The Big Bang is a Coordinate Singularity for $$k = -1$$k=-1 Inflationary FLRW Spacetimes

We show that the big bang is a coordinate singularity for a large class of $$k = -1$$ inflationary FLRW spacetimes which we have dubbed ‘Milne-like.’ By introducing a new set of coordinates, the big bang appears as a past boundary of the universe where the metric is no longer degenerate—a result which has already been investigated in the context of vacuum decay (Coleman and De Luccia in Phys Rev D 21:3305–3315, 1980). We generalize their results and approach the problem from a more mathematical perspective. Similar to how investigating the geometrical properties of the $$r = 2m$$ event horizon in Schwarzschild led to a better understanding of black holes, we believe that investigating the geometrical properties of the big bang coordinate singularity for Milne-like spacetimes could lead to a better understanding of cosmology. We show how the mathematics of these spacetimes may help illuminate certain issues associated with dark energy, dark matter, and the universe’s missing antimatter.