The QCD axion and other possible new light particles

I will show how the properties of the QCD axion can be calculated precisely using results from chiral perturbation theory and lattice QCD, and discuss the relevance of these results to ongoing experimental searches. The QCD axion can also act as dark matter, and I will report recent progress in predicting the axion mass that allows it to make up the full dark matter relic abundance. I will also discuss other classes of possible new light particles, and show that there are strong constraints on the allowed parameter space from observations of the evolution of the stars. In particular, I will show that consistently including finite temperature effects strengthens constraints on the couplings of new particles to the visible sector by more than an order of magnitude.