Simon tells us about the strong lensing time delay measurements of the Hubble constant performed by TDCOSMO.

In the recent paper he has relaxed assumptions about the density profiles around the lenses. Specifically, in this analysis it is no longer assumed that the density follows a power law, or NFW profile plus stars (with an assumption that the mass follows the light). This naturally widens the error bars on the measurement because the “mass sheet degeneracy” is no longer pinned down.

In order to pin this degeneracy back down they use kinematic data from the lenses to model the density profile. They also calibrate their model on an additional external data set of strong lenses. When using just TDCOSMO lenses the central value stays the same, but when adding the new “SLACS” lenses, the measured value of H0 drops to be almost exactly the same as the Planck value. Cat, meet pigeons.

Simon also goes into what the future will look like and what data is needed to bring the accuracy of TDCOSMO back to what it was before these assumptions were relaxed.

Simon: https://sibirrer.github.io
The paper: https://arxiv.org/abs/2007.02941
The analysis pipeline: https://github.com/TDCOSMO/hierarchy_analysis_2020_public