If the universe were infinitely old, why is the night sky still full of hot stars against cold dark space? That contradiction leads to the heat death of the universe, also called the big chill, a state of maximum entropy where no work can be done. This episode traces the idea from Jean Sylvain Bailly's 1777 speculation about cooling planets, through Lord Kelvin's 1862 extrapolation of the new laws of thermodynamics and the paradox that proved the universe had a beginning, to the contributions of Helmholtz and Rankine.

We then follow modern cosmology: how dark energy and accelerating expansion hasten the cooling, what ends the current Stelliferous Era, Hawking radiation and the roughly 10 to the 100 years it takes a galaxy-mass black hole to evaporate, and the Dark Era of diffuse photons and leptons that follows. We examine the pushback from ...