Schutz arXiv preprint shows literal thermal warm dark matter requires ~10^4 relativistic degrees of freedom or non-standard early-universe history
Schutz's preprint reveals that canonical warm dark matter cannot arise from a simple thermal relic without invoking extreme, unrelated early-universe physics. It urges replacement of WDM proxies with more expressive, simulation-calibrated parameterizations ahead of high-precision small-scale structure data. The work underscores how benchmark models can silently embed unphysical initial conditions that future observations must test directly.
The paper dissects the assumptions baked into WDM transfer functions. Standard calculations assume radiation domination and Standard Model degrees of freedom at decoupling. Schutz shows that reaching the required phase-space density without overclosing the universe forces extreme coincidences unrelated to dark matter itself. These include a vastly enlarged thermal bath or a separate reheating mechanism that coincidentally sets the correct temperature ratio.
Current Lyman-alpha forest and Milky Way satellite counts already push WDM masses near 10 keV. Any literal thermal origin therefore collides with both particle-physics bounds and cosmological expansion history. The analysis highlights that many simulation suites still use WDM as a proxy for self-consistent models whose suppression differs quantitatively and qualitatively from the thermal benchmark.
Schutz advocates replacing the single-parameter WDM mass with simulation-based inference that marginalizes over thermal history. This shift matters because upcoming Euclid, Roman, and DESI small-scale clustering data will otherwise be interpreted through an increasingly unphysical template.
The central limitation is the absence of explicit forecast forecasts for how much parameter space remains after next-generation surveys. Strengthening the claim requires coupling the analytic argument to full hydrodynamical runs that vary both dark-matter temperature and expansion history simultaneously.
Euclid: No WDM-like suppression signature detected in the Lyman-alpha forest power spectrum at k~10 h/Mpc by 2028 would exclude thermal relics above 5 keV at 3 sigma under standard assumptions.
Sources (2)
- [1]Primary Source(https://arxiv.org/abs/2608.24990)
- [2]Supporting Source(https://arxiv.org/abs/1504.03264)