:We should describe the notable models, but I think the issue whether cosmic time zero is a "real" moment in time is a non-issue: any serious physicist knows that in all variants this pertains to a model of which we don't know how well it fits reality (which really means: how well it conforms to present and future observations), and they also know that in models with a genuine mathematical singularity the accepted (experimentally confirmed) laws of physics break down anyway, rendering claims of this being "real" experimentally unfalsifiable and hence meaningless.  Lambiam