  Stability analysis of the Bianchi type I universe in pure gravity theory is
studied in details. We first derive the non-redundant field equation of the
system by introducing the generalized Bianchi type I metric. This non-redundant
equation reduces to the Friedmann equation in the isotropic limit. It is shown
further that any unstable mode of the isotropic perturbation with respect to a
de Sitter background is also unstable with respect to anisotropic
perturbations. Implications to the choice of physical theories are discussed in
details in this paper.
