Here is the second part of my series of articles taking the question of Multiverse under consideration.
As I mentioned in the previous article, these ideas are taken from the book written by U.S. physicist Brian Greene who is well known for his contribution to the development of String Theory. He is one of a group of physicists proposed the idea of so-called Flop-transitions and Conifold-transitions which is saying that the very space-time structure is being torn on incredibly tiny scales from time to time. These concepts are very interesting to think about so maybe I will do a post briefly describing them in the future. In this article, however, I shall be talking about the second type of Multiverse that has to do with the concept of inflation which was first proposed by Alan Guth, whereafter enhanced by Andrei Linde.
I don’t think it is going to be surprising for you that this type is called Inflationary Multiverse.
For those who are not familiar with the concept of inflation I must provide a brief explanation to make things a little bit clearer. The main idea of inflation is that right after the Big Bang (10 to the power of -35 seconds) our Universe underwent an extremely short (presumably ended at 10 to the power of -33 seconds) but at the same time extraordinarily rapid expansion period. According to this model the Universe expanded in that fraction of a second more than it’s done in the next 13.8 billion years.
You might ask how it is possible. According to the model right after the Big Bang our Universe was filled up with what’s now known as the inflaton field which had possessed high energy until a random quantum fluctuation caused it to drop down to its lower energy state whereby releasing a huge amount of its potential energy. This energy acted as an extremely strong repulsive force pushing all the existing particles (there were not even atoms at that time) away from each other. Presumably, inflation occurred when the Universe was in so-called false-vacuum state whereafter decaying to a true-vacuum state through the process known in physics as state-transition, which stopped the inflation. When this period had ended the Universe continued to expand with a normal rate and is still doing that, though the expansion rate is actually increasing the last 7 billion years due to dark energy. Nevertheless, current rate is not even close to that of inflation.
The question arises, why do we need so bizarre model of Big Bang and what could provide evidence for it? One of the serious problems inflation solves is horizon problem. It was identified when physicists studied initial Big Bang model. It turns out that with the standard Big Bang model the further back in time we follow Universe’s chronology the lesser time its various parts have to interact with each other. Eventually, we reach the moment when for making the Universe isotropic (as we observe it) radiation must have moved faster than the speed of light which we know is not possible. Hence, we should observe anisotropic Universe, which we can definitely say is not the case. Cosmic Microwave Background (CMB) has almost the same temperature with only extremely tiny variations no matter which direction we observe it. That means that the Universe is certainly highly isotropic on large scales and this directly follows from the inflation idea. And although neither horizon problem solution nor Universe’s isotropy demand inflation, it is certainly the best explanation physicists have at the moment.
The next question that might appear in your head is how this whole inflation model leads to the idea of Multiverse. There is one aspect which I deliberately left out of consideration speaking of inflation. The state-transition process halting the inflation shouldn’t have occurred everywhere. Instead, it may have happened in a certain finite region leaving other parts with continuing inflation. That particular region became the Universe we inhabit after all.
The next moment another region could have underwent a state-transition giving rise to another Universe. The next moment again another Universe came into existence and so on and so forth. You might think that since different Universes are being born one after another this process must eventually come to an end. But don’t forget that while one Universe appears in a certain region the inflation continues in other regions at the same time, so this process never stops. Admittedly, inflation cosmology is giving rise to the countless Universes within a single framework.
The last important thing I want to point out is that most likely each of those Universes has its own distinctive physical features such as the masses of various particles, the amount of dark energy and the like. So most probably, almost all the existing Universes would not be hospitable to life and the question of why we humans find ourselves in this particular Universe would be quite easy to address. Here I give you a reference to Brian Greene’s TED talks where he describes the idea of Multiverse.
It should also be mentioned that there are several inflation model approaches, for example those of Alan Guth and of Andrei Linde, which differ from each other in details. All those models give their own spin to the Multiverse idea but remarkably, each of them most likely leads to the eternal inflation and to the Multiverse consequently.
Hopefully, you found this article useful in some way.