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Is there a border we will never cross?  Are there places we will never reach,  

no matter how hard we try? It turns out,  there are. Even with sci-fi technology,  

we are trapped in a limited pocket of the Universe  

and the finite stuff within it. How much  universe is there for us and how far can we go?

If you look at the night sky, you  might assume it will be there forever.  

Stars are born and die again in a cycle that  feels endless. But it is not. Take the milky way:  

Up to 200,000 light years in diameter,  containing some 100 to 400 BILLION stars.  

How many stars do you think are born  here each year? Thousands? Millions?

The answer is around three. Three new stars per  year. 95% of all the stars that will ever exist in  

the universe have already been born and we live  at the tail end of the age of star formation.  

We are at the beginning of the  end of the universe as we know it,  

the formation of new stars  will continue to slow down.

But there is more. It turns out the  universe is rushing away from us.

The Milky Way is not alone - together with  the Andromeda galaxy, and more than fifty  

dwarf galaxies,it forms the Local Group, a  region of space about ten million light years  

in diameter. Our galactic neighbourhood.  Hundreds of galaxy groups like the local  

group make up the Laniakea Supercluster, which  itself is only one of a myriad of superclusters.  

In total there are around two trillion galaxies  that make up the current observable universe.

Unfortunately, even if we could travel at light  speed, around 94% of the galaxies we can see  

are already unreachable for us forever. Let this  number sink in for a moment. The simple fact that  

there is a limit for us, and that there is so much  universe that a human will never be able to touch,  

is kind of frightening. Why are all of  these galaxies out of reach already?

Well, it all has to do with why there are  galaxies in the first place: the big bang.

We are simplifying here, but in a nutshell  about 10^-36 seconds after the big bang,  

the young universe was a very small bubble of  energy. It was not completely uniform though,  

some parts of it were a tiny, tiny bit denser  than others, which had massive consequences.  

In a process called cosmic inflation,  the observable universe expanded rapidly,  

from the size of a marble to trillions of  kilometers, in a trillionth of a second.

This was so fast that all those tiny differences  in density were stretched from subatomic distances  

into galactic distances. Which is why the whole  universe consists of more and less dense regions.  

Pockets of the universe, filled with a bit  more stuff than the space around them. After  

that short but powerful inflation ended, gravity  began trying to pull everything back together.  

Inside the denser pockets gravity emerged  victorious and so over time, they grew into  

groups of galaxies, like the one we live in today.  The Local Group is our pocket of the universe.

But at the larger scales, outside the  denser pockets, the expansion of space  

never stopped. This means that our Local  Group is surrounded by a lot of stuff,  

but none of those structures and galaxies  are gravitationally bound to us. The more the  

universe expands, the larger the distance between  us and the other gravitational pockets becomes.

Even worse for us the expansion of the universe is  accelerating. We don’t know why this happens so we  

came up with the concept of dark energy. You can  imagine it like an invisible effect that speeds up  

the expansion of the universe. We will explain  these concepts in more detail in another video  

though, for now all you need to know is that  the universe is expanding faster and faster!

This expansion means that there is a cosmological  horizon around us. Everything beyond it, is  

traveling faster, relative to us, than the speed  of light. So everything that passes the horizon,  

is irretrievably out of reach forever and we  will never be able to interact with it again.  

In a sense it’s like a black hole’s event  horizon, but all around us. 94% of the  

galaxies we can see today have already  passed it and are lost to us forever.  

Wait, if we can’t interact with  them, how come we can still see them?

Well, the way we are able to see something is  via light. And although the speed of light is  

the fastest way to travel through the universe,  it needs time to get from one place to another.  

Every second light reaches us from  trillions of galaxies that have  

passed the horizon because when their light  was emitted, they were much closer to us.  

We are looking at their ancient past  and see their ancient positions.  

So the observable universe is much larger than  the universe we can actually interact with. In  

a sense, the universe is pulling off a great show  for us, showing us things that are out of reach  

forever. We have no idea what these galaxies look  like today and we will never know. But we will be  

able to observe them for a long time as their  light hits our telescopes. Interestingly this  

means that currently the observable universe still  appears to be growing as more and more light,  

released by super distant galaxies billions  of years ago is arriving at our doorsteps.

Still. All the pockets of the universe outside  the local group will one day pass our cosmological  

horizon . Once they do, their light won’t be able  to reach us anymore and from our perspective,  

they will fade away into darkness. Every second  of your life 60,000 stars pass the horizon.  

Since you started watching this video around 22  million stars have moved out of our reach forever.

Ok, but if 94% of the observable universe is  beyond the cosmic horizon and gone forever,  

that still leaves us with 6% that is technically  in reach, which is still a ton of stuff:  

All the galaxy pockets that are less than 18  billion light-years away. They are still moving  

away, but slow enough that we could physically  reach them, although chances are shrinking with  

every second that passes. Everything that is  more than around 5 million light years away  

is moving away from us. But the closest galaxy  groups are receding the slowest so there is a  

time window to jump galaxy groups. The challenge  is extreme though even for type 3 civilizations.

Even at the speed of light, a trip to  the Maffei Group, the closest pocket of  

galaxies outside the local group, would take 11  million years. If some sort of super motivated,  

super advanced civilization  takes this challenge on,  

its potential sphere of influence could  expand to hundreds or thousands of galaxies.  

Although as time passes and the universe  grows, they would be separated forever.

It is pretty safe to assume that  humans will not make this journey,  

at least not with technologies that  are even remotely on the horizon.  

For us, the Local Group is most likely the  largest structure that we will ever be a part of.  

Just traveling between the stars would  be an achievement of epic proportions.  

We would already be incredibly successful if we  colonize our cosmic backyard. Which accounts for  

0.0 0 0 - 0 0 0 - 0 0 0- 01%  of the observable universe.

As dark energy pushes the rest of the universe  away from us the Local Group will become more  

tightly bound. All its galaxies, big and  small, will merge together to form one  

giant elliptical galaxy with the unoriginal  name „Milkdromeda" in a few billion years.  

This process might even smash huge gas clouds  together and respark star formation for some time!

And this new light will be very welcome because  at some point, the galaxies outside Milkdromeda  

will be so far away that they become too faint  to detect. Once this happens, no information  

outside of the Local Group will reach us ever  again. The universe will recede from view.

A being born in the far future in Milkdromeda  will think that the universe consists of nothing  

but its own galaxy. When they look far into  empty space, they will only see more emptiness  

and darkness. They won’t see cosmic background  radiation, and they won’t be able to learn about  

the Big Bang. They may have no way of knowing  what we know today: the nature of the expanding  

universe, when it began, and how it will end. They  might think the universe is static and eternal.  

Milkdromeda will be an island in the  darkness, slowly getting darker and darker.

Still, with its trillion stars, the Local  Group is certainly a big enough playground  

to entertain humanity for a while. After  all, we still haven’t figured out how to  

leave our solar system and we have dozens  of billions of years at the very least,  

to explore our galaxy. And we have the incredible  luck to exist at the perfect moment in time  

to see not only our future, but also our most  distant past, just by looking into the night sky.  

As isolated as the Local Group is it is our  home. And it really is a spectacular place.

Time for some behind the scenes content.

You might have noticed that we sorta already  made this video a few years ago. But the original  

had a regrettable mistake. In it, we said it  would be physically impossible to ever leave  

the local group to reach other galaxy groups.  But our travel limit is actually much larger,  

as we showed in this video. It’s not  physically impossible to go further  

but just extremely unlikely that it’ll ever  happen. So why did we leave the video up?

Well it has to do with the nature of the  mistake. We talked to Astrophysicists  

and they thought it didn’t matter.  The parts they considered important  

were correct and they thought  we should just leave it and move  

on. Which is one of the funny things about working  with astrophysicists. Rounding up or down a few  

billion is not too important to them, as long as  you’re kind of in the right ballpark, because of  

the humongous numbers they work with. So with  that feedback, we decided to spare the video.

But since 2016, and really thanks to you  birbs, we were able to grow Kurzgesagt and add  

fact checkers to the team and develop an in  depth process with experts and our detailed  

sources documents to avoid these sorts of mistakes  and also to make our work transparent. We can’t  

avoid making mistakes from time to time,  but we can work hard on getting better.

And this video kept bugging us. So we finally  decided to replace it, not just reupload,  

but remake it better and longer and add  fun universe facts to make it worthwhile.  

Sorry for taking so long. We’ll leave the old  video up but pin a comment and change the title.

If you want to support us and our sometimes  exhausting but hopefully worthwhile methods,  

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Inspired by this video’s topic we created a Milky  Way Poster and a Local Group Poster and we are  

working on a series about the entire universe.  Visit our shop for more science-y and kurzgesagt-y  

products designed with love and produced with  care. In any case, thank you for watching.