Hans Oude Groeniger: Quiescence for the exceptional Bianchi cosmologies

This talk is about the initial singularity of Bianchi type VI_-1/9 spacetimes that solve Einstein's equations coupled to an orthogonal stiff fluid. Bianchi type VI_-1/9 is known as exceptional because the dynamics of orthogonal perfect fluid cosmologies of this type have an additional degree of freedom compared to the other lower Bianchi types. This is a result of a certain degeneracy of the momentum constraints for this Bianchi type and type of matter. However, the presence of a stiff fluid effectively kills one degree of freedom, allowing for a detailed understanding of the asymptotics. As a result, we can prove quiescent behaviour for these spacetimes, i.e. the convergence of the eigenvalues of the expansion-normalized Weingarten map. I will discuss the degeneracy in the momentum constraints, several possible causes of quiescence, how to represent the dynamics of these spacetimes, as well as the main result which concerns the different types of quiescence we show to occur in these spacetimes as well as their genericity.