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 $R^{1+\epsilon}$, where $\epsilon$ 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.