Friction in inflaton equations of motion

The possibility of a friction term η ̇ \.φ in the equation of motion for a scalar field is investigated in nonequilibrium field theory. The central result is that this equation of motion does not possess a time-derivative expansion, and so the friction coefficient η is not well defined. It is nevertheless possible to obtain an effective friction coefficient numerically. In λΦ⁴ theory, this differs greatly from existing estimates based on linear response theory, and is actually negative for some values of temperature and coupling constant, indicating that the breakdown of the time-derivative expansion may be quantitatively rather significant.