SIDM versus CDM from strong-lensing subhalo mass functions and concentrations at 10⁷–10⁸ M☉
A population-level test exists with Euclid, JWST and LSST lens samples, but the inference needs coupled gravothermal and baryonic modelling that is not validated; it sits at the old small-scale-structure difficulty (45). Leverage 46, just below that entry's 49, because a measured σ/m(v) would be the first non-gravitational information on dark matter and fixes mediator masses. Application value 5 is similar to the old entry (4).
The problem
Use gravitational lenses to test whether dark matter particles can bounce off one another, by weighing tiny invisible clumps.
- 1Dark matter clumps inside galaxy halos
- 2Bouncing particles change clump density
- 3Lenses bend light around clumps
- 4Count and weigh clumps from lens kinks
- 5Constrain how strongly particles bounce
The physics question. In standard cold dark matter (CDM), particles interact only gravitationally. In self-interacting dark matter (SIDM), they also scatter off one another with a cross-section per unit mass σ/m, which can depend on the relative velocity v. Models where σ/m is large in small halos (low velocity) and small in clusters (high velocity) are viable and attractive. The task is to constrain σ/m(v) over 10 to 3000 km/s.
Built from
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- SIDM versus CDM from strong-lensing subhalo mass functions and concentrations at 10⁷–10⁸ M☉This problem
Path to understanding
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- ΛCDM, the standard model of cosmologyTheory · host
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- SIDM versus CDM from strong-lensing subhalo mass functions and concentrations at 10⁷–10⁸ M☉The problem
State of the art
The unexpectedly high concentration of the dark substructure in the lens SDSSJ0946+1006 (Minor et al., arXiv:2011.10627; 2–3.7×10⁹ M☉ within 1 kpc, so above the 10⁷–10⁸ M☉ target scale) is difficult for CDM and has been interpreted as a signature of gravothermal collapse in SIDM. Yang & Yu (arXiv:2102.02375) showed with gravothermal fluid models that SIDM halos in clusters can reproduce compact small-scale lens features. Kaplinghat, Tulin & Yu (arXiv:1508.03339) showed that a velocity-dependent σ/m of about 2 cm²/g in dwarfs falling to about 0.1 cm²/g in clusters fits halo cores from dwarfs to clusters. Enzi et al. (arXiv:2411.08565) find it a >5σ outlier from ΛCDM expectations and suggestive of SIDM core collapse, while He et al. (arXiv:2506.07978) propose a less dense, luminous alternative, so even this single object is disputed. Population-level substructure measurements now exist for the mass function: JWST flux ratios of 31 lensed quasars (Keeley et al., arXiv:2511.07765) and joint arc plus flux-ratio fits of 28 lenses (Gilman et al., arXiv:2511.07513) bound the half-mode mass and warm dark matter, but they have not yet measured the subhalo concentration–mass relation that distinguishes SIDM from CDM. Euclid Q1 (arXiv:2503.15302, March 2025) found hundreds of strong-lens candidates; Euclid DR1 is not expected before mid-2027. No population-level measurement of σ/m(v) from subhalo concentrations exists as of October 2026.
Realistic applications
A measured σ/m(v) would be the first non-gravitational information on dark-sector interactions, constraining mediator masses and couplings. The lens-modelling and halo-evolution tools transfer to other substructure tests.
What counts as solved
Resolved when a lens-population analysis with at least a few hundred systems measures the mass function and concentration of 10⁷–10⁸ M☉ subhalos to precision that excludes either CDM or SIDM at ≥3σ, validated against hydrodynamic and gravothermal simulations with baryons.
How it gets verified
ObservationSettled by new measurements or observations.
Needs a lens-population analysis of a few hundred systems measuring the mass function and concentration of 1e7 to 1e8 solar-mass subhalos well enough to exclude either CDM or SIDM at 3 sigma or more, validated against hydrodynamic and gravothermal simulations with baryons.
Claims under verification
+ add a claimNo claimed results on the Verification board for this problem yet.
Where the work is
Modelling and simulation of halo evolution and lens fitting dominate, with new lens data as the other half.
Bottlenecks
ranked by votes- #1ComputeCalibrating SIDM halo evolution with baryons
Gravothermal collapse timescales depend on halo concentration, tidal stripping and the baryonic potential, and simulations differ by factors of a few.
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Connected problems
Progress
- Nov 10, 2025JWST lens-population substructuresource
Joint arc and flux-ratio modelling of 28 lenses constrains the subhalo mass function.
- Nov 13, 2024SDSSJ0946+1006 as a >5σ outliersource
Enzi et al. confirm the over-concentrated subhalo and discuss SIDM core collapse.
- Nov 20, 2020Dense substructure in SDSSJ0946+1006source
A highly concentrated dark subhalo is reported, difficult in CDM.
- Aug 13, 2015Velocity-dependent SIDM from dwarfs to clusterssource
A σ/m falling with velocity fits halo cores from dwarfs to clusters (PRL 2016).