字幕表 動画を再生する 英語字幕をプリント 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, 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 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.