On (in)stabilities of perturbations in mimetic models with higher derivatives

Abstract

Usually when applying the mimetic model to the early universe, higher\nderivative terms are needed to promote the mimetic field to be dynamical.\nHowever such models suffer from the ghost and/or the gradient instabilities and\nsimple extensions cannot cure this pathology. We point out in this paper that\nit is possible to overcome this difficulty by considering the direct couplings\nof the higher derivatives of the mimetic field to the curvature of the\nspacetime.\n