Short answers, pulled from the story.
In ancient Greece, thinkers developed a cosmogony focused on the origin of matter, space, and time. This transition from Chaos to Cosmos formed a form of philosophical cosmogony distinct from modern empirical science.
Like most physical models, these describe changes of state rather than initial conditions. Initial states are given by experimental measurements or by hypothesis in standard practice.
As the model follows smaller times, density exceeds the validity of general relativity at this point in time called the Planck time. The observable universe is much more homogeneous than an extrapolated Big Bang can account for.
Sean M. Carroll specializes in theoretical cosmology and field theory while explaining competing explanations for origins. One view holds that when the universe started to expand, the Big Bang occurred which evidently began it.
The Sumerian tablet containing parts of the Eridu Genesis offers another early model describing the creation of the world out of a primeval sea known as Abzu. These narratives dealt with many similar questions about origins without relying on experimental measurements.