4 Problems of the Big Bang Theory

Last time, we saw that the Big Bang Theory is so successful, it predicts CMB radiation, also matches with the, um, Big Bang nucleosynthesis, predicts a abundance of elements perfectly well, and it also matches with the expansion of the universe. However, there are four problems that even Big Bang theory cannot explain that we’re gonna take a look at today. Let’s get started. The first problem is initial density perturbation. So this is a CMB map taken by NASA’s Planck satellite, and I told that CMB’s uniform in degree scale, but if we look at the micro-Kelvin scale, I told that the CMB is uniform in Kelvin scale.

But if we look at it in micro-Kelvin scale, there are fluctuations like this, there are red parts and blue parts, this is in micro-Kelvin level. So, these are very tiny fluctuations, but fluctuation is there. Now where did this fluctuation came from, come from? That’s one of the problems and this is the important problem, because those fluctuations are origin of us. In these high density regions, red parts, they are slightly over density and this, this is where our stars collapse, and stars are born and galaxies are born and planets are born and then we are born. So, this is related to the origin of us. Second problem is a flatness problem.

In our high school mathematics, we do mathematics in a Euclidean space. Euclidean space is where parallel, right, stays in parallel, that means “flat space.” But the geometry of the universe doesn’t necessarily, I can, isn’t necessarily be flat. For example, it can be open, where parallel light is gonna separate eventually, or close the universe like the surface of a sphere that parallel light um, closes each other in the end. But if we compare these three patterns with the observation of the cosmic microwave background, we know the closest one is this one. It turned out the geometry of the universe is almost perfectly flat. Why is it so flat? Is there a reason? So that’s the flatness problem.

The third problem is a horizon problem. In Kelvin scale, uh, CMB is very very uniform, it’s 2.73 degree. But why this is so uniform? If we calculate, do the calc, do the mathematics, size of the particle horizon at that time is about two degrees. Particle horizon is a maximum distance the light can travel at the time of the CMB radiation, and so nothing, the fastest thing is the speed of the light, right? So nothing can travel beyond two degrees. So in a sense, this beyond two degrees the CMB radiation cannot mix each other.

So in a sense if the particle horizon size is two degrees, it is natural to have hot region, region, and then cold region, um, in in CMB, because they didn’t have time to mix. It’s something like this. If you put cold water and hot water in the bathtub, eventually they become the same temperature, but at the beginning if there’s not enough time, there will be a hot part and a cold part in the bathtub, right? The universe at the beginning should have been like that because particle horizon size was two degrees Then, why CMB it’s so uniform, why it’s this one temperature? This is called horizon problem. The last problem is a monopole problem.

If you look at the magnets, usually we see N and S right, in a pair. But theoretically there can be something like, only N magnet, something like this. Electronically this exists for example electron is one example. But we never found this monopole, magnet, magnetical monopole. But grand unified theory predicts about one monopole per horizon, and I told that if the horizon size of the universe is two degrees the two degrees there, so universe is 360 degrees right? And there are many two degrees in 360 degrees, so there should be many monopoles like this. But so far, human, we haven’t found any monopole at all. So why is that? This is the monopole problem.

So, the Big Bank theory cannot explain these problems. Initial density perturbation, or flatness problem, horizon problem, monopole problem. So this indicates we need new physics here. And that’s where inflation theory comes in. Explaining inflation theory can explain all these four problems. See you next time.