Lyman-α forest bounds on warm and fuzzy dark-matter masses with DESI-era P1D and IGM marginalisation
A bound exists and the route to a robust one is simulation and data, with no new theory, so it sits in the 30s below the SIDM lensing test (46). Leverage 40: the bound cuts warm sterile-neutrino and ultralight-axion model space and is an input to every small-scale-structure argument. Application value 5.
The problem
Use distant quasar light passing through hydrogen gas to rule out dark matter that is too light or too fast.
- 1Quasar light crosses hydrogen clouds
- 2Forest of lines maps small-scale clumps
- 3Warm or fuzzy dark matter smooths clumps
- 4Hot gas smooths them too
- 5Fit both to bound the dark-matter mass
The question. For thermal-relic warm dark matter (WDM), find the smallest allowed particle mass m_WDM (currently a few keV). For ultralight scalar ("fuzzy") dark matter, find the smallest allowed boson mass m_a (currently of order 10⁻²⁰ eV). Both suppress the matter power spectrum on small scales: WDM by free streaming (particles move fast enough to escape small clumps), fuzzy dark matter by quantum pressure, since the particle's de Broglie wavelength is astronomically large.
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- Lyman-α forest bounds on warm and fuzzy dark-matter masses with DESI-era P1D and IGM marginalisationThis problem
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Path to understanding
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- ΛCDM, the standard model of cosmologyTheory · host
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- Lyman-α forest bounds on warm and fuzzy dark-matter masses with DESI-era P1D and IGM marginalisationThe problem
State of the art
Iršič et al. used XQ-100 and HIRES/MIKE spectra to bound WDM (arXiv:1702.01764; 4.1 keV from HIRES/MIKE alone, stronger combined) and fuzzy dark matter (arXiv:1703.04683; m_a > 1.4×10⁻²¹ eV from HIRES/MIKE with a conservative IGM thermal history). Rogers & Peiris (arXiv:2007.12705; PRL 126, 071302) found m_a > 2×10⁻²⁰ eV at 95% in an emulator analysis marginalising over the IGM, and Villasenor et al. (arXiv:2209.14220) used 1,080 Cholla hydrodynamic simulations spanning thermal histories to derive updated WDM bounds. The DESI early-data P1D (arXiv:2306.06311) is the first of a new generation, and an April 2026 dark-matter-only preprint (Wang, arXiv:2604.06038) shows that for mixed fuzzy/cold dark matter the flux power depends on the velocity treatment, not only on the matter power spectrum, so particle-only proxies are inadequate. Iršič et al. (arXiv:2309.04533) extended high-redshift data to m_WDM > 5.7 keV (95%), or 4.1 keV with restricted scales, and a June 2026 analysis gives m_FDM > 1.9×10⁻²¹ eV for pure fuzzy dark matter (Liu et al., arXiv:2606.06969). DESI DR1 P1D has been measured, but no DESI-based WDM or FDM bound had been published by October 2026 (a DR1 thermal-relic analysis constrains only N_eff and light relics). An independent cross-check now exists: JWST lensed-quasar flux ratios give m_WDM > 5.6–6.4 keV from 31 lenses (Keeley et al., arXiv:2511.07765) and 6.5–7.4 keV with extended arcs (Gilman et al., arXiv:2511.07513). Published Lyman-α bounds still move by factors of about 1.5–3 with IGM thermal-history assumptions.
Realistic applications
A cutoff in small-scale power would indicate non-CDM physics (warm or wave dark matter), while a firm bound restricts sterile-neutrino and axion-like particle parameter spaces and informs structure-formation simulations. The IGM thermal-history inference is reused in reionisation studies.
What counts as solved
Resolved when two independent pipelines (different simulation suites and emulators) using DESI DR1/DR2 Lyman-α spectra give consistent bounds on m_WDM and m_a, with IGM and systematics marginalised to the same level, and when the bound is cross-checked against an independent probe such as strong-lensing flux ratios (now at m_WDM ≳ 6 keV) or the 21-cm power spectrum.
How it gets verified
ReproductionA training run, experiment or computation rerun by someone else.
Two independent pipelines (different simulation suites and emulators) must run on DESI DR1/DR2 Lyman-alpha spectra and give consistent bounds on m_WDM and m_a, with IGM and systematics marginalised to the same level. The spec fixes the dataset release, the P1D estimator, the emulator validation and the cross-check against an independent probe such as strong-lensing flux ratios.
Claims under verification
+ add a claimNo claimed results on the Verification board for this problem yet.
Where the work is
Simulation emulators and IGM modelling dominate; DESI supplies the data.
Bottlenecks
ranked by votes- #1ComputeIGM thermal-history marginalisation
Hydrodynamic emulators must span temperature, pressure smoothing and reionisation heating; bounds change by factors of 2–3 with assumptions.
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Connected problems
Progress
- Nov 11, 2025Independent lensing boundsource
JWST flux ratios of 31 lensed quasars give m_WDM > 5.6–6.4 keV.
- Sep 8, 2023High-redshift WDM boundsource
Iršič et al. obtain m_WDM > 5.7 keV (95%) from high-redshift Lyman-α data.
- Jul 24, 2020Ultralight axion boundsource
Rogers & Peiris bound ultralight axion dark matter with an emulator marginalising over IGM.
- Feb 6, 2017WDM bound from Lyman-αsource
Iršič et al. bound the WDM free-streaming scale with XQ-100 and HIRES/MIKE.