In quantum field theory in curved spacetime, there is a unique quantum state over a background de Sitter space which is invariant under all the isometries. This is a thermal state, which is at the de Sitter temperature of where H is the Hubble constant. This state is called the Bunch–Davies state.
This explains the origin of cosmological perturbation fluctuations in inflationary models.
- ^ Bunch, Timothy Stephen; Davies, Paul (1978). "Quantum Field Theory In De Sitter Space: Renormalization By Point Splitting". Proceedings of the Royal Society of London. A 360: 117.