 Bunch–Davies vacuum

The Bunch-Davies vacuum selects the mode that approaches the positive-frequency ground-state mode of angular frequency $ck$ in the far past $\tau \to -\infty$.

The Bunch-Davies vacuum is a concept in the context of quantum field theory, particularly in relation to the study of inflation in cosmology. It represents a specific vacuum state defined for quantum fields in de Sitter spacetime, which is the solution to Einstein's equations for a universe experiencing exponential expansion.