Beyond (and back to) Palatini quadratic gravity and inflation

Understand

We study single-field slow-roll inflation embedded in Palatini $F(R)$ gravity where $F(R)$ grows faster than $R^2$.

• Surprisingly, the consistency of the theory requires the Jordan frame inflaton potential to be unbounded from below.
• Even more surprisingly, this corresponds to an Einstein frame inflaton potential bounded from below and positive definite.
• We prove that for all such Palatini $F(R)$'s, there exists a universal strong coupling limit corresponding to a quadratic $F(R)$ with the wrong sign for the linear term and a cosmological constant in the Jordan frame.