Fuzzy Dark Matter from IR Confinement

An ultra light axion dark matter (DM) may avoid certain problems of the cold DM paradigm with observations at galactic scales.  Such an axion, often referred to as "Fuzzy DM (FDM)," may get its tiny mass from large masses suppressed by non-perturbative stringy effects.  We examine an alternative possibility that the mass of FDM is generated by infrared confining dynamics, in analogy with the QCD axion. We find that cosmological constraints are suggestive of a period of mild of inflation that reheats the Standard Model (SM) sector only. A typical prediction of the scenario, broadly speaking, is a larger effective number of neutrinos compared to the SM value Neff≈3, as inferred from precision measurements of the cosmic microwave background. Some of the new degrees of freedom may be identified as "sterile neutrinos," which may be required to explain certain neutrino oscillation anomalies.