Sterile Neutrinos and Warm Dark Matter in the Era of Precision Cosmology*

Dark matter is well established as a necessary majority constituent of the cosmic matter density. While cold dark matter is a viable benchmark model, warm dark matter (WDM) models, constrained by current observational data, provide an important comparison. In the present work, we analyze WDM with a focus on the interplay between particle dark matter spin and thermal history. Our analysis reveals significant corrections to the linear matter power spectrum across a range of thermal WDM particle masses, particularly in two primary classes: spin-1/2 particles (e.g., thermalized sterile neutrinos, axinos) and thermal spin-3/2 particles (e.g., gravitinos or nonsupersymmetric particles).

New transfer function fits for thermal WDM candidates are presented, extending the mass regime beyond previous work and spanning scales relevant to current and forthcoming observational capability. Notably, we show that the commonly used spin-1/2 thermal WDM particle exhibits a colder transfer function than previously assumed, changing current constraints at the 20% level. Additionally, we investigate the entropy requirements for these WDM models to successfully account for observed dark matter densities.

One example of non-thermal WDM, sterile neutrinos, offer multiple potential cosmological signatures. Using numerical tools, including our new WDM transfer functions, we trace the cosmic production and behavior of sterile neutrino dark matter, producing constraints on the mass-mixing angle parameter space. Specifically, we analyze Shi-Fuller-based production and models with enhanced active sector self-interaction, which raise the intriguing possibility of sterile neutrinos comprising the majority of dark matter. Combining our analysis with recent constraints derived from strong lensing and structural observations, we decisively rule out the Shi-Fuller model at a 95% confidence level.