  The functional renormalisation group is used for the BCS-BEC crossover in
gases of ultracold fermionic atoms. In a simple truncation, we see how
universality and an effective theory with composite bosonic di-atom states
emerge. We obtain a unified picture of the whole phase diagram. The flow
reflects different effective physics at different scales. In the BEC limit as
well as near the critical temperature, it describes an interacting bosonic
theory.
