What we mean when we say that the Son of God descended into the time and space of creation:
"The Son does embark on a space odyssey, leaving the Father for a period of time, literally coming down, and later returning to his proper abode in the trinity. Such an understanding arises from taking a metaphor too literally. It must be understood that God, precisely as trinity, is present at all times and places. To speak of a mission of one of the divine persons is to say that the eternal emanation is made manifest in time and has a particular temporal effect. The purpose of any such mission is to deepen the reality of the indwelling of God in human souls. Thus, when we say that the Son is sent, or that He descends, this does not indicate something new on the part of God, but designates that the temporal effect of the eternal emanation has a beginning and endures in time."
(Zachary Hayes, OFM, in his introduction to Bonaventure's Disputed Questions on the Mystery of the Trinity (Franciscan Institute, 2000)