appear to be running away from us.

And Slipher reached his landmark conclusion thanks

to measurements of redshift, one of the most important weapons in an astronomer's armoury.

First, take light from a galaxy and break it up into its constituent colours - much

like Grosseteste correctly assumed raindrops do to create rainbows. Second,

look for the dark bands hidden in this spectrum that represent missing colours swallowed by the

various chemical elements that make up the galaxy. Finally, measure how far this pattern

of lines has been shunted towards the red end of the spectrum. The more pronounced this “redshift”,

the faster the galaxy is receding from us. This was only one half of the puzzle, however.

The final, missing piece would be provided by Edwin Hubble in 1929. He measured the distances

to galaxies, before comparing them to the speeds with which the galaxies are fleeing. In doing

so he found a very strict pattern now known as Hubble’s Law. The further a galaxy is from us,

the faster it appears to be running away. How fast? According to modern measurements,

about 23 kilometres per second for every million light years.

And so Hubble immediately knew that Lemaître was right. The universe is expanding after all,

just as the visionary Grossteste had suspected centuries before.

Despite how often it is talked about, it's not always immediately obvious why the

fact that more distant galaxies are fleeing from us faster automatically means that the

universe must be expanding. So let's nail the link with a more familiar example

of something else that expands: bread. Bread with raisins in it - to be precise.

Imagine mixing and kneading the dough, before placing it in the oven for an hour to bake. In

that time it will double in size to give you a tasty treat. But now imagine placing yourself on

one of the raisins and looking around you at the other raisins as the dough rises.

A raisin that was initially one centimetre away from you will end up two centimetres away at the

end of the baking time. It will have moved one centimetre in an hour. If a raisin was already

two centimetres away from you to begin with then it will end up four centimetres away, moving at

an apparent speed of two centimetres per hour. A third raisin initially three centimetres away

would finish the bake six centimetres distant, apparently moving at three centimetres per hour.

In other words, the bigger the initial gap between you and raisin, the faster you’ll see it move away

from you. Why? Because the dough is expanding. It is not that the raisins are moving through

the dough. Nor is more dough somehow being added. Instead the gap between the raisins is stretched

by the existing dough’s expansion. The more dough there was between you and a raisin to begin with,

the more pronounced the effect of its expansion. Hubble’s Law offers up an identical explanation

for galaxies. As Slipher realised, most appear to be running away from us, but the galaxies

themselves aren’t fleeing through space. Instead, the space between the galaxies is expanding and

carrying them ever further from us. The more space there was to begin with - in other words

the further a galaxy is from us - the faster it will appear to move away. No new space is being

added, merely existing space stretched. This is allowed by General Relativity - space and

time are malleable, inconstant things.

And it is the expansion of the universe that is also responsible for the more pronounced redshift

of more distant galaxies spotted by Slipher. As the light waves travelled towards Earth,

they were stretched as the space they travelled through expanded. Of all the colours of the

rainbow, red light has the longest waves. The more space the light had to travel through to

get here, the closer to the red end of the spectrum the spectral lines will appear.

This is a good illustration of another subtle point that often vexes people when it comes

to fully understanding an expanding universe. People often ask about what happens to energy

as the universe expands. Energy conservation is one of the most famous laws of physics,

stating that energy can't be created or destroyed and that the total energy of

a system must stay the same. However, energy is not conserved in an expanding universe.

The energy conservation rule holds for the kind of physics covered by Isaac

Newton's three famous laws of motion where particles move through a benign background

space that isn't changing. However, space is constantly changing in an expanding universe

and so the total energy of the particles moving through it is not conserved in the same way.

Redshifted light is a perfect example. As the expansion stretches out the light waves, they

lose energy. The total energy of all the photons reaching Earth decreases, it is not conserved.

And this expansion of the universe also leads to another curious effect. Light

from the most ancient events takes longer to arrive as it has had to travel a long

way through an expanding universe to get here. The result is that the oldest objects in the

universe appear to evolve almost five times more slowly than the same events today.

The fact that the universe is expanding is clear, but when exactly did this expansion start? Well,

if the universe is getting bigger day by day then it was smaller yesterday. It was smaller

still a century ago and yet more minuscule nearly a millennium ago when Grossteste’s

ghost was supposedly seeing-off the Pope. And so how far back does this expansion go?

It is Hubble's Law that tells us how much expansion there has been since the

Big Bang. Rewinding the clock on this expansion tells us when the expansion

started. At this earliest moment in the universe's history, every part of

the modern cosmos was concentrated down into an infinitely small speck. This little piece

of nothingness is what Lemaître called the “Primeval Atom” - today,

astronomers call it the Big Bang. And rewinding Hubble’s Law timestamps the beginning of this

expansion at around 13.8 billion years ago.

The very name of the

event - The Big Bang - calls to mind some kind of explosion, one that continues to drive the ongoing

expansion of the universe even to this day. Understandably, people then ask astronomers

for the location of the explosion. To point them to the place in the universe where the Big Bang

banged. Where is the centre of the universe? After all, if a bomb exploded in a room then

investigators sent in in the aftermath could piece together the necessary clues from the shrapnel and

debris to work out where in the room the bomb went off. So why can’t the same be done with the Big

Bang? Well, the Big Bang created the universe. If a bomb exploded, and in doing so created a room,

then it would make no sense to ask where in that room the bomb detonated. After all,

the room didn’t exist before the explosion.

In an expanding

universe everyone thinks that they are at the centre of the expansion,

when in fact there is no centre at all. And so - It's clear that the universe isn’t

expanding from anywhere, but then what is it expanding into? It is one of the other questions

most frequently asked of astronomers, but also one that turns out to be trickier to answer than it

first appears, with deep and profound consequences for the way we understand the universe…

The view is magnificently monochrome as you hurtle high above the surface of the

Moon. Prehistoric craters, smooth lava plains, soaring mountains and spindly volcanic rilles

jut and spread for as far as the eye can see. An ancient, empty wasteland touched only by twelve

pairs of American boots in billions of years. Flying over the jagged lunar landscape is more

than just breathtaking. It is also a journey through the history of science. You’ll find

crater after crater named after the most towering figures ever to contemplate the cosmos. Indeed,

this roll-call of celestial greatness includes Einstein, Hubble, Slipher and Lemaître.

And it also includes two craters that sit on opposite sides of the Moon - one on the

northern nearside and the other on the southern far side. Their geographical juxtaposition is apt

because the two physicists they are named after - Wilhelm de Sitter and Hermann Minkowski - also

lend their names to opposing possibilities for the shape of our universe. Which one

turns out to be correct governs whether or not the cosmos will ever end - and

has important consequences for our question of what exactly the universe is expanding into.

Minkowski was once Einstein's professor. They didn't always see eye to eye,

however. “He's a lazy dog who never bothered about mathematics at all,” Minkowski once said

of the most famous scientist who has ever lived. Indeed, Minkowski was far closer to the legendary

German mathematician David Hilbert, who wrote a touching obituary of his friend. Referring

to their shared scientific work, Hilbert said: “It seemed to us a garden full of flowers. In it,

we enjoyed looking for hidden pathways and discovered many a new perspective that

appealed to our sense of beauty, and when one of us showed it to the other and we marvelled