The Zoo Plot Meets the Swampland: Mutual (In)Consistency of Single-Field Inflation, String Conjectures, and Cosmological Data

William H. Kinney, Sunny Vagnozzi, Luca Visinelli

Published 2018-08-20Version 1

We consider single-field inflation in light of string-motivated "swampland" conjectures suggesting that effective scalar field theories with a consistent UV completion must have field excursion $\Delta \phi \lesssim M_{\rm Pl}$, in combination with a sufficiently steep potential, $V_\phi/V \gtrsim M_{\rm Pl}$. Here, we show that the swampland conjectures are inconsistent with existing observational constraints on single-field inflation. We map the allowed swampland region onto the $n_S$-$r$ "zoo plot" of inflationary models, and find that consistency with the Planck satellite and BICEP2/Keck Array requires $V_\phi/V \lesssim 0.1 M_{\rm Pl}$, in strong tension with swampland conjectures. Extension to non-canonical models such as DBI Inflation does not significantly weaken the bound.