Amplitude constraints on dark energy

This talk gives a short introduction to the ``UV/EFT correspondence", which uses scattering amplitudes to relate the Effective Field Theory (EFT) coefficients probed by low-energy measurements to properties of the underlying high-energy (UV) completion. This includes recent ``positivity bounds" on EFT coefficients, which are the low-energy signatures of causality and unitarity in the UV. To illustrate their phenomenological impact, I apply these bounds to a simple EFT for dark energy and compare with recent cosmological observations.