On the Emergence of Einstein's Gravity from f(R) Gravity through Cosmological Evolution

$f(R)$-Gravity, a simple generalization of Einstein's General theory of Relativity has been considered in the context of Cosmology as one of the approaches to explain phenomena such as early-time inflation and late-time accelerated expansion of the Universe purely from the Gravity sector. In this work, we have considered a class of $f(R)$-Gravity theories with $f(R)=R+\alpha R^n$ and it's dual scalar tensor theory in the Einstein frame. We have shown that in an isotropic and homogeneous background, for both positive and negative integral values of $n$, the extra scalar degree of freedom of the $f(R)$-theory (manifested as the scalar field in the Einstein frame action) dynamically freezes out due to cosmological evolution, resulting in the survival of only the Einstein-Hilbert term and a cosmological constant at most. This implies that all gravity models given as $R + \alpha R^n$ inevitably evolve into pure Einstein gravity with a cosmological constant term through cosmological evolution.