  Light sterile neutrinos represent a well-motivated extension of the
3-neutrino paradigm. However, the impressive agreement between standard
cosmology and data casts doubts on their existence. Here we present a class of
scenarios that robustly avoids this tension. In these models the sterile
neutrinos are light, chiral states of a new sector interacting with the
Standard Model via the right-handed neutrino portal and, crucially,
active-sterile neutrino oscillations require a phase transition in the hidden
sector. We explore the hidden-couplings/critical-temperature plane and identify
regions where several sterile neutrinos can be accommodated. A late phase
transition is usually preferred and may also ward off a potential threat posed
by the formation of topologically stable defects.
