1 00:00:02,820 --> 00:00:06,549 Welcome to the Coursera class on Galaxies & Cosmology. 2 00:00:06,549 --> 00:00:11,636 My name is George Djorgovski and I am a professor of California Institute of 3 00:00:11,636 --> 00:00:15,162 Technology. These classes offered to the second year 4 00:00:15,162 --> 00:00:18,011 physics and astronomy students of Caltech, 5 00:00:18,011 --> 00:00:23,572 therefore, it requires certain amount of preparation and we'll come to that in a 6 00:00:23,572 --> 00:00:26,691 moment. But first of all, let us define what is 7 00:00:26,691 --> 00:00:30,432 cosmology. It is the science of the universe as a 8 00:00:30,432 --> 00:00:36,008 whole and its major constituents, how they work, and how they evolve, how 9 00:00:36,008 --> 00:00:39,935 they form? It is different from cosmetology and if 10 00:00:39,935 --> 00:00:44,647 you made that mistake, this'll be a really good time to stop. 11 00:00:44,647 --> 00:00:46,719 Well. It's not an easy task. 12 00:00:46,719 --> 00:00:49,884 Universe is very big and we can not reach it all. 13 00:00:49,884 --> 00:00:54,471 We can just sit here and watch and so we have to make some assumptions. 14 00:00:54,471 --> 00:00:59,768 The basic assumption that we make is that laws of physics are same everywhere and 15 00:00:59,768 --> 00:01:03,385 at all times. This is a very reasonable assumption and 16 00:01:03,385 --> 00:01:07,713 if it weren't true, it would be very hard to do any kind of science. 17 00:01:07,713 --> 00:01:11,460 But we can actually test some aspects of this proposition. 18 00:01:11,460 --> 00:01:16,928 Moreover, sometimes, while studying the universe we discover things in physics 19 00:01:16,928 --> 00:01:21,971 that we didn't know about. notable examples are nature and existence 20 00:01:21,971 --> 00:01:27,368 of dark matter and dark energy and we'll talk about them later in the class. 21 00:01:27,368 --> 00:01:32,055 Unlike most other sciences, cosmology has only one object to study. 22 00:01:32,055 --> 00:01:35,180 There is only one universe in which we live. 23 00:01:35,180 --> 00:01:40,578 And all we can do is sit and watch or in technical terms, we can look at the 24 00:01:40,578 --> 00:01:45,742 surface of the past light cone. So there are parts of the universe that 25 00:01:45,742 --> 00:01:51,142 are a priori, not observable to us. We pretty much sure that they do exist 26 00:01:51,142 --> 00:01:55,890 and they're probably not very different from the one part that we do see, but 27 00:01:55,890 --> 00:02:00,715 it's something to keep in mind. And also, things are far away and 28 00:02:00,715 --> 00:02:06,134 therefore, or they're faint and small and we need most advanced technology to do 29 00:02:06,134 --> 00:02:09,182 this. This is why cosmology really flourished 30 00:02:09,182 --> 00:02:13,043 as a science in the latter part of the twentieth century, 31 00:02:13,043 --> 00:02:18,191 because before then we simply did not have good enough tools to do it right. 32 00:02:18,191 --> 00:02:23,610 But even so, we have to be always aware of possible biases and selection effects. 33 00:02:23,610 --> 00:02:28,420 For example, there could be some very faint galaxies that we're missing. 34 00:02:29,800 --> 00:02:35,731 Cosmology responds to the basic human need to understand the big picture. 35 00:02:35,731 --> 00:02:39,388 Where is it all coming from? How does it work? 36 00:02:39,388 --> 00:02:43,074 Where's it going? And it evolve through time. 37 00:02:43,074 --> 00:02:49,901 In a pre-scientific days, creation myths were made to answer these questions, but 38 00:02:49,901 --> 00:02:56,154 they really were just made up stories. starting from beginnings of modern 39 00:02:56,154 --> 00:03:00,010 science in the Renaissance and the Enlightenment, 40 00:03:00,010 --> 00:03:02,863 things started to get little more reverse. 41 00:03:02,863 --> 00:03:08,299 First, the Copernican revolution which was a major shift in the way people think 42 00:03:08,299 --> 00:03:13,056 about universe or the world. That positive that we're not at a special 43 00:03:13,056 --> 00:03:15,706 place. The earth is not the center of the 44 00:03:15,706 --> 00:03:21,142 universe, it's just a random spot. Cosmology actually uses this in so called 45 00:03:21,142 --> 00:03:24,540 cosmological principle that we'll define in a bit. 46 00:03:24,540 --> 00:03:30,409 But to, not only that, universe is not static and unchanging, we see the change. 47 00:03:30,409 --> 00:03:36,889 And just like biology and planet earth, universe evolves on its own and following 48 00:03:36,889 --> 00:03:42,607 laws of physics, major constituents of the universe, galaxies, stars and them 49 00:03:42,607 --> 00:03:47,340 have also evolved and we can understand how that works. 50 00:03:47,340 --> 00:03:54,355 And just like anything else in science, it became subject of study and therefore 51 00:03:54,355 --> 00:03:59,433 it is always improvable. We are not declaring anything with 52 00:03:59,433 --> 00:04:04,948 absolute certainty, but we're actually trying to understand what's going on. 53 00:04:04,948 --> 00:04:10,095 And like any other science, other science, the more we push further, the 54 00:04:10,095 --> 00:04:13,625 more we learn, the more new questions we open up. 55 00:04:13,625 --> 00:04:17,669 And that's the nature of science and this is very good. 56 00:04:17,669 --> 00:04:20,550 So, where, really cosmology started as a 57 00:04:20,550 --> 00:04:26,535 branch of astronomy, astronomy as a whole and certainly cosmology became a branch 58 00:04:26,535 --> 00:04:29,934 of physics. Because we use, laws of physics and 59 00:04:29,934 --> 00:04:35,181 principles of physics to obtain our measurements to understand them to 60 00:04:35,181 --> 00:04:40,280 interpret them make predictions. Alright, now let us find out how well 61 00:04:40,280 --> 00:04:45,340 prepared you are for this class. First we'll do few quizzes. 62 00:04:45,340 --> 00:04:51,030 Let us now resume our journey to the universe and dust off some of the 63 00:04:51,030 --> 00:04:56,720 astronomical units of measurement. The most basic unit in astronomy and 64 00:04:56,720 --> 00:05:01,128 cosmology is distance to the, from the sun to the earth. 65 00:05:01,128 --> 00:05:07,520 It's one astronomical unit and it's about 150 million kilometers. 66 00:05:07,520 --> 00:05:14,442 There is of course the light year which is the distance that light travels over 67 00:05:14,442 --> 00:05:20,420 one year and one light year is approximately ten to eighteen cm. 68 00:05:20,420 --> 00:05:23,950 Now astronomers almost never use light-years. 69 00:05:23,950 --> 00:05:30,394 We use parsecs, which is the distance from which one astronomical unit is seen 70 00:05:30,394 --> 00:05:35,671 as an angle of one arcsecond. And that turns out to be roughly 200,000 71 00:05:35,671 --> 00:05:40,577 astronomical units or 3 * 10^18 centimeters. 72 00:05:40,577 --> 00:05:46,821 So when we talk parsecs or kilo parsecs and mega parsecs and giga parsecs, you 73 00:05:46,821 --> 00:05:52,470 can multiply that by 3.26 to get number, corresponding number in light years. 74 00:05:52,470 --> 00:05:57,653 Two basic properties of objects that we need to understand are masses and 75 00:05:57,653 --> 00:06:01,015 luminosities. And for convenience and historical 76 00:06:01,015 --> 00:06:07,458 reasons, we use solar mass as a unit. And that is approximately 2 * 10^33 grams 77 00:06:07,458 --> 00:06:13,062 and solar luminosity which is close to 4 * 10^33 ergs/s. 78 00:06:13,062 --> 00:06:16,844 So you can convert them to other units as you need to. 79 00:06:16,844 --> 00:06:22,097 You should really remember these numbers, because we'll constantly refer to 80 00:06:22,097 --> 00:06:26,020 distances and masses and luminosities using these units. 81 00:06:26,020 --> 00:06:33,840 Now what we observe are fluxes. We mostly observe electromagnetic 82 00:06:33,840 --> 00:06:39,048 radiation, although we now have forms of astronomy that are not dependent on 83 00:06:39,048 --> 00:06:43,846 electromagnetic radiation like cosmic rays, neutrinos and [INAUDIBLE] or 84 00:06:43,846 --> 00:06:47,889 gravitational waves. But flux is what's measured and we have 85 00:06:47,889 --> 00:06:53,441 detectors that can operate in full range of wavelengths from radio to gamma rays 86 00:06:53,441 --> 00:06:57,347 and they are usually measured over some finite bandpass. 87 00:06:57,347 --> 00:07:02,830 So, spectral energy distribution which is the shape describes the spectrum of an 88 00:07:02,830 --> 00:07:09,274 object is defined to be energy per unit time per unit second per unit for 89 00:07:09,274 --> 00:07:15,834 frequency or wavelength and we never observe that, that's a differential unit. 90 00:07:15,834 --> 00:07:22,906 Instead of that, it's always observed or some finite bandpass like optical filter 91 00:07:22,906 --> 00:07:30,415 or bandwidth in [INAUDIBLE] astronomy. One unit that is often used is jansky 92 00:07:30,415 --> 00:07:37,353 which was introduced by radio astronomers and it's 10^-23 ergs per second per, per 93 00:07:37,353 --> 00:07:41,516 square per hertz. Optical and [INAUDIBLE] astronomers are 94 00:07:41,516 --> 00:07:47,578 beginning to use janskies as well although, we are typically talking about 95 00:07:47,578 --> 00:07:53,206 microjanskies and nanojanskies. So this is often called flux density and 96 00:07:53,206 --> 00:07:58,914 get really the power of object. One has to integrate it over all 97 00:07:58,914 --> 00:08:04,580 bandwidth and then multiply it by the area that's correct. 98 00:08:04,580 --> 00:08:10,542 Astronomers use magnitudes which are a logarithmic measure of flux defined by 99 00:08:10,542 --> 00:08:14,995 the formula written here. The magnitude is -4 decibals. 100 00:08:14,995 --> 00:08:20,640 The minus sign tells you that the higher the number, means the lower the flux. 101 00:08:20,640 --> 00:08:26,785 And because its a log, its a relative measure of flux relative to some unit 102 00:08:26,785 --> 00:08:30,554 flux. In log that comes as a additive constant. 103 00:08:30,554 --> 00:08:36,536 Typically, flux is measured over some finite bandpass like B-band filter 104 00:08:36,536 --> 00:08:43,993 centered on 5500 angstroms and then log of that times -2.5 plus a constant zero 105 00:08:43,993 --> 00:08:48,820 [INAUDIBLE] second gives you the actual magnitude. 106 00:08:48,820 --> 00:08:54,153 If for some reason you could integrate oh, the flux over the entire spectrum, 107 00:08:54,153 --> 00:08:58,780 then this would be called the bolometric magnitude. 108 00:08:58,780 --> 00:09:04,829 Now, magniute zero points are another part of astronomical craziness that's 109 00:09:04,829 --> 00:09:09,360 unfortunately well established and hard to change. 110 00:09:09,360 --> 00:09:14,915 For historical reasons, again, Vega which is alpha Lyrae was declared, declared to 111 00:09:14,915 --> 00:09:19,567 have zero magnitude and then everything is measured relative to it. 112 00:09:19,567 --> 00:09:23,803 Notice that because we're talking about logarithms of fluxes, 113 00:09:23,803 --> 00:09:28,594 talking about ratios of fluxes, and so it's all relative to some unit. 114 00:09:28,594 --> 00:09:32,274 Well, that unit for magnitude systems is usually Vega. 115 00:09:32,274 --> 00:09:35,260 And unfortunately, its spectrum is not flat, 116 00:09:35,260 --> 00:09:39,568 it looks like this. There is also a more rational kind of 117 00:09:39,568 --> 00:09:44,453 magnitudes that's been introduced. It's called AB new magnitudes, which are 118 00:09:44,453 --> 00:09:48,840 with the fixed zero point as the formula on the bottom shows. 119 00:09:48,840 --> 00:09:52,595 But typically, people use Vega based magnitudes. 120 00:09:52,595 --> 00:09:59,227 And it's a handy way to remember it that zero magnitude star corresponds to almost 121 00:09:59,227 --> 00:10:04,741 1000 photons per square centimeter per second per angstrom and you can scale 122 00:10:04,741 --> 00:10:09,499 from there. Now, magnitudes are measured. 123 00:10:09,499 --> 00:10:14,683 It's apparent magnitude. What we'd like to do is know how luminous 124 00:10:14,683 --> 00:10:19,710 an object really is, and therefore, we need to know how far it is. 125 00:10:19,710 --> 00:10:25,915 So an absolute magnitude was introduced, which is the apparent magnitude object 126 00:10:25,915 --> 00:10:30,628 would have if you observe it from a distance of ten parsecs. 127 00:10:30,628 --> 00:10:35,812 Why, why ten and not one is anybody's guess, but this is the definition. 128 00:10:35,812 --> 00:10:42,011 And so if you put sun or sun-like star, ten parsecs away from us, it would have 129 00:10:42,011 --> 00:10:47,507 apparent magnitude of roughly +5. Different in different bands because the 130 00:10:47,507 --> 00:10:51,540 spectrum shape is different from that one of Vega. 131 00:10:51,540 --> 00:10:56,561 So as a handy measure of the distance, sometimes we use the difference of 132 00:10:56,561 --> 00:11:01,926 apparent and absolute magnitude which is called the distance modules and it's 133 00:11:01,926 --> 00:11:06,260 equal to five times log of the distance divided by ten parsecs. 134 00:11:06,260 --> 00:11:12,005 So this is how you can convert. Well, let us begin with what's probably 135 00:11:12,005 --> 00:11:18,900 the first cosmological experiment, and it's Olbers paradox, which is stated in a 136 00:11:18,900 --> 00:11:23,096 misleadingly simple fashion. Why is the sky dark at night? 137 00:11:23,096 --> 00:11:28,102 And here is why this is a paradox. We'll assume that the universe is 138 00:11:28,102 --> 00:11:33,991 infinite and it's more or less uniformly filled with stars and it never changes, 139 00:11:33,991 --> 00:11:39,733 which is pretty much what people thought back in the early 19th century. 140 00:11:39,733 --> 00:11:43,490 Well then, the flux from a star declines as a square 141 00:11:43,490 --> 00:11:49,099 of the distance and its surface area on the sky also declines a square of the 142 00:11:49,099 --> 00:11:52,695 distance. So the surface brightness, which is light 143 00:11:52,695 --> 00:11:57,729 per unit area in the sky or solid angle I should say, remains constant. 144 00:11:57,729 --> 00:12:03,410 Now if you have infinite amount of stars, sooner or later, line, line, your line of 145 00:12:03,410 --> 00:12:09,091 sight is going to actually intersect one of those and so all of the sky should 146 00:12:09,091 --> 00:12:14,530 always be bright as a surface of the sun and obviously this is not true. 147 00:12:14,530 --> 00:12:21,199 So trying to understand why this is, was one of the first important paradoxes in 148 00:12:21,199 --> 00:12:25,135 cosmology. Notice that exactly same reasoning can 149 00:12:25,135 --> 00:12:30,679 apply to gravity because it too declines as a square of the distance. 150 00:12:30,679 --> 00:12:36,705 But if everything is symmetric then there'll be infinite forces pulling in 151 00:12:36,705 --> 00:12:42,490 all directions in same way so you would still feel no net force. However, 152 00:12:42,490 --> 00:12:48,319 stars are units and they're randomly distributed in their fluctuation and 153 00:12:48,319 --> 00:12:52,982 fluctuations of infinite gravitational forces are also infinite. 154 00:12:52,982 --> 00:12:57,920 So there'll be infinite tidal shears trying to tear you apart. 155 00:12:57,920 --> 00:13:02,378 Well, alright how's this resolved? One possibility is that, well, maybe 156 00:13:02,378 --> 00:13:06,771 universe is not transparent. Okay, maybe that, say that it was filled 157 00:13:06,771 --> 00:13:10,574 with interstellar dust. You will not see the light but the 158 00:13:10,574 --> 00:13:14,443 energy's still there. So still be infinitely bright, in some 159 00:13:14,443 --> 00:13:17,678 wave length. [SOUND] Another solution that was 160 00:13:17,678 --> 00:13:23,583 proposed early in the twentieth century after people realize that the universe is 161 00:13:23,583 --> 00:13:27,286 expanding because of the redshift, the decline of 162 00:13:27,286 --> 00:13:32,889 energy of protons due to the cosmological expansion, you will actually not have 163 00:13:32,889 --> 00:13:36,222 same amounts of energy from more distant stars. 164 00:13:36,222 --> 00:13:39,485 Well, that helps but it's not enough by itself. 165 00:13:39,485 --> 00:13:45,230 Another very clever explanation was that in, matter in the universe is not really 166 00:13:45,230 --> 00:13:49,770 distributed randomly, but it's clusters and in fact this is true. 167 00:13:49,770 --> 00:13:56,238 And it's clustered in such a way that its fractal dimension is less than -2, 168 00:13:56,238 --> 00:13:59,683 therefore, the integral will not leverage. 169 00:13:59,683 --> 00:14:05,899 This is a very clever solution but universe turns out not to be a fractal. 170 00:14:05,899 --> 00:14:12,230 It is kind of close but not really. All of these things will help but the 171 00:14:12,230 --> 00:14:16,936 real explanation is, the universe is finite page and finite extent. 172 00:14:16,936 --> 00:14:22,214 We can only see this many light years out of the years since the Big Bang. 173 00:14:22,214 --> 00:14:27,634 And therefore, there was a finite number of stars that can contribute to the 174 00:14:27,634 --> 00:14:32,626 brightness of the night sky. And there simply isn't enough energy that 175 00:14:32,626 --> 00:14:38,046 was ever generated through the age of the universe by stars to make the star sky 176 00:14:38,046 --> 00:14:41,821 infinitely bright. But actually the sky is bright only not 177 00:14:41,821 --> 00:14:47,220 very much and it's in microwaves and has black body temperature of 2.7 degree 178 00:14:47,220 --> 00:14:51,996 Kelvin and that's certainly not bright enough to cause any damage. 179 00:14:51,996 --> 00:14:57,256 So, it turns out that actually all star light, ever made by all stars through 180 00:14:57,256 --> 00:15:03,001 history of universe so far is just a few percent of the energy density of a cosmic 181 00:15:03,001 --> 00:15:07,500 microwave background which has a very subtle effect in of itself. 182 00:15:10,860 --> 00:15:15,991 Alright. So, this is it for now and the next time, we'll start talking about 183 00:15:15,991 --> 00:15:20,360 history of cosmology, from the earlier days to the present time.