What is the force of "friction" (for want of a better word) between the two four-dimensional universes that have come together to create our three-dimensional surface (universe). As a three-dimensional analog think of two rectangular solids moving toward each other. One is moving right with it's bottom surface exactly the same height as the other's (moving left) top surface. At some point (the beginning of our universe and time) they meet and form a "surface of friction". They continue on in their initial direction creating a larger surface in contact and greater friction.