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

TL;DR

This paper demonstrates that string-inspired swampland conjectures are incompatible with current observational data on single-field inflation, challenging their validity in cosmological models.

Contribution

It provides a detailed analysis showing the inconsistency between swampland conjectures and observational constraints, especially for concave inflationary potentials.

Findings

Swampland conjectures imply steep potentials incompatible with data.

Observational constraints favor shallow potentials with specific parameters.

Non-canonical models like DBI inflation do not resolve the tension.

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 $\Delta \phi \lesssim M_{\rm Pl}$, in combination with a sufficiently steep potential, $M_{\rm Pl} V_\phi/V \gtrsim {\cal O}(1)$. 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 $M_{\rm Pl} V_\phi/V \lesssim 0.1$ and $-0.02 \lesssim M_{\rm Pl}^2 V_{\phi\phi}/V < 0$, in strong tension with swampland conjectures. Extension to non-canonical models such as DBI Inflation does not significantly weaken the bound.