Kimmy tells us about a subtle but very interesting tension between Planck lensing data and line of sight Baryon Acoustic Oscillation (BAO) data.

She and her coauthors discovered this via an intriguing mismatch between Planck and South Pole Telescope (SPT) lensing results. The Planck and SPT power spectrum amplitudes matched, but when combined with BAO and Big Bang Nucleosynthesis the inferred Hubble parameters were slightly different.

Like great data-detectives they tracked the source of this discrepancy down to the mismatch between Planck lensing and line of sight BAO. Why the line of sight BAO might be causing this is unclear. On the Planck lensing side, it has something to do with the shape of the lensing power spectrum, e.g. the location of the peak – because SPT only measures the power spectrum’s tail and so is only sensitive to the amplitude.

The result is definitely interesting, and unknown by the community until now (as far as I’m aware). Whether it is a red herring or a vital clue in the hunt to solve the Hubble mystery remains to be seen. But it should provide fuel for both model builders and hunters of systematic errors trying to solve this mystery.

Paper: https://arxiv.org/abs/2004.10207
Kimmy: https://web.stanford.edu/~wlwu/index.html