Although there is a great deal of evidence for the big bang, it does raise an interesting question. If the universe began with the big bang, what caused the big bang? That’s a bit of a stumper, because we aren’t entirely sure. Observationally, we have a good understanding of the universe as far back as nucleosynthesis, which occurred when the universe was about 10 seconds old. We also have some initial observational evidence of the inflationary period, which occurred in the earliest moments of the universe. As for the cause of the universe, we just have ideas at this point. One of the more popular ones is that the universe quite literally came from nothing.
The “nothing” in this case is not the type of nothing we might typically imagine. Here, “nothing” refers to the idea that there are no particles, energy, space, or anything else that could be considered “real” in a traditional sense. Instead there is only a quantum potential, or a nebulous cosmic wavefunction. You might argue that a quantum state subject to some set of physical laws is hardly “nothing”, and I’d be inclined to agree with you, but in cosmology the term “nothing” has kind of stuck. It’s similar to the term “big bang” being used for an event that was neither big nor explosive.
That said, there has been some interesting theoretical work on just what this quantum nothing might have been like. One such work was recently published in Physical Review D. In this paper the authors looked at what is known as the Wheeler-DeWitt equation, which is a way to describe the entire universe within the framework of quantum mechanics. The model is not without its problems, but it is a way to study the intersection between general relativity (gravity) and quantum theory.
Within the Wheeler-DeWitt formalism, it is possible for quantum fluctuations to occur. Basically, within the quantum nothing fluctuations of spacetime can appear, though they would dissolve back into the quantum-ness. The team showed that in certain cases it is possible for such a fluctuation to expand rapidly, where the state of the quantum system acts as a cosmological constant to cause inflation. Once that occurs, the fluctuation is to big to dissolve back into a quantum state, and has thus become “real”. What this shows is that it is possible for a quantum state to give rise to a universe with an early inflationary period, similar to the way our universe seemed to arise.
It should be kept in mind that this is still very speculative. This work is really a demonstration of a possibility, and not confirmation of how our universe came to be. We are only beginning to gather evidence on the inflationary period of the big bang, and we’ll need a solid understanding of that period before we can explore any quantum origin of the Cosmos.
In the end, these ideas might prove successful, or it may turn out that nothing comes of it.
Paper: Dongshan He, Dongfeng Gao, and Qing-yu Cai. Spontaneous creation of the universe from nothing. Phys. Rev. D 89, 083510 (2014)