2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/161315We derive the metric for a Bianchi type I space-time with energy density that is dominated by that of a perfect fluid with equation of state $p=wρ$ and whose anisotropy is seeded by a fixed norm spacelike vector field. We solve for the evolution of perturbations about this space-time. In particular, the Jeans instability in an expanding flat Friedmann-Robertson-Walker universe is modified by the presence of the vector field so that energy density perturbations develop direction-dependent growth. We also briefly consider observational limits on the vector field vacuum expectation value, $m$. We find that, if $m$ is constant during recombination and thereafter, $m \lesssim 10^{14} GeV$.11 pages, two column; references and new section on observational limits added, typos corrected and clarifications made in textGeneral Relativity and Quantum Cosmologytext