Ghosts in extended quasidilaton theories

Abstract

We report on our independent investigations of the puzzle of cosmological perturbations in extended quasidilaton. We confirm the claims of presence of the Boulware-Deser ghost. We use both the language of cosmological perturbations with broken diffeomorphisms and the Stückelberg approach.

Beyond dRGT as mimetic massive gravity

“…In the general case one has to use a more complicated ansatz ofg µν = α(x) · g µν + γ(x) · (∂ µ φ)(∂ ν φ) with two remaining scalar Lagrange multipliers, λ(x) ≡ λ µν g µν and µ(x) ≡ λ µν (∂ µ φ)(∂ ν φ). Under certain circumstances, coefficients α and β would not be fully determined by equations of motion with the gravitational model getting coupled to a constrained k-essence, akin to the field φ in the action (7). We leave details for a future work.…”

“…However, it turned out that it does not seem to allow for reasonable cosmological models [4], even after some further extensions [5]. Some other extensions potentially could be more successfull, however they bring the ghost back [6,7]. It is possible to construct viable models in bimetric setup [8] which is arguably better justified than having a fixed fiducial metric, also from fundamental perspectives.…”