So Paul, we mentioned stars as being a source of ultraviolet photons, but I also noticed your spectrum of a quasar had a lot of ultraviolet photons too. So how about quasars being the things that reionise the universe? Yes, as we talked about in the last lesson, quasars put out pretty just amounts of our devices in parts of their accretion disks. So, shortly there, a very good contender for producing these very hard and energetic photons that could reionise the universe. This occurred to be a long time ago, and so they started trying to search to find if there are enough bright quasars in the early universe to do this. Now, normally the way you find quasars, we talked about finding by the radio emission, but most quasars don't emit radio waves. So, normally the way quasars are found is look for very bright blue objects. When we get quasars at these enormous distances, that's not going to work anymore, because if you remember, all the light that's below 10.2 electron volts is being eaten up by all these absorption lines, so that absorbs a large fraction of that light. So, in fact, it turns out when the quasars get to be under certain distance about a redshift of 304, they become very red. The flux, the radiate we pick up at optical blue wave vectors absorbed by all this gas, when there's still the flux at long wave, let's get through. So, the Sloan Digital Sky Survey was a huge survey of the Northern sky, and they mapped hundreds of billions of objects looking for things that look like stars, but had incredibly red colors. So, I guess this is their biggest or most distant object they saw, this tiny little red smudge right in the center, it doesn't look like much, but it's had a redshift of 6.4, that's a long way back. This is a real needle that haste act problem. They would look at maybe a billion stars looking for 304, things these sort of distances. It's one and a billion problem, it's actually much worse than a needle and a haste, that could result in a million problems. Right, so, alright, so they found this one at 6.4, I know there were ones they found at O'ots and Watts of quasars in the Sloan Digital Sky Survey, were there enough to cause the universe to be re-ionized? Well, they plotted the number of quasars versus time, so versus redshift, they have to correct for the fact that space is expanding, but this is after correct for that, how many quasars there are per what would be a unit volume today, it was called a co-moving volume, and you find there are very many quasars today, as we talked about earlier, there are a lot of quasars out of the matter, redshift of 2, but there really weren't very many out of redshift of 5 or 6 or so on, which is when you might expect either this realization to occur. So, you showed us your spectrum of quasars from right here, and I guess the universe is re-ionized, right, completely re-ionized then at that point. Yes, it seems to be changing in this area, but there's fewer and fewer objects to go back in time. So, there are quasars up here and all the way back down here, which are still not showing solid absorption, they have the narrow individual absorption lines, but they're not showing continuous, gun-peedersome absorption, so something must have re-ionized most of the gas of the universe before the age of the quasars. This is the age of the quasars, about 10 billion light years ago, before that. So, that means quasars can't really do this, there just weren't enough of them early enough, they came along too late. Right, and so there is an interesting surprise though that came out of these really high redshift ones, which is looking at that limonal for forest. You can see, as we look at a redshift of 5.7, 5.8, there's a bunch of spectrum here, spectra of quasars, ordered by their redshift, and you can see they're becoming more and more redshifted as you go further and further, and these are the most distant quasars known, and you can see that they're all chopped up here, but something happens. Yes, so down here, this is the limonal for emission line, so what we would expect remember if the universe was full of neutral hydrogen is no light, shortwards of this. And the ones down here, they don't have much light, but they have some. This is the limonal for us who've been talking about light gets chewed up by individual clouds, but in between the clouds, the gas has been ionized, but something strange happens by the time you get up to a little bit above redshift 6 maybe. You really do seem to be picking up nothing here, so I can be a bit more picking up over there, but really very little indeed, especially the top one over there, so it's actually looking like the prediction, it looks like up these redshift's of these 6.4 or so, there really is neutral hydrogen everywhere. So that's interesting, so what are the problems of course that's going to be, if we want to go out and find even more distant quasars, maybe there are more distant quasars, and we just simply can't see them, because this is of course where we're looking in the optical light, they're not meeting any optical light, they're only a meeting in for red light. Yeah, we'd really like to see if this trend continues, it kind of looks like we've identified when the universe became ionized up here, no flux. So that means the universe hydrogen gets neutral, down here we are seeing flux, albeit not very much, so somewhere between redshift 6 and redshift 5 or so, that seems to be when it happened, but we're right to check that by finding more distant quasars, and that's really hard. But thanks to new infrared detectors, which used to be tiny little pieces of weird types of things like... Mercury, cadmium, teleride, indium, antimonide. Yeah, all sorts of weird elements put together to make these detectors, well they've made those much bigger, and so they've been able to go out and search many thousands of square degrees in the infrared now, and they found one really distant objects, but only one. And it's this object, this little red thing there. This is not a visible light image, visible light image, it's just completely blank. What we've done here is what they've showed red here is actually infrared wavelengths, and so you're looking for something that shows up infrared, but it's absolutely gone, optical wavelengths. So we can take a spectrum of this with a very big telescope, and what do we see? Well, we see a normal quasar, and then, wam! Below that magic, 10.2 electrons, nothing. So it seems like there is a real trend here, which beyond a redshift of six or so, the universe seems to be full of neutral hydrogen, like we sort of expected it should be, and that seems to be when the era of reionization occurred. This looks like it's all done and dusted. The universe does not do very hot. At redshift about 1000, it cooled down enough to become neutral. It stayed neutral all the way down to redshift about six when something, but not quazars, put out enough ultraviolet to ionize it again. Lessons simple. So Paul, we seem to have this indication that the universe went through this change at a redshift of six, but there's a problem. And that problem comes from the observations of that cosmic microwave background, because when we look at the cosmic microwave background, which we understand exquisitely well, we realize that it doesn't quite look pristine. Instead, it's like it has a fog in front of it. And that fog is because 9% of the photons of the cosmic microwave background appear to be scattered. We're pretty sure they'd be scattering off electrons. And electrons, of course, there because the universe is reionized. So in itself, it doesn't sound like a problem. Sure, but you can calculate when this must happen. So as you know, if guests neutral the microwave background doesn't interact with it, so you need electrons to scatter photons. You've got to imagine a microwave background photon flying through. It starts with the neutral gas, so it's transparent. And then this time goes on, space is getting bigger and bigger. And then at some point, it becomes ionized, presumably by these whatever this mysterious ultraviolet sources are. But if that happens too late, density is very low, because density is dropping very fast, as time goes on. And so while there are electrons around there, there won't be enough of them in the path then a given microwave background photon to scatter it. So the second term is that we need these electrons to be produced, as I said, to be ionized pretty early, while the density is still high enough for the fraction of the microwave background photons. So if you go through and you calculate how long and how dense so you, you know, the longer you have to go, the denser. So it's really two things compounding on each other. You can figure it out. And so when you do that calculation, it turns out the universe needs to be re-ionized, or needs to be ionized or re-ionized from a ratio to zero to about 10. Now that 10 is a lot bigger than what seems to be indicated by the quasars. So if, as we thought from the quasars, everything would ionize around redshift six, that wouldn't be enough to produce this. Basically by that point, the density has got so low, it would only be scattering a smaller percent, maybe you know, three or four percent, of the microwave background photons. To get the percentage up as high as we observe, it has to have been ionized up about redshift 10. So we have a problem here. We do have yet another mystery.