Higher-order gravity and the cosmological background of gravitational waves

The cosmological background of gravitational waves can be tuned by the higher-order corrections to the gravitational Lagrangian. In particular, it can be shown that assuming R1+ϵ, where ϵ indicates a generic (eventually small) correction to the Hilbert-Einstein action in the Ricci scalar R, gives a parametric approach to control the evolution and the production mechanism of gravitational waves in the early Universe.