Discussion
The model is similar to the closed Friedmann-Lemaitre-Robertson-Walker universe, in that spatial slices are positively curved and are topologically three-spheres . However, in the FRW universe, the can only expand or contract: the only dynamical parameter is overall size of the, parameterized by the scale factor . In the Mixmaster universe, the can expand or contract, but also distort anisotropically. Its evolution is described by a scale factor as well as by two shape parameters . Values of the shape parameters describe distortions of the that preserve its volume and also maintain a constant Ricci curvature scalar. Therefore, as the three parameters assume different values, homogeneity but not isotropy is preserved.
The model has a rich dynamical structure. Misner showed that the shape parameters act like the coordinates of a point mass moving in a triangular potential with steeply rising walls with friction. By studying the motion of this point, Misner showed that the physical universe would expand in some directions and contract in others, with the directions of expansion and contraction changing repeatedly. Because the potential is roughly triangular, Misner suggested that the evolution is chaotic.
Read more about this topic: Mixmaster Universe
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