Welcome back everybody. So now we're making the transition out of this era of the universe when, you know, basically it was a primordial soup. That everything up until recombination, the universe is pretty smooth. It's a, it's a fairly homogeneous gas of particles. But once recombination occurs, and the photons and the matter go, kind of go their separate ways now gravity can start beginning to become a player in the, the forces that shape the universe. In particular, before recombination, if I had any ripple, any kind of small chunk of matter that had been, you know, pulled together or that was, you know, from the quantum fluctuations that was, you know, slightly over dense compared to its surroundings. The radiation pressure would pretty much smooth it away. As the waves of radiation and matter full waves rippled through it. Nothing could sort of ever, gravity could never get hold of it before it got pushed apart. But now, after recombination, you have the possibility of gravity being able to take hold of some of these perturbations. So after about 100 million years, and the universe is about 1 10th the size it is today, the temperature was about 30K above zero which is pretty cold. And now, what we're going to see is, is that those perturbations, the tiny ripples in the gas, left, you know, that, were there from, recombination. Or, sorry. Were there from, actually, the era of inflation. And have, you know, been trying to grow with time. Now the, fact that you have more matter in one region than in its surrounding, allows gravity to begin to pull that matter in on itself. The grav-, its own gravity. Its self gravity actually can take those tiny ripples and begin to amplify them. So we now see something really remarkable happening, which is the beginning of what we call cosmic structure. So [COUGH] ] what we get is, those perturbations growing in time, becoming over-dense filaments compared to their surroundings. And then within those filaments we actually get, you know, there's ripples on ripples. We get individual places that are really beginning to accumulate matter, and they will become the first stars. So somewhere around a z of 20 or so we begin to see, and z, remember, is the red shift. The cosmic microwave background the recombination occurs at around z of 1000 or so or z of 2000. We are now at z of zero, so looking back in time z increases. So at around z of 20, we see the first generation of stars begin to form. And these are very different than anything that's around today because there were no metals. There's no, what we call metals, there's no heavy elements. And heavy elements really control a lot, even though there's not many of them in a star today, they actually control a lot of the behavior of the star. But these first generations of stars are really just hydrogen and helium. And they do what stars do. They start fusing. But because there's no heavy element, they lead very different lives. And in particular, many of them are huge, right? It's very rare to find the star more than 100 solar masses today, or the star that has more mass than 100 times the sun. Whereas, probably they were quite common, these behemoth were quire common during the early universe. So we, simulations show us that we have lots of stars between 100 and even 300 times the mass of the sun. And these stars are very big, they're very bright. And they produce ionizing radiation, ultraviolet photons. And those ultraviolet photons go streaming out into the environment. And they ionize the gas, the gas of neutral hydrogen. Remember, that we started to get neutral hydrogen at recombination about 300,000 years after the Big Bang. And here we are, you know, a few tens of millions of years after the Big Bang, and what we see is, the newly born stars beginning to reionize, or beginning to turn that hydrogen gas back into electrons and protons. And that's going to be very important because right now, the universe is pretty much the space empty space, well, it's not quite empty, of course. But, you know, interstellar, intergalactic space is full of ionized hydrogen. So there's only this brief window that we call the dark ages, between recombination and these first stars forming. That the universe was actually mostly neutral hydrogen. Okay, so these stars form and then they go supernova, so they're starting to blow some of the processed elements that were created inside those stars. The heavier elements that are forming inside of stars, that we talked about in week two. They're starting to now seed the universe with heavier elements. So that's important. So it's from this point onward that maybe you get the possibility of forming things like planets and such. What's also happening during this time, though, is that we're also getting the first black holes forming. Some of these things that these places, these overdensities, where gravity is just getting a hold of, and pulling material inward, are going to go right into black holes. And so those black holes what are, these are the beginnings of the supermassive black holes that we looked at with galaxies. So, it looks like that the formation of galaxies, and formation of these super massive black holes probably go together. As these density perturbations that are growing via gravity are beginning to draw more material in. And if there's any angular momentum. If there's any rotation, we're also going to get the beginnings of the, the, the discs of galaxies as well. So the, the galaxies are just beginning to start at this, at the end of this phase. They are, we're just beginning to see the formation of the super massive black holes at the same time. And we're even beginning to see some of the interactions between, sort of, the, these proto galaxies. We can even see some, indications that there were collisions ongoing. We know that collisions are very important for galaxies. And they seem to happen pretty early on. And you'd expect a lot of collisions between galaxies during this phase, because space is more crowded, right? There's, you know, space has been expanding since the beginning of the universe. So, these early phases, everything is more packed in. So, this, what we're seeing in this early, this epoch after recombination. Is the beginning of stars, the first stars forming and the first galaxies forming. So it's really the beginning of what we call large scale structure in the universe. Okay.
