1 00:00:00,012 --> 00:00:05,710 So far, we have seen how do we measure the spacial scale of the universe, which 2 00:00:05,710 --> 00:00:11,358 is simply one over a Hubble constant. What about its temporal scale? What's the 3 00:00:11,358 --> 00:00:17,031 age of the universe? It turns out that we actually can't measure the age of the 4 00:00:17,031 --> 00:00:21,484 universe directly. However, what we can do is measure lower 5 00:00:21,484 --> 00:00:25,023 limits to the ages of all these things we can find. 6 00:00:25,023 --> 00:00:30,425 And, then presumably the age of the universe is just a little more than that. 7 00:00:30,425 --> 00:00:34,392 Several different possibilities exist. The first one. 8 00:00:34,392 --> 00:00:38,927 Is the globular star clusters, These are believed to be formed over very short 9 00:00:38,927 --> 00:00:43,437 time interval in the early universe and all stars in them have essentially the 10 00:00:43,437 --> 00:00:46,350 same age. Understanding the ages of star clusters 11 00:00:46,350 --> 00:00:50,721 or stellar populations is the basic fundamental thing in stellar evolution 12 00:00:50,721 --> 00:00:53,378 theory. So in principle we understand how the 13 00:00:53,378 --> 00:00:56,202 changes, by looking at their color magnitude. 14 00:00:56,202 --> 00:01:00,552 The second kind of thing that we can look at are white dwarfs. 15 00:01:00,552 --> 00:01:05,862 Again, these are inert, incandescent cores of low-mass stars that are now 16 00:01:05,862 --> 00:01:09,792 slowly cooling down. The cooling theory is fairly well 17 00:01:09,792 --> 00:01:15,637 understood, and by observing apparent luminosity function of white dwarf stars, 18 00:01:15,637 --> 00:01:19,842 you can estimate how old the, how old are the oldest ones. 19 00:01:19,842 --> 00:01:25,067 Another thing we can do, is estimate the age of the heavy elements, created in 20 00:01:25,067 --> 00:01:28,367 stars. Very high mass, elements, will tend to be 21 00:01:28,367 --> 00:01:32,817 radioactively unstable. And, if we can find long lived isotopes, 22 00:01:32,817 --> 00:01:37,817 of some of those elements, and measure their alter abundances, we can estimate 23 00:01:37,817 --> 00:01:42,142 the age of those elements, which presumably came from supernova 24 00:01:42,142 --> 00:01:45,069 explosions. Some of the very first stars. 25 00:01:45,069 --> 00:01:49,728 This is very similar to the carbon dating that's often used on planet Earth. 26 00:01:49,728 --> 00:01:54,745 That is somewhat model-dependent, because it depends on when the Supernova were 27 00:01:54,745 --> 00:01:59,615 exploding, but the upper limit of that would, again give the lower limit, the 28 00:01:59,615 --> 00:02:03,323 age of the universe. And finally, we could model Stellar 29 00:02:03,323 --> 00:02:07,048 populations, in a similar way that we model, star clusters. 30 00:02:07,048 --> 00:02:11,392 But that turns out to be much complicated, and, it's really not used to 31 00:02:11,392 --> 00:02:15,704 constrain the age the universe. The fundamental measurement here, is 32 00:02:15,704 --> 00:02:20,660 estimating the ages of globular clusters, which are almost as old, as our galaxy 33 00:02:20,660 --> 00:02:24,032 itself. This is based on a well established and 34 00:02:24,032 --> 00:02:29,207 well tested theory of stellar evolution, which predicts so called Isograms which 35 00:02:29,207 --> 00:02:34,532 is color magnitude diagrams and the paths that, main sequence and the giant branch 36 00:02:34,532 --> 00:02:39,132 will have, at any given time. They look like this, the main sequence 37 00:02:39,132 --> 00:02:44,279 turn-off, moves to ever lower masses. Lower luminosity as time goes on, and 38 00:02:44,279 --> 00:02:48,686 then stars ascend the giant branch. So, by measuring where the main sequence 39 00:02:48,686 --> 00:02:53,042 turnoff is, and where the giant branch is, we can fit the models and find out 40 00:02:53,042 --> 00:02:56,555 how old the cluster is. Likewise, the difference between the 41 00:02:56,555 --> 00:03:00,389 turnoff, and the position of the horizontal branch is another age 42 00:03:00,389 --> 00:03:03,741 indicator. The isograms will depend slightly, on the 43 00:03:03,741 --> 00:03:06,515 chemical composition of stars in the cluster. 44 00:03:06,515 --> 00:03:11,337 So there are many uncertainties, in those estimates but the biggest one of them 45 00:03:11,337 --> 00:03:15,292 all, is the uncertainty in the distance to the globular clusters. 46 00:03:15,292 --> 00:03:20,282 Stellar evolution models are pretty good but in detail, there's still some minor 47 00:03:20,282 --> 00:03:24,639 uncertainties that need to be resolved. The exact effects of the different 48 00:03:24,639 --> 00:03:29,099 abundances of chemical elements and diffusion from stellar course in which 49 00:03:29,099 --> 00:03:31,742 they are made towards the surface and so on. 50 00:03:31,742 --> 00:03:36,227 Those also contribute to the uncertainty and the estimate of ages of globular 51 00:03:36,227 --> 00:03:37,962 clusters. Nevertheless, 52 00:03:37,962 --> 00:03:42,974 measuring ages of globular clusters was probably one of the more reliable 53 00:03:42,974 --> 00:03:48,138 cosmological measurements for many years, certainly more so than any other 54 00:03:48,138 --> 00:03:53,403 cosmological parameters until the 1990s. The key point here was to calibrate 55 00:03:53,403 --> 00:03:59,178 exactly where the main sequences are, and this became possible only after Hipparcos 56 00:03:59,178 --> 00:04:04,195 satellite measured the actual paraloxes and distances to some metal core stars 57 00:04:04,195 --> 00:04:08,522 which defined the metal core main sequence that can be then applied to 58 00:04:08,522 --> 00:04:12,291 globular star clusters. Different groups have done that and 59 00:04:12,291 --> 00:04:17,245 typically the results they get, are the ages of globular clusters, on the order 60 00:04:17,245 --> 00:04:21,233 of 12 to 13 billion years. Which is very close to what we now know 61 00:04:21,233 --> 00:04:25,292 is the actual age of the universe of about 13.7 billion years. 62 00:04:25,292 --> 00:04:29,876 Here is a probability distribution of globular cluster ages that take into 63 00:04:29,876 --> 00:04:33,075 account all of the sources of uncertainty and so on. 64 00:04:33,075 --> 00:04:38,167 Clusters could have different ages and probably do so that will contribute to 65 00:04:38,167 --> 00:04:41,527 the spread as well. So the peak of this distribution is 66 00:04:41,527 --> 00:04:45,345 indeed where it should be. A little more than 13 giga years ago 67 00:04:45,345 --> 00:04:50,462 which allowed for a few hundred million years for galaxy and clusters to form. 68 00:04:50,462 --> 00:04:54,313 Of course there are no clusters older than the universe itself. 69 00:04:54,313 --> 00:04:58,976 The fact that the distribution has a high end tail is simply indicative of the 70 00:04:58,976 --> 00:05:02,986 errors in age estimates. The next method is estimating the ages of 71 00:05:02,986 --> 00:05:06,397 white dwarfs. They are fairly well understood, and they 72 00:05:06,397 --> 00:05:10,867 are cooling a very orderly fashion. So the faintest white dwarfs that we can 73 00:05:10,867 --> 00:05:15,402 find are probably the coolest, the ones that had been cooling for the longest 74 00:05:15,402 --> 00:05:20,142 period of time, and they can constrain the age of the cluster, or galactic disk, 75 00:05:20,142 --> 00:05:24,092 in which they are found. Again, theories fairly well understood. 76 00:05:24,092 --> 00:05:28,247 In over-all sense, but there's still details that need to be ironed out. 77 00:05:28,247 --> 00:05:32,666 Probably the best way to do this is in star clusters, where again, we know the 78 00:05:32,666 --> 00:05:35,499 population of white dwarfs all have the same age. 79 00:05:35,499 --> 00:05:39,818 And because they're really faint, and clusters are crowded, we need Hubble 80 00:05:39,818 --> 00:05:43,167 Space Telescope to do this. Here is an example of the first 81 00:05:43,167 --> 00:05:48,303 measurement of this kind in the upper right you see a little segment called the 82 00:05:48,303 --> 00:05:51,706 main sequence. In the center is the white dwarf cooling 83 00:05:51,706 --> 00:05:54,726 sequence, and you can see it stops at some point. 84 00:05:54,726 --> 00:05:59,224 Those are the oldest and the coolest white dwarfs in the cluster and their 85 00:05:59,224 --> 00:06:03,672 position determines the edge. So this method produced results which are 86 00:06:03,672 --> 00:06:08,380 perfectly consistent with those from. Main sequence, isochrome feeding, and 87 00:06:08,380 --> 00:06:11,644 that's, very different physics, so it's very encouraged. 88 00:06:11,644 --> 00:06:16,314 Here is the observed luminosity function, of y dwarfs, in this particular cluster. 89 00:06:16,314 --> 00:06:19,137 Meaning, their distribution of the luminosity's. 90 00:06:19,137 --> 00:06:23,419 You can see that there is a, fairly sharp cutoff, at the faint end, which is, 91 00:06:23,419 --> 00:06:27,698 indicative, what the h really is. An entirely different approach, uses ages 92 00:06:27,698 --> 00:06:31,641 of chemical elements. Heavy chemical elements that must have 93 00:06:31,641 --> 00:06:36,613 been produced in super nova explosions of say per supernovae often have unstable 94 00:06:36,613 --> 00:06:39,904 isotopes. And their radioactive decay can be used 95 00:06:39,904 --> 00:06:44,951 to age data the elements themselves. Because the age of the universe is about 96 00:06:44,951 --> 00:06:48,559 13 billion years. You need radioactive decays that are 97 00:06:48,559 --> 00:06:53,026 commensurate with it. Again, measured in billion, billions of 98 00:06:53,026 --> 00:06:56,291 years. There are such isotopes like thorium and 99 00:06:56,291 --> 00:07:00,365 uranium, there is also ranium and osmium, and a few others. 100 00:07:00,365 --> 00:07:05,781 And so you need, something that has a radioactive half-life, decay time of life 101 00:07:05,781 --> 00:07:08,797 time. But that, unfortunately, also means that 102 00:07:08,797 --> 00:07:13,707 it will be really hard to measure, in the laboratory, what the half-life decay time 103 00:07:13,707 --> 00:07:16,207 is. So doing this for variety of different 104 00:07:16,207 --> 00:07:20,432 isotopes then provides the estimate of the age of the oldest, supernova 105 00:07:20,432 --> 00:07:24,252 explosions, all this chemical elements in our galaxy. 106 00:07:24,252 --> 00:07:29,523 The abundances of these elements are done by very high resolution, high signal to 107 00:07:29,523 --> 00:07:32,653 noise spectroscopy of all the stars we can find. 108 00:07:32,653 --> 00:07:37,129 The [INAUDIBLE] are very subtle and that measurements are very hard. 109 00:07:37,129 --> 00:07:41,702 Nevertheless, they've been done over the years and results are shown. 110 00:07:41,702 --> 00:07:47,122 So for one particular star shown here. There are ratios of several isotopes that 111 00:07:47,122 --> 00:07:51,402 are being used to estimate the age of the elements that made a star. 112 00:07:51,402 --> 00:07:56,467 And the average of it is remarkably close to what we now know is the actual age of 113 00:07:56,467 --> 00:08:00,008 the universe. And it's also essentially the same as the 114 00:08:00,008 --> 00:08:03,833 age as measured for globular clusters and for white dwarfs. 115 00:08:03,833 --> 00:08:08,162 This is just 1 star, doing it for many stars improves the result. 116 00:08:08,162 --> 00:08:11,945 And so, to recap. We have, now, a fairly good idea of what 117 00:08:11,945 --> 00:08:15,922 the age of the universe it. Or at least the lower limit to it. 118 00:08:15,922 --> 00:08:18,919 Using several completely different methods. 119 00:08:18,919 --> 00:08:23,574 Which rely on different physics, different assumption, and different 120 00:08:23,574 --> 00:08:25,902 measurements. And they all agree. 121 00:08:25,902 --> 00:08:29,470 It's because of that we think we got it right. 122 00:08:29,470 --> 00:08:34,754 Moreover this is also in perfect agreement with age of the universe 123 00:08:34,754 --> 00:08:40,611 deduced from measurement of other cosmological parameters to which we'll 124 00:08:40,611 --> 00:08:44,599 come later. Next time we will address the question 125 00:08:44,599 --> 00:08:50,361 of, is the universe actually expanding? You think we would have probably figured 126 00:08:50,361 --> 00:08:52,959 this one out by now, but it's good to be sure.