Cosmology is a special type of science. In typical scientific research, one prepares an initial condition and measure the final outcome after an experiment. However, in cosmology, we do not have control of the initial conditions – they are prepared by Nature.
Especially, the structures in our Universe originates from the quantum fluctuations during the cosmic inflation. It is usually argued that “Bunch-Davies” states are the natural initial conditions of those quantum fluctuations. However, no precise statement has been proposed on how fast the inflationary fluctuations approach to Bunch-Davies after inflation starts, or after the inflationary process is disturbed by features in the potential. Also, the Bunch-Davies approach is not precise enough for nonlinear fluctuations.
How to determine the physical initial condition of inflationary fluctuations?
The idea is study the stability of those initial states. If the state were stable, they are the physical vacua. Otherwise, there is a calculable decay rate for the process, telling us after how long time the fluctuations returns to the most table state.
Hongliang Jiang and I, and later Hongliang Jiang, Siyi Zhou and I calculate such a decay rate. We also show that the i-epsilon prescription can be better defined in our procedure.