Soft de Sitter Effective Theory

Understanding the quantum evolution of a de Sitter universe on superhorizon scales is notoriously difficult. To address this challenge, we introduce the Soft de Sitter Effective Theory (SdSET), which holds for superhorizon modes whose (comoving) momentum is far below the UV cutoff, the (comoving) horizon. SdSET makes power counting manifest while preserving the low energy symmetries, simplifying many aspects of perturbation theory outside the horizon. As an example, we will give a simple derivation of the all orders conservation of the adiabatic modes and gravitational waves.