This expression can also be expressed in terms of the vacuum expectation value of the commutator of the free scalar field. In this case, G adv ( x , y ) = i ⟨ ⟨ 0 | [ Φ Φ ( x ) , Φ Φ ( y ) ] | 0 ⟩ ⟩ Θ Θ ( y 0 − − x 0 ) .