Adam tells us about what he and collaborators considered to be the leading candidate for a systematic error in the SHOES measurement of the expansion rate of the Universe. This is “Cepheid crowding”, the possibility that background sources change our interpretation of Cepheid brightness, ruining one step in the SHOES distance ladder.

They devise a nice way to test whether the crowding is correctly accounted for and find that it is. So crowding cannot be the “explanation” of an error in the distance ladder measurement of H0.

He also stresses that both the early and late universe measurements of H0 are now backed up by multiple different measurements. Therefore, if the resolution isn’t fundamental physics, then no single systematic can entirely solve the tension.

We also discuss a few topics around the H0 tension, including what resolution of the tension he would pick as most likely if forced to gamble (answer: a deviation from vanilla ΛCDM in the early universe).

Paper: https://arxiv.org/abs/2005.02445
Adam: https://www.stsci.edu/~ariess/