ncts

Much of our current understanding of the early universe is based on cosmological solutions to the Einstein field equations. These solutions are obtained after imposing a spatial homogeneity assumption, which reduces the dynamics to a finite system of coupled ordinary differential equations. An open question, however, is whether such a homogeneous reduction is valid: while one may expect the cosmological principle to hold on the largest scales, our everyday experience does suggest some amount of inhomogeneity in the universe. The validity of the homogeneous reduction hinges on whether such inhomogeneities can compound nonlinearly and lead to a significant departure from the spatially homogeneous solutions on the cosmological timescale. In this talk, I will present some recent joint work with Shih-Fang Yeh, where we invalidate the homogeneous ansatz for a class of big bang solutions to the Einstein-Euler equations in which the matter content of the universe is modeled by a compressible fluid. Our result exploits a combination of two features: the tendency for compressible fluids to develop shock waves and the conformal structure of the big bang solutions near the primordial singularity.