New interpretation of the Minkowski limit of $R^2$ gravity

Abstract

It is well-established that the Minkowski limit of pure $f(R)=R^2$ gravity breaks down, unlike that of full Starobinsky theory $f(R)=R+αR^2$. We provide a novel interpretation of this phenomenon using the recent thermal analogy between scalar-tensor gravity and Eckart's relativistic dissipative fluids. In this framework, we show that approaching the Minkowski background corresponds to a diverging effective ``gravitational temperature''. This perspective naturally rephrases the strong coupling problem as a thermal singularity, demonstrating that $R^2$ gravity departs infinitely far from General Relativity rather than recovering it.