Johannes R. Eskilt

Here is my prediction for 2026:
Another CMB experiment will find isotropic cosmic birefringence at 3+sigma, and the world will start taking cosmic birefringence very seriously

2) You can't blame dust! LFI and WMAP has negligible amount of dust. And ACT measurements do not use the foreground to calibrate. Hence, dust EB will again have a small impact on the measured birefringence angle.

1) It is not just Planck HFI that favors a non-zero birefringence angle. Planck LFI, WMAP and ACT also increase the significance (all towards a positive value).

I've spent three days now making a plot that would summarize the past 10 years of cosmic birefringence research

Different datasets with widely different systematics (astrophysical + instrumental) favor a positive and non-zero cosmic birefringence angle! Excited to fill in more colors in the future!

YouTube video by APCTP [APCTP Colloquium] Parity Violation in Cosmology | Prof. Eiichiro Komatsu

He already gave a talk on the results! youtu.be/vvxSsnZivTQ?...

Currently at 4.6sigma, so we are not far from crossing it!

bsky.app/profile/joha...

Combining these results, we get 4.6 sigma. Getting close to 5 sigma!
Graph credit: E. Komatsu

In 2022, we were very excited to see 3.6sigma evidence of Cosmic Birefringence in Planck+WMAP. We've been waiting for confirmation from other experiments since, and now we are slowly getting it!

New paper is finding 2.9sigma evidence in ACT DR6. This is getting serious!
arxiv.org/abs/2509.13654

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