Constraining feeble neutrino interactions with ultralight dark matter

If ultralight $(\ll$ eV), bosonic dark matter couples to right handed neutrinos, active neutrino masses and mixing angles depend on the ambient density. When Majorana mass, induced by background, is small compared Dirac neutrinos are "pseudo-Dirac" fermions that undergo oscillations between nearly degenerate sterile states. We present a complete cosmological history for such scenario find severe limits from variety of terrestrial observables. For scalar in "fuzzy" regime ($\sim 10^{-20}$ these exclude couplings order $10^{-30}$, corresponding Yukawa interactions comparable gravitational force surpassing equivalent time variation scalar-induced electron proton couplings.