Mixmaster universe

The Mixmaster universe (named after Sunbeam Mixmaster, a brand of Sunbeam Products electric kitchen mixer) is a solution to Einstein field equations of general relativity studied by Charles Misner in an effort to better understand the dynamics of the early universe. He hoped to solve the horizon problem in a natural way by showing that the early universe underwent an oscillatory, chaotic epoch.

The model is similar to the closed Friedmann–Lemaître–Robertson–Walker universe, in that spatial slices are positively curved and are topologically three-spheres ${\displaystyle S^{3}}$. However, in the FRW universe, the ${\displaystyle S^{3}}$ can only expand or contract: the only dynamical parameter is overall size of the ${\displaystyle S^{3}}$, parameterized by the scale factor ${\displaystyle a(t)}$. In the Mixmaster universe, the ${\displaystyle S^{3}}$ can expand or contract, but also distort anisotropically. Its evolution is described by a scale factor ${\displaystyle a(t)}$ as well as by two shape parameters ${\displaystyle \beta _{\pm }(t)}$. Values of the shape parameters describe distortions of the ${\displaystyle S^{3}}$ that preserve its volume and also maintain a constant Ricci curvature scalar. Therefore, as the three parameters ${\displaystyle a,\beta _{\pm }}$ assume different values, homogeneity but not isotropy is preserved.

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