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

Jurek tells us about the prospects for constraining axion (aka ultralight aka fuzzy) dark matter with future 21cm intensity mapping survey such as SKA and HIRAX.

Axion models arising from specific energy scales predict that an axion with a given mass will only provide a certain fraction of the total dark matter. It seems plausible that with SKA we will be able to detect ultralight dark matter even if it arises from a GUT scale axion model. An observational noise model for SKA was included to make this claim, but as of yet no theoretical uncertainty is included in the calculation.

Paper: https://arxiv.org/abs/2003.09655

Omar tells us about the excellent quality lensing map he’s produced with the Atacama Cosmology Telescope collaboration. You honestly won’t believe how well this lensing map correlates with the cosmic infrared background (sorry about the clickbait). The map will be incredibly useful to cross-correlate with any dataset of tracers inside the relatively large window of ACT observations.

Omar also explains how he and the collaboration, for the first time, removed the annoying thermal Sunyaev–Zeldovich contamination that ordinarily produces a ~10% bias in sigma8 (and thus any other cosmological parameter correlated with sigma8).

Paper: https://arxiv.org/abs/2004.01139
Omar: http://www.damtp.cam.ac.uk/person/od261
Map page on NASA LAMBDA: https://lambda.gsfc.nasa.gov/product/act/act_dr4_derived_maps_get.cfm

Deanna tells us about what we could learn from future measurements of the spectral distortions in the CMB, as well as how spectral distortions complement current and future measurements of CMB anisotropies. She also discusses CLASS (v3.0), the code you can use to calculate predictions for both.

There is a guaranteed spectral distortion signal to detect within ΛCDM and the possibility to constrain many possible deviations, including primordial black holes and decaying dark matter. In fact, we can detect the signal even if the PBHs and/or decaying dark matter only make up one part in a million of the total dark matter!

Paper: https://arxiv.org/abs/1910.04619​
Deanna: https://www2.ulb.ac.be/sciences/physth/people_DCHooper.html
Twitter: https://twitter.com/DCHooper91​
CLASS: https://lesgourg.github.io/class_public/class.html