Hubble sinks in the low-redshift swampland

Abstract

Local determinations of the Hubble constant 𝐻0 favor a higher value than Planck based on cosmic microwave background and Λ cold dark matter (Λ⁢CDM). Through a model-independent expansion, we show that low redshift (𝑧≲0.7) data comprising baryon acoustic oscillations, cosmic chronometers, and Type Ia supernovae have a preference for quintessence models that lower 𝐻0 relative to Λ⁢CDM. In addition, we confirm that an exponential coupling to dark matter cannot alter this conclusion in the same redshift range. Our results leave open the possibility that a coupling in the matter-dominated epoch, potentially even in the dark ages, may yet save 𝐻0 from sinking in the string theory swampland.