Why is z=1 below Tc in Bose Einstein condensation?

[eas123]

Hi.

For a Bose gas, my textbook states that below the critical temperature, which is given by n(λth)³/(2S+1)=2.612, the fugacity z=e^βμ≈1.

Why is this? The most basic explanation possible would be ideal, as I only need the rough idea.

Thanks!

 

[DrDu]

Or chemical potential μ=0, i.e. the energy of the ground state. Would it be lower, not even the ground state would be populated. Would it be higher, the population of the ground state would be infinite.