Birkhoff-theorem and FLRW
18 Feb 2025The Birkhoff-theorem states that spherically symmetric vacuum solutions in relativity need to be static. On the other hand, a completely empty FLRW-universe is a spherically symmetric vacuum solution, but has a constant expansion velocity \(\dot{a} = \mathrm{const}\). Does that mean that the gravitational field is evolving? Can you clear up the apparent contradiction?