Paper detail

A local resolution of the Hubble tension: The impact of screened fifth forces on the cosmic distance ladder

The discrepancy between the values of the Hubble constant $H_0$ derived from the local distance ladder and the cosmic microwave background provides a tantalising hint of new physics. We explore a potential resolution involving screened fifth forces in the local Universe, which alter the Cepheid calibration of supernova distances. In particular, if the Cepheids with direct distance measurements from parallax or water masers are screened but a significant fraction of those in other galaxies are not, neglecting the difference between their underlying period--luminosity relations biases the local $H_0$ measurement high. This difference derives from a reduction in the Cepheid pulsation period and possible increase in luminosity under a fifth force. We quantify the internal and environmental gravitational properties of the Riess et al. distance ladder galaxies to assess their degrees of screening under a range of phenomenological models, and propagate this information into the $H_0$ posterior as a function of fifth force strength. We consider well-studied screening models in scalar--tensor gravity theories such as chameleon, K-mouflage and Vainshtein, along with a recently-proposed mechanism based on baryon--dark matter interactions in which screening is governed by local dark matter density. We find that a fifth force strength $\sim5-30\%$ that of gravity can alleviate (though not resolve) the $H_0$ tension in some scenarios, around the sensitivity level at which tests based on other distance ladder data can constrain this strength. Although our analysis is exploratory and based on screening models not necessarily realised in full theories, our results demonstrate that new physics-based local resolutions of the $H_0$ tension are possible, supplementing those already known in the pre-recombination era.