Multifield Ultralight Dark Matter

Ultralight – or fuzzy – dark matter hypothesises that dark matter consists of an extraordinarily light fundamental particle with a rest mass at least a trillion, trillion times less than that of an electron. A particle this light can exhibit wavelike behaviour over astronomical distances and, as a consequence, this scenario leads to distinctive phenomena  (relative to “regular” dark matter) on sub-galactic scales. 

That said, searches for these phenomena have now ruled out ultralight dark matter for most of the range of masses for which it makes sense. However, while adding complexity to models usually reduces their credibility with ultralight dark matter it might be the other way round – theories of fundamental particle physics (including string theory) which predict the existence of very light fields tend to suggest there is more than just one of them. We show that multifield ultralight dark matter is likely to evade the constraints on single-field models, and is thus a viable and interesting dark matter candidate. 

Abstract

Ultralight dark matter (ULDM) is usually taken to be a single scalar field. Here we explore the possibility that ULDM consists of N light scalar fields with only gravitational interactions. This configuration is more consistent with the underlying particle physics motivations for these scenarios than a single ultralight field. ULDM halos have a characteristic granular structure that increases stellar velocity dispersion and can be used as observational constraints on ULDM models. In multifield simulations, we find that inside a halo the amplitude of the total density fluctuations decreases as 1/√N and that the fields do not become significantly correlated over cosmological timescales. Smoother halos heat stellar orbits less efficiently, reducing the velocity dispersion relative to the single field case and thus weakening the observational constraints on the field mass. Analytically, we show that for N equal-mass fields with mass m the ULDM contribution to the stellar velocity dispersion scales as 1/(Nm³). Lighter fields heat the most efficiently and if the smallest mass m_L is significantly below the other field masses the dispersion scales as 1/(N²m_L³).

• Gosenca, Eberhardt, Wang, Eggemeier, Kendall, Zagorac and Parkinson
• Multifield Ultralight Dark Matter  
• Phys Rev D 107 083014  or ArXiV:2301.07114

Simulated galactic halos with one [top] and four [bottom] ultralight dark matter fields — the density is shown on a logarithmic scale and the reduced granularity in the multifield case is clearly visible.