  We consider the nonminimally coupled lambda phi^4 scalar field theory in de
Sitter space and construct the renormalization group improved renormalized
effective theory at the one-loop level. Based on the corresponding quantum
Friedmann equation and the scalar field equation of motion, we calculate the
quantum radiative corrections to the scalar spectral index n_s, gravitational
wave spectral index n_g and the ratio r of tensor to scalar perturbations. When
compared with the standard (tree-level) values, we find that the quantum
contributions are suppressed by lambda N^2 where N denotes the number of
e-foldings. Hence there is an N^2 enhancement with respect to the naive
expectation, which is due to the infrared enhancement of scalar vacuum
fluctuations characterising de Sitter space. Since observations constrain
lambda to be very small lambda ~ 10^(-12) and N ~ 50-60, the quantum
corrections in this inflationary model are unobservably small.
