We study the instability of de Sitter space-time (dS) under thermal radiation in different vacua. For this purpose we model the interaction between thermal radiation and unknown ultraviolet physics as a scattering process inside the horizon. Then we argue that the mode function solution of a scalar field in four-dimensional dS can be separated into the incoming and outgoing modes. Different vacua for dS are realized by different combinations of positive frequency modes assigned to each solution. For a minimally coupled massless scalar field, we explicitly compute the behavior of the mode function and the corresponding energy-momentum tensor in the Unruh vacuum near the horizon, and find that the horizon area increases (decreases) in time when the incoming (outgoing) mode contributes to thermal flux.
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