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"Current theories about matter are valid only up to a finite temperature, and hence valid only after the first fraction of a second following the Singularity, the initial moment entailed by general relativity. … Closer to the Singularity there comes a moment, presumably the Planck Time... when general relativity must be replaced by a quantum theory of gravity. Theories of space and time applicable before Planck Time are not known; even the meaningfulness of space and time is uncertain. ...The intitial Singularity itself is a third limit, at least if there is such an initial Singularity. The Big Bang theory results in unreallistic numbers, like infinite density and so on. ...it has been shown that such peculiarities are unavoidable in general relativity, once certain very general assumptions are made, like causality and positive energy density (Hawking and Penrose 1970)."
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Singularity