Solar System and Atomic Clock Bounds on Locally Coupled Swampland Scalars

Abstract

We study how local measurements constrain light scalar fields that are relevant in late time cosmic acceleration and are often discussed in connection with swampland criteria. Starting from a scalar-tensor framework, we define the swampland slope and curvature variables in the canonically normalized Einstein frame and relate them to Solar System tracking, Lunar Laser Ranging, equivalence principle tests and atomic clock comparisons. These measurements do not constrain the scalar velocity by itself, but products of that velocity with microscopic couplings to matter, gravity and atomic parameters. For coupling directions visible to local experiments, the scalar is driven into an ultra slow present regime. This severely restricts the possibility of realizing an $\mathcal{O}(1)$ de Sitter gradient through unscreened visible sector couplings alone. The refined de Sitter alternative remains possible, but in the single field hilltop realization it requires proximity to the maximum or a tuned suppression of the growing mode.