The Second Law of Thermodynamics is the surest thing in science, as the astronomer Arthur Eddington famously said a century ago. How strange, then, that its relationship with known cosmology remains mysterious. The early universe must have had lower entropy and it must have been a uniform gas, but these two facts don’t fit together well. Wallace offers a simple explanation: uniformity notwithstanding, the early universe was not in equilibrium. In the fifteen billion years since, entropy has increased due to both universal expansion and local clumping, the latter because clumped systems—stars, galaxies, bodies—are dissipative: we disperse more energy than we concentrate, thus following the dictates of the Second Law. Wallace is a philosopher by training, and the master narrative he summarizes here is philosophically stunning. Reality is non-equilibrium, and always has been. A
Wallace, David. 2011. “Gravity, entropy, and cosmology: in search of clarity.” The British Journal for the Philosophy of Science 61(3): 513-540. doi: 10.1093/bjps/axp048.