1 00:00:00,837 --> 00:00:05,333 In the last video we learned that 380,000 years after the Big 2 00:00:05,333 --> 00:00:09,600 Bang which is still roughly 13.7 billion years ago 3 00:00:09,600 --> 00:00:17,000 every atom in space that was roughly 3000 kelvin temperature 4 00:00:17,000 --> 00:00:22,933 was emitting this electromagnetic radiation 5 00:00:22,933 --> 00:00:27,400 there are points in space or there are points in the universe 6 00:00:27,400 --> 00:00:29,867 that that radiation is only just now reaching us, it 7 00:00:29,867 --> 00:00:33,733 has been traveling for 13.7 billion years, so when 8 00:00:33,733 --> 00:00:35,800 we look at radiation that has been traveling for 9 00:00:35,800 --> 00:00:40,800 that long we can look at any direction and we will see this uniform radiation 10 00:00:40,800 --> 00:00:44,234 that radiation has been red-shifted into the 11 00:00:44,234 --> 00:00:46,556 microwave range from the higher frequencies that 12 00:00:46,556 --> 00:00:48,600 it was actually emitted at 13 00:00:48,600 --> 00:00:51,267 Now a question that might pop in your brain is 14 00:00:51,267 --> 00:00:53,400 what happens if we wait a billion years ? 15 00:00:53,400 --> 00:00:55,867 because if we wait a billion years, if we have a 16 00:00:55,867 --> 00:00:59,533 billion 380.000 years after the beginning of the 17 00:00:59,533 --> 00:01:02,933 universe, this stuff won't just be atoms anymore 18 00:01:02,933 --> 00:01:06,333 it will have started to condense to actual stars 19 00:01:06,333 --> 00:01:12,467 the universe at this point, at every point in space 20 00:01:12,467 --> 00:01:15,952 will no longer be this uniform, it will actually start 21 00:01:15,952 --> 00:01:22,133 having condensation into stars, so if we move 22 00:01:22,133 --> 00:01:24,867 forward a little bit the universe will expand 23 00:01:24,867 --> 00:01:26,400 maybe I will just draw a half of it since it's 24 00:01:26,400 --> 00:01:28,267 expanding, it has obviously expanded much more 25 00:01:28,267 --> 00:01:30,467 but now all of a sudden we actually have stars 26 00:01:30,467 --> 00:01:33,467 these are no longer just uniformed atoms spread 27 00:01:33,467 --> 00:01:35,806 through the universe, we actually have condensation 28 00:01:35,806 --> 00:01:39,892 into stars and so if you look at what is being emitted 29 00:01:39,892 --> 00:01:42,533 from the points of space from which we're only 30 00:01:42,533 --> 00:01:45,000 now getting this cosmic background radiation 31 00:01:45,431 --> 00:01:48,815 if we wait a billion years, the light that we see 32 00:01:48,815 --> 00:01:52,467 from those points in space will not look like this 33 00:01:52,467 --> 00:01:55,133 uniform radiation, it will start to look a little 34 00:01:55,133 --> 00:01:58,200 bit more like the more mature parts of the universe 35 00:01:58,200 --> 00:02:01,000 we'll essentially be looking at the universe a billion 36 00:02:01,000 --> 00:02:04,823 years after the Big Bang when stars are formed, 37 00:02:04,829 --> 00:02:07,033 other structures are formed, so the question is in 38 00:02:07,048 --> 00:02:09,962 a billion years will this cosmic microwave background 39 00:02:09,962 --> 00:02:13,143 radiation disappear ? I'm using billion just because that's 40 00:02:13,143 --> 00:02:15,733 you know, just a arbitrarily used number 41 00:02:15,733 --> 00:02:19,800 but will it eventually disappear ? and the answer to that is 42 00:02:19,800 --> 00:02:25,067 yes and no, so to think about it, it is true that this 43 00:02:25,067 --> 00:02:28,533 point in space will mature, it will mature in a billion 44 00:02:28,533 --> 00:02:34,400 years, it will no longer be this uniform haze of hot 45 00:02:34,400 --> 00:02:36,671 hydrogen atoms but what we have to think about is 46 00:02:36,764 --> 00:02:43,333 there are further points in the universe at that same time 47 00:02:43,333 --> 00:02:46,836 there are further points that were also emitting this 48 00:02:46,836 --> 00:02:52,600 radiation and the original photons from those original 49 00:02:52,600 --> 00:02:56,067 points still haven't gotten to us, so from those further 50 00:02:56,067 --> 00:02:58,538 outpoints, right now the observable universe is 51 00:02:58,538 --> 00:03:02,333 we can only see electromagnetic radiation that's been 52 00:03:02,333 --> 00:03:05,267 travelling for 13.7 billion years and in another billion 53 00:03:05,267 --> 00:03:07,600 years the universe will be a billion years older and then 54 00:03:07,600 --> 00:03:10,067 there will be radiation that has been traveling for 14.7 55 00:03:10,067 --> 00:03:13,800 billion years and so we will start to observe that 56 00:03:13,800 --> 00:03:16,133 and we'll start to observe that radiation from the 57 00:03:16,133 --> 00:03:18,800 same time period of the universe, it will just be from 58 00:03:18,800 --> 00:03:24,667 further out. Now what I want to make clear is since those 59 00:03:24,667 --> 00:03:26,667 points were even further out, where that radiation 60 00:03:26,667 --> 00:03:28,800 was emitted, the stuff that we will see in a billion years 61 00:03:28,800 --> 00:03:31,000 it will be even more red-shifted. 62 00:03:36,846 --> 00:03:40,868 So at that point the cosmic backgroung radiation we see 63 00:03:40,868 --> 00:03:44,862 will have longer wavelengths than the radio spectrum 64 00:03:44,862 --> 00:03:47,800 it will be redder and I should say redder because we're 65 00:03:47,800 --> 00:03:58,933 already more red than the microwave radiation of course 66 00:03:58,933 --> 00:04:01,533 that's a funny thing because microwave radiation is already 67 00:04:01,533 --> 00:04:04,600 more red than actual visible red light, it has a longer 68 00:04:04,600 --> 00:04:07,873 wavelength. Now this will keep happening 69 00:04:07,873 --> 00:04:13,467 we will keep getting radiation as we go further and further 70 00:04:13,467 --> 00:04:16,133 into the future, we will keep getting radiation from further 71 00:04:16,133 --> 00:04:20,333 outpoints in space and it will get more and more red-shifted 72 00:04:20,333 --> 00:04:23,467 the actual wavelengths of that electromagnetic light 73 00:04:23,467 --> 00:04:25,533 will be bigger and bigger and bigger 74 00:04:25,533 --> 00:04:28,600 until we really aren't able to even see it as an electomagnetic light 75 00:04:28,600 --> 00:04:30,667 because it will be red-shifted to infinity, it will have an 76 00:04:30,667 --> 00:04:35,467 infinite wavelength and to make that point clear I want to 77 00:04:35,467 --> 00:04:38,000 show you there will even be points, at some point 78 00:04:38,000 --> 00:04:40,067 there will a kind of threshold where we can't even get 79 00:04:40,067 --> 00:04:42,600 radiation from further out 80 00:04:42,600 --> 00:04:45,133 let me show you, let me draw a diagram of that 81 00:04:45,133 --> 00:04:51,000 so let's say that this is the universe 13.7 billion years ago 82 00:04:51,000 --> 00:04:53,333 right with that radiation what we now see as cosmic 83 00:04:53,333 --> 00:04:57,067 microwave background radiation right when it started to be emitted 84 00:04:57,067 --> 00:04:59,600 and let's say that this is the point in the universe 85 00:04:59,600 --> 00:05:02,867 where we are now, so this is "us" 86 00:05:02,867 --> 00:05:07,600 let's say that this is the point in the universe that where 87 00:05:07,600 --> 00:05:10,733 we now observe the background radiation 88 00:05:10,733 --> 00:05:13,333 this is one of the points, we obviously could form a circle 89 00:05:13,333 --> 00:05:16,511 around us, it could be any of these points over here 90 00:05:16,511 --> 00:05:22,533 where the photons, the electromagnetic radiaton that 91 00:05:22,533 --> 00:05:26,067 were emitted from this point 380 000 years after the 92 00:05:26,067 --> 00:05:29,267 beginning of the universe is only just now reaching us 93 00:05:29,267 --> 00:05:32,733 so this is the point in the universe from which we're 94 00:05:32,733 --> 00:05:35,226 observing the cosmic background radiation 95 00:05:35,226 --> 00:05:37,933 and let me be very clear, that point in the universe 96 00:05:37,933 --> 00:05:41,733 has now matured into things that look, into stars 97 00:05:41,733 --> 00:05:44,933 and galaxies and planets and if they were to look at 98 00:05:44,933 --> 00:05:47,467 our point in space, they are also going to see cosmic 99 00:05:47,467 --> 00:05:49,467 background radiation from us, it's not like this is 100 00:05:49,467 --> 00:05:51,600 some type of permanently old place, it's just that the 101 00:05:51,600 --> 00:05:55,242 light we're getting from them right now is old light 102 00:05:55,242 --> 00:05:58,800 light that they at that point in space emitted way before 103 00:05:58,800 --> 00:06:01,400 it was able to mature into actual structures 104 00:06:01,400 --> 00:06:07,133 so this is the point in space from which we're 105 00:06:07,133 --> 00:06:14,067 receiving cosmic background radiation right now. 106 00:06:14,067 --> 00:06:16,558 Now let's take another point in space that's, 107 00:06:16,558 --> 00:06:19,267 whatever this distance is, now it is estimated to be 108 00:06:19,267 --> 00:06:24,801 around 46 billion light years, at that time when things 109 00:06:24,801 --> 00:06:27,800 were just beginning to be emitted, this was only about 110 00:06:27,800 --> 00:06:33,733 36 million light years. This is a very rough estimate I shouldn't 111 00:06:33,733 --> 00:06:36,400 even write it down because that's really based on how 112 00:06:36,400 --> 00:06:39,200 fast we assume the universe is expanding and all that type of thing 113 00:06:39,200 --> 00:06:42,667 but is was just a lot smaller than 46 billion light years 114 00:06:42,667 --> 00:06:44,956 now let's go that same distance again from this point 115 00:06:44,956 --> 00:06:48,300 space, so let's see let me make it clear this is 116 00:06:48,300 --> 00:06:55,800 380,000 years ago. Now let's fast farword, sorry not 117 00:06:55,800 --> 00:07:02,000 380,000 ago 380,000 years after the Big Bang 118 00:07:02,000 --> 00:07:07,333 which is approximately 13.7 billion years ago. 119 00:07:07,333 --> 00:07:11,311 So that's then, now let's look at now, I will just draw it a 120 00:07:11,311 --> 00:07:13,867 little it bigger, it's actually going to be much much 121 00:07:13,867 --> 00:07:18,787 bigger now, if we do it a little bit bigger so when I 122 00:07:18,787 --> 00:07:21,733 draw it like this, this is where we are now this point 123 00:07:21,733 --> 00:07:25,933 in space from which we are only now receiving that 124 00:07:25,933 --> 00:07:29,376 cosmic background radiation is over here and then this 125 00:07:29,376 --> 00:07:32,933 other point in space is going to be over here, and we 126 00:07:32,933 --> 00:07:35,715 saw in the video on the actual size of the observable 127 00:07:35,715 --> 00:07:38,600 universe, not just what it appears to be based on how 128 00:07:38,600 --> 00:07:41,133 long the light has been traveling, this is now on the 129 00:07:41,133 --> 00:07:44,887 order of 46 or 47 billion light years 130 00:07:47,595 --> 00:07:49,800 so this distance is also going to be 131 00:07:49,800 --> 00:07:55,333 46 billion light years. 132 00:07:55,333 --> 00:08:00,667 Now every point in space back then was emitting this 133 00:08:00,667 --> 00:08:03,267 radiation, we have this uniform radiation, it was 134 00:08:03,267 --> 00:08:06,333 just hydrogen atoms everywhere, these hot hydrogen atoms. 135 00:08:06,333 --> 00:08:08,933 Now we're only, maybe I should just do it in the colour 136 00:08:08,933 --> 00:08:12,867 of the radiation, I'm just showing you it's coming this guy. 137 00:08:12,867 --> 00:08:15,933 Well only now, 13.7 billion years in the future 138 00:08:15,933 --> 00:08:20,133 we're receiving photons from this guy, only now we're 139 00:08:20,133 --> 00:08:23,000 receiving it and frankly this green guy only now is 140 00:08:23,000 --> 00:08:26,133 going to be receiving photons. When he looks at the 141 00:08:26,133 --> 00:08:28,267 points in space when the things that he thinks are 142 00:08:28,267 --> 00:08:30,932 the points in space out there, he will see that uniform 143 00:08:30,932 --> 00:08:37,133 radiation and likewise this guy over here will only now 144 00:08:37,133 --> 00:08:39,400 be receiving photons from the point in space from 145 00:08:39,400 --> 00:08:43,467 where we are now, he will see the universe where we 146 00:08:43,467 --> 00:08:46,600 are now as how it was 380,000 years after the Big Bang 147 00:08:46,600 --> 00:08:49,183 and same thing from that point in space, the photons 148 00:08:49,183 --> 00:08:52,200 will only just now reach. Now let's think about it 149 00:08:52,200 --> 00:09:01,800 it took this guy's photons 13.7 billion years 150 00:09:01,800 --> 00:09:06,200 to reach this point over here which is now 46 billion 151 00:09:06,200 --> 00:09:10,000 light years away from us and the universe continues 152 00:09:10,000 --> 00:09:13,533 to expand, depending on the, if the universe expands 153 00:09:13,533 --> 00:09:17,133 fast enough there is no way that that photon that got 154 00:09:17,133 --> 00:09:19,800 to this guy will eventually get to this, the universe 155 00:09:19,800 --> 00:09:22,333 is expanding faster that the light can never even 156 00:09:22,333 --> 00:09:26,600 catch up to us and this light will never ever get to us 157 00:09:26,600 --> 00:09:29,133 and so there is some threshold, some distance from 158 00:09:29,133 --> 00:09:32,400 which will never get light at during this time period 159 00:09:32,400 --> 00:09:35,000 or actually from which we will never ever get any 160 00:09:35,000 --> 00:09:37,733 electromagnetic radiation, so the simple answer is 161 00:09:37,733 --> 00:09:43,467 the cosmic background radiation from this point 162 00:09:43,467 --> 00:09:46,667 yes it will start to mature, it won't be as uniform 163 00:09:46,667 --> 00:09:49,867 if we go fast forward 400 million years or billion years 164 00:09:49,867 --> 00:09:52,504 but we will get uniform radiation from further out 165 00:09:52,504 --> 00:09:54,667 but it will be even more red-shifted and the further 166 00:09:54,667 --> 00:09:56,933 forward we will get into the future the background 167 00:09:56,933 --> 00:09:59,467 radiation we get will be from further and further out 168 00:09:59,467 --> 00:10:01,908 and it will be more and more and more red-shifted 169 00:10:01,908 --> 00:10:04,800 until some point where it's going to be so red-shifted 170 00:10:04,800 --> 00:10:07,733 we won't even observe it as electromagnetic radiation 171 00:10:07,733 --> 00:10:10,067 and there is some threshold where we won't even 172 00:10:10,067 --> 00:10:11,915 observe anything any more because beyond that 173 00:10:11,915 --> 99:59:59,999 the light wasn't able to actually get to us.