Exploring Intermediate and Natural Inflation under a Nonminimal Derivative Coupling: The John-George Framework

Intermediate and natural inflation are investigated within a nonminimal derivative coupling scalar field model, namely the John–George framework. The derivative coupling introduces enhanced gravitational friction, modifying inflaton dynamics and enabling sustained accelerated expansion even for sub-Planckian parameter values. The field equations and slowroll parameters are derived in a flat Friedmann–Lemaître–Robertson–Walker spacetime. For intermediate inflation, the effective potential is reconstructed assuming a power-law scalar field evolution, while a periodic potential with a decay constant, f describes the natural inflation. Numerical analysis of inflationary observables yields ns = 0.96397 and r = 0.0019 for intermediate inflation, and ns = 0.961176 and r = 0.00651 for natural inflation at N = 60efolds. These predictions are in excellent agreement with Planck and BICEP/Keck constraints. The enhanced friction mechanism slows the inflaton evolution and suppresses the tensor-to-scalar ratio, bringing the model well within observational bounds. Parameter analysis indicates that m ~ 0.5–1 and f ~ 0.5 provide optimal agreement with data. The John–George framework, therefore, constitutes a viable and competitive alternative to standard inflationary scenarios.