iiinova — where your digital self keeps evolving

Reposted by Johannes R. Eskilt

Ethan Siegel @startswithabang.bsky.social · 18/08/2026

Cosmic birefringence: the signal that could upend cosmology The biggest observational anomaly in cosmology is the Hubble tension. But second-biggest is much less well-known: the cosmic birefringence signal in the CMB. (And yes, YOU can understand it!) bigthink.com/starts-with-... #space #astro

bigthink.com

Cosmic birefringence: the signal that could upend cosmology

We assume there's no preferred frame of reference in the Universe, and that light behaves the same in all directions. What if we're wrong?

0214

Johannes R. Eskilt @johanres.bsky.social · 10/08/2026

I am excited to see what the BICEP3 and Simons Observatory find in the near future! We need more independent measurements with good control over instrumental systematics. 4/4

092

Johannes R. Eskilt @johanres.bsky.social · 10/08/2026

Systematics: ACT found an unexplained discrepancy in the polarization angle within same polarization array, and we still do not have a full understanding of dust EB. These two can bias our cosmic birefringence measurement if we model them wrong. 3/n

193

Johannes R. Eskilt @johanres.bsky.social · 10/08/2026

We show that dust EB cannot explain the signal in the joint analysis, and that retrieving β=0 would require both the dust EB modeling and instrumental priors to fail simultaneously. However, systematics need to be better understood before drawing any conclusions. 2/n

182

Johannes R. Eskilt @johanres.bsky.social · 10/08/2026

First joint analysis of cosmic birefringence from ACT and Planck giving a 4.8 sigma measurement of birefringence using reported systematic priors! arxiv.org/abs/2608.06480 We find a positive birefringence angle in all combinations of the two datasets - Positive EB everywhere! 1/n

1172

Johannes R. Eskilt @johanres.bsky.social · 31/12/2025

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

030

Johannes R. Eskilt @johanres.bsky.social · 17/10/2025

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.

000

Johannes R. Eskilt @johanres.bsky.social · 17/10/2025

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).

110

Johannes R. Eskilt @johanres.bsky.social · 17/10/2025

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!

120

Johannes R. Eskilt @johanres.bsky.social · 20/09/2025

Currently at 4.6sigma, so we are not far from crossing it! bsky.app/profile/joha...

210

Johannes R. Eskilt @johanres.bsky.social · 20/09/2025

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

060

Johannes R. Eskilt @johanres.bsky.social · 20/09/2025

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

76812

Johannes R. Eskilt @johanres.bsky.social · 19/03/2025

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

000

Johannes R. Eskilt @johanres.bsky.social · 18/03/2025

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 😅

110

Johannes R. Eskilt @johanres.bsky.social · 18/03/2025

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

act.princeton.edu

130

Johannes R. Eskilt @johanres.bsky.social · 18/03/2025

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

161