Markovian dynamics in de Sitter

The equilibrium state of fields in the causal wedge of a dS observer is thermal, though realistic observers have only partial access to the state. To them, out-of-equilibrium states of a light scalar field appear to thermalize in a Markovian fashion. We show this by formulating a systematic expansion for tracing out the environment. As an example, we calculate the O(λ) correction to the result of Starobinsky and Yokoyama for the relaxation exponents of λϕ⁴ theory. (paper)