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We generalize gravitational particle production in a radiation-dominated$CPT$-symmetric universe to non-standard, but also $CPT$-symmetric earlyuniverse cosmologies. We calculate the mass of a right-handed "sterile"neutrino needed for it to be the cosmological dark matter. Since genericallysterile neutrinos mix with the Standard Model active neutrinos, we usestate-of-the-art tools to compute the expected spectrum of gamma rays andhigh-energy active neutrinos from ultra-heavy sterile neutrino dark matterdecay. We demonstrate that the sterile neutrinos are never in thermalequilibrium in the early universe. We show that very high-energy Cherenkovtelescopes might detect a signal for sterile neutrino lifetimes up to around10$^{27}$ s, while a signal in high-energy neutrino telescopes such as IceCubecould be detectable for lifetimes up to 10$^{30}$ s, offering a better chanceof detection across a vast landscape of possible masses.

Sterile Neutrino Dark Matter from Generalized $CPT$-Symmetric Early-Universe Cosmologies