Does inflation squeeze cosmological perturbations?

Understand

There seems to exist agreement about the fact that inflation squeezes the quantum state of cosmological perturbations and entangles modes with wavenumbers $\vec k$ and $-\vec k$.

• Paradoxically, this result has been used to justify both the classicality as well as the quantumness of the primordial perturbations at the end of inflation.
• We reexamine this question and point out that the definition of two-mode squeezing of the modes $\vec k$ and $-\vec k$ used in previous work rests on choices that are only justified for systems with time-independent Hamiltonians and finitely many degrees of freedom.
• We argue that for quantum fields propagating on generic time-dependent Friedmann-Lema\^itre-Robertson-Walker backgrounds, the notion of squeezed states is subject to ambiguities, which go hand in hand with the ambiguity in the definition of particles.