= Solution
The https://www-thphys.physics.ox.ac.uk/people/AndreiStarinets/TUTORIALS_NOTES_GR/gr-cosmology-exam-paper-2011-solutions.pdf#page=21[official notes, PDF p. 21, handwritten p. 14] give the acceleration signs and the instability conclusion for the <Einstein static universe>. One wording correction is necessary: their word “oscillatory” for the overdense perturbation is a slip. Starting near rest, an overdense model contracts and its <matter density> rises, whereas an underdense model expands and its <matter density> falls; both move away from the static balance.
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