  The infrared effects for light minimally coupled scalar fields with quartic
self-interaction in de Sitter space is investigated using the 2PI effective
action formalism. This formalism partially resums infinite series of loop
diagrams, and enables us to circumvent the IR divergence problem for a massless
minimally coupled scalar field in de Sitter space. It is anticipated that
nonperturbative infrared effects generate a curvature-induced mass and
self-regulate the IR divergence. However, due to its nonperturbative nature,
the renormalization prescription is a nontrivial task. To calculate physical
quantities, an appropriate renormalization prescription is required. In this
paper, we will show that the MS-like scheme is possible at the Hartree-Fock
truncation of the 2PI effective action, and infinite series of divergent terms
are needed as counterterms. The phase structure and the quantum backreaction to
Einstein's field equation are calculated.

High Energy Physics - Theory;General Relativity and Quantum Cosmology;High Energy Physics - Phenomenology