We study cosmological perturbations in mimetic matter scenario with a general higher derivative function. We show that the model suffers from the ghost and the gradient instabilities. We perform the analysis in both comoving and Newtonian gauges and confirm that the Hamiltonians and the associated instabilities are consistent with each other in both gauges. The existence of instabilities is independent of the specific form of higher derivative function which generates gradients for mimetic field perturbations. It is verified that these instabilities are not associated with the higher derivative instabilities such as the Ostrogradsky ghost.
H. Firouzjahi, M. Gorji and A. Mansoori
Thu, 9 Mar 17