The zoo plot meets the swampland: mutual (in)consistency of single-field inflation, string conjectures, and cosmological data

Abstract

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 $\hat{I}”\ddot{I}•\lesssim M_{\mathrm{Pl}}$, in combination with a sufficiently steep potential, $M_{\mathrm{Pl}}{V}_{\ddot{I}}•/V\gtrsim \mathcal{O}(1)$. Here, we show that the swampland conjectures are inconsistent with existing observational constraints on single-field inflation. Focusing on the observationally favoured class of concave potentials, 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 $M_{\mathrm{Pl}}{V}_{\ddot{I}}•/V\lesssim 0.1$ and $-0.02\lesssim M_{\mathrm{Pl}}^2{V}_{\ddot{I}•\ddot{I}•}/V<0$, in strong tension with swampland conjectures. Extension to non-canonical models such as DBI Inflation does not significantly weaken the bound.

5 pages, 1 figure. The busy reader should skip directly to Fig. 1, where the swampland-allowed region is mapped onto the $n_S$-$r$ plane. v2: added discussion on the refined swampland conjecture, basic conclusions unchanged. v3: version accepted for publication in Class. Quant. Grav