The Lyman-alpha forest is the universe's barcode: thousands of absorption lines stamped on quasar spectra by wisps of neutral hydrogen along the sightline, tracing the matter distribution at redshifts near 3 on scales smaller than any other cosmological probe reaches. Its one-dimensional flux power spectrum, measured by eBOSS and now DESI, reads the small-scale matter power spectrum at wavenumbers around 1 h/Mpc (Palanque-Delabrouille et al. 2020).
The barcode reads low. A persistent 3 to 5 sigma tension separates the forest measurements from the Planck ΛCDM extrapolation: the high-redshift small scales carry less fluctuation power than the CMB-calibrated model predicts, a deficit in amplitude or effective tilt that has survived survey generations and independent analysis pipelines (Rogers et al. 2024). The standard repair options each carry a price tag: neutrino masses large enough to suppress the power conflict with other cosmological mass bounds; warm dark matter erases the small-scale structure that dwarf-galaxy counts require; and a running spectral index strains the simple inflationary models that motivated the power-law in the first place. The deficit sits in the same direction as the S8 tension, less structure than the CMB extrapolation predicts, but at smaller scales and higher redshift.
The standing sharpens with DESI's growing quasar sample, which will fix the forest power spectrum at percent precision and force the repair options into direct confrontation with their respective constraints.