Instabilities in mimetic matter perturbations

Abstract

We study cosmological perturbations in mimetic matter scenario with a general\nhigher derivative function. We calculate the quadratic action and show that\nboth the kinetic term and the gradient term have the wrong sings. We perform\nthe analysis in both comoving and Newtonian gauges and confirm that the\nHamiltonians and the associated instabilities are consistent with each other in\nboth gauges. The existence of instabilities is independent of the specific form\nof higher derivative function which generates gradients for mimetic field\nperturbations. It is verified that the ghost instability in mimetic\nperturbations is not associated with the higher derivative instabilities such\nas the Ostrogradsky ghost.\n