Warm dark matter

Warm dark matter (WDM) is a hypothesized class of dark‑matter particles that possess non‑relativistic velocities at the time of structure formation, but are significantly faster than the particles constituting cold dark matter (CDM). In cosmological models, WDM is invoked to address certain small‑scale discrepancies between CDM predictions and observed galaxy‑scale structures while preserving the successful large‑scale predictions of CDM.

Definition and properties

• Velocity dispersion: At the epoch of matter‑radiation equality, WDM particles have thermal velocities that suppress the growth of density fluctuations below a characteristic free‑streaming length, typically of order a few hundred kiloparsecs.
• Mass range: The free‑streaming scale implies particle masses in the keV range (≈1–10 keV) for thermal relics; non‑thermal production mechanisms can result in different effective masses but remain in a comparable regime.
• Interaction: Like CDM, WDM interacts gravitationally but is otherwise non‑baryonic and does not emit, absorb, or reflect electromagnetic radiation.

Theoretical candidates

• Sterile neutrinos: Right‑handed neutrinos that do not participate in standard weak interactions, possibly produced via oscillations (Dodelson‑Widrow mechanism) or resonant production (Shi‑Fuller mechanism).
• Thermal relics: Hypothetical particles that decoupled while relativistic, then cooled to semi‑non‑relativistic speeds.
• Other models: Light gravitinos, axinos, or hidden‑sector particles with appropriate production histories.

Cosmological implications

• Structure formation: The suppression of power on small scales leads to reduced numbers of low‑mass dark‑matter halos, potentially alleviating the “missing satellites” and “cusp‑core” problems observed in dwarf galaxies.
• Lyman‑α forest: Absorption features in the spectra of distant quasars constrain the matter power spectrum at sub‑megaparsec scales; current analyses place lower limits on the WDM particle mass (e.g., m > 3–5 keV for thermal relics).
• Reionization: Delayed formation of the smallest halos can affect the timing and duration of cosmic reionization, providing additional observational tests.

Observational constraints

• X‑ray searches: Decays of sterile‑neutrino WDM could produce mono‑energetic X‑ray lines; tentative detections (e.g., a ∼3.5 keV line) have been reported but remain under debate.
• Galaxy counts: Counts of satellite galaxies in the Local Group and faint dwarf galaxies in deep surveys are consistent with CDM; any deviation attributable to WDM must be statistically significant.
• Cosmic microwave background (CMB): Primary CMB anisotropies are largely insensitive to WDM; however, secondary effects (e.g., lensing) can provide indirect limits.

Current status
Warm dark matter remains a viable, though constrained, alternative to cold dark matter. The most stringent limits from Lyman‑α forest measurements and X‑ray observations disfavour thermal relic masses below ≈4 keV, narrowing the parameter space for WDM models. Ongoing and future surveys—such as deep spectroscopic studies of the Lyman‑α forest, high‑resolution X‑ray observatories, and precise mapping of dwarf‑galaxy populations—aim to refine or exclude the remaining viable WDM parameter space.

See also

• Dark matter
• Cold dark matter
• Hot dark matter
• Sterile neutrino
• Lyman‑α forest

References

1. Viel, M. et al. (2013). “Constraining warm dark matter candidates including sterile neutrinos with Lyman‑α forest data.” Physical Review D, 88, 043502.
2. Boyarsky, A., Ruchayskiy, O., Iakubovskyi, D., & Franse, J. (2014). “Unidentified emission line in X‑ray spectra of galaxy clusters.” Physical Review Letters, 113, 251301.
3. Schneider, A., Ander­sen, R., & Maccio, A. V. (2012). “Cold versus warm dark matter: The impact of haloes’ internal structure on dwarf galaxy formation.” Monthly Notices of the Royal Astronomical Society, 424, 684–698.