Linear perturbations in Eddington-inspired Born-Infeld gravity

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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 κ, the scalar modes are stable in the infinite wavelength limit (k=0) but unstable for k≠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 κ.