Constraints on Axion Inflation from the Weak Gravity Conjecture
  We derive constraints facing models of axion inflation based on decay
constant alignment from a string-theoretic and quantum gravitational
perspective. In particular, we investigate the prospects for alignment and
`anti-alignment' of $C_4$ axion decay constants in type IIB string theory,
deriving a strict no-go result in the latter case. We discuss the relationship
of axion decay constants to the weak gravity conjecture and demonstrate
agreement between our string-theoretic constraints and those coming from the
`generalized' weak gravity conjecture. Finally, we consider a particular model
of decay constant alignment in which the potential of $C_4$ axions in type IIB
compactifications on a Calabi-Yau three-fold is dominated by contributions from
$D7$-branes, pointing out that this model evades some of the challenges derived
earlier in our paper but is highly constrained by other geometric
considerations.
