Quantum decoherence of gravitational waves during inflation

Inflationary perturbations couple to all degrees of freedom present in the early Universe, therefore it is realistic to view these perturbations as an open quantum system interacting with an environment. Due to this interaction, the perturbations undergo quantum decoherence, which can affect their statistical properties. Several works have investigated this effect for primordial density perturbations, but we will study the effects of quantum decoherence on primordial tensor perturbations (i.e., gravitational waves) due to a quadratic interaction with an environment. The evolution of tensor perturbations can be modeled using the Lindblad equation, from which we obtain the time evolution of the two-point correlators (or their Fourier transform, i.e. power spectra). This allows us to compute the gravitational wave power spectrum and its contribution due to decoherence. Looking at such decoherence contributions to the tensor power spectrum, it is clear that there are three different scenarios depending on the time dependence of the interaction strength. First, the power spectrum increases like a power law on large scales due to decoherence and remains unchanged at small scales. Second, the power spectrum increases like a power law on small scales due to decoherence, but remains unchanged at large scales. Lastly, decoherence has (almost) no effect on the power spectrum. This shows that decoherence can give very distinct features to the primordial gravitational wave power spectrum. To constrain quantum decoherence, we look at known observational constraints for the power spectrum (−1.37 < n T < 0.42 at 95% CL by [1]) and compare them with our results. We can see that some regions of the parameter space are preferred, but the currently available constraints are very broad and obtained using only two specific scales (CMB and ground-based laser interferometer). Therefore, we cannot definitively dismiss any of the decoherence scenarios we discuss. In future work we can use data from new observational tools (like Pulsar Timming Array, LISA, Einstein Telescope, and Cosmic Explorer) to look for observational signatures from decoherence or further constrain our decoherence scenarios. On top of this, a further future improvement can come from the fact that decoherence needs to last long enough for the perturbations to become classical, allowing one to put a lower bound on the interaction strength between the system and the environment.Inflationary perturbations should be seen as an open quantum system interacting with an environment. The evolution can be modeled using the Lindblad equation. Studying the how the tensor perturbations are decohered by the environment leads us to study how the gravitational wave power spectrum changes due to this interaction.