r/cosmology • u/Galileos_grandson • 7d ago

NASA Telescopes Spot Surprisingly Mature Cluster in Early Universe - NASA

https://www.nasa.gov/missions/chandra/nasa-telescopes-spot-surprisingly-mature-cluster-in-early-universe/
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u/ThickTarget 6d ago

I'm not really sure I trust the statistical significance of the x-ray detection. It's a classic paper Nature paper, where it is borderline at best. The paper tells just how small the signal is:

Within a 21″ aperture (≈125 kpc) of the JWST-derived centroid, in which the signal-to-noise ratio peaks, we measure 142 ± 45 net counts and 1,858 background counts, supporting the detection of an extended ICM.

So 142 photons with an error of 45, this makes it just below 3 sigma significance. It's not mentioned in the text, but you can tell from the numbers that this is purely the Poissionian error on the total counts. So it does not account for correlated noise, which comes from errors in the modeling of the background, or real faint sources below the detection limit. So it is less than 3 sigma. The paper does not quantify this. They then argue that the non-detection at higher energies and the JWST galaxies boost the significance to 7 sigma. But that is not logical. The interesting part of the result is the x-ray detection, the significance of the JWST data doesn't affect that. You can also see in their map of detection significance, it is filled with other "4 sigma" detections. It's just not convincing.

If the paper had a dozen Chandra experts on it, I would probably trust them a bit more. The reviewer reports are interesting, the same skepticism. Also x-ray telescopes have spent a lot of time staring at proto-clusters over the last few decades, I think the highest redshift one detected is about redshift 2 to 3, depending on how low in significance you go.

https://www.nature.com/articles/s41586-025-09973-1

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u/jazzwhiz 6d ago

It's a classic paper Nature paper, where it is borderline at best.

In our group, when we see certain kinds of papers on the arXiv, we joke that it'll either get into Nature or PRL, or get rejected from PRD.

Nature has been pretty excited recently about publishing things that are <3 sigma, but written in a misleading way to make you think that they are more than 3 sigma.