Johannes R. Eskilt

ACT's 2.5sigma cosmic birefringence measurement should not be viewed in isolation. Combining their measurement with previous WMAP+Planck result increases the significance to 4+ sigma (assuming Gaussian dist+independent measurements).

Graph credit: E. Komatsu

With Planck+WMAP, we reached 3.6 sigma. If another CMB experiment with different systematics and people doing the analysis reaches the same significance, I won't just be excited, I'll actually believe cosmic birefringence is real. But I might just be young and naive 😅

I would expect you to be more excited about their 2.5 sigma birefringence results! It's a bit hidden, but they find a birefringence angle of 0.20 \pm 0.08 deg (Eq 14 in act.princeton.edu/sites/g/file...). Very consistent with our Planck+WMAP results! arxiv.org/abs/2205.13962

Although #ACT DR6s headline is that LCDM is still king, they do find a 2.5 sigma of an unexplained polarization angle (aka cosmic birefringence)! I am amazed that people are not more excited by this! Their beta = 0.20 \pm 0.08 is very consistent with our Planck+WMAP results! arxiv.org/abs/2205.13962