No absorption in de Sitter space

We study the wave equation for a minimally coupled massive scalar in D-dimensional de Sitter space. We compute the absorption cross section to investigate the property of the cosmological horizon in the southern diamond of de Sitter space. It is found that there is no absorption of radiation by the cosmological horizon of de Sitter space at the classical level. This means that the de Sitter space is usually in thermal equilibrium with the external scalar perturbation. It confirms that the cosmological horizon not only absorbs radiation of the scalar but also emits that previously absorbed by itself at the same rate, keeping the curvature radius of de Sitter space fixed. Finally, we discuss the dS/CFT correspondence in the context of the wave equation approach.