Abstract

Contributed Talk - Splinter GalaxyClusters

Tuesday, 08 September 2026, 16:50   (MW-1050)

Can Post-Inflationary Warm Wave Dark Matter Explain the eBOSS Lyman-alpha Power Spectrum Tension?

Mihir Kulkarni, Jens Niemeyer, Mustafa Amin
Georg-August Universität Göttingen, Rice University

Recent analyses of eBOSS Lyman-alpha forest data by Rogers & Poulin (2025) reveal a ~5σ tension with Planck CMB+BAO+SNe in the inference of the linear matter power spectrum amplitude and tilt at k ~ 1 h/Mpc. Among possible resolutions, a small fraction (~1-5%) of ultralight axion dark matter with mass ~10⁻²⁵ eV provides an excellent fit. Here we investigate whether post-inflationary ultralight axions, so-called "warm wave" dark matter, could explain this suppression without invoking a mixed dark matter scenario. When ultralight axions are produced after inflation via a causal mechanism, the initial field configuration is dominated by sub-horizon modes peaked at a characteristic wavenumber k*. This leads to two competing effects on the matter power spectrum: a free-streaming suppression of the adiabatic perturbations on scales smaller than the free-streaming length, and a scale-dependent gravitational growth of an initial white-noise isocurvature enhancement sourced by the finite coherence scale of the field. Together, these features imprint a characteristic suppression in the power spectrum amplitude and tilt at observable scales. Drawing on the analytic framework of Amin, May & Mirbabayi (2025), we map out the parameter space in (m_ax, k*) that reproduces the suppression preferred by eBOSS, and discuss consistency with high-resolution Lyman-alpha forest data and early galaxy abundance constraints from JWST.