We show that decoupled hidden sectors can have observational consequences .
As a representative model example , we study dark matter production in the Higgs portal model with one real singlet scalar s coupled to the Standard Model Higgs via \lambda _ { hs } \Phi ^ { \dagger } \Phi s ^ { 2 } and demonstrate how the combination of non-observation of cosmological isocurvature perturbations and astrophysical limits on dark matter self-interactions imply stringent bounds on the magnitude of the scalar self-coupling \lambda _ { s } s ^ { 4 } .
For example , for dark matter mass m _ { s } = 10 MeV and Hubble scale during cosmic inflation H _ { * } = 10 ^ { 12 } GeV , we find 10 ^ { -4 } \lesssim \lambda _ { s } \lesssim 0.2 .