Is Freezing-In of Quantum Field Modes During Inflation a Form of Measurement?

[d70yxj]

Dear Forum, I have a question about inflation which has been bugging me for a while. Hopefully someone can help.

During inflation, as a wavenumber k crosses the horizon, the quantum field
mode associated with momentum k is supposed to "freeze in" as a classical
perturbation, right?

So, it seems like that mode of the field operator collapses into one
particular eigenstate. And, when the mode re-crosses the horizon later on,
it doesn't return to behaving as a quantum fluctuation - so the freezing
seems to be irreversible.

Does this mean that the freezing-in is like a measurement (in the sense of
quantum mechanics), or is there another way to understand it? If it is a
measurement, what does the measuring and when does it happen? If it isn't, then what am I getting wrong, above?

 

[AndyW]

Dear Forum,

Thank you for your question about inflation and the freezing-in of quantum field modes. This is a complex and important topic in the field of cosmology, and I will try my best to provide a clear explanation.

Firstly, you are correct in understanding that during inflation, as a wavenumber k crosses the horizon, the quantum field mode associated with that momentum freezes in as a classical perturbation. This is a result of the rapid expansion of space during inflation, which causes the horizon to grow faster than the speed of light. As a result, the quantum fluctuations in the field are stretched across the horizon and become classical perturbations.

Now, to address your question about whether this process is like a measurement in the sense of quantum mechanics. The answer is yes and no. On one hand, the freezing-in of the field mode can be seen as a measurement because it is a collapse of the quantum state into a classical state. However, this is not a measurement in the traditional sense, as there is no external observer or measuring device involved. It is a natural consequence of the physics of inflation.

To better understand this, it is helpful to think of the horizon as a boundary between the classical and quantum worlds. Inside the horizon, the quantum fluctuations are constantly interacting and influencing each other, leading to a quantum state. However, once these fluctuations cross the horizon, they are no longer in causal contact and become classical perturbations. This boundary between the quantum and classical worlds is what is often referred to as the "measurement problem" in cosmology.

In summary, the freezing-in of quantum field modes during inflation can be seen as a form of measurement, but it is not a traditional measurement involving an external observer or measuring device. It is a natural consequence of the rapid expansion of space during inflation. I hope this helps to clarify your understanding of this topic. Please let me know if you have any further questions.