1 00:00:00,060 --> 00:00:02,400 The following content is provided under a Creative 2 00:00:02,400 --> 00:00:03,790 Commons license. 3 00:00:03,790 --> 00:00:06,030 Your support will help MIT OpenCourseWare 4 00:00:06,030 --> 00:00:10,120 continue to offer high quality educational resources for free. 5 00:00:10,120 --> 00:00:12,660 To make a donation or to view additional materials 6 00:00:12,660 --> 00:00:16,590 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,590 --> 00:00:17,940 at ocw.mit.edu. 8 00:00:20,800 --> 00:00:22,450 NICHOLAS DIBELLA: Last week I talked 9 00:00:22,450 --> 00:00:26,590 about the basics of the Big Bang Theory, which essentially 10 00:00:26,590 --> 00:00:30,070 says that there was a beginning of time. 11 00:00:30,070 --> 00:00:33,040 We call it the Big Bang. 12 00:00:33,040 --> 00:00:36,811 And at this moment in time every piece 13 00:00:36,811 --> 00:00:38,560 of matter and energy in the whole universe 14 00:00:38,560 --> 00:00:40,932 was concentrated at a single point. 15 00:00:40,932 --> 00:00:43,390 So the density of the universe was extremely high, in fact, 16 00:00:43,390 --> 00:00:44,482 in principle, infinite. 17 00:00:44,482 --> 00:00:45,940 And the temperature of the universe 18 00:00:45,940 --> 00:00:47,620 was also much higher because, as we 19 00:00:47,620 --> 00:00:49,810 discussed last time, when you compress 20 00:00:49,810 --> 00:00:51,370 something it gets hotter. 21 00:00:51,370 --> 00:00:53,560 So if you rewind the history of the universe 22 00:00:53,560 --> 00:00:57,430 to the very beginning, the universe will be very, very hot 23 00:00:57,430 --> 00:01:01,950 and at the Big Bang should, in principle, be infinite. 24 00:01:01,950 --> 00:01:03,700 And then after the Big Bang the universe 25 00:01:03,700 --> 00:01:07,355 has-- universe started to expand, and expand, expand 26 00:01:07,355 --> 00:01:11,615 and here we find ourselves today. 27 00:01:11,615 --> 00:01:13,406 Does anyone have any questions about-- yes. 28 00:01:13,406 --> 00:01:16,920 AUDIENCE: If it's at that high temperature and all 29 00:01:16,920 --> 00:01:18,670 that matter in the universe compressed 30 00:01:18,670 --> 00:01:22,670 into [? a neutron, ?] whatever-- 31 00:01:22,670 --> 00:01:25,170 wouldn't it create some sort of fusion or something 32 00:01:25,170 --> 00:01:25,785 [INAUDIBLE]? 33 00:01:25,785 --> 00:01:27,160 NICHOLAS DIBELLA: Oh, well, yeah, 34 00:01:27,160 --> 00:01:30,460 so very early in-- very early-- 35 00:01:30,460 --> 00:01:35,660 so the question is since the temperature is so high 36 00:01:35,660 --> 00:01:38,480 and the early universe, wouldn't all the atom-- 37 00:01:38,480 --> 00:01:40,019 all the nuclei fuse together and-- 38 00:01:40,019 --> 00:01:40,810 AUDIENCE: Yes, yes. 39 00:01:40,810 --> 00:01:41,200 NICHOLAS DIBELLA: Oh, yeah. 40 00:01:41,200 --> 00:01:42,366 That's exactly what happens. 41 00:01:42,366 --> 00:01:45,670 In the very early universe you start off 42 00:01:45,670 --> 00:01:49,370 bas-- you start off with protons and electrons and so forth. 43 00:01:49,370 --> 00:01:51,940 And these protons, when you go back far enough in time-- 44 00:01:51,940 --> 00:01:52,644 these protons-- 45 00:01:52,644 --> 00:01:54,310 yeah, because the temperature is so high 46 00:01:54,310 --> 00:01:56,351 they're going be so energetic that they literally 47 00:01:56,351 --> 00:01:57,430 fuse with each other. 48 00:01:57,430 --> 00:02:00,340 And so you form-- you start to form the light elements. 49 00:02:00,340 --> 00:02:03,980 So a proton and proton will form a deuterium atom. 50 00:02:03,980 --> 00:02:07,120 So that's an isotope of-- well, they will form atoms yet. 51 00:02:07,120 --> 00:02:08,919 They'll form a deuterium nucleus. 52 00:02:08,919 --> 00:02:11,030 So that's a two-proton nucleus. 53 00:02:11,030 --> 00:02:12,880 And then a two-proton nucleus could 54 00:02:12,880 --> 00:02:15,270 collide with a very energetic proton, 55 00:02:15,270 --> 00:02:18,080 then form a three-proton nucleus and so forth and so forth. 56 00:02:18,080 --> 00:02:21,820 And this-- yeah, this happens in the very early universe. 57 00:02:21,820 --> 00:02:24,690 It's called the big bang nucleosynthesis era. 58 00:02:24,690 --> 00:02:29,830 Because this is where nuclei were being created and you can 59 00:02:29,830 --> 00:02:34,150 actually-- you can actually calculate the abundances-- 60 00:02:34,150 --> 00:02:36,690 the proportions of elements-- 61 00:02:36,690 --> 00:02:40,160 of that light elements that were created in this era. 62 00:02:40,160 --> 00:02:44,064 And then you can compare it with the abundances 63 00:02:44,064 --> 00:02:46,480 that you actually observe it by looking at stars and stuff 64 00:02:46,480 --> 00:02:47,040 in the sky. 65 00:02:47,040 --> 00:02:49,979 And you actually find amazing agreement with the Big Bang-- 66 00:02:49,979 --> 00:02:50,770 the Big Bang model. 67 00:02:50,770 --> 00:02:54,925 And-- so that's also very good evidence for the Big Bang. 68 00:02:54,925 --> 00:02:57,800 So last time I talked about the existence of-- yeah? 69 00:02:57,800 --> 00:03:00,350 AUDIENCE: [INAUDIBLE] it's just weird [INAUDIBLE] 70 00:03:00,350 --> 00:03:03,920 fusion of all this matter into one area [INAUDIBLE] 71 00:03:03,920 --> 00:03:04,840 why would it explode? 72 00:03:04,840 --> 00:03:07,140 NICHOLAS DIBELLA: Why would it-- so why-- 73 00:03:07,140 --> 00:03:08,300 so why the bang? 74 00:03:08,300 --> 00:03:09,180 What banged? 75 00:03:09,180 --> 00:03:09,890 How did it bang? 76 00:03:09,890 --> 00:03:11,712 AUDIENCE: [INAUDIBLE] explode if it's 77 00:03:11,712 --> 00:03:13,503 in the process of heating right [INAUDIBLE] 78 00:03:13,503 --> 00:03:16,472 in such a small area-- the temperature [INAUDIBLE]?? 79 00:03:16,472 --> 00:03:18,130 NICHOLAS DIBELLA: Why would it explode? 80 00:03:18,130 --> 00:03:20,894 AUDIENCE: Yeah, why would all of the matter shoot [INAUDIBLE].. 81 00:03:20,894 --> 00:03:22,060 NICHOLAS DIBELLA: Yeah, so-- 82 00:03:22,060 --> 00:03:24,400 I like-- essentially your question is why the bang? 83 00:03:24,400 --> 00:03:25,450 AUDIENCE: Yeah. 84 00:03:25,450 --> 00:03:28,960 NICHOLAS DIBELLA: Yeah, that's something I'll come to later. 85 00:03:28,960 --> 00:03:30,931 Yeah. 86 00:03:30,931 --> 00:03:34,191 Any other questions? 87 00:03:34,191 --> 00:03:34,890 Yes. 88 00:03:34,890 --> 00:03:38,020 AUDIENCE: If the space between galaxies expanding-- 89 00:03:38,020 --> 00:03:41,610 is expanding why isn't the space inside the galaxies? 90 00:03:41,610 --> 00:03:44,110 NICHOLAS DIBELLA: Why isn't the space inside, yeah, so the-- 91 00:03:44,110 --> 00:03:46,600 I'm just repeating the questions because microphone's here, 92 00:03:46,600 --> 00:03:49,700 so the question is if the space between the galaxies 93 00:03:49,700 --> 00:03:53,690 is increasing, then why isn't the space between people 94 00:03:53,690 --> 00:03:55,220 increasing, for example. 95 00:03:55,220 --> 00:04:01,460 There's a funny quote from a Woody Allen why isn't-- 96 00:04:01,460 --> 00:04:04,040 I forget the exact quote-- but the gist of it 97 00:04:04,040 --> 00:04:09,860 is Brooklyn is expanding. 98 00:04:09,860 --> 00:04:10,460 Yes. 99 00:04:10,460 --> 00:04:13,170 So your question is why-- yeah, so your question is, 100 00:04:13,170 --> 00:04:16,910 why aren't objects that we see around us expanding? 101 00:04:16,910 --> 00:04:18,615 And the answer is that-- 102 00:04:21,230 --> 00:04:23,540 so this expansion-- the physical theory 103 00:04:23,540 --> 00:04:26,120 that describes this expansion is general relativity. 104 00:04:26,120 --> 00:04:28,280 Which I;ll describe in a little bit. 105 00:04:28,280 --> 00:04:33,710 And the expansion only applies over a large scales. 106 00:04:33,710 --> 00:04:37,882 When you actually work it out it only applies over large scales. 107 00:04:37,882 --> 00:04:39,590 The equation that you write down are only 108 00:04:39,590 --> 00:04:42,320 valid for large things like galaxies 109 00:04:42,320 --> 00:04:44,420 and the large structure of the universe. 110 00:04:44,420 --> 00:04:46,700 And to describe smaller things-- 111 00:04:46,700 --> 00:04:51,034 to describe stars or even a single galaxy, 112 00:04:51,034 --> 00:04:53,450 the equation that you use in describing the whole universe 113 00:04:53,450 --> 00:04:54,740 are no longer valid. 114 00:04:54,740 --> 00:04:56,470 And so you have to use others-- 115 00:04:56,470 --> 00:04:59,060 other equations. 116 00:04:59,060 --> 00:05:00,780 And when you use those equations, 117 00:05:00,780 --> 00:05:02,750 you don't get that space is expanding. 118 00:05:02,750 --> 00:05:05,810 You have to-- yeah, so real-- so if you want to describe 119 00:05:05,810 --> 00:05:07,370 everything in the universe-- 120 00:05:07,370 --> 00:05:10,860 have to patch together these different solutions. 121 00:05:10,860 --> 00:05:17,300 And so that's why I'm not drifting away from you 122 00:05:17,300 --> 00:05:20,440 or something like that. 123 00:05:20,440 --> 00:05:21,580 Any the other questions? 124 00:05:25,400 --> 00:05:27,900 So as I mentioned-- 125 00:05:27,900 --> 00:05:29,680 so the physical theory that you need 126 00:05:29,680 --> 00:05:35,230 to describe-- to really describe the Big Bang Theory 127 00:05:35,230 --> 00:05:37,660 and to really describe the expansion of space 128 00:05:37,660 --> 00:05:40,540 is called general relativity. 129 00:05:40,540 --> 00:05:43,510 And in general relativity was a theory 130 00:05:43,510 --> 00:05:46,750 that Einstein proposed in 1916 and it's basically 131 00:05:46,750 --> 00:05:50,650 a theory of space, time and gravity. 132 00:05:50,650 --> 00:05:52,790 In fact they're all related to each other. 133 00:05:52,790 --> 00:05:54,535 So general relativity. 134 00:06:12,390 --> 00:06:20,091 Space, time, gravity. 135 00:06:20,091 --> 00:06:22,740 I'll have a lot more to say about general relativity 136 00:06:22,740 --> 00:06:25,800 in the next two lectures-- well, a lot more in-- 137 00:06:25,800 --> 00:06:27,800 when I talk about time travel, which will either 138 00:06:27,800 --> 00:06:29,091 be next week or the week after. 139 00:06:29,091 --> 00:06:30,740 I haven't decided. 140 00:06:30,740 --> 00:06:32,360 But for now it just-- 141 00:06:32,360 --> 00:06:34,880 just know that it's a theory that 142 00:06:34,880 --> 00:06:37,530 explains gravity and explains space and time-- 143 00:06:37,530 --> 00:06:39,900 they're all related to each other. 144 00:06:39,900 --> 00:06:44,900 Now, general relativity is one of the two most 145 00:06:44,900 --> 00:06:47,559 successful theories that have ever been proposed-- 146 00:06:47,559 --> 00:06:49,100 two most successful physical theories 147 00:06:49,100 --> 00:06:50,900 that have ever been proposed. 148 00:06:50,900 --> 00:06:53,240 Every single-- oh, the other one is quantum theory, 149 00:06:53,240 --> 00:06:58,520 by the way, which I'll also talk about in the future, 150 00:06:58,520 --> 00:07:00,020 but not today. 151 00:07:00,020 --> 00:07:03,590 Every test that has been put forth 152 00:07:03,590 --> 00:07:07,820 to test general relativity, general relativity has passed. 153 00:07:07,820 --> 00:07:11,720 It can make predictions to the 10th decimal place, or even 154 00:07:11,720 --> 00:07:14,650 past the 10th decimal place, and it gets it. 155 00:07:14,650 --> 00:07:17,090 That that's how good general relativity is 156 00:07:17,090 --> 00:07:22,180 and so this theory is actually quite weird. 157 00:07:22,180 --> 00:07:24,140 You'll see when I talk about it. 158 00:07:24,140 --> 00:07:26,222 But it's enormously successful, and its success 159 00:07:26,222 --> 00:07:27,680 is the reason that we talk about it 160 00:07:27,680 --> 00:07:30,680 and that we even think about it. 161 00:07:30,680 --> 00:07:37,580 So when general relativity was first proposed in 1916 people 162 00:07:37,580 --> 00:07:41,720 wanted to know what are some of the consequences of the theory 163 00:07:41,720 --> 00:07:45,470 and, in particular, what does general relativity have 164 00:07:45,470 --> 00:07:48,050 to say about the universe. 165 00:07:48,050 --> 00:07:55,800 And so in 1922, a mathematician by the name of-- 166 00:07:55,800 --> 00:07:59,250 a Russian mathematician by the name of Alexander Friedmann 167 00:07:59,250 --> 00:08:02,450 solved it-- solved the universe. 168 00:08:02,450 --> 00:08:06,610 So let's see-- 169 00:08:06,610 --> 00:08:19,245 Friedmann applied GR to the universe. 170 00:08:25,300 --> 00:08:31,360 Now the universe is a pretty messy place. 171 00:08:31,360 --> 00:08:34,570 I mean, just look around. 172 00:08:34,570 --> 00:08:35,419 It's just messy. 173 00:08:35,419 --> 00:08:39,560 It just seems like it would be so hard to analyze and its-- 174 00:08:39,560 --> 00:08:42,820 the universe is also pretty big. 175 00:08:42,820 --> 00:08:47,679 So you have to be a pretty confident guy 176 00:08:47,679 --> 00:08:48,720 to say, hey, I've got a-- 177 00:08:48,720 --> 00:08:50,178 I'm going-- I've got a theorem, I'm 178 00:08:50,178 --> 00:08:52,739 going to explain the universe. 179 00:08:52,739 --> 00:08:54,280 It seems mind-boggling to me at first 180 00:08:54,280 --> 00:08:55,613 that it should even be possible. 181 00:08:55,613 --> 00:08:58,240 Because I'm having trouble coming up 182 00:08:58,240 --> 00:09:01,770 with a model for girls or something, 183 00:09:01,770 --> 00:09:03,890 and so to model the universe that's just-- 184 00:09:03,890 --> 00:09:05,740 it's just mind-boggling. 185 00:09:05,740 --> 00:09:09,340 So it's a hard question, I mean, to actually describe 186 00:09:09,340 --> 00:09:10,900 the universe. 187 00:09:10,900 --> 00:09:13,810 But, fortunately, physicists have a trick 188 00:09:13,810 --> 00:09:15,400 in tackling hard problems. 189 00:09:15,400 --> 00:09:16,810 They have a trick. 190 00:09:16,810 --> 00:09:20,020 And the trick is when you're faced with a hard problem, 191 00:09:20,020 --> 00:09:22,650 you just solve a simpler one. 192 00:09:22,650 --> 00:09:24,930 That's the trick. 193 00:09:24,930 --> 00:09:27,310 So want Friedmann did was he made 194 00:09:27,310 --> 00:09:29,760 a couple of assumptions about the universe-- 195 00:09:29,760 --> 00:09:31,610 a couple of assumptions. 196 00:09:31,610 --> 00:09:36,370 The first assumption that he made is that the universe-- 197 00:09:36,370 --> 00:09:45,900 let's see-- so the first assumption he made 198 00:09:45,900 --> 00:09:48,690 is that the universe is homogeneous, 199 00:09:48,690 --> 00:09:52,770 which means that it's uniform. 200 00:09:52,770 --> 00:09:55,510 If you travel from one place to another, 201 00:09:55,510 --> 00:09:56,700 It doesn't change very much. 202 00:09:56,700 --> 00:09:59,160 The density of the universe is roughly constant, doesn't 203 00:09:59,160 --> 00:10:01,660 change very much. 204 00:10:01,660 --> 00:10:13,972 So two assumptions-- the first is homogeneous. 205 00:10:20,720 --> 00:10:26,020 Now, in my remark about the messiness of the universe-- 206 00:10:26,020 --> 00:10:28,630 it certainly doesn't look like the universe is homogeneous. 207 00:10:28,630 --> 00:10:30,875 I mean, just look around you. 208 00:10:30,875 --> 00:10:32,500 Yeah, there are some people over there, 209 00:10:32,500 --> 00:10:35,890 there's some chairs over there, there's a chalkboard over here, 210 00:10:35,890 --> 00:10:38,440 I just heard a plane up there. 211 00:10:38,440 --> 00:10:42,340 The universe surely doesn't look homogeneous. 212 00:10:42,340 --> 00:10:48,600 And so you might think that this plan of Friedmann's to solve 213 00:10:48,600 --> 00:10:50,170 the universe would be doomed. 214 00:10:50,170 --> 00:10:52,180 It wouldn't work at all. 215 00:10:52,180 --> 00:10:59,380 But this bias that we have in thinking that's the universe 216 00:10:59,380 --> 00:11:03,160 isn't uniform, it's not homogeneous-- 217 00:11:03,160 --> 00:11:05,890 it's really limited to the-- 218 00:11:05,890 --> 00:11:08,890 it's really due to our observations of the universe 219 00:11:08,890 --> 00:11:10,870 over small scales. 220 00:11:10,870 --> 00:11:12,640 I mean, we're pretty small things 221 00:11:12,640 --> 00:11:14,950 compared to the universe. 222 00:11:14,950 --> 00:11:17,512 I mean, this room is very small compared to the universe. 223 00:11:17,512 --> 00:11:18,970 But it turns out that when you look 224 00:11:18,970 --> 00:11:20,386 at the universe over large scales, 225 00:11:20,386 --> 00:11:23,210 when you look at the universe over large distances, 226 00:11:23,210 --> 00:11:26,205 this assumption of homogeneity is actually pretty good. 227 00:11:26,205 --> 00:11:29,470 I mean, astronomers have looked around the universe 228 00:11:29,470 --> 00:11:35,560 and they've noticed that it's a pretty boring place in terms 229 00:11:35,560 --> 00:11:38,380 of large scale structure. 230 00:11:38,380 --> 00:11:42,080 I mean, you see a galaxy here, you see galaxy there, 231 00:11:42,080 --> 00:11:45,010 you see another galaxy there, it starts 232 00:11:45,010 --> 00:11:47,370 to get boring after a while. 233 00:11:47,370 --> 00:11:51,940 And the density doesn't really fluctuate a whole lot-- doesn't 234 00:11:51,940 --> 00:11:52,990 really vary a whole lot. 235 00:11:52,990 --> 00:11:58,510 So this homogeneity assumption is pretty good. 236 00:11:58,510 --> 00:12:03,790 The second assumption he made is that the universe is isotropic. 237 00:12:09,180 --> 00:12:13,440 And isotropic just means that if I look in this direction, 238 00:12:13,440 --> 00:12:15,760 I look in this direction, I look in this direction-- 239 00:12:15,760 --> 00:12:18,510 every direction I look the universe looks the same. 240 00:12:18,510 --> 00:12:21,090 That's all that isotropic means. 241 00:12:21,090 --> 00:12:23,520 Of course, over small scales it doesn't 242 00:12:23,520 --> 00:12:27,780 look like that, because I see you over there, 243 00:12:27,780 --> 00:12:29,310 but I don't see him over there. 244 00:12:29,310 --> 00:12:31,204 I see somebody else. 245 00:12:31,204 --> 00:12:35,730 [INAUDIBLE] once again this isotropy assumption 246 00:12:35,730 --> 00:12:38,580 is actually a pretty good assumption 247 00:12:38,580 --> 00:12:40,590 over the large scales. 248 00:12:40,590 --> 00:12:43,530 And last week I talked about the cosmic microwave background 249 00:12:43,530 --> 00:12:46,110 radiation. 250 00:12:46,110 --> 00:12:48,570 It turns out that if you look at this radiation, 251 00:12:48,570 --> 00:12:50,340 regardless of where you look, it's-- 252 00:12:50,340 --> 00:12:53,310 the temperature of the radiation is pretty constant. 253 00:12:53,310 --> 00:12:55,470 It's pretty much the same. 254 00:12:55,470 --> 00:12:59,874 So this isotropy assumption is a pretty good assumption. 255 00:13:04,620 --> 00:13:07,020 It's possible-- I mean, it's possible 256 00:13:07,020 --> 00:13:13,640 for there to be one of these [? mets, ?] but not the other. 257 00:13:13,640 --> 00:13:17,370 You can have an isotropic universe 258 00:13:17,370 --> 00:13:22,050 that's not uniform, for example, if you had something like-- 259 00:13:22,050 --> 00:13:26,280 so here's two and not one-- 260 00:13:26,280 --> 00:13:28,260 two and not one. 261 00:13:30,926 --> 00:13:32,800 You can have isotropic universe, for example, 262 00:13:32,800 --> 00:13:38,430 if you had, say, a series of shells where the first shell is 263 00:13:38,430 --> 00:13:41,310 more dense than the second shell, which 264 00:13:41,310 --> 00:13:42,540 is more dense than the-- 265 00:13:42,540 --> 00:13:49,050 this third shell and so once you go out the density goes down, 266 00:13:49,050 --> 00:13:51,600 but it looks the same regardless of where you look. 267 00:13:51,600 --> 00:13:54,430 So you can have isotropy without homogeneity. 268 00:13:54,430 --> 00:13:57,530 You can also have homogeneity without isotropy. 269 00:13:57,530 --> 00:14:00,870 You can have something like-- 270 00:14:00,870 --> 00:14:05,910 the matter can be distributed in something like this. 271 00:14:05,910 --> 00:14:09,425 You can be sitting over here and you look up, 272 00:14:09,425 --> 00:14:11,550 you see some matter, you look in another direction, 273 00:14:11,550 --> 00:14:12,425 you don't see manner. 274 00:14:15,640 --> 00:14:16,630 So you can have-- 275 00:14:16,630 --> 00:14:20,700 so they're not equivalent to each other. 276 00:14:20,700 --> 00:14:25,920 Although, if the whole universe is isotropic, then it has to be 277 00:14:25,920 --> 00:14:27,930 homogeneous. 278 00:14:27,930 --> 00:14:30,530 But I-- we'll just make this assumption of-- 279 00:14:30,530 --> 00:14:33,390 about just one point in the universe 280 00:14:33,390 --> 00:14:35,556 when I made those assumptions. 281 00:14:35,556 --> 00:14:37,680 It's also possible, of course, that the universe is 282 00:14:37,680 --> 00:14:39,630 neither homogeneous nor isotropic, 283 00:14:39,630 --> 00:14:41,940 that's another possibility. 284 00:14:41,940 --> 00:14:44,760 But when you look at the universe 285 00:14:44,760 --> 00:14:50,432 it is roughly homogeneous, it is roughly isotropic. 286 00:14:50,432 --> 00:14:52,440 Oh, and by the way, this-- 287 00:14:52,440 --> 00:14:55,300 these two assumptions-- homogeneous plus-- 288 00:14:55,300 --> 00:15:00,360 homogen-- homogeneous plus isotropic form 289 00:15:00,360 --> 00:15:02,571 what's called the cosmological principle. 290 00:15:15,350 --> 00:15:18,140 Cosmological principle. 291 00:15:18,140 --> 00:15:20,390 Last week I talked about one-- 292 00:15:20,390 --> 00:15:22,280 I talked very briefly about one alternative 293 00:15:22,280 --> 00:15:25,220 to the Big Bang model called the steady state theory where 294 00:15:25,220 --> 00:15:28,050 the universe actually exists. 295 00:15:28,050 --> 00:15:30,670 It didn't have a beginning, it's eternal. 296 00:15:30,670 --> 00:15:32,660 In the steady state theory the universe 297 00:15:32,660 --> 00:15:35,600 is both quite homogeneous, and isotropic 298 00:15:35,600 --> 00:15:38,510 and it's also the same in time. 299 00:15:38,510 --> 00:15:41,490 So it's also homogeneous in time in a sense. 300 00:15:41,490 --> 00:15:44,332 And so in the steady state model it actually-- 301 00:15:44,332 --> 00:15:46,040 the steady state model actually satisfies 302 00:15:46,040 --> 00:15:49,160 what's called the perfect cosmological principle, which 303 00:15:49,160 --> 00:15:55,130 is even more than that and which is even more elegant or more 304 00:15:55,130 --> 00:15:57,920 pleasing than this standard cosmological principle. 305 00:15:57,920 --> 00:16:00,440 But it doesn't look like the steady state theory's right 306 00:16:00,440 --> 00:16:01,580 because steady state theory doesn't 307 00:16:01,580 --> 00:16:03,440 imply the existence of the cosmic microwave 308 00:16:03,440 --> 00:16:05,900 background which we observe, which the Big Bang 309 00:16:05,900 --> 00:16:07,054 Theory predicts. 310 00:16:07,054 --> 00:16:08,720 So we throw the steady state theory out. 311 00:16:08,720 --> 00:16:11,990 But this perfect cosmological principle actually-- 312 00:16:11,990 --> 00:16:15,340 it's found a new life in certain classes 313 00:16:15,340 --> 00:16:17,090 of extensions of the Big Bang model, which 314 00:16:17,090 --> 00:16:19,490 I'll talk about later-- 315 00:16:19,490 --> 00:16:25,540 so-called inflationary theories of the universe. 316 00:16:25,540 --> 00:16:28,767 Does anybody have any questions about these two assumptions 317 00:16:28,767 --> 00:16:29,600 that Friedmann made? 318 00:16:32,074 --> 00:16:33,740 So Friedmann made these two assumptions. 319 00:16:33,740 --> 00:16:36,382 He said, hey, let's make the universe-- 320 00:16:36,382 --> 00:16:37,840 let's consider the simpler problem. 321 00:16:37,840 --> 00:16:40,006 The universe itself is pretty complicated, but let's 322 00:16:40,006 --> 00:16:41,480 consider this simpler problem. 323 00:16:41,480 --> 00:16:43,910 We make the universe homogeneous and isotropic, 324 00:16:43,910 --> 00:16:45,260 and let's solve this. 325 00:16:45,260 --> 00:16:46,910 So let's put this into the equations 326 00:16:46,910 --> 00:16:50,570 of general relativity, which, of course, I won't show you. 327 00:16:50,570 --> 00:16:53,799 Let's put this into the equations and see what we get. 328 00:16:53,799 --> 00:16:54,590 And so he did that. 329 00:17:01,385 --> 00:17:03,830 He that and he came up with three possible solutions 330 00:17:03,830 --> 00:17:05,076 for the universe. 331 00:17:12,040 --> 00:17:18,079 Three possible solutions as three possible universes. 332 00:17:31,780 --> 00:17:35,500 Oh, it turns out that the type of universe 333 00:17:35,500 --> 00:17:41,860 that you have, that is, the type of history that the universe 334 00:17:41,860 --> 00:17:44,950 has, the type of shape it has, the type of size it has 335 00:17:44,950 --> 00:17:47,496 depends on the density of the universe. 336 00:17:47,496 --> 00:17:49,120 Well, it depends on the average density 337 00:17:49,120 --> 00:17:53,530 of the universe relative to a certain density 338 00:17:53,530 --> 00:17:56,500 called the critical density. 339 00:17:56,500 --> 00:17:58,990 So let me just write down this critical density 340 00:17:58,990 --> 00:18:00,880 before I talk about these universes. 341 00:18:00,880 --> 00:18:11,180 So the critical density is a density that's a very small, 342 00:18:11,180 --> 00:18:12,810 but it's very important. 343 00:18:12,810 --> 00:18:19,430 It's about one-- it's about 10 to the minus 26 kilograms per 344 00:18:19,430 --> 00:18:24,330 cubic meter, which [? is ?] approximately-- 345 00:18:24,330 --> 00:18:29,615 does anybody have a feeling for how small that number is? 346 00:18:29,615 --> 00:18:31,009 AUDIENCE: Really small. 347 00:18:31,009 --> 00:18:32,300 NICHOLAS DIBELLA: Really small. 348 00:18:32,300 --> 00:18:32,799 Yes. 349 00:18:32,799 --> 00:18:35,400 AUDIENCE: [INAUDIBLE] 26 decimal places. 350 00:18:35,400 --> 00:18:36,460 NICHOLAS DIBELLA: Yeah. 351 00:18:36,460 --> 00:18:42,170 So the mass of a proton is about 1.67 times 10 352 00:18:42,170 --> 00:18:44,330 to the minus 27 kilograms. 353 00:18:44,330 --> 00:18:48,500 So this critical density is about 10 protons 354 00:18:48,500 --> 00:18:50,070 per cubic meter. 355 00:18:50,070 --> 00:18:54,200 So cubic meter-- meter-- 356 00:18:54,200 --> 00:18:54,950 [INAUDIBLE] 357 00:18:58,040 --> 00:19:01,876 10 protons in that amount of space, which is pretty small. 358 00:19:01,876 --> 00:19:09,610 It's-- per cubic meter. 359 00:19:11,692 --> 00:19:13,150 That's [? just ?] critical density. 360 00:19:16,390 --> 00:19:18,430 So if the density of the universe 361 00:19:18,430 --> 00:19:21,140 is greater than this critical density, 362 00:19:21,140 --> 00:19:24,310 then you have this first type of universe that he discovered. 363 00:19:24,310 --> 00:19:26,170 So let's suppose-- let's say density is 364 00:19:26,170 --> 00:19:28,900 greater than critical density. 365 00:19:28,900 --> 00:19:33,640 I'm going to-- actually I'm going 366 00:19:33,640 --> 00:19:35,280 to give this critical density a symbol. 367 00:19:35,280 --> 00:19:37,780 I'm just going to write down a symbol just to make it easier 368 00:19:37,780 --> 00:19:38,488 to write it down. 369 00:19:41,850 --> 00:19:45,120 I'm just going to give it this symbol. 370 00:19:45,120 --> 00:19:47,730 This is the Greek letter rho that's c. 371 00:19:47,730 --> 00:19:50,086 I said I wouldn't do math. 372 00:19:50,086 --> 00:19:52,710 I'm not doing any math, I'm just giving things different names. 373 00:19:52,710 --> 00:19:57,690 So I-- you can't accuse me of doing math, so I rho c. 374 00:19:57,690 --> 00:20:01,610 So if the density of the universe, which I'll call rho-- 375 00:20:01,610 --> 00:20:04,770 if this-- if the density of the universe is greater than this 376 00:20:04,770 --> 00:20:13,290 critical density, then it turns out that-- 377 00:20:13,290 --> 00:20:15,660 well, first let me say something about all these 378 00:20:15,660 --> 00:20:19,330 three possible universes. 379 00:20:19,330 --> 00:20:21,330 In each one of them there's a beginning of time. 380 00:20:21,330 --> 00:20:25,680 Each one of them starts out with some kind of a big bang. 381 00:20:25,680 --> 00:20:29,430 But the future evolution, the future behavior of the universe 382 00:20:29,430 --> 00:20:30,840 depends on this density. 383 00:20:30,840 --> 00:20:32,760 So if the density of the universe right 384 00:20:32,760 --> 00:20:35,230 now is greater than this critical density, 385 00:20:35,230 --> 00:20:38,980 then it turns out that the universe will eventually-- 386 00:20:38,980 --> 00:20:41,700 so it's expanding right now, but if the density is greater 387 00:20:41,700 --> 00:20:43,199 than the critical density, then it's 388 00:20:43,199 --> 00:20:47,730 going to eventually starts to contract because the gravity 389 00:20:47,730 --> 00:20:49,100 due to these-- 390 00:20:49,100 --> 00:20:51,245 due to the matter and the energy in the universe 391 00:20:51,245 --> 00:20:53,120 is going to be too great because it's greater 392 00:20:53,120 --> 00:20:53,840 than this critical density. 393 00:20:53,840 --> 00:20:56,340 It's can going to be to great that eventually it just starts 394 00:20:56,340 --> 00:20:59,080 making the universe-- collapsing onto itself. 395 00:20:59,080 --> 00:21:01,660 So if the critical density is smaller than the density 396 00:21:01,660 --> 00:21:05,590 of the universe, then the universe eventually 397 00:21:05,590 --> 00:21:06,690 re-contracts-- 398 00:21:06,690 --> 00:21:07,750 it contracts. 399 00:21:11,334 --> 00:21:18,880 The universe collapses-- collapses in some kind of a Big 400 00:21:18,880 --> 00:21:20,910 Crunch-- 401 00:21:20,910 --> 00:21:22,840 with the Big Bang as the beginning, 402 00:21:22,840 --> 00:21:24,710 Big Crunch being the end-- 403 00:21:24,710 --> 00:21:25,960 everything crunching together. 404 00:21:34,940 --> 00:21:36,780 And this is also tru-- also this is true 405 00:21:36,780 --> 00:21:39,690 if the density is greater than the critical density, 406 00:21:39,690 --> 00:21:45,560 then general relativity implies that the universe is finite. 407 00:21:45,560 --> 00:21:48,170 So not only does the universe eventually collapse, 408 00:21:48,170 --> 00:21:49,503 but the universe is also finite. 409 00:22:00,190 --> 00:22:03,190 And the type of finite that I mean-- 410 00:22:03,190 --> 00:22:05,440 there are at least a couple of times-- types of finite 411 00:22:05,440 --> 00:22:06,970 that I can think of. 412 00:22:06,970 --> 00:22:08,770 The type of finite that I mean is a type 413 00:22:08,770 --> 00:22:10,990 of finite that the surface of the balloon is 414 00:22:10,990 --> 00:22:13,690 or that the surface of a bottle is. 415 00:22:13,690 --> 00:22:15,070 So if I'm sitting-- 416 00:22:15,070 --> 00:22:17,050 if I'm a two-dimensional creature-- 417 00:22:17,050 --> 00:22:21,310 if I'm like an inch worm from last time and I'm-- 418 00:22:21,310 --> 00:22:23,276 I hear voices. 419 00:22:23,276 --> 00:22:25,012 AUDIENCE: Outside. 420 00:22:25,012 --> 00:22:26,970 NICHOLAS DIBELLA: Outside, on the second floor? 421 00:22:26,970 --> 00:22:31,960 There's a window there, oh, but it can be from the bottom and-- 422 00:22:31,960 --> 00:22:35,070 if I'm sitting-- sorry, I have ADD. 423 00:22:35,070 --> 00:22:40,660 Sorry-- If I'm sitting on the surface of this bottle 424 00:22:40,660 --> 00:22:43,510 and I start walking in one direction, 425 00:22:43,510 --> 00:22:45,925 then I'm eventually going to reach my starting point 426 00:22:45,925 --> 00:22:48,550 where I started-- if I just keep walking in the same direction. 427 00:22:48,550 --> 00:22:51,210 I mean, you can just look at it, it's closed-- 428 00:22:51,210 --> 00:22:54,660 same thing with a balloon or some other kinds of surfaces. 429 00:22:54,660 --> 00:22:57,100 That's the kind of finite that I mean. 430 00:22:57,100 --> 00:23:02,110 If I started walking in one direction for a very long time, 431 00:23:02,110 --> 00:23:03,852 or if I took a spaceship or whatever 432 00:23:03,852 --> 00:23:06,310 and just started traveling in the same direction for a very 433 00:23:06,310 --> 00:23:08,860 long time, then I would eventually 434 00:23:08,860 --> 00:23:11,170 get back to my starting point. 435 00:23:11,170 --> 00:23:14,710 It's hard to imagine how something like that 436 00:23:14,710 --> 00:23:16,570 would work in the real universe because this 437 00:23:16,570 --> 00:23:17,590 isn't the real universe. 438 00:23:17,590 --> 00:23:20,410 We could just see two dimensions living inside 439 00:23:20,410 --> 00:23:22,530 of three dimensions. 440 00:23:22,530 --> 00:23:25,330 How does three dimensions close on itself? 441 00:23:25,330 --> 00:23:27,490 It's hard to imagine, but this is what 442 00:23:27,490 --> 00:23:28,660 general relativity implies. 443 00:23:28,660 --> 00:23:31,090 I mean, we might not be able to visualize it in our heads. 444 00:23:31,090 --> 00:23:33,790 We might not be able to see it, but we can still 445 00:23:33,790 --> 00:23:35,355 understand how it works. 446 00:23:35,355 --> 00:23:36,730 I keep walking in this direction, 447 00:23:36,730 --> 00:23:38,774 I eventually get back to my spot as 448 00:23:38,774 --> 00:23:40,690 if I was walking on the earth and eventually I 449 00:23:40,690 --> 00:23:41,523 returned to my spot. 450 00:23:44,140 --> 00:23:46,960 There's not any kind of a boundary. 451 00:23:46,960 --> 00:23:50,470 There's no boundary in this kind of universe. 452 00:23:50,470 --> 00:23:51,670 There's not a sign-- 453 00:23:51,670 --> 00:23:53,530 if I were to keep walking in that direction, 454 00:23:53,530 --> 00:23:54,790 I wouldn't reach a boundary. 455 00:23:54,790 --> 00:23:56,170 I wouldn't reach a sign that says 456 00:23:56,170 --> 00:23:57,960 end of the universe, dead end. 457 00:23:57,960 --> 00:23:59,260 That's not what would happen. 458 00:23:59,260 --> 00:24:01,930 That's not the kind of finite that general relativity 459 00:24:01,930 --> 00:24:03,370 implies the universe might be. 460 00:24:06,155 --> 00:24:08,254 [INAUDIBLE] a question? 461 00:24:08,254 --> 00:24:10,380 AUDIENCE: But if the universe were 462 00:24:10,380 --> 00:24:13,320 that kind of finite, what would the edge of the universe look 463 00:24:13,320 --> 00:24:14,320 like? 464 00:24:14,320 --> 00:24:15,695 NICHOLAS DIBELLA: If the universe 465 00:24:15,695 --> 00:24:17,695 were the kind of finite that it's not, you mean? 466 00:24:17,695 --> 00:24:18,490 AUDIENCE: Yeah. 467 00:24:18,490 --> 00:24:21,675 NICHOLAS DIBELLA: What would the edge look like? 468 00:24:21,675 --> 00:24:22,300 I mean that's-- 469 00:24:25,030 --> 00:24:27,780 I mean, I'm not sure I-- 470 00:24:27,780 --> 00:24:31,567 like, what would it look like, I mean, you would-- 471 00:24:31,567 --> 00:24:34,150 if you were if you were standing in front of it and you just-- 472 00:24:34,150 --> 00:24:35,330 AUDIENCE: Yeah. 473 00:24:35,330 --> 00:24:35,970 NICHOLAS DIBELLA: Well, I mean-- 474 00:24:35,970 --> 00:24:37,360 so if that's the edge of the universe, 475 00:24:37,360 --> 00:24:39,110 then you obviously can't get outside of it 476 00:24:39,110 --> 00:24:40,270 because it's the universe. 477 00:24:40,270 --> 00:24:41,770 It might not look like anything. 478 00:24:41,770 --> 00:24:43,728 I mean, there might not be anything over there. 479 00:24:43,728 --> 00:24:45,100 There might just be empty space. 480 00:24:45,100 --> 00:24:47,170 So if you just walk there, you just 481 00:24:47,170 --> 00:24:48,820 might not be able to penetrate past it. 482 00:24:48,820 --> 00:24:50,780 But, I know, I know-- 483 00:24:50,780 --> 00:24:52,960 I mean the universe isn't that kind of a finite. 484 00:24:52,960 --> 00:24:54,820 So I'm not exactly sure if it's like really 485 00:24:54,820 --> 00:25:00,550 a sensible question, but it's an interesting thing 486 00:25:00,550 --> 00:25:04,967 to talk about, but I'm not really sure how you really 487 00:25:04,967 --> 00:25:06,550 answered because the assumptions are-- 488 00:25:10,715 --> 00:25:14,915 what's the word I'm looking for-- the assumptions are bad. 489 00:25:14,915 --> 00:25:15,415 Yeah. 490 00:25:15,415 --> 00:25:20,620 Did You have a question or are you just tired? 491 00:25:20,620 --> 00:25:22,100 Anymore question? 492 00:25:22,100 --> 00:25:22,600 Yes. 493 00:25:25,750 --> 00:25:29,350 You're also doing the fake question-- raise hand. 494 00:25:29,350 --> 00:25:31,360 No more questions? 495 00:25:31,360 --> 00:25:33,220 This kind of a universe is called closed 496 00:25:33,220 --> 00:25:37,150 because it closes onto itself the same kind of way 497 00:25:37,150 --> 00:25:40,090 that a water bottle closes on the same kind of way 498 00:25:40,090 --> 00:25:42,604 that a balloon closes on. 499 00:25:42,604 --> 00:25:43,645 So this is called closed. 500 00:25:51,940 --> 00:25:53,500 Let's go to a new board. 501 00:25:59,754 --> 00:26:01,170 We can also consider the case when 502 00:26:01,170 --> 00:26:03,900 the density of the universe is less than the critical density. 503 00:26:07,770 --> 00:26:10,670 So rho, the density of the universe now, 504 00:26:10,670 --> 00:26:13,560 is less than the critical density now. 505 00:26:13,560 --> 00:26:18,109 And in this case the universe expands forever. 506 00:26:18,109 --> 00:26:19,275 It's always going to expand. 507 00:26:36,120 --> 00:26:39,330 And the universe is also infinite-- 508 00:26:39,330 --> 00:26:40,980 it's not finite, it's infinite. 509 00:26:51,560 --> 00:26:54,920 So if you kept walking in one direction 510 00:26:54,920 --> 00:26:58,070 you would not get back to where you started from-- bringing it 511 00:26:58,070 --> 00:27:00,410 back to that point. 512 00:27:00,410 --> 00:27:02,090 I suppose if the universe is infinite-- 513 00:27:02,090 --> 00:27:04,131 I'll have whole lot more to say about this when I 514 00:27:04,131 --> 00:27:05,764 talk about parallel universes. 515 00:27:05,764 --> 00:27:07,430 But if the universe is infinite and kept 516 00:27:07,430 --> 00:27:09,840 walking in the same direction and 517 00:27:09,840 --> 00:27:12,070 then you might eventually get back to-- 518 00:27:12,070 --> 00:27:14,660 you might eventually get to some galaxy 519 00:27:14,660 --> 00:27:16,910 where there's a complete duplicate of yourself 520 00:27:16,910 --> 00:27:18,493 and you might think that you came back 521 00:27:18,493 --> 00:27:20,320 to your starting point. 522 00:27:20,320 --> 00:27:22,804 But you wouldn't really come back to your starting point. 523 00:27:22,804 --> 00:27:25,220 I mean-- and if I left here and I kept going, 524 00:27:25,220 --> 00:27:29,840 eventually I would reach a parallel universe where there's 525 00:27:29,840 --> 00:27:32,750 a complete duplicate of this, and there's this mark right 526 00:27:32,750 --> 00:27:35,600 here, and I would come to it and I would 527 00:27:35,600 --> 00:27:37,110 think I came back to my point. 528 00:27:37,110 --> 00:27:40,970 But if the universe is open, then it's 529 00:27:40,970 --> 00:27:42,250 not really the same point. 530 00:27:42,250 --> 00:27:44,770 That just something funny that I thought of. 531 00:27:47,871 --> 00:27:52,190 So the universe expands forever, the universe is infinite. 532 00:27:52,190 --> 00:27:54,650 There is a final case. 533 00:27:54,650 --> 00:27:59,560 And that's if the universe is-- 534 00:27:59,560 --> 00:28:00,950 if the density of the universe is 535 00:28:00,950 --> 00:28:03,481 equal to this critical density. 536 00:28:03,481 --> 00:28:05,480 Oh, by the way, this is called an open universe, 537 00:28:05,480 --> 00:28:06,650 it doesn't close on itself. 538 00:28:13,780 --> 00:28:20,010 The final case rho equals rho c. 539 00:28:20,010 --> 00:28:23,350 And in this case the universe also expands forever 540 00:28:23,350 --> 00:28:26,560 and the universe is also infinite, 541 00:28:26,560 --> 00:28:31,510 but there's a slight difference. 542 00:28:31,510 --> 00:28:41,585 So universe same as above, but we call it flat. 543 00:28:45,070 --> 00:28:48,795 We call it flat because-- 544 00:28:48,795 --> 00:28:51,600 so I haven't really talked about the geometry of the universe, 545 00:28:51,600 --> 00:28:56,010 but I've hinted at what that might be like. 546 00:28:56,010 --> 00:28:58,560 So in the closed universe the geometry of the universe 547 00:28:58,560 --> 00:29:03,600 is like a sphere, although you have 548 00:29:03,600 --> 00:29:05,410 to use your imagine to-- you have 549 00:29:05,410 --> 00:29:08,595 to use your imagination to really visualize 550 00:29:08,595 --> 00:29:10,550 what it would be like. 551 00:29:10,550 --> 00:29:16,530 But it's like a sphere in the sense that if you drew a circle 552 00:29:16,530 --> 00:29:18,900 on it, for example-- if you-- or if you-- sorry-- 553 00:29:18,900 --> 00:29:22,340 if you drew a triangle on it and you-- 554 00:29:22,340 --> 00:29:25,250 if you added up the angles on the triangle, 555 00:29:25,250 --> 00:29:28,710 then the angles [? wouldn't ?] add up to 180 degrees. 556 00:29:28,710 --> 00:29:31,090 They would actually add up to-- 557 00:29:31,090 --> 00:29:33,015 is it more than 180 or less than 180? 558 00:29:37,910 --> 00:29:39,765 I think it's more than 180 degrees. 559 00:29:39,765 --> 00:29:44,130 I think it's-- yeah, so in a closed universe if you made 560 00:29:44,130 --> 00:29:47,615 a really big triangle-- if you just-- if you found that-- 561 00:29:47,615 --> 00:29:49,740 if you made a really big triangle and then added up 562 00:29:49,740 --> 00:29:52,330 the angles in the triangle, you wouldn't get 100 degrees. 563 00:29:52,330 --> 00:29:55,660 You'd actually get more than 180 degrees. 564 00:29:55,660 --> 00:30:01,200 In an open universe and if you drew a very large triangle 565 00:30:01,200 --> 00:30:03,570 and you added up the angles in the triangle, 566 00:30:03,570 --> 00:30:05,560 you also wouldn't get 180 degrees. 567 00:30:05,560 --> 00:30:08,430 Now, you would get less than 180 degrees. 568 00:30:08,430 --> 00:30:10,340 I-- a picture might actually help. 569 00:30:10,340 --> 00:30:13,920 So let's say-- let's go back to the closed case. 570 00:30:13,920 --> 00:30:15,390 So if you have a sphere-- can you 571 00:30:15,390 --> 00:30:16,695 see this-- what I'm writing? 572 00:30:16,695 --> 00:30:21,520 So if he had the circle, let's pretend it's a sphere, 573 00:30:21,520 --> 00:30:23,520 If he had a sphere and he drew a triangle on it, 574 00:30:23,520 --> 00:30:25,575 it would be curved. 575 00:30:28,054 --> 00:30:30,220 It'd be curved [INAUDIBLE] something [INAUDIBLE]---- 576 00:30:30,220 --> 00:30:31,250 anyway like this. 577 00:30:31,250 --> 00:30:35,570 You can just-- you can go home and buy a balloon 578 00:30:35,570 --> 00:30:36,862 and then draw a triangle on it. 579 00:30:36,862 --> 00:30:39,070 And then look at the angle and see what it looks like 580 00:30:39,070 --> 00:30:41,320 and see if it looks like what I'm drawing over there. 581 00:30:41,320 --> 00:30:42,695 And then you can add up-- you can 582 00:30:42,695 --> 00:30:45,050 measure the angles on the triangle and add them all up, 583 00:30:45,050 --> 00:30:47,591 and you can see that they add up to greater than 180 degrees. 584 00:30:47,591 --> 00:30:50,530 I think that's right. 585 00:30:50,530 --> 00:30:52,380 I think that's right. 586 00:30:52,380 --> 00:30:55,040 It's one of the other. 587 00:30:55,040 --> 00:30:59,270 Similarly, in this opening case, the geometry of the universe 588 00:30:59,270 --> 00:31:01,970 is, well, in some sense open. 589 00:31:01,970 --> 00:31:04,770 It's actually like-- it's like a saddle. 590 00:31:08,830 --> 00:31:10,480 I'm bad at drawing. 591 00:31:10,480 --> 00:31:11,480 It's like a saddle. 592 00:31:19,550 --> 00:31:23,190 It's some-- does anybody know-- 593 00:31:23,190 --> 00:31:24,770 can anybody draw a saddle? 594 00:31:29,060 --> 00:31:30,060 I'll send you a picture. 595 00:31:30,060 --> 00:31:32,130 I'll send you a picture of a saddle later, 596 00:31:32,130 --> 00:31:34,650 so you can get a better grip of this. 597 00:31:34,650 --> 00:31:38,880 Or, maybe next point-- next time I'll just bring my laptop. 598 00:31:38,880 --> 00:31:42,286 And if drew a triangle on a saddle-- 599 00:31:42,286 --> 00:31:43,267 AUDIENCE: [INAUDIBLE]? 600 00:31:43,267 --> 00:31:44,225 NICHOLAS DIBELLA: Sure. 601 00:31:44,225 --> 00:31:45,326 AUDIENCE: Sweet. 602 00:31:45,326 --> 00:31:46,580 AUDIENCE: [INAUDIBLE]. 603 00:31:46,580 --> 00:31:49,751 NICHOLAS DIBELLA: [INAUDIBLE] draw a saddle. 604 00:31:49,751 --> 00:31:51,240 AUDIENCE: Like the shape like-- 605 00:31:51,240 --> 00:31:55,007 NICHOLAS DIBELLA: Just like the shape of a saddle. 606 00:31:55,007 --> 00:31:56,090 AUDIENCE: No, [INAUDIBLE]. 607 00:31:56,090 --> 00:31:57,311 NICHOLAS DIBELLA: OK. 608 00:31:57,311 --> 00:32:01,260 Yeah, I should have-- next time I'll bring my laptop. 609 00:32:01,260 --> 00:32:04,910 Yeah, so if it drew a triangle on a saddle then the triangle-- 610 00:32:04,910 --> 00:32:06,660 the angles wouldn't add up to 180 degrees. 611 00:32:06,660 --> 00:32:09,270 They would actually add up to less than 180 degrees. 612 00:32:09,270 --> 00:32:11,982 So you can buy a saddle for yourself at home 613 00:32:11,982 --> 00:32:13,440 and do this, and get a magic marker 614 00:32:13,440 --> 00:32:15,150 and just draw a triangle, and then add up the angles 615 00:32:15,150 --> 00:32:16,630 and you can see for yourself. 616 00:32:16,630 --> 00:32:19,870 And so I'll just try to draw it here. 617 00:32:19,870 --> 00:32:23,464 Add up the angles, see what you get. 618 00:32:23,464 --> 00:32:25,380 In an open universe, this is what the geometry 619 00:32:25,380 --> 00:32:28,660 of the universe would be like. 620 00:32:28,660 --> 00:32:31,560 I mean, the kinds of saddles that we would buy 621 00:32:31,560 --> 00:32:33,480 or that we would get in some kind of way-- 622 00:32:33,480 --> 00:32:35,130 those would be two-dimensional surfaces 623 00:32:35,130 --> 00:32:36,546 and we have to use our imagination 624 00:32:36,546 --> 00:32:39,390 to imagine the three-dimensional saddle, 625 00:32:39,390 --> 00:32:41,760 but we could still talk about what we would observe. 626 00:32:41,760 --> 00:32:43,510 We might not be able to imagine the shape, 627 00:32:43,510 --> 00:32:46,022 but we could still talk about what we would observe. 628 00:32:46,022 --> 00:32:47,730 If we drew a triangle, add up the angles, 629 00:32:47,730 --> 00:32:48,813 you don't get 180 degrees. 630 00:32:48,813 --> 00:32:50,890 If you start walking in one direction, 631 00:32:50,890 --> 00:32:54,670 you're not going get back to your starting point. 632 00:32:54,670 --> 00:32:57,480 So, now, on this last case, this third case, which I 633 00:32:57,480 --> 00:32:58,479 called flat-- 634 00:32:58,479 --> 00:33:01,020 it's similar to the opening case in that the universe expands 635 00:33:01,020 --> 00:33:02,910 forever and that the universe is infinite, 636 00:33:02,910 --> 00:33:05,550 but the kind of geometry is like a flat shape. 637 00:33:05,550 --> 00:33:09,100 It's like a plane. 638 00:33:09,100 --> 00:33:12,000 So in a flat universe-- 639 00:33:12,000 --> 00:33:13,500 I know how to draw a plane. 640 00:33:13,500 --> 00:33:17,480 I know how to draw a rectangle. 641 00:33:17,480 --> 00:33:21,174 In a flat universe, the universe has a, kind of, 642 00:33:21,174 --> 00:33:21,840 shape like this. 643 00:33:21,840 --> 00:33:26,535 And if you were to draw a big triangle in this universe-- 644 00:33:26,535 --> 00:33:29,310 if you were to find three points and draw a triangle, 645 00:33:29,310 --> 00:33:32,460 and then you added up the angles, 646 00:33:32,460 --> 00:33:34,500 you would get the happy 180 degrees. 647 00:33:38,790 --> 00:33:42,930 So what Friedmann-- when Friedmann made these two 648 00:33:42,930 --> 00:33:46,400 assumptions of homogeneous universe and isotropic universe 649 00:33:46,400 --> 00:33:49,050 and then just put them into the equations of general 650 00:33:49,050 --> 00:33:52,320 relativity, he found out that there could be three possible 651 00:33:52,320 --> 00:33:54,085 universes-- 652 00:33:54,085 --> 00:33:55,710 the closed universe, the open universe, 653 00:33:55,710 --> 00:33:57,730 and the flat universe. 654 00:33:57,730 --> 00:34:00,810 So the important question becomes which universe is ours? 655 00:34:04,510 --> 00:34:05,478 AUDIENCE: [INAUDIBLE]. 656 00:34:05,478 --> 00:34:06,930 NICHOLAS DIBELLA: No problem. 657 00:34:06,930 --> 00:34:12,510 So an important question becomes which what universe is ours? 658 00:34:12,510 --> 00:34:15,060 General relativity, as I said, it's 659 00:34:15,060 --> 00:34:17,584 one of the two most successful physical theories ever, 660 00:34:17,584 --> 00:34:18,750 and so we believe it's true. 661 00:34:18,750 --> 00:34:20,590 And with this-- these two assumptions 662 00:34:20,590 --> 00:34:23,679 of homogeneity and isotropy, we also believe those are true. 663 00:34:23,679 --> 00:34:25,980 And so we probably believe that-- 664 00:34:25,980 --> 00:34:28,440 I mean, you should then believe that the universe would 665 00:34:28,440 --> 00:34:32,040 be one of these three types. 666 00:34:32,040 --> 00:34:34,320 Does anybody have a guess for which one 667 00:34:34,320 --> 00:34:36,640 best approximates our universe? 668 00:34:36,640 --> 00:34:37,139 Yes. 669 00:34:37,139 --> 00:34:37,919 AUDIENCE: One. 670 00:34:37,919 --> 00:34:38,550 NICHOLAS DIBELLA: One, OK. 671 00:34:38,550 --> 00:34:39,550 So somebody guessed one. 672 00:34:39,550 --> 00:34:42,170 Do you have a guess? 673 00:34:42,170 --> 00:34:43,580 AUDIENCE: Same one. 674 00:34:43,580 --> 00:34:45,840 NICHOLAS DIBELLA: You're also guessing one. 675 00:34:45,840 --> 00:34:47,989 Anybody else have a guess for which universe best 676 00:34:47,989 --> 00:34:49,080 describes ours? 677 00:34:49,080 --> 00:34:49,580 Yes. 678 00:34:49,580 --> 00:34:50,260 AUDIENCE: Two. 679 00:34:50,260 --> 00:34:52,429 NICHOLAS DIBELLA: You say two. 680 00:34:52,429 --> 00:34:54,239 Anybody have a third guess? 681 00:34:54,239 --> 00:34:54,739 Yes. 682 00:34:54,739 --> 00:34:55,929 AUDIENCE: In for two. 683 00:34:55,929 --> 00:34:57,637 NICHOLAS DIBELLA: You're also in for two. 684 00:34:57,637 --> 00:35:00,420 How about-- let's see a show of hands for one. 685 00:35:00,420 --> 00:35:06,130 See-- one, two, three. 686 00:35:06,130 --> 00:35:09,080 Let's see a show of hands for two. 687 00:35:09,080 --> 00:35:11,280 One, two, three, four, five, six, 688 00:35:11,280 --> 00:35:16,454 seven, eight, nine, plus or minus a couple. 689 00:35:16,454 --> 00:35:17,120 How about three? 690 00:35:17,120 --> 00:35:19,220 Who thinks three? 691 00:35:19,220 --> 00:35:21,074 One, two, three. 692 00:35:21,074 --> 00:35:21,740 Three for three. 693 00:35:25,670 --> 00:35:27,950 It turns out that the minority was right. 694 00:35:27,950 --> 00:35:34,220 This third one gives the best description of the universe. 695 00:35:34,220 --> 00:35:36,620 So the question of which one best describes the universe 696 00:35:36,620 --> 00:35:41,390 was pretty much an open question until about 1998. 697 00:35:41,390 --> 00:35:42,980 Nobody really knew which one it was. 698 00:35:42,980 --> 00:35:45,080 We had set some limits for this density, 699 00:35:45,080 --> 00:35:46,370 but we didn't really know. 700 00:35:46,370 --> 00:35:49,340 Like, we knew-- we had some range of what the density could 701 00:35:49,340 --> 00:35:52,550 be, but we didn't actually know which one it was. 702 00:35:52,550 --> 00:35:57,906 But, finally, in 1998, people discovered that the universe-- 703 00:35:57,906 --> 00:36:00,950 the expansion of the universe is actually accelerating 704 00:36:00,950 --> 00:36:05,090 and that immediately ruled out the first one. 705 00:36:05,090 --> 00:36:08,120 Because, in this case, the universe is expanding, 706 00:36:08,120 --> 00:36:14,230 but the rate of the expansion isn't increasing. 707 00:36:14,230 --> 00:36:16,520 The expansion isn't accelerating because-- 708 00:36:16,520 --> 00:36:18,110 it's acceler-- it's expanding now, 709 00:36:18,110 --> 00:36:21,680 but eventually it's going to stop expanding 710 00:36:21,680 --> 00:36:24,830 and then just collapse on itself in a big crunch. 711 00:36:24,830 --> 00:36:26,270 But there was a discovery in 1998 712 00:36:26,270 --> 00:36:29,810 that the universe is accelerating its expansion. 713 00:36:29,810 --> 00:36:34,460 And what people did was to look at these systems 714 00:36:34,460 --> 00:36:38,030 in the universe called Type 1a supernova, which are basically 715 00:36:38,030 --> 00:36:40,070 explosions of stars. 716 00:36:40,070 --> 00:36:43,880 And these explosions have a brightness. 717 00:36:43,880 --> 00:36:47,300 If you were to see how bright they are-- 718 00:36:47,300 --> 00:36:50,060 they always explode in the same way basically. 719 00:36:50,060 --> 00:36:53,450 They're always going to be the same brightness regardless 720 00:36:53,450 --> 00:36:56,990 of where it happens or what led up to it. 721 00:36:56,990 --> 00:37:00,165 And so for this reason we call these supernova-- 722 00:37:00,165 --> 00:37:03,050 these supernovas standard candles because it's-- 723 00:37:03,050 --> 00:37:05,900 you can just buy a candle and light it up wherever you want, 724 00:37:05,900 --> 00:37:08,310 it's always going to be the same. 725 00:37:08,310 --> 00:37:11,570 And so what they did was they looked at these Type 1A 726 00:37:11,570 --> 00:37:14,300 supernovas, as they're called, and they 727 00:37:14,300 --> 00:37:16,040 measured the distance-- 728 00:37:16,040 --> 00:37:17,660 distances to a bunch of them. 729 00:37:17,660 --> 00:37:19,130 And they also measured how far away 730 00:37:19,130 --> 00:37:22,180 they're moving from us and from this 731 00:37:22,180 --> 00:37:26,084 they were actually able to calculate the expansion rates-- 732 00:37:26,084 --> 00:37:27,500 the expansion rate of the universe 733 00:37:27,500 --> 00:37:28,970 at two different times. 734 00:37:28,970 --> 00:37:31,730 And they found out that the expansion rate now 735 00:37:31,730 --> 00:37:35,240 is greater the expansion rate that was some previous time. 736 00:37:35,240 --> 00:37:38,905 And so that implied that the universe is expanding 737 00:37:38,905 --> 00:37:41,158 and the expansion is accelerating. 738 00:37:44,580 --> 00:37:46,940 So we knew the expansion was accelerating by 1998. 739 00:37:46,940 --> 00:37:49,605 But It was still controversial at that time. 740 00:37:49,605 --> 00:37:51,980 Some people didn't really think that the supernovas would 741 00:37:51,980 --> 00:37:55,970 be good standard candles and some people 742 00:37:55,970 --> 00:37:58,460 had criticisms with the way that they 743 00:37:58,460 --> 00:38:00,320 were interpreting the data. 744 00:38:00,320 --> 00:38:07,460 But, more recently, the 2000s, people 745 00:38:07,460 --> 00:38:10,550 had more precise ways of determining which one 746 00:38:10,550 --> 00:38:12,560 of these three cases was true. 747 00:38:12,560 --> 00:38:16,070 And-- so I said-- so I talked about this cosmic microwave 748 00:38:16,070 --> 00:38:19,250 background radiation and it turns out that regardless 749 00:38:19,250 --> 00:38:20,840 of where you look you see it. 750 00:38:20,840 --> 00:38:24,890 And I also mentioned that it's approximately isotropic, 751 00:38:24,890 --> 00:38:27,860 meaning that the temperature is about the same regardless 752 00:38:27,860 --> 00:38:29,120 of where you look. 753 00:38:29,120 --> 00:38:33,820 So it's approximately isotropic, but it's slightly anisotropic. 754 00:38:33,820 --> 00:38:38,660 There's a slight deviation from isotropy, a slight fluctuation 755 00:38:38,660 --> 00:38:39,830 from isotropy. 756 00:38:39,830 --> 00:38:45,710 And from this slight fluctuation you can completely determine-- 757 00:38:45,710 --> 00:38:49,090 you can easily determine that this is the case-- 758 00:38:49,090 --> 00:38:50,929 it's a flat universe. 759 00:38:50,929 --> 00:38:52,970 Unfortunately, I can't really go into the details 760 00:38:52,970 --> 00:38:54,870 of how they did that. 761 00:38:54,870 --> 00:38:59,900 But you can feel free to ask me after class about it. 762 00:38:59,900 --> 00:39:07,059 So here we are today, 2008. 763 00:39:07,059 --> 00:39:09,350 We know that there's-- we're pretty sure that there was 764 00:39:09,350 --> 00:39:11,100 a beginning of time-- 765 00:39:11,100 --> 00:39:11,610 big bang. 766 00:39:11,610 --> 00:39:14,340 Big Bang model says there's a beginning of time. 767 00:39:14,340 --> 00:39:15,960 And now we have a pretty good idea 768 00:39:15,960 --> 00:39:17,790 of where the universe is heading. 769 00:39:17,790 --> 00:39:20,070 We know about the fate of the universe. 770 00:39:20,070 --> 00:39:22,650 Because we're pretty sure now that the universe is flat. 771 00:39:22,650 --> 00:39:25,260 The universe is expanding, and accelerating 772 00:39:25,260 --> 00:39:26,869 and has this kind of geometry. 773 00:39:26,869 --> 00:39:29,160 So it will expand forever, and space will live forever, 774 00:39:29,160 --> 00:39:32,880 and time will live forever and the universe will have no end. 775 00:39:32,880 --> 00:39:35,730 And this is basically where we are today. 776 00:39:35,730 --> 00:39:40,290 And I'll take-- no-- 777 00:39:40,290 --> 00:39:41,294 we started at 1:30? 778 00:39:41,294 --> 00:39:42,210 AUDIENCE: [INAUDIBLE]. 779 00:39:42,210 --> 00:39:42,580 NICHOLAS DIBELLA: OK. 780 00:39:42,580 --> 00:39:44,100 Yeah, I take a break now. 781 00:39:44,100 --> 00:39:45,225 I take a five minute brake. 782 00:39:48,147 --> 00:39:49,730 I thought we might've started at 2:00. 783 00:39:49,730 --> 00:39:51,240 And I was looking at my clock-- 784 00:39:51,240 --> 00:39:51,790 2:15? 785 00:39:51,790 --> 00:39:53,792 AUDIENCE: [INAUDIBLE]. 786 00:39:53,792 --> 00:39:55,750 NICHOLAS DIBELLA: Yeah, you feel free to ask me 787 00:39:55,750 --> 00:39:58,513 any questions during the break. 788 00:39:58,513 --> 00:40:02,620 AUDIENCE: [INAUDIBLE] universe was like? 789 00:40:02,620 --> 00:40:08,870 NICHOLAS DIBELLA: So-- so we have these models-- 790 00:40:08,870 --> 00:40:09,370 all right. 791 00:40:09,370 --> 00:40:11,590 So-- I mean, I'm not describing any of the mathematical details 792 00:40:11,590 --> 00:40:12,140 of it. 793 00:40:12,140 --> 00:40:13,460 But we have-- 794 00:40:13,460 --> 00:40:19,890 I mean, there is, of course, math that goes along with this 795 00:40:19,890 --> 00:40:27,530 and so what people do is they look at this-- 796 00:40:27,530 --> 00:40:31,450 they look at this variation of temperature of this microwave 797 00:40:31,450 --> 00:40:34,960 radiation and, basically, what they do is a fit. 798 00:40:34,960 --> 00:40:40,330 They do a fit to the curvature of the universe. 799 00:40:40,330 --> 00:40:42,870 So this curvature-- this has no curvature. 800 00:40:42,870 --> 00:40:44,200 This has negative curvature. 801 00:40:44,200 --> 00:40:46,030 Open means it has negative curvature. 802 00:40:46,030 --> 00:40:48,070 And closed means it has positive curvature. 803 00:40:48,070 --> 00:40:51,320 And there is a parameter that you have in your equations, 804 00:40:51,320 --> 00:40:53,800 and when you do a best fit it turns out 805 00:40:53,800 --> 00:40:55,870 that the universe is flat-- 806 00:40:55,870 --> 00:41:01,130 to within a really good range it's fits flats. 807 00:41:01,130 --> 00:41:03,510 I know that doesn't really tell you a whole lot more, 808 00:41:03,510 --> 00:41:06,820 but that's basically what they do-- is they do a best fits 809 00:41:06,820 --> 00:41:09,470 and it turns out the universe, from this best fit, 810 00:41:09,470 --> 00:41:10,600 is very likely flat. 811 00:41:14,020 --> 00:41:16,020 AUDIENCE: So how fast is the universe expanding? 812 00:41:16,020 --> 00:41:17,145 NICHOLAS DIBELLA: How fast? 813 00:41:19,710 --> 00:41:24,160 So that question-- so maybe a better question is how fast are 814 00:41:24,160 --> 00:41:25,420 galaxies moving away from us? 815 00:41:25,420 --> 00:41:28,150 That might be a better question. 816 00:41:28,150 --> 00:41:34,250 Yeah, so there's-- so Hubble, I mentioned last time-- 817 00:41:34,250 --> 00:41:37,450 so Hubble discovered that most things are moving away from us, 818 00:41:37,450 --> 00:41:39,340 most of the galaxies are moving away from us. 819 00:41:39,340 --> 00:41:41,770 He actually discovered a law that's a little more 820 00:41:41,770 --> 00:41:43,030 precise than that. 821 00:41:43,030 --> 00:41:46,320 He discovered that the speed-- 822 00:41:46,320 --> 00:41:48,430 so this-- I said I was going to write down 823 00:41:48,430 --> 00:41:51,250 a minimal of equations, but this isn't technically 824 00:41:51,250 --> 00:41:54,190 part of class. 825 00:41:54,190 --> 00:41:56,770 So Hubble discovered that the speed that galaxies move away 826 00:41:56,770 --> 00:42:06,070 from us, v, is equal to some constants times the distance 827 00:42:06,070 --> 00:42:11,290 that the galaxy is from us, so time is, say, d. 828 00:42:11,290 --> 00:42:13,894 And this constant, h-- 829 00:42:13,894 --> 00:42:16,540 this h-- it's called Hubble's constant. 830 00:42:22,696 --> 00:42:25,740 And it's approximately equal to-- 831 00:42:25,740 --> 00:42:26,902 I always forget the units-- 832 00:42:26,902 --> 00:42:28,610 well, I always have to think about units. 833 00:42:28,610 --> 00:42:39,770 It's some weird units-- something like 70 kiloparsecs 834 00:42:39,770 --> 00:42:41,646 per-- 835 00:42:41,646 --> 00:42:43,270 I'm going to forget about these units-- 836 00:42:43,270 --> 00:42:47,790 I'm-- these are the units it's usually given in, but you can 837 00:42:47,790 --> 00:42:49,950 always convert it into more familiar units. 838 00:42:49,950 --> 00:42:53,160 And in familiar units it's equal to approximately 1 839 00:42:53,160 --> 00:42:57,585 over 14 billion years. 840 00:43:01,770 --> 00:43:03,810 This con-- so this constant is related 841 00:43:03,810 --> 00:43:07,330 to the age of the universe. 842 00:43:07,330 --> 00:43:10,290 And so this is what that constant is, 843 00:43:10,290 --> 00:43:12,880 so it has units 1 over seconds. 844 00:43:12,880 --> 00:43:18,960 And then if I-- if you tell me how far away a galaxy is, then 845 00:43:18,960 --> 00:43:22,090 you can easily find out how fast it's moving away from you 846 00:43:22,090 --> 00:43:24,570 just based on this simple formula. 847 00:43:24,570 --> 00:43:25,320 And then you can-- 848 00:43:25,320 --> 00:43:26,420 AUDIENCE: [INAUDIBLE]? 849 00:43:26,420 --> 00:43:27,628 NICHOLAS DIBELLA: Yeah, yeah. 850 00:43:27,628 --> 00:43:31,290 So-- yeah, so it turns out that the speed that a galaxy is 851 00:43:31,290 --> 00:43:34,200 moving away from us is proportional to how far away it 852 00:43:34,200 --> 00:43:40,080 is and you-- that-- you can actually derive that from 853 00:43:40,080 --> 00:43:42,130 general relativity, but I won't. 854 00:43:42,130 --> 00:43:45,100 So-- no, but it's easy to-- 855 00:43:45,100 --> 00:43:47,040 I think that's pretty easy to imagine though. 856 00:43:47,040 --> 00:43:48,110 I mean, you could take-- 857 00:43:48,110 --> 00:43:52,030 I mean, you can take a balloon, look at two points on it-- 858 00:43:52,030 --> 00:43:54,170 two points on the balloon and start to blow it up, 859 00:43:54,170 --> 00:43:57,246 and then you can-- then you can ask, well, how-- 860 00:43:57,246 --> 00:43:58,990 to tell you how something that's one 861 00:43:58,990 --> 00:44:01,031 inch away from a given point and something that's 862 00:44:01,031 --> 00:44:03,470 two inches away from a given point and then say, well, 863 00:44:03,470 --> 00:44:07,880 how fast does the first guy move compare with the second guy? 864 00:44:07,880 --> 00:44:09,880 And the second guy-- the guy that's farther away 865 00:44:09,880 --> 00:44:12,088 is going to move faster when you blow up the balloon. 866 00:44:12,088 --> 00:44:14,770 And so it makes sense from that perspective. 867 00:44:14,770 --> 00:44:17,460 But, of course, the universe isn't-- 868 00:44:17,460 --> 00:44:18,910 I mean, the universe isn't closed, 869 00:44:18,910 --> 00:44:22,700 it's actually this geometry. 870 00:44:22,700 --> 00:44:25,780 And so it might be a little hard to see in this case. 871 00:44:25,780 --> 00:44:30,055 I mean, how do you blow up a sheet of paper? 872 00:44:32,686 --> 00:44:34,750 Maybe a little hard to imagine in that case. 873 00:44:46,112 --> 00:44:48,088 Ah, let's see. 874 00:45:01,426 --> 00:45:02,501 AUDIENCE: [INAUDIBLE]? 875 00:45:02,501 --> 00:45:03,750 NICHOLAS DIBELLA: What's that? 876 00:45:03,750 --> 00:45:06,124 AUDIENCE: If it's-- and if it's flat how come we can move 877 00:45:06,124 --> 00:45:06,624 around? 878 00:45:06,624 --> 00:45:08,790 NICHOLAS DIBELLA: How come we can move-- no, I mean, 879 00:45:08,790 --> 00:45:09,690 it's not really flat. 880 00:45:09,690 --> 00:45:11,940 I mean, that's just an analogy to understand. 881 00:45:11,940 --> 00:45:14,970 It's flat in the sense-- 882 00:45:14,970 --> 00:45:18,720 so it's like a three-dimensional kind of flat. 883 00:45:18,720 --> 00:45:22,710 I mean, it's easy to understand living on a circle-- 884 00:45:22,710 --> 00:45:24,870 I'm sorry-- living on sphere, living on a balloon-- 885 00:45:24,870 --> 00:45:26,250 walking around like that-- 886 00:45:26,250 --> 00:45:30,440 I mean, that's a closed shape, that's a spherical shape. 887 00:45:30,440 --> 00:45:32,384 And then you can imagine extending that 888 00:45:32,384 --> 00:45:33,300 to one more dimension. 889 00:45:33,300 --> 00:45:34,680 You might not be able to see it in your head, 890 00:45:34,680 --> 00:45:35,660 but you can imagine-- 891 00:45:35,660 --> 00:45:37,500 just keep walking and then eventually 892 00:45:37,500 --> 00:45:40,987 get back to where you started from. 893 00:45:40,987 --> 00:45:42,820 And you can also imagine drawing a triangle, 894 00:45:42,820 --> 00:45:45,360 and then adding up the angles and then getting 895 00:45:45,360 --> 00:45:46,620 a number greater than-- 896 00:45:46,620 --> 00:45:50,610 did I say greater than 180-- one of those. 897 00:45:50,610 --> 00:45:52,950 In a flat universe-- 898 00:45:52,950 --> 00:45:54,420 in the flat universe the geometry 899 00:45:54,420 --> 00:45:57,390 is similar to the geometry on top of a-- 900 00:45:57,390 --> 00:46:00,490 similar to the geometry of a sheet of paper. 901 00:46:00,490 --> 00:46:03,000 But you have to add one more dimension, 902 00:46:03,000 --> 00:46:05,160 so if you kept walking in one direction, 903 00:46:05,160 --> 00:46:08,280 you would never get back to where you started from. 904 00:46:08,280 --> 00:46:10,530 And if you tried-- but if you tried drawing a triangle 905 00:46:10,530 --> 00:46:13,095 and adding up the angles, you would get 180 degrees. 906 00:46:15,960 --> 00:46:17,250 Other than that it's-- 907 00:46:17,250 --> 00:46:19,840 I mean, other than that you might not be able to visualize, 908 00:46:19,840 --> 00:46:21,714 but you can certainly understand what happens 909 00:46:21,714 --> 00:46:24,240 in that kind of universe. 910 00:46:24,240 --> 00:46:25,601 And it's also infinite, so-- 911 00:46:25,601 --> 00:46:27,475 I mean, this is just a finite sheet of paper. 912 00:46:27,475 --> 00:46:30,070 If the universe is flat, which looks like it is, 913 00:46:30,070 --> 00:46:32,450 then this is infinite, so it goes out at every-- 914 00:46:32,450 --> 00:46:33,464 it's pretty big-- 915 00:46:33,464 --> 00:46:34,380 infinity's pretty big. 916 00:46:38,810 --> 00:46:40,088 Question? 917 00:46:40,088 --> 00:46:46,364 AUDIENCE: [INAUDIBLE] but you said the universe started 918 00:46:46,364 --> 00:46:50,029 and it's never going to end or [INAUDIBLE]?? 919 00:46:50,029 --> 00:46:51,320 NICHOLAS DIBELLA: That's right. 920 00:46:51,320 --> 00:46:54,450 The universe had a beginning, but it won't have an end 921 00:46:54,450 --> 00:46:56,790 according to this Big Bang model, which 922 00:46:56,790 --> 00:46:59,070 is the conventional wisdom that we've 923 00:46:59,070 --> 00:47:02,490 gained within the past century. 924 00:47:02,490 --> 00:47:03,450 AUDIENCE: [INAUDIBLE]? 925 00:47:07,300 --> 00:47:10,168 NICHOLAS DIBELLA: Well, I mean, my-- 926 00:47:10,168 --> 00:47:12,168 AUDIENCE: [INAUDIBLE] just seems like it's wrong 927 00:47:12,168 --> 00:47:15,055 because [INAUDIBLE] seems like cheating almost. 928 00:47:15,055 --> 00:47:17,300 NICHOLAS DIBELLA: Seems like cheating. 929 00:47:17,300 --> 00:47:18,600 I mean-- [? by ?] mean-- 930 00:47:18,600 --> 00:47:20,580 so this is an intuition that you personally 931 00:47:20,580 --> 00:47:22,060 have-- if it has a beginning doesn't have-- 932 00:47:22,060 --> 00:47:22,976 it has to have an end. 933 00:47:22,976 --> 00:47:24,960 Yeah, it's an intuitive lot of people have. 934 00:47:24,960 --> 00:47:27,261 But why should the universe care about our intuition? 935 00:47:27,261 --> 00:47:30,290 AUDIENCE: [INAUDIBLE]. 936 00:47:30,290 --> 00:47:31,540 NICHOLAS DIBELLA: What's that. 937 00:47:31,540 --> 00:47:32,545 AUDIENCE: [INAUDIBLE]. 938 00:47:32,545 --> 00:47:33,920 NICHOLAS DIBELLA: Yeah, there are 939 00:47:33,920 --> 00:47:39,500 a lot of people that before we had the experimental evidence 940 00:47:39,500 --> 00:47:42,460 to know which one's right, a lot of people were saying, well, 941 00:47:42,460 --> 00:47:46,880 this closed universe model, this model that you prefer-- 942 00:47:46,880 --> 00:47:48,330 it's too simple to be wrong. 943 00:47:48,330 --> 00:47:49,640 How could it be wrong? 944 00:47:49,640 --> 00:47:55,784 And then when we found it was flat they were upset, I guess. 945 00:47:55,784 --> 00:48:03,852 AUDIENCE: [INAUDIBLE] I think that it's possible for it 946 00:48:03,852 --> 00:48:04,640 to have an end. 947 00:48:04,640 --> 00:48:07,100 It's-- I don't think it's [INAUDIBLE].. 948 00:48:07,100 --> 00:48:09,580 Everything can end. 949 00:48:09,580 --> 00:48:12,520 NICHOLAS DIBELLA: Well, I mean, yeah, general relativity-- 950 00:48:12,520 --> 00:48:15,050 I mean-- yeah, I mean, general relativity 951 00:48:15,050 --> 00:48:16,480 says that it can have an end. 952 00:48:16,480 --> 00:48:18,890 Friedmann derived that it might have an end depending 953 00:48:18,890 --> 00:48:20,035 on what the density is. 954 00:48:20,035 --> 00:48:21,410 But it doesn't have that density, 955 00:48:21,410 --> 00:48:23,606 so it doesn't have an end. 956 00:48:23,606 --> 00:48:24,148 AUDIENCE: OK. 957 00:48:24,148 --> 00:48:25,564 NICHOLAS DIBELLA: I mean, it could 958 00:48:25,564 --> 00:48:27,180 have been-- it could have had an end, 959 00:48:27,180 --> 00:48:30,680 but [INAUDIBLE] were different it could of had an end, 960 00:48:30,680 --> 00:48:32,700 but it's too small. 961 00:48:32,700 --> 00:48:36,430 So it's not going to have an end. 962 00:48:36,430 --> 00:48:38,788 However, humanity might have an end. 963 00:48:38,788 --> 00:48:41,010 AUDIENCE: [INAUDIBLE]. 964 00:48:41,010 --> 00:48:43,760 NICHOLAS DIBELLA: But humanity's different from the universe, 965 00:48:43,760 --> 00:48:44,930 I think, in any case. 966 00:49:09,230 --> 00:49:10,210 AUDIENCE: [INAUDIBLE]? 967 00:49:10,210 --> 00:49:10,877 NICHOLAS DIBELLA: Sure. 968 00:49:10,877 --> 00:49:12,778 AUDIENCE: You said that time doesn't end. 969 00:49:12,778 --> 00:49:15,586 And it doesn't end, why? [? Because the ?] universe 970 00:49:15,586 --> 00:49:16,756 accelerates [INAUDIBLE]? 971 00:49:16,756 --> 00:49:18,048 NICHOLAS DIBELLA: Right, time-- 972 00:49:18,048 --> 00:49:19,422 AUDIENCE: But at a certain point, 973 00:49:19,422 --> 00:49:21,450 when it hits the speed of light [INAUDIBLE] 974 00:49:21,450 --> 00:49:23,650 eventually time end [INAUDIBLE]? 975 00:49:23,650 --> 00:49:24,990 NICHOLAS DIBELLA: Oh. 976 00:49:24,990 --> 00:49:27,080 So-- I mean-- 977 00:49:27,080 --> 00:49:31,030 so general relativity definitely implies that some 978 00:49:31,030 --> 00:49:33,500 points in the universe. 979 00:49:33,500 --> 00:49:35,120 If you go far enough-- 980 00:49:35,120 --> 00:49:37,690 just-- I mean, the simple Hubble Law, that simple equation 981 00:49:37,690 --> 00:49:41,277 I wrote down, implies that far enough away from us 982 00:49:41,277 --> 00:49:43,360 points of space will be moving away from us faster 983 00:49:43,360 --> 00:49:44,200 than the speed of light. 984 00:49:44,200 --> 00:49:45,200 You actually reach the speed of light 985 00:49:45,200 --> 00:49:47,283 and then points will be moving away from us faster 986 00:49:47,283 --> 00:49:48,790 than the speed of light. 987 00:49:48,790 --> 00:49:54,910 But that doesn't-- yeah, so that doesn't mean that time stops. 988 00:49:54,910 --> 00:49:58,900 So this statement that time stops, basically, 989 00:49:58,900 --> 00:50:01,240 for things moving at the speed of light, 990 00:50:01,240 --> 00:50:03,880 this is something that's derived from special relativity. 991 00:50:03,880 --> 00:50:14,380 And it applies to objects that are 992 00:50:14,380 --> 00:50:19,620 in a fixed space-time, a fixed space-time that's 993 00:50:19,620 --> 00:50:21,250 not expanding. 994 00:50:21,250 --> 00:50:24,190 And in an expanding universe-- 995 00:50:24,190 --> 00:50:26,680 I mean, general relativity is a little more lax, 996 00:50:26,680 --> 00:50:30,610 I guess, in allowing faster than light travel. 997 00:50:30,610 --> 00:50:33,010 I mean no signal is really traveling faster 998 00:50:33,010 --> 00:50:34,450 than the speed of light-- 999 00:50:34,450 --> 00:50:36,534 I mean, relative to the expansion of the universe, 1000 00:50:36,534 --> 00:50:38,491 nothing travels faster than the speed of light. 1001 00:50:38,491 --> 00:50:39,970 But the universe itself, the space 1002 00:50:39,970 --> 00:50:42,490 itself can travel faster than light. 1003 00:50:42,490 --> 00:50:44,474 There's no problem with that. 1004 00:50:44,474 --> 00:50:45,640 AUDIENCE: I have a question. 1005 00:50:45,640 --> 00:50:48,520 I read somewhere, I can't remember, 1006 00:50:48,520 --> 00:50:52,809 the-- some people thought that something travel faster 1007 00:50:52,809 --> 00:50:55,812 than the speed of light, time goes backwards not forwards. 1008 00:50:55,812 --> 00:50:57,020 NICHOLAS DIBELLA: Yeah, yeah. 1009 00:50:57,020 --> 00:51:00,960 So-- well, first of all, yeah, so first of all, 1010 00:51:00,960 --> 00:51:04,254 relativity says that if you start out at a speed lower 1011 00:51:04,254 --> 00:51:05,670 than the speed of light, you can't 1012 00:51:05,670 --> 00:51:07,139 get past the speed of light. 1013 00:51:07,139 --> 00:51:09,180 You can't accelerate to get to the speed of light 1014 00:51:09,180 --> 00:51:10,300 and then pass it, because that would be required 1015 00:51:10,300 --> 00:51:11,508 an infinite amount of energy. 1016 00:51:11,508 --> 00:51:13,260 But you might imagine, yeah-- maybe you 1017 00:51:13,260 --> 00:51:15,420 were always faster than light for all your life or something 1018 00:51:15,420 --> 00:51:16,253 something like that. 1019 00:51:18,600 --> 00:51:21,137 And if that's the case, then, yeah, you 1020 00:51:21,137 --> 00:51:23,720 can conceive of particles that have this property of traveling 1021 00:51:23,720 --> 00:51:25,261 faster than the speed of light called 1022 00:51:25,261 --> 00:51:26,821 tachyons, that's their name. 1023 00:51:26,821 --> 00:51:28,320 If they do travel faster than light, 1024 00:51:28,320 --> 00:51:29,750 then they could travel back in time. 1025 00:51:29,750 --> 00:51:30,810 But we'll have a time travel lecture, 1026 00:51:30,810 --> 00:51:32,610 it'll be either next week or the week after. 1027 00:51:32,610 --> 00:51:33,720 So we'll talk about the questions like that. 1028 00:51:33,720 --> 00:51:35,136 AUDIENCE: I have another question. 1029 00:51:35,136 --> 00:51:38,670 Since light is pure energy [INAUDIBLE],, 1030 00:51:38,670 --> 00:51:41,010 wouldn't you be stripped of energy? 1031 00:51:41,010 --> 00:51:42,090 NICHOLAS DIBELLA: If you go faster than the speed of light 1032 00:51:42,090 --> 00:51:43,470 would you be stripped of energy? 1033 00:51:45,721 --> 00:51:47,220 Why would you be stripped of energy? 1034 00:51:47,220 --> 00:51:51,084 AUDIENCE: Well, because light is pure and [INAUDIBLE] 1035 00:51:51,084 --> 00:51:53,024 energy [INAUDIBLE]. 1036 00:51:53,024 --> 00:51:55,440 NICHOLAS DIBELLA: Well, why can't things faster than light 1037 00:51:55,440 --> 00:51:56,148 also have energy? 1038 00:51:56,148 --> 00:51:59,010 AUDIENCE: [INAUDIBLE] can't go beyond the speed of light-- 1039 00:52:01,625 --> 00:52:02,500 NICHOLAS DIBELLA: No. 1040 00:52:02,500 --> 00:52:03,624 There's no reason for that. 1041 00:52:06,630 --> 00:52:08,970 But I also like to think of energy more as a property 1042 00:52:08,970 --> 00:52:10,440 than as a substance. 1043 00:52:10,440 --> 00:52:12,210 I mean, things have energy. 1044 00:52:12,210 --> 00:52:13,320 Light has energy. 1045 00:52:13,320 --> 00:52:14,940 Light isn't energy, light has energy. 1046 00:52:14,940 --> 00:52:19,380 It's-- the subtle distinction, but I thought about it a lot 1047 00:52:19,380 --> 00:52:22,070 actually, yeah. 1048 00:52:22,070 --> 00:52:23,320 And I think that the property. 1049 00:52:23,320 --> 00:52:25,470 I think that the property is better. 1050 00:52:25,470 --> 00:52:26,070 Did you have a question in the back? 1051 00:52:26,070 --> 00:52:26,690 Yes. 1052 00:52:26,690 --> 00:52:28,148 AUDIENCE: Yeah, that's why Einstein 1053 00:52:28,148 --> 00:52:30,956 said you can't go faster than the speed light [INAUDIBLE]?? 1054 00:52:32,964 --> 00:52:35,130 NICHOLAS DIBELLA: The universe is allowed to expand. 1055 00:52:35,130 --> 00:52:37,410 Space is allowed to expand faster than light. 1056 00:52:37,410 --> 00:52:40,260 There's no problem with that. 1057 00:52:40,260 --> 00:52:43,530 But what is a problem is that is if you had a space that wasn't 1058 00:52:43,530 --> 00:52:46,410 expanding, for example, and then something 1059 00:52:46,410 --> 00:52:48,480 goes faster than light. 1060 00:52:48,480 --> 00:52:50,610 That won't happen ever. 1061 00:52:50,610 --> 00:52:53,260 But spa-- so here-- 1062 00:52:53,260 --> 00:52:55,860 here's a statement that's always going to be true. 1063 00:52:55,860 --> 00:53:01,155 If you come in-- if you go on a race with lights-- 1064 00:53:01,155 --> 00:53:03,280 let's suppose there's a flashlight right over here, 1065 00:53:03,280 --> 00:53:06,379 and you turn it on, and then I were to go next to it, 1066 00:53:06,379 --> 00:53:07,920 light would always beat me in a race. 1067 00:53:07,920 --> 00:53:10,640 That's something that's going to be true. 1068 00:53:10,640 --> 00:53:13,950 But in an expanding universe you can think of it-- 1069 00:53:13,950 --> 00:53:20,130 an expanding universe-- you can think of the galaxies and all 1070 00:53:20,130 --> 00:53:25,340 the objects sitting in the universe as drifting away. 1071 00:53:25,340 --> 00:53:27,000 And you can't you can't travel faster 1072 00:53:27,000 --> 00:53:30,300 than light relative to this drift. 1073 00:53:30,300 --> 00:53:32,010 So if there's already a drift like this-- 1074 00:53:35,977 --> 00:53:37,560 if there's a drift like this-- suppose 1075 00:53:37,560 --> 00:53:40,346 the universe is carrying me, then I 1076 00:53:40,346 --> 00:53:41,970 can have a speed relative to this drift 1077 00:53:41,970 --> 00:53:44,261 and that speed can't be faster than the speed of light. 1078 00:53:44,261 --> 00:53:49,330 There's-- you can think of it like that, if that makes sense. 1079 00:53:49,330 --> 00:53:50,880 Does that make sense a little bit? 1080 00:53:56,260 --> 00:53:58,260 General relativity allows space to travel faster 1081 00:53:58,260 --> 00:53:58,800 than the speed of light. 1082 00:53:58,800 --> 00:53:59,716 AUDIENCE: [INAUDIBLE]. 1083 00:53:59,716 --> 00:54:00,270 Yeah. 1084 00:54:00,270 --> 00:54:01,453 NICHOLAS DIBELLA: Yeah, question? 1085 00:54:01,453 --> 00:54:02,411 AUDIENCE: [INAUDIBLE]-- 1086 00:54:02,411 --> 00:54:04,640 NICHOLAS DIBELLA: Have you thought more about-- 1087 00:54:04,640 --> 00:54:05,716 AUDIENCE: [INAUDIBLE]. 1088 00:54:09,540 --> 00:54:11,649 The universe is infinite, correct? 1089 00:54:11,649 --> 00:54:13,190 NICHOLAS DIBELLA: It looks like that. 1090 00:54:13,190 --> 00:54:14,648 At least, it's definitely very big. 1091 00:54:14,648 --> 00:54:16,740 AUDIENCE: How can something infinite expand? 1092 00:54:16,740 --> 00:54:19,400 NICHOLAS DIBELLA: How can something-- 1093 00:54:19,400 --> 00:54:23,470 so this-- so the universe isn't expanding into anything. 1094 00:54:23,470 --> 00:54:25,120 It's not expanding into anything. 1095 00:54:25,120 --> 00:54:26,927 It universe just is. 1096 00:54:26,927 --> 00:54:29,510 What is true-- when I say that the universe is expanding, what 1097 00:54:29,510 --> 00:54:32,690 I mean is that points on the universe-- 1098 00:54:32,690 --> 00:54:35,270 the distances between points on the universe is increasing. 1099 00:54:35,270 --> 00:54:36,179 That's what I mean. 1100 00:54:36,179 --> 00:54:37,970 The universe isn't expanding into anything, 1101 00:54:37,970 --> 00:54:39,740 but the distances between points is increasing. 1102 00:54:39,740 --> 00:54:40,948 That's what the expand means. 1103 00:54:40,948 --> 00:54:44,760 That-- so then there's no problem. 1104 00:54:44,760 --> 00:54:46,290 Yeah, it's confusing. 1105 00:54:46,290 --> 00:54:48,294 Yeah, it's confusing. 1106 00:54:48,294 --> 00:54:48,960 Let me get back. 1107 00:54:48,960 --> 00:54:49,680 AUDIENCE: Thank you [INAUDIBLE]. 1108 00:54:49,680 --> 00:54:50,222 NICHOLAS DIBELLA: What's that? 1109 00:54:50,222 --> 00:54:51,780 AUDIENCE: Thanks for being patient [INAUDIBLE].. 1110 00:54:51,780 --> 00:54:52,821 NICHOLAS DIBELLA: Oh, no. 1111 00:54:52,821 --> 00:54:53,730 No problem. 1112 00:54:53,730 --> 00:54:56,830 So let's get back. 1113 00:54:56,830 --> 00:54:57,907 Where was I? 1114 00:55:11,330 --> 00:55:15,870 So it looks like the universe is flat and, therefore, looks 1115 00:55:15,870 --> 00:55:18,160 like the universe will expand forever 1116 00:55:18,160 --> 00:55:21,220 and there will be no time. 1117 00:55:21,220 --> 00:55:26,085 Well, you might say, I guess, that's good for the universe. 1118 00:55:26,085 --> 00:55:27,550 But what about me? 1119 00:55:27,550 --> 00:55:28,830 Am I going to live forever? 1120 00:55:28,830 --> 00:55:31,680 Or, are my ancestors going to live forever? 1121 00:55:31,680 --> 00:55:33,145 Is Earth going to live forever? 1122 00:55:33,145 --> 00:55:34,520 Is the sun going to live forever? 1123 00:55:34,520 --> 00:55:35,895 What about all these other stars, 1124 00:55:35,895 --> 00:55:38,500 are those going to live forever? 1125 00:55:38,500 --> 00:55:43,200 Well, the situation looks better for the universe than it does-- 1126 00:55:43,200 --> 00:55:46,320 than it looks for us. 1127 00:55:46,320 --> 00:55:49,450 Because-- so the sun-- well, let's start with the sun. 1128 00:55:49,450 --> 00:55:54,720 So the sun shines because it has this core of nuclear fuel-- 1129 00:55:54,720 --> 00:55:57,630 hydrogen burning into helium at a temperature 1130 00:55:57,630 --> 00:56:00,900 of millions of degrees. 1131 00:56:00,900 --> 00:56:04,020 But, eventually, this nuclear fuel is going to run out 1132 00:56:04,020 --> 00:56:07,890 and the sun won't be able to provide 1133 00:56:07,890 --> 00:56:11,130 for us the way it provides now. 1134 00:56:11,130 --> 00:56:12,930 Actually, eventually, it'll engulf us all. 1135 00:56:16,200 --> 00:56:21,510 And not tomorrow, not the next day, but in billions 1136 00:56:21,510 --> 00:56:24,069 of years down the line when the star-- 1137 00:56:24,069 --> 00:56:25,860 when the sun gets near the end of its life, 1138 00:56:25,860 --> 00:56:28,050 it's going to engulf us all. 1139 00:56:28,050 --> 00:56:31,599 And so the earth will be eaten up-- 1140 00:56:31,599 --> 00:56:33,390 same thing is true for all the other stars. 1141 00:56:33,390 --> 00:56:35,223 Eventually they will run out of nuclear fuel 1142 00:56:35,223 --> 00:56:39,180 and they'll die out, they'll burn out and eventually turn 1143 00:56:39,180 --> 00:56:43,560 into something like a white dwarf, which 1144 00:56:43,560 --> 00:56:49,980 is a dead star basically, or a black hole, which 1145 00:56:49,980 --> 00:56:53,970 is a region of space that sucks in everything and nothing 1146 00:56:53,970 --> 00:56:57,120 can escape from it, not even light can escape from it. 1147 00:56:57,120 --> 00:57:02,174 And so the universe eventually is going to be pretty dead. 1148 00:57:02,174 --> 00:57:04,170 The universe is still going to be alive, 1149 00:57:04,170 --> 00:57:06,545 and it still going to exist, time will still persist, 1150 00:57:06,545 --> 00:57:08,670 but everything in it will eventually-- pretty dead. 1151 00:57:08,670 --> 00:57:12,420 Everything will burn out or-- 1152 00:57:12,420 --> 00:57:14,940 actually, eventually, even the black holes 1153 00:57:14,940 --> 00:57:21,900 are going to decay eventually, according to most theories. 1154 00:57:21,900 --> 00:57:24,990 Eventually everything will decay eventually, even protons, 1155 00:57:24,990 --> 00:57:29,670 according to some of our theories. 1156 00:57:29,670 --> 00:57:35,640 And so this-- the prospect of an interesting universe 1157 00:57:35,640 --> 00:57:39,510 for eternity seems rather bleak, even though the universe 1158 00:57:39,510 --> 00:57:41,262 exists forever. 1159 00:57:41,262 --> 00:57:42,720 Now, you might counter to this ar-- 1160 00:57:42,720 --> 00:57:44,610 you might counter to this by saying, well, 1161 00:57:44,610 --> 00:57:48,120 your analysis of the universe-- it didn't take into account 1162 00:57:48,120 --> 00:57:49,710 the existence of intelligence. 1163 00:57:49,710 --> 00:57:52,168 I mean, we're intelligent beings maybe we can do something. 1164 00:57:52,168 --> 00:57:53,490 Maybe we can fix the universe. 1165 00:57:53,490 --> 00:57:54,489 We can fix the sun. 1166 00:57:54,489 --> 00:57:57,030 If it runs out of nuclear fuel, let's just give it some more. 1167 00:57:57,030 --> 00:57:57,940 Let's fix it. 1168 00:57:57,940 --> 00:57:59,070 Let's fix these stars. 1169 00:57:59,070 --> 00:58:00,077 Let's fix everything. 1170 00:58:00,077 --> 00:58:00,910 I mean, we're smart. 1171 00:58:00,910 --> 00:58:06,149 We were smart now, imagine us in 1,000 years or 10,000 years. 1172 00:58:06,149 --> 00:58:06,690 I don't know. 1173 00:58:06,690 --> 00:58:09,140 Maybe. 1174 00:58:09,140 --> 00:58:11,580 But I do think it's an interesting question, though, 1175 00:58:11,580 --> 00:58:16,507 of the role that intelligence plays in a changing universe. 1176 00:58:16,507 --> 00:58:18,945 AUDIENCE: [INAUDIBLE]. 1177 00:58:18,945 --> 00:58:20,195 NICHOLAS DIBELLA: What's that? 1178 00:58:20,195 --> 00:58:21,059 AUDIENCE: I don't think we'll even 1179 00:58:21,059 --> 00:58:22,309 be humans anymore [INAUDIBLE]. 1180 00:58:22,309 --> 00:58:24,850 NICHOLAS DIBELLA: Yeah, we might be transhumans at that point 1181 00:58:24,850 --> 00:58:26,730 or some kind of a higher consciousness. 1182 00:58:26,730 --> 00:58:34,040 AUDIENCE: [INAUDIBLE] thousands of years [INAUDIBLE].. 1183 00:58:34,040 --> 00:58:35,350 NICHOLAS DIBELLA: Or we'll-- 1184 00:58:35,350 --> 00:58:36,683 AUDIENCE: Actually [INAUDIBLE]-- 1185 00:58:36,683 --> 00:58:39,600 NICHOLAS DIBELLA: Or we'll be ashes, which would be-- 1186 00:58:39,600 --> 00:58:43,004 AUDIENCE: [INAUDIBLE]. 1187 00:58:43,004 --> 00:58:43,920 NICHOLAS DIBELLA: Yes. 1188 00:58:43,920 --> 00:58:46,630 AUDIENCE: I don't think we could evolve [INAUDIBLE]---- 1189 00:58:46,630 --> 00:58:48,088 NICHOLAS DIBELLA: Sorry, excuse me. 1190 00:58:48,088 --> 00:58:50,772 AUDIENCE: I don't think we can evolve at this point because-- 1191 00:58:50,772 --> 00:58:54,580 I don't why [INAUDIBLE]. 1192 00:58:54,580 --> 00:59:03,282 I mean, if you had a disability, you die [INAUDIBLE] technology 1193 00:59:03,282 --> 00:59:04,554 can fix you, so-- 1194 00:59:04,554 --> 00:59:05,720 NICHOLAS DIBELLA: Yeah, so-- 1195 00:59:05,720 --> 00:59:08,110 AUDIENCE: [INAUDIBLE] die [INAUDIBLE].. 1196 00:59:10,910 --> 00:59:12,410 NICHOLAS DIBELLA: So you don't think 1197 00:59:12,410 --> 00:59:14,040 that we're evolving anymore because now we have technology? 1198 00:59:14,040 --> 00:59:14,870 AUDIENCE: Well-- 1199 00:59:14,870 --> 00:59:15,200 NICHOLAS DIBELLA: Basically. 1200 00:59:15,200 --> 00:59:16,540 AUDIENCE: [? Slower ?] [? than-- ?] 1201 00:59:16,540 --> 00:59:18,790 NICHOLAS DIBELLA: Well, we could say that technology-- 1202 00:59:18,790 --> 00:59:22,399 I mean, if-- so here-- this is the universe all the stuff. 1203 00:59:22,399 --> 00:59:23,190 This is all nature. 1204 00:59:23,190 --> 00:59:26,392 And technology, in principle, is part of nature. 1205 00:59:26,392 --> 00:59:28,100 And you can say technology, in principle, 1206 00:59:28,100 --> 00:59:29,730 is part of evolution. 1207 00:59:29,730 --> 00:59:33,950 And so you could say that if you want to. 1208 00:59:33,950 --> 00:59:36,620 But I just thought it was interesting to mention 1209 00:59:36,620 --> 00:59:38,060 that point. 1210 00:59:38,060 --> 00:59:42,530 You might-- I personally-- 1211 00:59:42,530 --> 00:59:44,390 I think it would be great if humanity 1212 00:59:44,390 --> 00:59:46,230 can live for a long time or live forever. 1213 00:59:46,230 --> 00:59:47,470 That there would be great. 1214 00:59:47,470 --> 00:59:50,390 And, maybe, we can keep home, we can keep earth, 1215 00:59:50,390 --> 00:59:52,280 as a reservation. 1216 00:59:52,280 --> 00:59:53,705 It could be a historical landmark. 1217 00:59:56,770 --> 00:59:57,270 Yes. 1218 00:59:57,270 --> 01:00:00,220 AUDIENCE: Somebody told me that in the next 50 years 1219 01:00:00,220 --> 01:00:02,395 [INAUDIBLE] something's going to happen to the sun-- 1220 01:00:02,395 --> 01:00:04,580 [INAUDIBLE] start sending solar storms at us. 1221 01:00:07,120 --> 01:00:08,120 NICHOLAS DIBELLA: Maybe. 1222 01:00:08,120 --> 01:00:10,220 AUDIENCE: Like wipe out all electricity. 1223 01:00:10,220 --> 01:00:11,240 NICHOLAS DIBELLA: I mean, there are a lot of bad things 1224 01:00:11,240 --> 01:00:13,787 that could happen at any given moment in time. 1225 01:00:13,787 --> 01:00:16,650 AUDIENCE: [INAUDIBLE]. 1226 01:00:16,650 --> 01:00:19,710 NICHOLAS DIBELLA: Well, I mean, worse than normal. 1227 01:00:19,710 --> 01:00:20,450 I don't know. 1228 01:00:20,450 --> 01:00:22,592 I don't-- I don't think we have much-- 1229 01:00:22,592 --> 01:00:24,800 I don't think-- just from what I've read and what I-- 1230 01:00:24,800 --> 01:00:26,258 I don't think we have much to worry 1231 01:00:26,258 --> 01:00:27,690 about the sun in the near future. 1232 01:00:27,690 --> 01:00:30,090 I think we're able to sleep tonight, 1233 01:00:30,090 --> 01:00:33,140 but tomorrow night we should worry. 1234 01:00:33,140 --> 01:00:40,800 So this whole-- the past two weeks 1235 01:00:40,800 --> 01:00:45,110 I've talked about the Big Bang and general relativity 1236 01:00:45,110 --> 01:00:51,680 and this is the conventional wisdom 1237 01:00:51,680 --> 01:00:54,261 in cosmology, the study of the universe as a whole. 1238 01:00:54,261 --> 01:00:55,760 The conventional wisdom of cosmology 1239 01:00:55,760 --> 01:01:02,500 that we've accumulated starting with Einstein-- 1240 01:01:02,500 --> 01:01:04,670 but I've dodged a very important question 1241 01:01:04,670 --> 01:01:06,820 that somebody raised earlier. 1242 01:01:06,820 --> 01:01:08,160 I forgot who raised it. 1243 01:01:08,160 --> 01:01:11,270 But-- so this big bang-- 1244 01:01:11,270 --> 01:01:13,440 what exactly was it? 1245 01:01:13,440 --> 01:01:15,076 What exactly was this big bang. 1246 01:01:15,076 --> 01:01:17,450 I mean, general relativity says there was this thing that 1247 01:01:17,450 --> 01:01:19,080 was the beginning of time. 1248 01:01:19,080 --> 01:01:22,300 And we know from observation that the [? universe ?] was 1249 01:01:22,300 --> 01:01:26,090 very hot very back in time, but what exactly 1250 01:01:26,090 --> 01:01:28,590 was this thing, this bang? 1251 01:01:28,590 --> 01:01:29,640 What was it exactly? 1252 01:01:33,360 --> 01:01:35,070 Here's what we know. 1253 01:01:35,070 --> 01:01:37,910 We know-- we're very confident that we 1254 01:01:37,910 --> 01:01:41,030 know what happened in the universe 1255 01:01:41,030 --> 01:01:45,110 up till about three seconds after this moment in time 1256 01:01:45,110 --> 01:01:46,890 that we call the Big Bang. 1257 01:01:46,890 --> 01:01:51,029 At about three seconds after this moment that's presumed-- 1258 01:01:51,029 --> 01:01:52,820 that was presumably the beginning of time-- 1259 01:01:52,820 --> 01:01:54,410 after about three seconds or-- 1260 01:01:54,410 --> 01:01:56,891 three seconds to three minutes or so, all the nuclei 1261 01:01:56,891 --> 01:01:57,890 were being built. This-- 1262 01:01:57,890 --> 01:02:02,050 the era of nucleosynthesis was happening 1263 01:02:02,050 --> 01:02:03,980 and we understand that. 1264 01:02:03,980 --> 01:02:07,130 But if you go further back in time, 1265 01:02:07,130 --> 01:02:10,130 then you go to higher temperatures 1266 01:02:10,130 --> 01:02:13,370 and higher temperatures means higher energy. 1267 01:02:13,370 --> 01:02:17,430 And, eventually, if you go far back enough, 1268 01:02:17,430 --> 01:02:20,210 then you're going to reach an energy that we just 1269 01:02:20,210 --> 01:02:23,240 don't understand anymore. 1270 01:02:23,240 --> 01:02:25,820 I mean, we have particle physics theories that 1271 01:02:25,820 --> 01:02:30,710 describe the behavior of particles at high energies, 1272 01:02:30,710 --> 01:02:32,877 but we don't know everything. 1273 01:02:32,877 --> 01:02:34,460 I mentioned last week-- we know a lot. 1274 01:02:34,460 --> 01:02:38,420 We know a whole lot more than we knew 100 years ago, 1275 01:02:38,420 --> 01:02:40,010 but we don't know everything. 1276 01:02:40,010 --> 01:02:42,290 In particular, we don't know exactly what happens 1277 01:02:42,290 --> 01:02:44,180 at very, very high energies. 1278 01:02:44,180 --> 01:02:45,840 We know what happens at high energies, 1279 01:02:45,840 --> 01:02:49,150 but we don't know what happens at very, very high energies. 1280 01:02:49,150 --> 01:02:54,680 And in very early universe you have these very high energies. 1281 01:02:54,680 --> 01:03:00,210 And-- so we have some theories to describe very high energies, 1282 01:03:00,210 --> 01:03:03,230 but we don't know if they're right. 1283 01:03:03,230 --> 01:03:05,840 We have these theories called grand unified theories that 1284 01:03:05,840 --> 01:03:08,900 combine the electric-- the electromagnetic force, 1285 01:03:08,900 --> 01:03:10,040 the nuclear forces-- 1286 01:03:10,040 --> 01:03:12,440 but we don't know if they're right. 1287 01:03:12,440 --> 01:03:14,690 And so really anything-- 1288 01:03:14,690 --> 01:03:18,420 any talk about the universe before about a second or so 1289 01:03:18,420 --> 01:03:19,210 is-- 1290 01:03:19,210 --> 01:03:20,740 maybe a little earlier-- 1291 01:03:20,740 --> 01:03:22,970 any talk of the universe before that time 1292 01:03:22,970 --> 01:03:25,960 is really speculation. 1293 01:03:25,960 --> 01:03:29,690 It's really speculation. 1294 01:03:29,690 --> 01:03:33,010 And if you go very far back in time these particles-- 1295 01:03:33,010 --> 01:03:35,680 these high energy particle physics theories that we have-- 1296 01:03:35,680 --> 01:03:39,064 they can't even describe what happens anymore 1297 01:03:39,064 --> 01:03:41,105 because then eventually you get down to a scale-- 1298 01:03:41,105 --> 01:03:43,610 you get down to an energy scale that-- 1299 01:03:43,610 --> 01:03:45,310 well, you get-- you go back in time 1300 01:03:45,310 --> 01:03:47,780 and then get up to an energy scale that's 1301 01:03:47,780 --> 01:03:50,660 so high that you actually have to take into account quantum 1302 01:03:50,660 --> 01:03:55,490 gravity, a scale where you have to take into account quantum 1303 01:03:55,490 --> 01:03:57,690 effects as well as gravity effects. 1304 01:03:57,690 --> 01:04:01,972 And, as of today, we don't have a theory of quantum gravity. 1305 01:04:01,972 --> 01:04:05,410 I'll talk in a later lecture about some proposed candidates 1306 01:04:05,410 --> 01:04:07,666 of a theory of quantum gravity like string theory, 1307 01:04:07,666 --> 01:04:09,790 but nobody knows what the correct theory of quantum 1308 01:04:09,790 --> 01:04:12,970 gravity is or if we can even find one. 1309 01:04:12,970 --> 01:04:16,600 And so really what this Big Bang was, 1310 01:04:16,600 --> 01:04:20,620 if there even was a Big Bang that was a beginning of time-- 1311 01:04:20,620 --> 01:04:24,470 what it really was is really anybody's guess. 1312 01:04:24,470 --> 01:04:26,480 I mean, if you ask an honest physicist, 1313 01:04:26,480 --> 01:04:28,870 I mean, what was the Big Bang-- 1314 01:04:28,870 --> 01:04:31,360 explain it to me in rigorous detail, 1315 01:04:31,360 --> 01:04:34,550 explain to me the particles, and the energies and everything-- 1316 01:04:34,550 --> 01:04:37,400 an honest physicist would tell you that we don't really know. 1317 01:04:37,400 --> 01:04:39,340 We don't really know what it was, 1318 01:04:39,340 --> 01:04:43,510 and it's entirely possible that the Big Bang 1319 01:04:43,510 --> 01:04:44,840 wasn't the beginning of time. 1320 01:04:44,840 --> 01:04:47,650 It's entirely possible that there was a preexisting 1321 01:04:47,650 --> 01:04:49,750 time before the Big Bang. 1322 01:04:49,750 --> 01:04:52,720 And as you can imagine people have theories, 1323 01:04:52,720 --> 01:04:54,550 they have speculations about what 1324 01:04:54,550 --> 01:04:58,130 happened before the Big Bang. 1325 01:04:58,130 --> 01:04:59,950 I mean, they're just they're just ideas, 1326 01:04:59,950 --> 01:05:01,930 but we can still think about it. 1327 01:05:01,930 --> 01:05:07,200 They're ideas that are results of theories that we have 1328 01:05:07,200 --> 01:05:10,510 that might be true, but we don't know if they're true. 1329 01:05:10,510 --> 01:05:17,110 For example, one type of theory, that's 1330 01:05:17,110 --> 01:05:19,330 an extension to the Big Bang model, 1331 01:05:19,330 --> 01:05:21,640 is called inflationary theory, which I'll talk 1332 01:05:21,640 --> 01:05:24,170 about more in the future time. 1333 01:05:24,170 --> 01:05:28,225 And so inflation was this period in the very early universe 1334 01:05:28,225 --> 01:05:30,986 when the universe expanded very fast, exponentially fast. 1335 01:05:30,986 --> 01:05:33,360 And there are a variety of types of inflationary theories 1336 01:05:33,360 --> 01:05:37,600 and in one type called chaotic [? eternal ?] inflation, 1337 01:05:37,600 --> 01:05:40,800 the Big Bang wasn't the beginning of time. 1338 01:05:40,800 --> 01:05:44,980 There, in fact, existed time before the Big Bang, 1339 01:05:44,980 --> 01:05:48,070 before what we observe as being the Big Bang. 1340 01:05:48,070 --> 01:05:50,490 And our universe isn't the only universe, 1341 01:05:50,490 --> 01:05:54,070 there's, in fact, a vast ensemble, a vast-- 1342 01:05:54,070 --> 01:05:56,650 a multitude of universes that live inside 1343 01:05:56,650 --> 01:06:01,120 of this vast multiverse, which is a cool word-- 1344 01:06:01,120 --> 01:06:04,090 multiverse. 1345 01:06:04,090 --> 01:06:05,740 And in this multiverse, according 1346 01:06:05,740 --> 01:06:10,120 to some versions of inflation, a number of big bangs 1347 01:06:10,120 --> 01:06:12,887 have occurred in the universe's history. 1348 01:06:12,887 --> 01:06:14,470 And our Big Bang was just one of them. 1349 01:06:17,410 --> 01:06:20,260 So in some version of inflationary theory-- 1350 01:06:20,260 --> 01:06:22,940 some versions of this theory the universe 1351 01:06:22,940 --> 01:06:25,360 is very big, much bigger than we can possibly 1352 01:06:25,360 --> 01:06:29,030 imagine, much bigger than we thought it was. 1353 01:06:29,030 --> 01:06:31,600 And there are much more bangs. 1354 01:06:31,600 --> 01:06:36,570 There-- this bang that we observe 1355 01:06:36,570 --> 01:06:38,830 is not necessarily the beginning of time. 1356 01:06:38,830 --> 01:06:42,570 And so mo-- so these-- so inflation implies 1357 01:06:42,570 --> 01:06:46,875 that there probably won't be an end, that inflation will exist, 1358 01:06:46,875 --> 01:06:52,187 inflation will happen at lots of places in this vast multiverse, 1359 01:06:52,187 --> 01:06:54,520 and lots of big bangs will keep happening, and happening 1360 01:06:54,520 --> 01:06:57,700 and happening-- you get a bubble and then inflation. 1361 01:06:57,700 --> 01:06:59,350 It's really a random process. 1362 01:06:59,350 --> 01:07:03,020 You get-- you start off with one region of space 1363 01:07:03,020 --> 01:07:06,259 and then, if it's lucky, you get a bang. 1364 01:07:06,259 --> 01:07:07,300 In another region space-- 1365 01:07:07,300 --> 01:07:10,240 if you look at another region of space, if it's lucky, 1366 01:07:10,240 --> 01:07:12,198 you get a bang, you get a bang, you get a bang, 1367 01:07:12,198 --> 01:07:13,330 and so forth, and so forth. 1368 01:07:13,330 --> 01:07:15,190 And so in inflation-- 1369 01:07:15,190 --> 01:07:18,850 in [? eternal ?] inflation the universe 1370 01:07:18,850 --> 01:07:21,895 is infinite in the future-- 1371 01:07:21,895 --> 01:07:24,110 I mean, has infinite future. 1372 01:07:24,110 --> 01:07:25,390 It's not going to end. 1373 01:07:25,390 --> 01:07:27,040 But it's still an open question though 1374 01:07:27,040 --> 01:07:28,860 of whether there was a real beginning. 1375 01:07:28,860 --> 01:07:31,954 It doesn't look like the Big Bang 1376 01:07:31,954 --> 01:07:33,370 that we observe was the beginning, 1377 01:07:33,370 --> 01:07:35,755 but was there really a beginning before this Big Bang. 1378 01:07:35,755 --> 01:07:37,630 I mean, there was more of time before the Big 1379 01:07:37,630 --> 01:07:41,920 Bang in these theories, but this was there a beginning of time? 1380 01:07:41,920 --> 01:07:44,350 And nobody knows. 1381 01:07:44,350 --> 01:07:46,090 Some people speculate that there was, 1382 01:07:46,090 --> 01:07:47,950 some people speculate that there wasn't. 1383 01:07:47,950 --> 01:07:51,880 But, as of now, nobody really knows. 1384 01:07:51,880 --> 01:07:59,710 There's another interesting theory that's pretty recent, 1385 01:07:59,710 --> 01:08:03,130 and it's actually a prediction of string theory. 1386 01:08:03,130 --> 01:08:05,600 And in this theory-- 1387 01:08:05,600 --> 01:08:10,480 in this model the universe-- 1388 01:08:10,480 --> 01:08:12,160 so string theory, as I mentioned, 1389 01:08:12,160 --> 01:08:15,710 is this proposed theory of quantum gravity, 1390 01:08:15,710 --> 01:08:17,770 and nobody knows if it's right. 1391 01:08:17,770 --> 01:08:19,660 But a lot of people work on it. 1392 01:08:19,660 --> 01:08:22,359 And there's a lot of controversy over whether it's actually 1393 01:08:22,359 --> 01:08:24,069 physics or whether it's science or not. 1394 01:08:24,069 --> 01:08:25,860 But, in any case, lots of people work on it 1395 01:08:25,860 --> 01:08:33,279 and they try to discover its consequences. 1396 01:08:33,279 --> 01:08:37,270 And some people have wondered, well, 1397 01:08:37,270 --> 01:08:41,290 what does string theory have to say about the early universe? 1398 01:08:41,290 --> 01:08:46,100 And some people have found that-- 1399 01:08:46,100 --> 01:08:47,580 so there exist in string theory-- 1400 01:08:47,580 --> 01:08:51,760 there exist these objects called branes-- 1401 01:08:51,760 --> 01:08:54,220 B-R-A-N-E-- short for membranes. 1402 01:08:54,220 --> 01:08:56,470 There exist these things called branes. 1403 01:08:56,470 --> 01:09:05,620 And these branes are three-dimensional objects 1404 01:09:05,620 --> 01:09:06,200 or they-- 1405 01:09:06,200 --> 01:09:09,470 well, you can have branes of various dimensions. 1406 01:09:09,470 --> 01:09:16,109 But the type of branes that are important for this, I think-- 1407 01:09:16,109 --> 01:09:18,550 I'm not an expert on this. 1408 01:09:18,550 --> 01:09:22,350 The types of branes that you need in this model 1409 01:09:22,350 --> 01:09:23,910 are three-dimensional branes. 1410 01:09:23,910 --> 01:09:29,939 So just-- [INAUDIBLE] three-dimensional branes 1411 01:09:29,939 --> 01:09:31,740 like our universe-- like our universe 1412 01:09:31,740 --> 01:09:35,220 might be stuck on a brane. 1413 01:09:35,220 --> 01:09:37,050 So a brane is an object that lives 1414 01:09:37,050 --> 01:09:38,819 in higher dimensional space. 1415 01:09:38,819 --> 01:09:41,620 For example, you can look at this sheet of paper. 1416 01:09:41,620 --> 01:09:44,180 It's a two dimensional object, but it lives inside 1417 01:09:44,180 --> 01:09:46,000 of a three dimensional space. 1418 01:09:46,000 --> 01:09:47,660 The brane would be something like a three-dimensional object 1419 01:09:47,660 --> 01:09:49,830 that lives inside of a higher dimensional space. 1420 01:09:49,830 --> 01:09:52,590 And string theory-- some versions of string theory space 1421 01:09:52,590 --> 01:09:55,170 is actually 10 dimensional. 1422 01:09:55,170 --> 01:09:57,494 You just need 10 dimensions for the theory 1423 01:09:57,494 --> 01:09:59,160 to make any sense at all, for the theory 1424 01:09:59,160 --> 01:10:00,810 to be consistent at all. 1425 01:10:00,810 --> 01:10:04,480 It's very constraining, it's very restrictive. 1426 01:10:04,480 --> 01:10:06,810 So in this cyclic model of the universe 1427 01:10:06,810 --> 01:10:09,042 you have the two branes. 1428 01:10:09,042 --> 01:10:11,490 Well, we have many branes, but, in particular, you 1429 01:10:11,490 --> 01:10:14,730 can consider two branes and they could collide with each other. 1430 01:10:17,670 --> 01:10:19,440 And if they collide with each other, 1431 01:10:19,440 --> 01:10:22,800 then the kinetic energy of these branes when they're colliding 1432 01:10:22,800 --> 01:10:25,890 can be converted into energy that 1433 01:10:25,890 --> 01:10:28,140 produces a variety of particles like quarks, 1434 01:10:28,140 --> 01:10:31,020 and protons, and electrons and so forth. 1435 01:10:31,020 --> 01:10:37,110 And so if you collide them you can 1436 01:10:37,110 --> 01:10:39,900 create a bang, which you can call the Big Bang if you 1437 01:10:39,900 --> 01:10:41,880 want-- a big bang if you want. 1438 01:10:41,880 --> 01:10:45,870 And you'll have a very high temperature state at this 1439 01:10:45,870 --> 01:10:53,070 collision and you have this-- a very high temperature state 1440 01:10:53,070 --> 01:10:55,000 and it can be-- 1441 01:10:55,000 --> 01:10:57,000 it-- you can make it, so that it's 1442 01:10:57,000 --> 01:11:00,270 completely indistinguishable from what we 1443 01:11:00,270 --> 01:11:01,620 observe as being the Big Bang. 1444 01:11:05,600 --> 01:11:09,230 And so-- until you have a whole series of these collisions 1445 01:11:09,230 --> 01:11:12,480 and so universes undergo big bangs and then crunches, 1446 01:11:12,480 --> 01:11:14,640 bangs and crunches, banging crunches and there-- 1447 01:11:14,640 --> 01:11:15,560 vast variety of them. 1448 01:11:18,660 --> 01:11:21,230 And so in this scenario there wasn't a beginning of time 1449 01:11:21,230 --> 01:11:24,940 and there was-- and there won't be an end. 1450 01:11:24,940 --> 01:11:27,810 But, of course, nobody knows that this is right. 1451 01:11:27,810 --> 01:11:31,680 And nobody knows if inflation is right. 1452 01:11:31,680 --> 01:11:32,780 These are speculations. 1453 01:11:32,780 --> 01:11:38,010 But they're none the less very interesting to think about 1454 01:11:38,010 --> 01:11:40,650 and in the future they can-- they're testable. 1455 01:11:40,650 --> 01:11:43,950 It's possible to test them and see which one would be right. 1456 01:11:43,950 --> 01:11:48,055 And actually both of these two models 1457 01:11:48,055 --> 01:11:49,680 can actually make a prediction for what 1458 01:11:49,680 --> 01:11:53,010 we observe about the CMB, the cosmic microwave background. 1459 01:11:53,010 --> 01:11:55,310 They can both make predictions about the fluctuations. 1460 01:11:55,310 --> 01:11:57,810 And for a long time inflation was the only one that was able 1461 01:11:57,810 --> 01:11:59,610 to predict the precise way that's-- 1462 01:11:59,610 --> 01:12:00,510 it's anisotropic. 1463 01:12:02,842 --> 01:12:04,550 For a long time inflation was only theory 1464 01:12:04,550 --> 01:12:07,590 that was able to predict the precise fluctuations 1465 01:12:07,590 --> 01:12:10,920 and the temperature that we observe in the microwaves. 1466 01:12:10,920 --> 01:12:12,830 But now this new theory-- 1467 01:12:12,830 --> 01:12:17,220 this relatively new theory is also able to explain it. 1468 01:12:17,220 --> 01:12:20,080 And so inflation looked like it was really 1469 01:12:20,080 --> 01:12:22,020 the unique theory for a while, but now there's 1470 01:12:22,020 --> 01:12:25,890 this new cyclic model that some people like. 1471 01:12:25,890 --> 01:12:29,480 And who knows what's right? 1472 01:12:29,480 --> 01:12:30,563 You had a question? 1473 01:12:30,563 --> 01:12:31,510 AUDIENCE: Oh, no. 1474 01:12:31,510 --> 01:12:40,796 NICHOLAS DIBELLA: Oh, OK [INAUDIBLE] so right now 1475 01:12:40,796 --> 01:12:42,220 so we-- 1476 01:12:42,220 --> 01:12:46,200 so just to recap, we have conventional ideas 1477 01:12:46,200 --> 01:12:49,470 about the way that the universe is 1478 01:12:49,470 --> 01:12:51,030 and of the [? conventional ?] ideas 1479 01:12:51,030 --> 01:12:53,250 about the past of the universe and its future-- 1480 01:12:53,250 --> 01:12:55,380 its origin and its future. 1481 01:12:55,380 --> 01:12:57,780 The conventional wisdom was that there was a beginning 1482 01:12:57,780 --> 01:12:59,220 and there won't be an end. 1483 01:12:59,220 --> 01:13:01,680 But if you think hard about it, if you think about what was 1484 01:13:01,680 --> 01:13:03,000 the Big Bang really why-- 1485 01:13:03,000 --> 01:13:05,610 what was the bang and what caused it to bang? 1486 01:13:05,610 --> 01:13:08,490 What happened before the bang? 1487 01:13:08,490 --> 01:13:13,044 Think harder about it then you realize that we're really still 1488 01:13:13,044 --> 01:13:13,710 pretty confused. 1489 01:13:17,815 --> 01:13:20,190 And so nobody really knows if there's a beginning of time 1490 01:13:20,190 --> 01:13:23,090 or if there is going to be an end. 1491 01:13:23,090 --> 01:13:24,910 But we're-- I think we're making progress. 1492 01:13:24,910 --> 01:13:27,750 It sounds like we're making progress. 1493 01:13:27,750 --> 01:13:31,638 Does anybody have any questions? 1494 01:13:31,638 --> 01:13:32,580 All right. 1495 01:13:32,580 --> 01:13:38,212 I actually have a little extra time, so if-- 1496 01:13:38,212 --> 01:13:42,870 let's see-- so I talked about-- 1497 01:13:42,870 --> 01:13:45,150 I have a little extra time, so I'll talk li-- 1498 01:13:45,150 --> 01:13:48,150 about some more stuff. 1499 01:13:48,150 --> 01:13:53,341 So I talked about [? this ?] density of the universe. 1500 01:13:57,190 --> 01:13:59,560 What kinds of stuff make up the universe? 1501 01:13:59,560 --> 01:14:02,820 The kind of stuff that we're familiar with. 1502 01:14:02,820 --> 01:14:07,230 The kind of stuff that we-- that make up what we see-- 1503 01:14:07,230 --> 01:14:10,500 normal matter-- things like electrons, protons 1504 01:14:10,500 --> 01:14:12,760 and neutrons. 1505 01:14:12,760 --> 01:14:15,570 We can actually measure how much of this ordinary stuff 1506 01:14:15,570 --> 01:14:19,037 there is in the universe, it's called baryonic matter. 1507 01:14:19,037 --> 01:14:21,370 We can actually measure how much of this baryonic matter 1508 01:14:21,370 --> 01:14:22,660 there is. 1509 01:14:22,660 --> 01:14:25,200 And it turns out that the amount of baryonic 1510 01:14:25,200 --> 01:14:27,180 matter in the universe, compared with all 1511 01:14:27,180 --> 01:14:28,880 this stuff in the universe-- 1512 01:14:28,880 --> 01:14:30,000 so I'll make a little-- 1513 01:14:34,140 --> 01:14:37,243 so density of the universe-- 1514 01:14:47,080 --> 01:14:55,000 so normal-- it's called baryonic. 1515 01:15:02,840 --> 01:15:06,050 The kind of stuff that we're made out of only 1516 01:15:06,050 --> 01:15:09,360 takes up about 4% of all of the stuff in the universe-- 1517 01:15:09,360 --> 01:15:10,059 just 4%. 1518 01:15:18,550 --> 01:15:20,470 The rest of the stuff in the US we 1519 01:15:20,470 --> 01:15:22,000 have no idea what that stuff is. 1520 01:15:25,042 --> 01:15:27,250 There's two more kinds of stuff in the universe-- two 1521 01:15:27,250 --> 01:15:31,421 more essential kinds of stuff in the universe. 1522 01:15:31,421 --> 01:15:33,170 There's just something called dark matter. 1523 01:15:41,910 --> 01:15:48,780 So dark matter is matter that we've indirectly detected 1524 01:15:48,780 --> 01:15:50,350 by looking at galaxies. 1525 01:15:50,350 --> 01:15:55,260 What we notice that-- we noticed that galaxies spiral around 1526 01:15:55,260 --> 01:15:58,230 at a rate that's-- 1527 01:15:58,230 --> 01:16:01,350 is inconsistent with the amount of mass 1528 01:16:01,350 --> 01:16:03,630 that we actually see in the galaxy. 1529 01:16:03,630 --> 01:16:08,910 So through this observation we've 1530 01:16:08,910 --> 01:16:11,709 inferred that there must be more mass somewhere, 1531 01:16:11,709 --> 01:16:12,750 but we just can't see it. 1532 01:16:12,750 --> 01:16:15,699 It doesn't Interact with light, so we call it dark matter. 1533 01:16:15,699 --> 01:16:18,240 And there's lots of other lines of evidence for the existence 1534 01:16:18,240 --> 01:16:21,120 of dark matter. 1535 01:16:21,120 --> 01:16:24,710 But dark matter has properties-- 1536 01:16:24,710 --> 01:16:26,970 a lot of properties different from ordinary matter. 1537 01:16:26,970 --> 01:16:30,120 And you can-- [? you ?] actually measure 1538 01:16:30,120 --> 01:16:32,220 the amount of dark matter in the universe, 1539 01:16:32,220 --> 01:16:35,475 the fractional amounts, and it turns out that it's about 22%. 1540 01:16:41,160 --> 01:16:44,090 We don't really know what dark matter is, we know it has mass. 1541 01:16:44,090 --> 01:16:45,530 That's one thing we know. 1542 01:16:45,530 --> 01:16:48,680 But besides that, we don't really know a whole lot. 1543 01:16:48,680 --> 01:16:51,290 Some particle physicists have ideas about what 1544 01:16:51,290 --> 01:16:53,570 dark matter might be. 1545 01:16:53,570 --> 01:16:58,040 Dark matter might be particles that have mass, 1546 01:16:58,040 --> 01:17:00,140 but they also interact through what's 1547 01:17:00,140 --> 01:17:02,030 called the weak nuclear force. 1548 01:17:02,030 --> 01:17:04,850 The weak nuclear force is one of the fundamental forces 1549 01:17:04,850 --> 01:17:07,580 of nature and it's responsible for decays of particles like 1550 01:17:07,580 --> 01:17:10,460 beta decay and all for-- all the forms of radio-- 1551 01:17:10,460 --> 01:17:11,610 a lot of forms of-- 1552 01:17:11,610 --> 01:17:16,870 all forms of radioactivity is due to the weak nuclear force. 1553 01:17:16,870 --> 01:17:19,610 So particle physicists have speculated that maybe the dark 1554 01:17:19,610 --> 01:17:24,350 matter is particles-- they're-- 1555 01:17:24,350 --> 01:17:27,680 dark matter is particles that have mass and interact 1556 01:17:27,680 --> 01:17:28,850 with the weak force. 1557 01:17:28,850 --> 01:17:38,900 So as an abbreviation call them weakly 1558 01:17:38,900 --> 01:17:45,980 interacting massive particles, or wimps. 1559 01:17:45,980 --> 01:17:48,424 And there are a couple of other-- 1560 01:17:48,424 --> 01:17:50,090 couple-- there are some other candidates 1561 01:17:50,090 --> 01:17:52,430 that people have thought of, but nobody really knows. 1562 01:17:52,430 --> 01:17:53,440 And some-- and people-- 1563 01:17:53,440 --> 01:17:55,760 a lot of people here on earth-- 1564 01:17:55,760 --> 01:17:58,010 I guess that's where most people are-- a lot of people 1565 01:17:58,010 --> 01:18:01,615 here on Earth have tried to detect dark matter, 1566 01:18:01,615 --> 01:18:03,240 and they're currently searching for it. 1567 01:18:03,240 --> 01:18:05,239 No one's actually directly detected dark matter, 1568 01:18:05,239 --> 01:18:07,465 so we don't really know what dark matter is. 1569 01:18:07,465 --> 01:18:08,840 So we have normal matter and dark 1570 01:18:08,840 --> 01:18:12,040 matter, the rest of the stuff-- 1571 01:18:12,040 --> 01:18:14,050 or actually a very small-- 1572 01:18:14,050 --> 01:18:19,370 a very small percent is actually lights-- 1573 01:18:19,370 --> 01:18:24,620 light and neutrinos, which is very, very light and very fast 1574 01:18:24,620 --> 01:18:25,575 neutral particles. 1575 01:18:25,575 --> 01:18:27,950 A very small proportion of all this stuff in the universe 1576 01:18:27,950 --> 01:18:29,734 is radiation, is light. 1577 01:18:29,734 --> 01:18:31,400 But I won't even bother to write it down 1578 01:18:31,400 --> 01:18:33,340 because it's so negligible. 1579 01:18:33,340 --> 01:18:39,720 And so we have normal matter, dark matter, radiation and then 1580 01:18:39,720 --> 01:18:42,242 the rest of the universe, the rest of the density-- 1581 01:18:42,242 --> 01:18:43,700 we have no idea what this stuff is. 1582 01:18:43,700 --> 01:18:44,658 We call it dark energy. 1583 01:18:52,080 --> 01:18:55,590 And it takes up about [INAUDIBLE]---- 1584 01:18:55,590 --> 01:18:58,290 73 or so percent. 1585 01:18:58,290 --> 01:19:00,630 So 73% of the stuff in the universe we 1586 01:19:00,630 --> 01:19:03,430 just have no idea what it is. 1587 01:19:03,430 --> 01:19:06,850 We know some properties of it, we have some properties of it. 1588 01:19:06,850 --> 01:19:10,722 For example, as space expands, the dark 1589 01:19:10,722 --> 01:19:12,180 matter-- the density of dark matter 1590 01:19:12,180 --> 01:19:14,940 just-- it doesn't change. 1591 01:19:14,940 --> 01:19:16,890 And with normal matter if you-- with normal 1592 01:19:16,890 --> 01:19:21,665 matter if you have a certain amount of matter in a balloon, 1593 01:19:21,665 --> 01:19:23,790 for example, and then you just blow up the balloon, 1594 01:19:23,790 --> 01:19:25,998 then, obviously, the density of the matter decreases. 1595 01:19:25,998 --> 01:19:27,190 It gets diluted. 1596 01:19:27,190 --> 01:19:29,130 But this dark energy-- 1597 01:19:29,130 --> 01:19:32,940 we call it dark energy-- dark energy doesn't get diluted. 1598 01:19:32,940 --> 01:19:35,620 it's just always there. 1599 01:19:35,620 --> 01:19:38,580 And we just have no idea what it is and makes up 1600 01:19:38,580 --> 01:19:41,230 most of the universe. 1601 01:19:41,230 --> 01:19:44,130 But I think it's pretty amazing that we've 1602 01:19:44,130 --> 01:19:48,570 been able to actually infer the composition of the universe. 1603 01:19:48,570 --> 01:19:49,600 4% is normal stuff. 1604 01:19:49,600 --> 01:19:52,690 22% is dark matter. 1605 01:19:52,690 --> 01:19:55,410 And 73% is this dark energy. 1606 01:19:55,410 --> 01:20:01,800 We only understand 4% of this stuff. 1607 01:20:01,800 --> 01:20:05,230 And I just-- what do we know? 1608 01:20:05,230 --> 01:20:06,750 I mean, we're just-- 1609 01:20:06,750 --> 01:20:12,180 it just makes-- makes me feel futile or trivial or something. 1610 01:20:12,180 --> 01:20:13,060 Yes, question? 1611 01:20:13,060 --> 01:20:14,460 AUDIENCE: [INAUDIBLE]? 1612 01:20:14,460 --> 01:20:18,360 NICHOLAS DIBELLA: Oh, I guess that's a rounding error. 1613 01:20:18,360 --> 01:20:22,494 I think this was like 4.4%, this might've been something else. 1614 01:20:22,494 --> 01:20:23,910 A very small percent is radiation, 1615 01:20:23,910 --> 01:20:26,770 but, yeah, I know that comes out to 99%. 1616 01:20:26,770 --> 01:20:29,030 I think I just made a rounding error. 1617 01:20:29,030 --> 01:20:31,570 But, I mean, obviously, yeah, should come out to 100%. 1618 01:20:31,570 --> 01:20:34,825 I mean, the law of addition to 100% for probabilities 1619 01:20:34,825 --> 01:20:35,325 still holds. 1620 01:20:39,570 --> 01:20:42,920 And I think I'll end there.