When the Environment Speaks: Quantum Signatures in Non-Attractor Inflation

We study the open quantum dynamics of the adiabatic curvature perturbation interacting with a massive entropic scalar environment during an inflationary scenario featuring a transient Ultra-Slow-Roll phase. Working within a Gaussian two-field effective Lagrangian, we employ the exact Transport Equations Method to track the full non-unitary, non-Markovian evolution of the system's covariance matrix across the SR-USR-SR transition. We find that the efficiency of decoherence is sensitive to the background kinematics at horizon crossing. Most importantly, the interaction with the environment leaves distinct observable imprints on the primordial scalar power spectrum: the characteristic interference dip preceding the USR-driven enhancement can be partially or completely erased, the growth slope modified, and oscillatory features induced near the peak. Propagated to second order, these distortions further imprint on the stochastic background of Scalar-Induced Gravitational Waves, breaking single-field predictions and yielding unique spectral signatures potentially accessible to LISA. Our results demonstrate that the quantum environment is not a passive spectator during inflation, but an active agent whose imprint on the primordial universe may be within reach of the next generation of cosmological observations.