# Instabilities in Mimetic Matter Perturbations

## TL;DR

This paper investigates the stability issues in mimetic matter cosmological perturbations, revealing persistent instabilities regardless of the specific higher derivative functions used, and clarifying their nature.

## Contribution

It demonstrates that instabilities in mimetic matter perturbations are inherent and not linked to Ostrogradsky ghosts, across different gauges and higher derivative functions.

## Key findings

- Both kinetic and gradient terms have wrong signs indicating instabilities.
- Instabilities are gauge-independent and persist regardless of higher derivative function form.
- Ghost instabilities are distinct from Ostrogradsky ghosts in mimetic perturbations.

## Abstract

We study cosmological perturbations in mimetic matter scenario with a general higher derivative function. We calculate the quadratic action and show that both the kinetic term and the gradient term have the wrong sings. 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 the ghost instability in mimetic perturbations is not associated with the higher derivative instabilities such as the Ostrogradsky ghost.