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Johannes R. Eskilt

@johanres.bsky.social

64 followers 41 following 16 posts

Cosmologist - University of Oslo. Trying my best to figure out if cosmic birefringence is real or not.

Posts Replies Media

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

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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

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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

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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 · 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

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

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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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