Johannes Røsok Eskilt and Patricia Diego-Palazuelo update us on the state of measurements of birefringence in the cosmic microwave background in 2022. The current most popular video on the channel was about a tentative hint of CMB birefringence back in 2020 and much can change in two years.

The tentative hint is very much still present. Johannes and Patricia are both members of Planck itself and have spent the last few years checking for systematics, taking the effects of EB correlations in the dust into account, adding WMAP data, adding additional frequency maps from Planck, and checking for a frequency dependence of the signal.

So far there are no smoking guns saying “oops, this is what is causing the signal”, so new physics is very much still on the table.

With the additional Planck maps and WMAP data included, as well as tighter modelling of the dust, the statistical significance has drifted up from 2.4σ two years ago to 3.6σ today, which is exactly what one would expect if this was a real signal.

The crucial test of this would be a ground based experiment like ACT, SPT or BICEP calibrating their misalignment angle directly with greater accuracy than Planck/WMAP can indirectly and looking directly for an EB signal in the CMB… if they still see something that would be extremely interesting!

Johannes: https://www.mn.uio.no/astro/english/people/aca/johanres/
Patricia: https://inspirehep.net/authors/1954027

Paper 1: https://arxiv.org/abs/2201.07682
Paper 2: https://arxiv.org/abs/2205.13962