We present new constraints on the scale dependence of the primordial power spectrum by combining Planck, ACT DR6, SPT-3G and eBOSS Lyman-α forest data, extending sensitivity to smaller comoving scales. While ACT results previously indicated a mild preference for positive running of the spectral index, our joint analysis constrains both the running αs and its running βs. Including eBOSS markedly tightens these constraints, yielding a >2σ indication of nonzero αs and/or βs, challenging predictions from vanilla slow-roll inflation potentials. Comparing reconstructed spectral parameters with theoretical models, we find that inflationary potentials with localised dips, bumps, or oscillations better reproduce the observed scale dependence. We release the public PIPE code to test arbitrary inflationary potentials against these datasets.
@article{arxiv.2511.01612,
title = {Is $\Lambda$CDM on the run? Reconciling the CMB with the Lyman-$\alpha$ Forest},
author = {Malcolm Fairbairn and Lucien Heurtier and María Olalla Olea-Romacho},
journal= {arXiv preprint arXiv:2511.01612},
year = {2025}
}