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  • Is there a border we will never crossAre there places we will never reach,  

  • no matter how hard we try? It turns outthere 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 diametercontaining 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 neighbourhoodHundreds 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 inflationthe 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 todayThe 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 universeit 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  nameMilkdromeda" 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 groupsBut 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 matterThe 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 timebut 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,  

  • you can watch, share and click the  bell and get something from our shop.  

  • 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 universeVisit our shop for more science-y and kurzgesagt-y  

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

Is there a border we will never crossAre there places we will never reach,  

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TRUE Limits Of Humanity – The Final Border We Will Never Cross

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    羅紹桀 に公開 2021 年 05 月 13 日
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