During inflation, quantum vacuum fluctuations of fields are enlarged and eventually evolve into observations related to cosmological perturbations, including the Cosmic Microwave Background and Large Scale Structure. It is expected that the cosmological perturbations experienced a quantum-to-classical transition for fitting the macroscopic observations, described as the decoherence obtained by tracing out the unobserved degree of freedom. However, inflation is also a natural source of producing highly squeezed quantum states of cosmological perturbations, and their non-classicality can actually be probed with some proposals in the recent literature. We thus revisit and quantify the cosmic decoherence, from explaining the classicalization of observables to constraining the probes of non-...[ Read more ]

During inflation, quantum vacuum fluctuations of fields are enlarged and eventually evolve into observations related to cosmological perturbations, including the Cosmic Microwave Background and Large Scale Structure. It is expected that the cosmological perturbations experienced a quantum-to-classical transition for fitting the macroscopic observations, described as the decoherence obtained by tracing out the unobserved degree of freedom. However, inflation is also a natural source of producing highly squeezed quantum states of cosmological perturbations, and their non-classicality can actually be probed with some proposals in the recent literature. We thus revisit and quantify the cosmic decoherence, from explaining the classicalization of observables to constraining the probes of non-classicality. For the decoherence by the minimal gravitational interaction, we note that the slow-roll unsuppressed temporal boundary (total-derivative) terms usually neglected in the literature can contribute much faster decoherence rate, and the relation between the boundary terms and the WKB approximation of the Wheeler-DeWitt equation is also studied. We also explore the possibility of using the cosmic decoherence to distinguish the perturbative quantum gravity with semi-classical gravity, such as the Schrödinger-Newton model.