Starobinsky Inflation in the Swampland

Understand

We argue that the Starobinsky model of inflation, realised via an $R^2$ term in the Lagrangian, can originate from quantum effects due to a tower of light species.

• By means of two separate arguments, we show how this implies that the scale of the $R^2$ term must be of order of the species scale $\Lambda_s$, namely the energy at which gravity becomes strongly coupled.
• We discuss the implications and challenges of this scenario for inflation, inflationary reheating, and string theory embeddings.
• In this context, we collect strong evidence to conclude that Starobinsky inflation lies in the Swampland.