The candidate map: WIMPs, warm, axion, fuzzy

‘Dark matter’ names a gravitational phenomenon, not yet a particle. The viable candidates span some ninety orders of magnitude in mass — and where a candidate sits on that axis decides whether it behaves like a particle or like a wave.

One name, many candidates

All the evidence of Topic 2.1 constrains dark matter's gravity, not its identity. That leaves a vast candidate landscape (figure), from ultralight bosons at $10^{-22}$ eV to WIMPs near $100$ GeV — and beyond, to macroscopic primordial black holes. Each corner makes different small-scale predictions.

The mass landscape

WIMPs ($\sim100$ GeV) — the long-favoured thermal relic; heavy, cold, clumps on all scales.
Sterile neutrinos / warm dark matter ($\sim$keV) — fast enough to free-stream out of small halos, giving a cutoff.
QCD axion ($\sim\mu$eV) — a well-motivated light boson, but still effectively cold cosmologically.
Fuzzy / ultralight axion ($\sim10^{-22}$ eV) — so light its wave nature is macroscopic. This program's subject.

The dark-matter candidate landscape by particle mass (log$_{10}$ eV): from wave-like ultralight/fuzzy at $10^{-22}$ eV, through the QCD axion and warm/sterile neutrinos, to particle-like WIMPs near $100$ GeV.

Particle-like vs wave-like

The dividing line is the de Broglie wavelength. When $\lambda_{\rm dB}=\hbar/(mv)$ is microscopic, dark matter is a gas of particles; when it is astrophysical, dark matter is a coherent wave and must be treated with the Schrödinger–Poisson equations (Topic 4).

Worked example — why $10^{-22}$ eV is "fuzzy"

Take $m=10^{-22}\,{\rm eV}/c^2=1.8\times10^{-58}$ kg moving at a dwarf-galaxy speed $v\sim10\ \mathrm{km\,s^{-1}}$:

$$\lambda_{\rm dB}=\frac{\hbar}{mv}=\frac{1.05\times10^{-34}}{(1.8\times10^{-58})(10^{4})}\approx6\times10^{19}\ {\rm m}\approx2\ {\rm kpc}.$$

The wavelength is kiloparsecs — the size of a galaxy's core. That is exactly why fuzzy dark matter reshapes galaxies while leaving large scales untouched.

Why fuzzy is testable — and where it sits with warm

Warm dark matter also cuts off small-scale power (by free-streaming), so FDM and WDM share a family resemblance in the mass function; they differ in the halo interior, where only FDM builds a solitonic core. The macroscopic de Broglie scale makes fuzzy dark matter uniquely testable on galactic scales — the regime our three codes target.

In our research

Our entire program tests the wave-like corner at $m\sim10^{-22}$ eV. The $\sim$kpc de Broglie scale computed above is what puts the FDM cutoff at $M_{1/2}\sim10^{10}\,M_\odot$ and sizes the solitonic cores we resolve — the two observables Tasks 1 and 2 measure.

Key references

Hui, Ostriker, Tremaine & Witten (2017), Ultralight scalars as cosmological dark matter, Phys. Rev. D 95, 043541 (arXiv:1610.08297).
Marsh (2016), Axion Cosmology, Phys. Rep. 643, 1 (arXiv:1510.07633).
Ferreira (2021), Ultra-light dark matter, A&A Rev. 29, 7 (arXiv:2005.03254).

PreviousCold dark matter and its small-scale problems NextThe Schive soliton profile and its derivation