The beginning is fundamentally unknowable with any current and future observations.
The universe arises out of quantum tunneling from ‘nothing’, and the process can be described by a Lorentzian path integral (e.g., Vilenkin's proposal).
The universe is eternal and has no beginning.
There is change in spacetime signature (e.g., Hartle-Hawking proposal of Euclidean to Lorentzian signature)
The observable universe budded off as a baby universe from a black hole.