Thermalisation is the process by which out-of-equilibrium systems reach equilibrium. Coherence refers to the phases of waves being a constant difference apart.
While reading a paper on axion stars, I came across the claim that thermalisation mechanisms can bring incoherent axions (think of them as classical waves) with very large occupation numbers (whatever that means in the context of waves) into coherence.
My questions are the following:
Why are incoherent waves (with very large occupation numbers, whatever that means) considered out of equilibrium?
How can thermalisation mechanisms (such as scattering) make incoherent waves become coherent?