The Big Bang Nucleosynthesis

Hello everyone, today we’re going to talk about the Big Bang nucleosynthesis, the first 1,000 seconds that shaped the universe. This is one of the evidences of the hot Big Bang theory. Let’s get started. So, we learn about the expansion of the universe, the universe is expanding, this is what Edwin Hubble discovered. So if you think about, if the universe is expanding, if we rewind the clock, the universe was smaller, right? Then, if the smaller the temperature was probably higher. So there might be a, universe was hot in the past, smaller and hotter, and that was right, so that’s the start of the hot Big Bang theory.

So, during that early stage of the universe, the universe was much smaller and temperature was much higher, about you know, several thousand degrees. Then you might think there was a nuclear reaction, reaction there, for example in the core of the sun there’s a nuclear reaction, the temperature is very high. In a similar way, in the early universe the temperature was very high, so there was a nuclear reaction. Um, at the beginning there was this kind of reaction was going on. So, neutron and then proton was going back and forth in these two kind(s) of, uh, reactions, so neutron becomes proton, and a proton becomes neutron, or this reaction is going both ways.

But then while this is happening in a hot plasma universe, the universe is also expanding. Then when the universe is expanding, temperature goes down, with the expansion, and then when the temperature becomes the 10 to the 10 Kelvin, was still very hot, but this is going down, lower temperature than before. Then, reaction time scale of this neutron and proton becomes longer than the age of the universe. So, what does it mean? That means this reaction doesn’t, doesn’t happen anymore, it takes longer time than the age of the universe at this temperature.

So, this is the moment called “freeze out,” and then this reaction stops, so neutron/proton ratio is fixed at that moment, with a value of 0.34, and this is about one second after the beginning of the universe. And then, as the universe continues to expand, the succeeding reaction also freezes out. For example, after proton/neutron reaction, uh, there will be deuterium created, and helium is created, lithium is created, the more heavier, more and more heavier elements are created. But these reactions, also one-by-one in this order freezes out as the temperature goes down. First this freezes out, this freezes out, this freezes out, so that reaction’s gonna stop as the universe expands.

And as a result, the ratio of each of these elements are fixed at the very beginning of the universe, because there are no more reactions like this. So this is theoretical, uh, prediction, so this is a big bi-nucleosynthesis model and this is a prediction. Let me explain this. So this is the abundance, relative to hydrogen, of each element. For example this helium abundance, and the lithium abundance is here. Um, then this is a total amount of barium, total amount of the baryon in the universe. Baryon are regular atoms. Well this value, Big Bang nucleosynthesis model cannot decide, but we have a measurement from the cosmic microwave background. So the value is about here.

00:04:17.000 –> 00:04:22.000 And these red curves are predictions from the Big Bang nucleosynthesis,

that’s theory that we look at it. And then observe the value so we also can use telescope to look at the sky, and then measure the abundance of, for example, helium or lithium are shown in this blue.

And as you can see, this observed value of the elements in this blue box are exactly perfect match to these red models here and here. This is amazing, so these red curves are computed on a table as a theory, and then you observe the universe and the abundance matches, the theory is perfectly correct. So this is called the big triumph of the Big Bang nucleosynthesis model. So this is one of the reason(s) people believe in this Big Bang nucleosynthesis model and the Big Bang theory. Who calculated this Big Bang nucleosynthesis model? This is so-called Alpher-Bethe-Gamow theory in 1948. So this is a, um, Alpher is a student of professor George Gamow, and this is Alpher’s PhD thesis.

And then George Gamow thought, so, those two were writing paper(s) together, and then this this George Gamow thought okay, this is Alpher and his name is Gamow, like “gamma,” so, um, he thought he invites professor Bethe. Then, if you, if he invites professor Bethe to the paper it becomes “Alpha-Beta-Gamma” paper. So he invited Bethe and it was published as a Alpher-Bethe-Gamow (pronounced Alpha-Beta-Gamma) theory. I heard this Alpher, his PhD student didn’t like inviting professor Bethe to his paper, but uh, later it became famous because it’s catchy naming.

Well it seems this George Gamow, professor Gamow, was a very unique person, and then he wrote a lot of, he was also successful as a science writer, and one of his famous book(s) is this New World of Mr. Thom- Thompson Tompkins, The New World of Mr. Tompkins. Uh, in this book this Mr. Tompkins, uh, traveled in a different world with different physical, physics parameters.

For example, in one world, uh, speed of the light is about his walking speed, and he sees (the your) his friends are Lorentz contracting or he tried to live on a neutron star, and then he suffers from a huge gravity, so this even, well this was written long time ago, but even today’s standards it’s (a) very interesting book(s). Okay, so, so far we have looked at the three evidence of the Big Bang. First, Hubble discovered expansion of the universe, so, if you rewind the clock, it’ll be a small, hot universe. Also we have cosmic microwave background Penzias and Wilson discovered cosmic microwave background, this is a remnant of the hot fireball of the Big Bang.

And then today we look at the Big Bang nucleosynthesis, this theory perfectly predicts abundance of helium and lithium, um, are just like observed in the universe. So these are the three main reasons many researchers believe in the hot Big Bang theory.