Borde–Guth–Vilenkin theorem

The Borde–Guth–Vilenkin (BGV) theorem is a theorem in physical cosmology which deduces that any universe that has, on average, been expanding throughout its history cannot be infinite in the past but must have a past spacetime boundary. It is named after the authors Arvind Borde, Alan Guth and Alexander Vilenkin, who developed its mathematical formulation in 2003.

Source: Wikipedia — Borde–Guth–Vilenkin theorem (CC BY-SA 4.0)