Inflation

Last time, we saw that there are four problems that Big Bang theory cannot explain. Today were gonna see inflation can solve these problems. Let’s get started. Let’s get started with the horizon problem. Uh, the horizon problem is, CMB is so uniform, for uniform, but why? That is the horizon problem. The size of the horizon at the time of the recombination where CMB came from is two degrees. So the universe shouldn’t be at the same temperature beyond two degrees. But, observational results show that the universe is the same temperature, 2.73 Kelvin, why?

So the scientists at that time thought,

the fact that CMB temperature uniform, may be telling us that particle horizon size was bigger, so that the universe can mix. That means, universe was much, much smaller than its size, and experienced much more rapid expansion. So that is inflation. Therefore, inflation in a nutshell is something like this. Exponential expansion of the universe, this is inflation, in 10 to the minus 32 seconds, universe became 10 to the 26 times larger. And the energy scale is much, much larger than dark energy, 10 to 27 times larger than dark energy. And if we have this inflation, there will be no horizon problem.

For example, it was a problem because the particles horizon size was two degrees and the universe here, and the universe here, cannot mix to be the same temperature, that was a problem. But, if the universe was much smaller before, then, uh then smaller than the particle horizon size, then everything can be mixed in the universe and then become the same temperature of 2.73 Kelvin. Then there will be no horizon problem, it’s okay to have universe with the same temperature. And this inflation also solves the monopole problem. The mono-, the origin of the monopole problem is we don’t find monopole like this one, even though the monopole is predicted, predicted to be one particle horizon.

But now, before inflation, universe was within one particle horizon, so only one monopole is expecting the entire universe, and if there’s only one monopole in the whole universe, it is perhaps okay not to find one. That may be the reason we haven’t find any magnetic monopole in this universe. And the next one is the flatness problem. So why the geometry of the universe is so flat? Inflation can explain this too. So, if we look at the curvature of the universe, say, it may be something like this, you know, it’s not flat, but it’s winding. But, inflation is gonna make this much, much larger.

And if you magnify a universe, and then look at tiny, tiny part of the universe, this tiny part looks flat, right? So, actually inflation theory predicts flatness of the universe. So the flat problem, flatness problem is also solved. What about initial density perturbation? So, at the micro-Kelvin scale, universe has fluctuation, this is the origin of stars and the planets and us. So, where did this initial density perturbation come from? To talk about this, we need to talk about Hawking radiation. Hawking radiation is generally for a black hole. near the, so in the universe there’s always with a certain probability that there’s a particle and an antiparticle antiparticle pair, is created and then disappears, this is happening at a certain probability.

And if this happens near the black hole, and then one of the particles happen to go inside the black hole, if you, once you go into the black hole, nothing can come out from the black hole, so the remaining particle will fly to us, as a radiation, and this is so-called Hawking radiation. And the universe, this can happen in the universe too, in sort of the outside-in way. In the universe we have particle horizon, event horizon in outside, so if there’s a creation of particle and antiparticle pair, and if one of the particles goes outside of the event horizon of the universe, they can be never observed.

So then, the other particle is going to fly to us as a radiation. So, the early universe also has a background radiation from this Hawking radiation. And then this probability of this particle/antiparticle creation is from quantum Gaussian fluctuation. So, uh, universe should have fluctuation, uh, from here, quantum fluctuation, and that is the source of the fluctuation of the cosmic microwave background here. So initial density perturbation is also predicted by inflation. So those four problems of the Big Bang, inflation can explain them all.

However, so this is a wonderful, wonderful theory. However, okay, before that. Who came up with the inflation theory in 1981? So those two gentlemen, Katsuhiko Sato and Alan Guth came up with the inflation theory. Um, historically it’s very interesting. So, let’s look at their papers. So this, Katsuhiko Sato’s paper was submitted in February 1980, right? However, unaccepted May 1981. But Alan Guth’s paper is submitted August 1980, so it’s later, right, February and August, so this was submitted later. But it was accepted January in 1981, so this paper is accepted before Sato’s paper. So this paper is, I mean, Sato’s paper is submitted first, but Alan Guth’s paper is accepted first.

Well anyway, they both published a theory of about inflation in 1981. And then, um, Katsuhiko Sato called it’s it’s the ex- exponential expansion of the universe. He didn’t use the word “inflation.” The word “inflation” is used by in this Alan Guth’s paper, so that’s why Alan Guth’s is, uh, paper is, I think it’s more famous. But once inflation theory is proved, then those two gentlemen, um, probably will most likely get Nobel Prize. Okay, I stop here today.