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 stabilities 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 not equal 0. 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.