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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?

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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

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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

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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!

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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

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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

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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

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