Linear perturbations in Eddington-inspired Born-Infeld gravity

We study the full linear perturbations of a homogeneous and isotropic spacetime in the Eddington-inspired Born-Infeld gravity. The stability of the perturbations are analyzed in the Eddington regime. We find that, for positive $\kappa$, the scalar modes are stable in the infinite wavelength limit ($k=0$) but unstable for $k\neq0$. The vector modes are stable and the tensor mode is unstable in the Eddington regime, independent of the wave vector $k$. However, these modes are unstable and hence cause the instabilities for negative $\kappa$.