1 00:00:00,012 --> 00:00:06,382 So far we have discussed large scale structure in a sense of where the galaxies 2 00:00:06,382 --> 00:00:09,044 are. But where are they moving? 3 00:00:09,044 --> 00:00:15,485 So let's find out what we mean by the term of peculiar velocity for galaxies is a 4 00:00:15,485 --> 00:00:22,118 velocity component in addition to their Hubble flow, due to the expansion of the 5 00:00:22,118 --> 00:00:25,441 universe. So in other words If the galaxy was at 6 00:00:25,441 --> 00:00:30,215 rest compared to, say, microwave background in comoving coordinates, it 7 00:00:30,215 --> 00:00:34,999 will have peculiar velocity of 0. The only component we could observe for it 8 00:00:34,999 --> 00:00:38,241 would be due to the radial expansion of the universe. 9 00:00:38,241 --> 00:00:42,267 Now even though velocities are three-dimensional vectors, we can only 10 00:00:42,267 --> 00:00:47,052 observe the radial component. Due to the velocity because galaxies move 11 00:00:47,052 --> 00:00:51,457 way too slow to, on the sky to actually be able to measure them. 12 00:00:51,457 --> 00:00:57,079 So if you're interested in some aspect of cosmological expansion then these peculiar 13 00:00:57,079 --> 00:01:02,054 velocities are like velocity noise atop of the actual Hubble hole signal. 14 00:01:02,055 --> 00:01:07,258 And so for example, if you're plotting a Hubble diagram which is some form of 15 00:01:07,258 --> 00:01:13,080 distance versus [unknown] you can have errors in distances but you can also have, 16 00:01:13,080 --> 00:01:16,951 well it's not really an error in velocity, but a shift. 17 00:01:16,951 --> 00:01:22,077 In other words it's error in a sense if you assume that velocity is All made due 18 00:01:22,077 --> 00:01:26,574 to the expansion of the universe. And the origin of this is obvious. 19 00:01:26,574 --> 00:01:31,728 If there is a large-scale density field, there has to be a large-scale peculiar 20 00:01:31,728 --> 00:01:35,366 velocity field. A reason for this is that during the age 21 00:01:35,366 --> 00:01:40,514 of the universe, galaxies will fall towards the nearest massive structures to 22 00:01:40,514 --> 00:01:43,607 them. Acquire velocity gradually from zero to 23 00:01:43,607 --> 00:01:48,410 whatever they have today. So in some sense, the pattern of peculiar 24 00:01:48,410 --> 00:01:52,969 velocities has to reflect a pattern of the density distribution. 25 00:01:52,969 --> 00:01:58,536 Now there is one peculiar velocity, some people say only one, which we know with a 26 00:01:58,536 --> 00:02:02,950 great deal of precision. And this is our motion, relative to the 27 00:02:02,950 --> 00:02:07,357 cosmic microwave background. This causes a dipole cosmic micro 28 00:02:07,357 --> 00:02:12,397 background is a little hotter in the direction that we're moving to, and the 29 00:02:12,397 --> 00:02:17,517 velocity is about 620 kilometers per second in a particular direction in the 30 00:02:17,517 --> 00:02:20,675 sky. It turns out this is fairly [unknown] with 31 00:02:20,675 --> 00:02:25,979 typical peculiar velocities in the large scale structure around us today, or for 32 00:02:25,979 --> 00:02:30,542 galaxies maybe a little less - couple hundred kilometers per second. 33 00:02:30,542 --> 00:02:35,550 But Virgo cluster has Hubble Recession velocity of only like 1000 or 1200 34 00:02:35,550 --> 00:02:40,233 kilometres per second. So a peculiar velocity component like this 35 00:02:40,233 --> 00:02:45,403 could really make a difference. So we have a simple equation that relates 36 00:02:45,403 --> 00:02:49,866 total observed velocity. The distance to the galaxy, Hubble 37 00:02:49,866 --> 00:02:55,562 constant and it's peculiar velocity. So we can easily measure a total velocity 38 00:02:55,562 --> 00:03:00,316 that's the redshift. We can ostensibly measure Hubble constant. 39 00:03:00,316 --> 00:03:06,357 But then the tricky part is the distances. So the errors in stimation of distances to 40 00:03:06,357 --> 00:03:12,795 galaxies will map directly into the errors in estimates of their peculiar velocities. 41 00:03:12,795 --> 00:03:19,246 You may recall that there is a number of distance indicators for galaxies which we 42 00:03:19,246 --> 00:03:24,533 can use to compute their distances. They're not given by the redshift. 43 00:03:24,533 --> 00:03:27,825 But each one of them has its intrinsic scatter. 44 00:03:27,825 --> 00:03:32,977 And each one of them will then yield errors in the estimates of the peculiar 45 00:03:32,977 --> 00:03:37,417 velocity. For most part peculiar velocities are 46 00:03:37,417 --> 00:03:42,955 comensurate with Hubble velocities, only in the nearest universe. 47 00:03:42,955 --> 00:03:49,527 And, if we go further say, beyond local supercluster, most of the velocities due 48 00:03:49,527 --> 00:03:54,972 to Hubble expansion. But the fractional error of the Hubble 49 00:03:54,972 --> 00:04:01,972 expansion times the velocity gives you the absolute error of the peculiar velocity 50 00:04:01,972 --> 00:04:07,584 and so the sensitivity is much higher. Sometimes we measure distances not to 51 00:04:07,584 --> 00:04:12,412 individual galaxies but clusters to which they belong and that's were we can hour 52 00:04:12,412 --> 00:04:17,240 its measurement over a large number of galaxies have improved the corresponding 53 00:04:17,240 --> 00:04:21,926 measurement for the entire cluster, so measuring distance to galaxies is very 54 00:04:21,926 --> 00:04:26,612 tricky as we probably recall from the various chapter, but there is another way 55 00:04:26,612 --> 00:04:30,730 in which we can measure peculiar velocities and that is from red shift 56 00:04:30,730 --> 00:04:35,397 surveys themselves. Now, first remember about the redshift 57 00:04:35,397 --> 00:04:39,067 space. We observe some structure of galaxies an 58 00:04:39,067 --> 00:04:45,073 individual galaxy and if it is a very massive structure it's realized like core 59 00:04:45,073 --> 00:04:49,063 of rich a cluster. Galaxies will have their own Thermo 60 00:04:49,063 --> 00:04:53,206 velocity due to the potential well of the cluster itself. 61 00:04:53,206 --> 00:04:58,516 Now we do not measure the component that conventional in the sky, but the radio 62 00:04:58,516 --> 00:05:03,117 component will be there. And so, the higher the velocity dispersion 63 00:05:03,117 --> 00:05:08,237 in the cluster, the more will galaxies scatter in velocity around their true 64 00:05:08,237 --> 00:05:13,102 Hubble expansion value. And if you assume, naively, that measured 65 00:05:13,102 --> 00:05:18,822 velocity is only Hubble velocity in converting to distance, then a spherical 66 00:05:18,822 --> 00:05:24,454 cluster will become allocated along the line of sight, due to these peculiar 67 00:05:24,454 --> 00:05:28,563 components. That is known as the finger of God effect. 68 00:05:28,563 --> 00:05:32,736 On the other hand. If we are still in a linear regime, that 69 00:05:32,736 --> 00:05:38,565 is, low density, contrast, galaxies are slowly falling toward some filament or 70 00:05:38,565 --> 00:05:44,394 sheet, then the opposite will happen, there will be some intrinsic width of that 71 00:05:44,394 --> 00:05:49,382 filament on the skies, it's perpendicular through our line of site. 72 00:05:49,383 --> 00:05:54,428 The galaxies falling towards it, on the side, on our side, will acquire a little 73 00:05:54,428 --> 00:05:59,444 external velocity, will look like, look like they are little further away than 74 00:05:59,444 --> 00:06:02,512 they are. Galaxies on the other side are falling 75 00:06:02,512 --> 00:06:04,706 back. This is what they look like. 76 00:06:04,706 --> 00:06:09,204 They look closer than they really are. The net effect of that is that the 77 00:06:09,204 --> 00:06:13,341 filament looks thinner in the radio direction than it really is. 78 00:06:13,341 --> 00:06:18,252 So statistically, you can do this for whole ensemble galaxies including 79 00:06:18,252 --> 00:06:22,825 [inaudible] survey. And what's shown here is a density plot 80 00:06:22,825 --> 00:06:29,737 that, that corresponds to radial component of the distance, as well as the orthogonal 81 00:06:29,737 --> 00:06:32,997 component. Which you can measure from angular 82 00:06:32,997 --> 00:06:37,292 suppression of the sky. So note that there's two features to this. 83 00:06:37,292 --> 00:06:42,466 A very small angular separations, which will largely be for galaxies that really 84 00:06:42,466 --> 00:06:46,460 are physically close together like in clusters or galaxies. 85 00:06:46,460 --> 00:06:50,175 You see a vertical elongation, the finger of God effect. 86 00:06:50,175 --> 00:06:54,852 But for most of the rest, you see flattening of the distribution which 87 00:06:54,852 --> 00:06:59,387 normally should be spherical. The flattening is due to that linear 88 00:06:59,387 --> 00:07:02,881 infall into the structures along the line of sight. 89 00:07:02,881 --> 00:07:07,687 So, how can we measure peculiar velocities, when all we are measuring is 90 00:07:07,687 --> 00:07:11,806 total velocities in our Redshift survey. It goes like this. 91 00:07:11,806 --> 00:07:17,413 You assume first that galaxies are where they're observed velocities imply. 92 00:07:17,413 --> 00:07:22,425 That gives you first approximation to the density distribution in space. 93 00:07:22,425 --> 00:07:28,094 Then, making some assumptions about mass to light ratios and dark matter and so on. 94 00:07:28,094 --> 00:07:33,518 You figure out how much would galaxies there would have acquired in terms of 95 00:07:33,518 --> 00:07:39,452 peculiar velocities through the history of the universe and then you subtract the 96 00:07:39,452 --> 00:07:43,132 component from the original measured velocity. 97 00:07:43,132 --> 00:07:48,552 You reevaluate the density field at that point and repeat this several times. 98 00:07:48,552 --> 00:07:53,499 At that point you should have, statistically at least, a good separation 99 00:07:53,499 --> 00:07:58,777 between true distribution in space and the peculiar velocity distribution. 100 00:07:58,777 --> 00:08:03,035 So you can do this without measurement or distances altogether. 101 00:08:03,035 --> 00:08:07,274 However, the prob, the problem is that there is a model dependence. 102 00:08:07,274 --> 00:08:12,747 So you have to assume something about, say amounts of dark matter in these structures 103 00:08:12,747 --> 00:08:15,713 and so on. So here is the local kinematics of the 104 00:08:15,713 --> 00:08:20,368 large-scale structure plot. We are falling towards virgo with the 105 00:08:20,368 --> 00:08:25,943 speed of about 270 kilometres per second. The whole local supercluster is moving in 106 00:08:25,943 --> 00:08:31,221 the direction of Hydra-Centaurus supercluster at somewhat higher speed. 107 00:08:31,221 --> 00:08:34,582 And things may acutally go even beyond that. 108 00:08:34,582 --> 00:08:40,000 So here's map of galaxies in the sky. The band in the middle is the galactic 109 00:08:40,000 --> 00:08:45,655 plane, the zone of avoidance, and you can see large structure of many elements 110 00:08:45,655 --> 00:08:48,770 labeled. The hatched blue area on the left 111 00:08:48,770 --> 00:08:54,572 corresponds to replication of the masses. Or mass responsible for our peculiar 112 00:08:54,572 --> 00:08:59,372 velocity which was called the great attractor for a period of time, but I 113 00:08:59,372 --> 00:09:04,172 think now people refer to it by it's original name of the hydra centaurus 114 00:09:04,172 --> 00:09:07,711 supercluster. So I mentioned it, given a peculiar 115 00:09:07,711 --> 00:09:12,733 velocity field, you can derive the underlying density field which must be 116 00:09:12,733 --> 00:09:16,787 responsible for it. And here examples of what it might look 117 00:09:16,787 --> 00:09:20,086 like in projection to the super galactic plane. 118 00:09:20,086 --> 00:09:23,477 Of course, the real thing is in three dimensions. 119 00:09:23,477 --> 00:09:29,312 But here in this density of But here in this landscape, the height corresponds to 120 00:09:29,312 --> 00:09:34,880 the local density, and the plot on the upper right shows the vector field, the 121 00:09:34,880 --> 00:09:39,341 velocities that correspond to those density distribution. 122 00:09:39,341 --> 00:09:43,001 So we're in the middle of this map, in the valley. 123 00:09:43,001 --> 00:09:47,394 A little bump next to us is the local super cluster, and the much bigger 124 00:09:47,394 --> 00:09:50,181 mountain to the left is the Great Detractor. 125 00:09:50,181 --> 00:09:54,450 The large mountain on the other side is Perseus-Pisces Supercluster. 126 00:09:54,450 --> 00:09:58,776 And so if you'll look at the velocity field just above it, you'll see that 127 00:09:58,776 --> 00:10:02,811 there's a [unknown] flow between these two massive structures. 128 00:10:02,811 --> 00:10:07,010 You can do this with any Redshift survey, and it, it was indeed done so. 129 00:10:07,010 --> 00:10:11,504 That's good, because these are tricky measurements, and it's good to do them as 130 00:10:11,504 --> 00:10:15,785 many independent times as possible. You may remember there is, there was a 131 00:10:15,785 --> 00:10:20,477 survey based on IRAS [inaudible] galaxies which is called PSCz Survey, and this is 132 00:10:20,477 --> 00:10:25,373 what their density distributional galaxy looks like projected on the super galactic 133 00:10:25,373 --> 00:10:27,864 plane. And here is what the corresponding 134 00:10:27,864 --> 00:10:31,303 velocity field's like. There are again two huge attractors. 135 00:10:31,303 --> 00:10:35,665 There's a big one, Hydra-Centaurus, and then there is one on the other side, the 136 00:10:35,665 --> 00:10:39,082 Perseus-Pisces Supercluster. But the flow may continue. 137 00:10:39,082 --> 00:10:43,916 Deeper Redshift surveys suggest that, that in fact this whole local volume is moving 138 00:10:43,916 --> 00:10:48,880 towards an even larger mass concentration, couple hundred mega parsecs out called the 139 00:10:48,880 --> 00:10:53,966 Shapley Concentration of clusters. Named so because Harlow Shapley noticed 140 00:10:53,966 --> 00:10:58,573 that there seems to be excess number of clusters in that are of the sky. 141 00:10:58,574 --> 00:11:04,085 The picture here showing, shows where some of those clusters are in projection, and 142 00:11:04,085 --> 00:11:07,561 they're also close by and large, in three dimensions. 143 00:11:07,561 --> 00:11:12,051 And that's about as large density fluctuation that can cause peculiar 144 00:11:12,051 --> 00:11:16,490 [unknown] as you expect. Not everybody agrees with that, though. 145 00:11:16,491 --> 00:11:22,388 Sasha Kaslinksy and collaborators used distant clusters of galaxies, measuring 146 00:11:22,388 --> 00:11:27,949 their distances from x-rays on, and they concluded that the flow continues even 147 00:11:27,949 --> 00:11:30,587 beyond. They call it the Dark Flow. 148 00:11:30,587 --> 00:11:35,607 Of the face value, this seems to be in contradiction without understanding of 149 00:11:35,607 --> 00:11:40,920 structure formation because we simply not expect that there will be a substantial 150 00:11:40,920 --> 00:11:45,463 loss in build up due to the structure larger than a couple hundreds mega 151 00:11:45,463 --> 00:11:49,306 parsecs. But if this is true, then there must be an 152 00:11:49,306 --> 00:11:54,379 explanation like that. One possibility that people have discussed 153 00:11:54,379 --> 00:11:59,747 is that in fact, there is a huge mass-concentration beyond the horizon of 154 00:11:59,747 --> 00:12:05,478 our universe due to the inflation. We are close at one point but now we don't 155 00:12:05,478 --> 00:12:08,595 see it. But the reason over all gradient in 156 00:12:08,595 --> 00:12:14,491 gravitational potential in that direction and some people called it the tilted 157 00:12:14,491 --> 00:12:18,328 universe. The universe is rolling downhill towards 158 00:12:18,328 --> 00:12:23,620 this unseen hypothetical huge mass concentration, and I'll emphasize the 159 00:12:23,620 --> 00:12:27,954 word, hypothetical. So, to summarize, measuring particular 160 00:12:27,954 --> 00:12:32,734 velocities is very difficult. If you have to use distances to subtract 161 00:12:32,734 --> 00:12:38,517 them directly from observed [unknown] then errors in distances are very important. 162 00:12:38,517 --> 00:12:43,788 And systematic errors that could be present in distance indicator relations 163 00:12:43,788 --> 00:12:49,134 like Their zero point may be function of something could really play havoc with 164 00:12:49,134 --> 00:12:52,209 that. The other possibility is an internally 165 00:12:52,209 --> 00:12:57,502 self consistent solution from a redshift survey alone, but that requires you to 166 00:12:57,502 --> 00:13:01,871 make an assumption about underlying mass distribution models. 167 00:13:01,871 --> 00:13:06,838 We are falling towards Virgo cluster, two or 300 kilometers per second. 168 00:13:06,839 --> 00:13:13,094 And local superclusters falling toward the Hydra-Centaurus about 4 or 500 kilometers 169 00:13:13,094 --> 00:13:16,337 per second. And maybe the whole thing is moving 170 00:13:16,337 --> 00:13:21,466 towards an even larger concentration. But most people think that our net 171 00:13:21,466 --> 00:13:26,884 peculiar velocity, the one that we measure with respect to the cosmic micro 172 00:13:26,884 --> 00:13:32,560 background, is pretty much due to the masses within the 50 mega parsecs for mass 173 00:13:32,560 --> 00:13:36,556 or so. Just not too far from local supercluster. 174 00:13:36,556 --> 00:13:42,021 This is not entirely settled, but it, it is an emerging [unknown] view. 175 00:13:42,021 --> 00:13:48,079 Another important result from analysis like this is that it appears that on large 176 00:13:48,079 --> 00:13:54,329 scales, indeed, the mass and the light are distributed pretty much in the same way. 177 00:13:54,329 --> 00:13:59,766 Galaxies are where the dark matter is. This is not the case of small scales, like 178 00:13:59,766 --> 00:14:03,756 scales of the galaxies, where that there is real a separation. 179 00:14:03,756 --> 00:14:07,071 But scales of large scale structures, this is true. 180 00:14:07,071 --> 00:14:12,047 Because, and we know this because the pecu path there, and peculiar velocities we see 181 00:14:12,047 --> 00:14:15,251 is exactly what we'd expect from the density field. 182 00:14:15,251 --> 00:14:22,854 If the dark matter was associated with galaxies, it was in same general, we 183 00:14:22,854 --> 00:14:27,822 think. Next time we will talk about phenomenon of 184 00:14:27,822 --> 00:14:32,660 biasing and how is clustering evolving in time.