So to sum it up, what we have discussed so far, the big bang produced only hydrogen, and helium, and no other heavy elements. There was just a little bit of heavy ones, but you can pretty much ignore them, so to a good approximation just hydrogen, and helium, and that led to the formation of some force stars, and inside the stars you formed Bigger elements like carbon, oxygen, silicone, all the way up to iron, and as long as these elements are stuck inside the stars, we still don't get to use them to form a body, so it doesn't explain where we came from. But fortunately, as you saw on a previous few minutes, these stars can actually explode at the end of their life. And that would release a lot of stuff back into the outer space. And then, what had been synthesized at the core of these stars gets released like carbon, nitrogen, oxygen and iron that eventually formed a body. So what is believed, is that our sun is probably the third generation star. So we had the initial stage of the star formation based on the hydrogen helium alone. Some of them exploded, which leads to more stuff, you accumulate them, form the stars once again, they also reach the end of life exploded, and the sun use that kind of material to form the initial core of the star. And that led to our own existence. So the explosion star like this is very important because that would be the origin of our own existence. Origin of some Nobel, Nobel prize as well. And so we are literally stardust. So we came from the star. And that’s what we are actually made of. But of course you should still ask the question, is that really true? Is there any evidence for that? So at least we can observe some of the past supernova that happened nearby in our own galaxy So this is the famous one called crab nebula and is believed to be a remnant of the past supernova that happened actually very recently in year 10, 40, 54. The reason we know the year. Is that there is some historical record in old Japanese literature, by Teika Fujiwara and in this old manuscript, you find a reference to a super nova. What the literally says is the guest star. There is a star, he could see which wasn't there before, that's why it's the guest star and that was the super nova explosion. And these days you can study the same object, in many different ways. By using optical telescope, just the usual telescope, you can observe x-rays. You can use infrared light like when we talked about seeing big the black hole at the center of the galaxy. And radio, which we have used also for the [INAUDIBLE] observed the big bang itself. By combining information from these different techniques you can get a better understanding of what is going on inside this object And this time sequence of the X-ray textures coming from Chandra satellite. By putting that into a movie file, you can see that there's still a throbbing going on. So, this happened like a thousand years ago when it exploded, but is still releasing material. That you can see in this throbbing dissolute movie and so we can see that the supernova does release material into space and that's how we came to exist. We believe that something like that one is going to happen soon, actually fairly nearby. Do you know which star is Betelgeuse in this picture? You probably can identify Orion constellation, Orion. So Betelgeuse is on the left top corner of this Orion constellation. And it weighs as much as 20 times the mass of the sun. And it's fairly nearby. It's only 640 light-years away. So by now, listening to my lectures you would immediately notice that this is actually a very close distance on the scale of the universe. And if the star is disclosed, you can even measure how big it is just by looking at it using telescope. This is the data that shows that the apparent size of Betelgeuse has shrunk by 15%, in a mere 15 years. So something is really happening with this star. So you can even observe the surface of the star, when the star is so close. And what you see in this picture here, is that the surface is also really throbbing now. You'll see parts that are swelling up, you see parts that's sinking in. So you see that surface is very dynamical at this moment. So that really shows us that's somethings going to happen to this star pretty soon Pretty soon the case may mean next 10000 years, but you know explosion may happen tomorrow as well. We don't really know we can’t predict, but it's clear that this going to happen sometimes soon into a big supernova. So putting things in perspective the way we can tell, how old a star is? And what it is the star’s made of? Well we can do, again going back to this sort of prism, by measuring the spectrum of colors coming from star, you see these lines, which shows some colors are missing. And this is called spectroscopy, which is very important technique in studying the astronomical objects. So this particular star, which is actually our sun, has a lot of these lines which shows that has many heavy elements. So these stars are called population one, the lot of metals, elements heavier than helium. This is clearly contaminated by the past supernova explosions. If you look hard enough, sometimes you find stars which seems much cleaner. Less contamination of these heavy elements. So they are called population two stars. If you go way back into the beginning of the universe, there must have been what is called population three stars. Which are made of Hydrogen and Helium alone, without any contamination from the previous generation of stars. We haven't found those, but people still looking. So, now we comes to this question, how do we know that big bang produced hydrogen, helium, but not more? We need a different tool, as we talked about. What we can look back to the point when universe was only 380,000 years old, by using radio telescopes, because we can detect the cosmic micro background. Which is the start light coming from the big bang itself we talked about it. But there's a wall, right, you can't go beyond that using telescopes, we can't see through that wall, we need some different technique to understand what's going on at the earlier moments of the universe. So you, we use a different tool, called particle accelerator. The idea is very simple, universe is so dense and hot, we would like to understand what happened back then, so that’s something we can try to do in our own laboratory. We try to create an environment where similar reactions can be created artificially. Which must have happened early on, at the beginning of the universe. So that way, we can go back to the moment, when the universe was only three minutes old. What we do is bring in literally, neutrons and protons together, smash them against each other, And see if they can form something heavier, like helium. So we can try to redo this kind of reaction that happened right after big bang, in our own laboratory, and that's the way we try to go beyond this wall of 300,000 years old universe. So, by studying these reactions in a laboratory, you can measure the probability for a reaction like this to happen. And once you've measured that you can make a prediction, on how much of the helium must have been synthesized at the beginning of the universe, and that process is called Big Bang Nucleosynthesis. And that would give you a prediction, that the ratio of hydrogen to helium in our universe must be roughly 3:1. And indeed you can measure the ratio of hydrogen, helium by looking at, again, the spectrum of light coming from the far away objects, and this 3:1 ratio agrees very well With observation. So that's how we can compare, what you would predict based on what you learn from particle accelerators versus what we can actually go back, and see using telescopes. And if they match up, that would give us enough, confidence that we understand what happened in the Big Bang itself. So by putting these information together, I you can put them in the computer, and simulate how the first stars had been born. And we will come back to this later, but at the initial stage you don't have that much concentration of gas, you have sort of a big blob of dark matter. We briefly talked about it in the first lecture. Once we have this blob of dark matter, gravity pulls the ordinary gas in and it starts to form this cloud of gas called the molecular cloud, and the core of the molecular cloud further collapses and start producing something rather dense. And that would eventually become a star. You can reproduce this process in a computer these days. And if you go further with this computer simulation, then this initial form of star would start to accumulate more stuff from outside, by the gravitational pull. But once they start to accumulate so densely, then it starts to actually ejected off. It was sort of blowing out, by the reaction, so that's something you can see in this computer simulation, so for a while dust just settles on the surface of this protostar. It's not quite a star yet, but as it keeps accumulating on the surface, you see this ejection of material, in the polar direction. So it stops growing at that stage, because whatever settles in further gets blown out, so the stars wouldn't grow anymore. So what do you learn is that, the first stars can grow only up to like forty solar masses. But that is big enough to synthesize the elements all the way up to Iron at this stage, and that's how I believe that initial elements had been synthesized inside the stars. So, going back to the discussion we had on the very beginning, we start with only hyrdogen and helium. So we ask the question, where we could have possibly come from because we need carbon, oxygen and so on. And the answer turns out to be that they were built in stars, and we are the star dust. But, you might have been listening carefully to me and ask the question, Well you told me that we can only go only up to an iron, can't go beyond that because you can't release energy by shedding mass beyond iron. That's at the bottom of this curve. How can you further climb up the hill? And we, we of course know that there are elements that are heavier than iron, like what you might buy in a jewelry store. So we need a mechanism, to produce those elements as well. And indeed, if you look at the so called abundance of these elements in the universe and that you do see all these heavier elements, like silver, gold, platinum, lead, they must have been produced somewhere. And, you know, it is the nature of the science, that you don't have answers to every question, yet. They’re still yet to be studied. We don't quite know where they came from. Many of us think, that these heavier elements have been formed when the star is exploding. We talked about element synthesized that occur, explosion releases. But explosion itself may be bring the reaction even further. And explosion is such a dynamic process, that you can even put energy in to let the reaction happen. So presumably that's the way these heavier elements had been formed. But we don't know for sure yet, so that's where you might be able to provide further studies to prove this idea. This is yet another curious fact if you think about this, lot of important things have been explaining, explained by the property of these individual particles coming in together, like protons and neutrons. What we know, is that proton neutron have very similar masses within only like 2 per mill. Basically they are exactly the same mass to a good approximation. But if you think of possible universe, where the proton may be say 20% heavier than neutron. Heavier. E=mc squared means more energy. So proton can then release energy by turning into a, a neutron in that case. So all the protons end up decaying, in neutron if it is heavier by 20 percent. So if that's the case, even if you manage to synthesize like a helium nucleus, protons in the nucleus would decay into neutrons. They're all electrically neutral. So you don't end up with anything, that can become the atomic nucleus at the end of the day. So no atoms would be possible at all. So the very existence of chemical elements, actually hinges on the fact that proton, and neutron are pretty much the same in their masses [INAUDIBLE]. So that's one, another mystery we don't quite understand. Why are the masses of particles, designed in such a way, that somehow all these important things that would sustain our life had become possible. And another question of course is that even if you do manage to form this atomic nuclei, you still need electrons to go around them to form atoms. So how were the atoms possible, why was that possible? And that's the next question, about the Higgs boson.
