On Stability and Isotropization of Kasner Solution in $R^2$ Gravity

The fate of the Universe that initially expands anisotropically in the theory with $R^2$ quantum-gravitational term in the Lagrangian is investigated. The stability of Kasner-like expansion, specifically in the class of Kantowski-Sachs spacetimes, is analyzed. Kasner solutions are found to be unstable, with the bifurcation line between the initial conditions that lead to collapsing universes and the ones that set the universe for continuing expansion that becomes isotropic, established analytically. Under suitable conditions, the isotropized spacetime enters the intermediate slow-rolling inflationary stage similar to Starobinsky inflation.