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比可观测宇宙更小的宇宙. Sal Khan 创建




every video until now we've been working from the assumption that the observable universe is smaller than the entire universe and if you go by the cosmic inflation Theory cosmic inflation theory and it was come up or it was founded by Alan Guth and I have a almost personal connection to Alan Guth when I was at MIT I always used to go to this a Chinese food truck and I always used to show up at the food truck like two seconds before Alan Guth like he was always one or two people behind me in line but anyway he was the founder of the cosmic inflation theory which is basically this idea the very early in the very early moments or the very early period after the Big Bang we went through kind of this major inflation in the expansion of space but anyway if based on cosmic on on the theory of cosmic inflation then the observable universe is on the order of or maybe another way to say it the entire universe is on the order of 10 to the 23 times the size size of the observable universe observable universe so that would be this is just a tiny tiny fraction I mean this is an unimaginably large number in fact it is unimaginable so already everything we've talked about this itself is a huge this is an incomprehensible amount of space but this is an incomprehensible multiple of this incomprehensible amount of space and that's just based on that theory but it is possible we cannot rule out even the idea that the actual universe is smaller smaller than the observable universe and now that that one that one is in some ways even more mind-blowing than the idea that the universe is this big the fact that what we're observing is actually larger than the actual universe and so you men tomato salad that's impossible but just just think about a little bit this is the observable universe and the way we've depicted it it's based on how long the light has taken to reach us we've already covered before that this point in space is now 46 billion light years away not 13.7 the way it looks right over here just took 13.7 billion years to reach us if if there's any photon that would take longer than 13.7 billion years to reach us it hasn't reached us yet because it could have only started 13.7 billion years ago so they're there they're on their way and they're they started at some point outside of our observable universe so our observable universe will grow over time but with that said let's imagine that the actual universe is a subset of this observable universe let's say it's let's say it's roughly half the diameter so let's say it looks like this let me don't make it a little bit of an oval maybe observe maybe the actual universe and this is just to be a little bit provocative and it's not impossible this is let's say that this is the actual universe actual universe and the way I drew it it makes it look like Earth is the center that we're the center of it but remember this is very likely to be the surface or it is it is curved it has a slight curvature but it's it could very well be the surface of a four-dimensional of a four dimensional object and maybe the simplest want to visualize is a four dimensional sphere so if you really wanted to visualize this right this whole volume and remember this whole picture I say it keeps looking two-dimensional but it has depth it is a volume of space an incredibly vast volume of space and so what I've done here is this is a this is a an ellipsoid right here it's an elliptical volume of space that I've bubbled out right over here but if we if this was really the entire universe and if the entire universe really were the surface of a four dimensional sphere then the reality is that this entire space could be represented like this it could be represented as the surface if this was a four dimensional sphere obviously the way I can only draw three dimensional spheres but let me show you that it has some it's not just a circle that it actually has some depth to it and I can even shade it right over here and so you can imagine you can imagine that this point that this point over here is actually the same thing as that point over there that they have wrapped around and that they're connected right at the back over here let me show the draw go behind that they're connected right over there and that this point and this point are actually the same point that they've wrapped around that they've wrapped around maybe they've wrapped around actually the way I've drawn it right here they would actually all repre right right back there at that point if I'm visualizing properly but they were if you go in any one direction you would come back on the other side of the surface if you go let's say that Earth is right here the way we've depicted it earth is the center but we see that when when you look at it like this there is no Center to the surface of a sphere even a four dimensional sphere so in this in this sense if you go in any one direction you'll come back out the other side so if you start from Earth and you go in that direction once you get there you're really here again and then you would come back to Earth and so if this were the case if the actual volume of the true universe was smaller than what it looks like the observable universe and what's all this stuff on the outside and to think about it think about what would happen think about what would happen if 13.7 billion years ago when we were in that primitive state that you know that that were that that background radiation those photons are being that electromagnetic those electromagnetic waves are being released let's say they get released and those photons on their first pass and I think you know where this is going on their first pass they would get to us they would get to us in about this looks like a distance of about I don't know this looks like about six billion years then they would pass this up and then they would get back to this point again in another six billion years and then they would come back here and so those that very first pass photons are going to be right over here and from our point of view we're not going to see them for a couple of billion years and so when we do see and we're going to perceive them we're gonna say wow it took it took fifteen 16 billion years for that photon to get to me that must be from something out here but the reality is it's a photon from something within a smaller physical universe within a smaller actual universe that's just taken several passes by us and we're just seeing the the pass after fourteen billion years we just think it's from something further out now the other thing is you say well you know if this was the case if if we could just go in one direction of the universe and then come out of the other side and if all of that was within the observable universe wouldn't we be able to tell wouldn't we be able to look in two directions and see the same thing from a different perspective and the answer there is to think about what happens or actually wouldn't we even be able to see our cells because if you if if we we emit some light and it would take maybe I don't know how far that is let's say that's six or seven billion light years to get right over here which would be right over there and then it would take another six or seven billion light years to get over there so maybe that background radiation we're seeing is actually background radiation emitted from that exact point in space that we are right now or from a very similar point in space to where we are right now part of the background radiation is from a similar point in space that we are right now so so how come we can't how come we can't just see ourselves well I kind of just answered the question that that second path if you're observing the same point in space if you're observing light from the same point in space on a previous pass that light was emitted long long long time ago maybe thirteen billion years ago and so it would be unrecognizable that region this region of space the region of space that we are in right now if we saw the same region of space thirteen billion years ago we just wouldn't recognize it now there are people attempting to see if there are some patterns see if you can model how the universe would change and if you see patterns and and and maybe the the actual universe is a subset of the observable we just haven't seen it yet but it's commits completely a possibility hopefully I didn't confuse you I actually find this kind of an interesting idea that these things that we think are this light that we that has taken thirteen let's say the light that's taken us eight billion years to reach us we think it's from something based on this scale eight billion light years out it's actually further because the universe is expanding so would have actually traversed more space than that but we think it's from something like that but it could have been something further in if the actual universe is smaller and it's just on its second pass it's actually coming back again and that's why it took eight billion years to reach us and we don't even recognize it because it looks very different than that region of space right now or that region of space after four billion years it looks completely different than it did when it first released anyway hopefully I didn't confuse you too much but I think this is a fascinating fascinating topic