We know why coffee cools

Much is made of the mysterious directionality of thermodynamic time and its contrast to the near temporal symmetry of the underlying dynamical laws of Nature. I point out that if you are reasonably modest with respect to the “why” of this paperʹs title, the answer is known and is not particularly complicated. The basic reason is that the universe is expanding (as first emphasized in this context by T. Gold) and as a result, at about 380 000 years of age, underwent a transition (because of expansion-induced cooling) from being dominated by short-range forces to being dominated by long-range forces. The most likely states under one regime (homogeneous) became extremely unlikely under the successor regime. Proceeding from that unlikely state many billions of years ago continues to fuel the negentropy flow that maintains our thermodynamic arrow.