The Bose distribution (also called Bose–Einstein distribution) gives the probability $$P(W)$$ to encounter a boson (for example a photon) with the energy $$W$$ in a quantum mechanical gas. Here, $$\mu$$ is the chemical potential corresponding to the Fermi energy at $$T=0$$. At absolute zero $$T = 0$$, all bosons are in a single state (in a ground state).