4.9σ Evidence for Axions & Warm Dark Matter - eBOSS Lyα vs Planck CMB - Keir Rogers & Vivian Poulin

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Keir and Vivian tell us about their recent work looking at the Lyman-α power spectrum from eBOSS quasars. Specifically, they look at how consistent this power spectrum is with the power spectrum one would expect from the relevant scales given Planck CMB observations within the ΛCDM model and find a whopping 4.9σ tension.

There have been hints of a tension between Lyman-α and CMB for a while, but not with this magnitude. Keir and Vivian focus their analysis on the scales and observables that the eBOSS Lyman-α are most sensitive to and see that the slope of the power spectrum of this range of scales is very different to what is seen on CMB scales. Within ΛCDM the slope of the power spectrum should be constant, hence the whopping tension.

Most excitingly, they also find three simple models that more or less completely solve the tension, without harming any other observations! These are a one-parameter power-law running of the "spectral index" (i.e. the slope of the power spectrum), and a small fraction of axion "ultralight" dark matter, or warm dark matter. For the axion and warm dark matter models there are two free parameters (the mass and the fraction of the total dark matter). The running and axion models in particular completely remove the tension, and the warm dark matter reduces the "tension" to 1.5σ.

Very exciting!

Keir: https://keirkwame.github.io/
Vivian: https://inspirehep.net/authors/1341720
Paper: https://arxiv.org/abs/2311.16377