Marcovian ness is the fact that you can predict what happens next just on the basis of the current moment. And so you can make a causal diagram going from events to other ens forward in time, that is to say, from the big bang to to day and toward the future. You can't do it backwards because when you go backward in time, the current state of the universe is not enough to predict the paststate of the universe. So we also need to assume a low entropy past. That gives us extra information about the past that we don't have about the future.