Weighing up axions as a window on dark matter

Calculations that need to consider the theory of quantum chromodynamics, which describes how the strong interaction holds quarks together, are daunting because of the nonlinearity of the strong force. Despite the numerical difficulties, Szabolcs Borsanyi et al. have managed to perform an accurate calculation of the mass of an axion. These particles are at the heart of many dark-matter theories. Key in this paper is the ability to calculate the equation of state and the so-called topological susceptibility of quantum chromodynamics over a very wide temperature range. With their determination of the axion mass, the authors make important predictions about the evolution of the Universe that will help to test dark-matter theories involving axions in the near future.