1 00:00:01,207 --> 00:00:11,564 Right now the prevailing theory of how the Universe came about is commonly called the Big Bang Theory. 2 00:00:11,564 --> 00:00:16,718 And really it's just this idea that the Universe that this Universe started as kind of an infintely small 3 00:00:16,718 --> 00:00:24,067 point, this infinitely small singularity, and then it just had a Big Bang, or it just expanded from that 4 00:00:24,067 --> 00:00:30,133 state to the Universe we know right now. And when I first imagined this, and I think it's also a by-product 5 00:00:30,133 --> 00:00:36,897 of how its named, Big Bang, you kind of imagine this type of explosion. You kind of imagine this type 6 00:00:36,897 --> 00:00:43,537 of explosion. That everything was infinitely packed in together and then it exploded. And then it exploded 7 00:00:43,537 --> 00:00:51,067 outward. Then as all the matter exploded outward it started to condense and then you have all these 8 00:00:51,067 --> 00:00:58,133 galaxies and super clusters of galaxies and they started to condense and then within them, planets condense 9 00:00:58,133 --> 00:01:03,933 and stars condense and then we have the type of Universe that we have right now. But this model for 10 00:01:03,933 --> 00:01:09,467 visualizing the Big Bang has a couple of problems. One is, when we talk about the Big Bang, we're not 11 00:01:09,867 --> 00:01:15,209 talking about the matter, just the mass or just the matter in the Universe being in one point, we're 12 00:01:15,209 --> 00:01:24,533 talking about actual space expanding, we are not just talking about something inside of space, like the physical mass expanding 13 00:01:24,533 --> 00:01:29,443 we are talking about space itself so when you have this type of a model you have all the stuff expanding 14 00:01:29,443 --> 00:01:39,067 is it expanding into something else? Maybe if the furthest out part of this matter is right over here 15 00:01:39,067 --> 00:01:41,733 what's this stuff over here? So you say: 16 00:01:41,733 --> 00:01:47,267 wouldn't that be space, how can you say space itself is expanding? 17 00:01:47,267 --> 00:01:51,851 And another idea that a Big Bang also implies is that if this is furthest stuff out there 18 00:01:51,851 --> 00:01:58,867 would this be the edge of the Universe? Does the Universe have an edge? 19 00:01:58,867 --> 00:02:03,533 And the answer to this question, that's why we are going to try to tackle this is, 20 00:02:03,533 --> 00:02:06,363 one: **the Universe does not have an edge** and 21 00:02:06,363 --> 00:02:10,400 two: **there is no outside space**, we are not expanding 22 00:02:10,400 --> 00:02:12,800 into another space. I am going to explain that, 23 00:02:12,800 --> 00:02:15,733 hopefully we'll see why this is the case right now. 24 00:02:15,733 --> 00:02:19,691 So the best way of viewing it is by analogy 25 00:02:19,741 --> 00:02:31,115 If I were to tell you that I have a two dimensional space 26 00:02:31,115 --> 00:02:44,200 that has a finite area, so it's not infinite and also has no edge 27 00:02:44,200 --> 00:02:47,467 this once again, when you first look at it, it seems difficult 28 00:02:47,467 --> 00:02:50,800 how do I construct something that has a finite area 29 00:02:50,800 --> 00:02:54,933 but still has no edge? Every time I try to draw an area it looks like I have to have some edges 30 00:02:54,933 --> 00:03:01,400 and then you might remember, what if that two dimensional space is curved? 31 00:03:01,400 --> 00:03:04,204 what happens? I think that the easiest example of that is the the surface of the sphere. 32 00:03:04,204 --> 00:03:13,200 let me draw a sphere over here. So this right here is a sphere. 33 00:03:13,200 --> 00:03:19,343 let me draw some longitude and latitude lines on this sphere 34 00:03:19,343 --> 00:03:21,619 on this sphere all of a sudden, I shade it in a little bit, make it look nice, 35 00:03:21,619 --> 00:03:28,353 this type of a sphere you have a finite area, you can imagine the surface of a balloon 36 00:03:28,353 --> 00:03:31,133 or the surface of a bubble or the surface of the Earth 37 00:03:31,133 --> 00:03:36,400 you have a finite area but you have no edge, if you keep going forever in one direction 38 00:03:36,400 --> 00:03:44,769 you just keep going all the way around and come back to the other side 39 00:03:44,769 --> 00:03:51,667 now, to imagine a three dimensional space that has the same properties, a finite area and, 40 00:03:51,667 --> 00:03:54,333 I don't want to say finite area anymore, because we are not talking about 41 00:03:54,333 --> 00:03:57,296 a three dimensional space anymore, let me draw over here 42 00:03:57,296 --> 00:04:08,088 so let's think about three dimensional space, 43 00:04:08,088 --> 00:04:18,933 and instead of area I want to talk about finite volume 44 00:04:18,933 --> 00:04:22,316 and no edge, how do I do that? 45 00:04:22,316 --> 00:04:26,467 when you think about it superficially, look, if you had finite volume it's going to be maybe 46 00:04:26,467 --> 00:04:33,000 contained in some type of cube, we clearly have edges in those situations 47 00:04:33,000 --> 00:04:38,067 you can even think about a finite volume as being the inside of a sphere, that clearly has an edge 48 00:04:38,067 --> 00:04:40,543 this entire surface over there. 49 00:04:40,543 --> 00:04:44,467 so how do you construct an three dimensional space that has finite volume and no edge? 50 00:04:44,467 --> 00:04:46,133 And than I am going to tell you right now 51 00:04:46,133 --> 00:04:50,000 it's very hard for us to visualize it, but in order to visualize it 52 00:04:50,000 --> 00:04:53,000 I am going to draw the same thing that I drew right here 53 00:04:53,000 --> 00:04:56,944 you have to imagine it, you always have to imagine it by analogy 54 00:04:56,944 --> 00:05:04,646 (unless you have a profound brain wired for more than three spatial dimensions) 55 00:05:04,646 --> 00:05:15,950 is a sphere, let me make it clear, this is two dimensional surface. 56 00:05:15,950 --> 00:05:19,941 right on the surface of the sphere you can only move in two directions. 57 00:05:19,941 --> 00:05:24,643 two perpendicular directions, you can move like that or you can move like that, 58 00:05:24,643 --> 00:05:27,267 you can move left and right or you can move up and and down 59 00:05:27,267 --> 00:05:33,200 two dimensional surface of a three dimensional 60 00:05:33,200 --> 00:05:45,656 sphere, so if we take it by analogy, let's imagine, and it's hard to imagine, a three dimensional surface 61 00:05:45,656 --> 00:06:01,338 and you can do it mathematically, the math here is actually not that difficult, three dimensional surface of a four dimensional sphere 62 00:06:01,338 --> 00:06:02,719 and I'm going to draw up the same way, 63 00:06:02,719 --> 00:06:05,877 so if we kind of view those three dimensions are just these two dimension of the surface 64 00:06:05,877 --> 00:06:12,912 it's the same thing, and if imagine that I'm not saying this is the actual shape of the Universe, 65 00:06:12,912 --> 00:06:19,971 we don't know the shape, but we do know it has a slight curvature, we don't know the actual shape 66 00:06:19,971 --> 00:06:22,897 but the sphere is the simpliest, there's other ones we can do, a toroid 67 00:06:22,897 --> 00:06:27,077 would also fit the bill, it would have a finite volume with no edge 68 00:06:27,077 --> 00:06:31,906 and I think, I want to make it clear, we actually don't know whether it has a finite volume 69 00:06:31,906 --> 00:06:37,154 it's still an open question, but I want to show you it can have a finite volume and also have 70 00:06:37,154 --> 00:06:46,733 no edge. And most people believe, and I want to say believe because you know we can, just go based on evidence and all that, 71 00:06:46,733 --> 00:06:52,800 that we are talking about something with finite volume, especially when you talk about the Big Bang theory 72 00:06:52,800 --> 00:07:00,467 that kind of some dimensions implies a finite volume, although it could be a super large, unfathomably large volume 73 00:07:00,467 --> 00:07:07,029 it is finite, now if you have this, imagine, this sphere, 74 00:07:07,029 --> 00:07:14,515 once again if you're on this surface of this four dimensional sphere, I can't draw a four dimensional sphere, but 75 00:07:14,515 --> 00:07:22,874 if you are on this surface of this four dimensional sphere if you go in any direction you'll come back to where you started 76 00:07:22,874 --> 00:07:27,704 if you go that way you'll come back around here, now the Universe is super huge so even light 77 00:07:27,704 --> 00:07:32,301 maybe light itself would take an unbelievable amount of time to traverse it 78 00:07:32,301 --> 00:07:38,199 and if this sphere itself is expanding, it may be expanding so fast that light 79 00:07:38,199 --> 00:07:42,983 might not ever be able to come back around it, but in theory if somethings were fast enough 80 00:07:42,983 --> 00:07:49,267 if something were to keep going around it could actually go back to this point. Now when we talk about 81 00:07:49,267 --> 00:07:53,933 a three dimensional surface it's a three dimensional surface of a four dimensional sphere 82 00:07:53,933 --> 00:07:58,667 that means that any of the three dimensions that are over here on the surface, I can only draw two 83 00:07:58,667 --> 00:08:05,267 but that means, if this is true, if the Universe is a three dimensional surface of a four dimensional 84 00:08:05,267 --> 00:08:12,263 sphere, that means if you go up and if you just keep going up, you'll eventually come back from the bottom. 85 00:08:12,263 --> 00:08:19,667 So if you keep going, you keep going all the way up you'll eventually come back to the point that you where 86 00:08:19,667 --> 00:08:24,067 it might be an unbelievably large distance but you'll eventually get back where you were 87 00:08:24,067 --> 00:08:28,267 if you go to the right you'll eventually come back all the way around to the point where you were 88 00:08:28,267 --> 00:08:34,533 and if you want to go into the page, let me draw it that way, if you go into the page 89 00:08:34,533 --> 00:08:39,467 you'd eventually come back from above the page, and come back to the point that you are 90 00:08:39,467 --> 00:08:43,800 so that's what this implication would be that you would actually get back to where you are 91 00:08:43,800 --> 00:08:47,000 so let's go back to the question of and expanding Universe. 92 00:08:47,000 --> 00:08:50,181 You know an expanding Universe that is not expanding into any other space 93 00:08:50,181 --> 00:08:53,246 it is all of the space but it is still expanding 94 00:08:53,246 --> 00:09:02,009 this is the model, so you can imagine shortly after the Big Bang, our four dimensional sphere look like this 95 00:09:02,009 --> 00:09:09,467 maybe it was a little small four dimensional sphere, maybe at the Big Bang it was this little unbelievably 96 00:09:09,467 --> 00:09:13,667 small sphere then a little bit later it is this larger sphere, let me just shade it in 97 00:09:13,667 --> 00:09:24,959 to show that it is kind of popping out of the page this sphere, and in the later time this sphere might look like this, 98 00:09:24,959 --> 00:09:31,736 now you temptation might be to say, well isn't this stuff outside of the sphere? 99 00:09:31,782 --> 00:09:34,867 isn't that some type of a space that is expanding into ? 100 00:09:34,867 --> 00:09:41,867 isn't that somehow part of the Universe? and I would say if you are talking in three dimension, no it's not. 101 00:09:41,867 --> 00:09:48,200 The entire Universe is this surface, it is the surface of this four dimensional sphere 102 00:09:48,200 --> 00:09:54,333 if you start talking about more dimensions, then yes, you could talk about maybe things outside of our 103 00:09:54,333 --> 00:10:00,770 three dimensional Universe, so as this expands in space-time, you know one way to view 104 00:10:00,770 --> 00:10:05,367 this fourth dimension is that it is time itself, things are just getting further and further apart. 105 00:10:05,367 --> 00:10:11,544 And I'll talk about the evidence in future videos for why the Big Bang is the best theory 106 00:10:11,544 --> 00:10:16,467 we have out there right now. But as you can imagine, if you have two points on this sphere that are that 107 00:10:16,467 --> 00:10:22,690 far apart, as this sphere expands, this four dimensional sphere, this bubble, blows up, or this balloon 108 00:10:22,690 --> 00:10:29,052 blows up, those two points; let me draw three points, those three points are just going to get further 109 00:10:29,052 --> 00:10:37,457 and further apart, that's actually one of the main points that, one of the first reasons why it makes 110 00:10:37,457 --> 00:10:41,451 sense to believe in the Big Bang is that everything is expanding not from some central point, but everything 111 00:10:41,451 --> 00:10:46,792 is expanding from everything, that if you go in any direction from any point in the Universe, everything else 112 00:10:46,792 --> 00:10:52,667 is expanding away and the further away you go it looks like the faster it's expanding away from you, 113 00:10:52,667 --> 00:10:58,077 so I'll leave there it's something for you to think about a little bit, and then we'll build on some of this 114 00:10:58,077 --> 00:11:02,077 to think about what it means to observe the observable Universe.