I will review the recently developed effective theories for gapless excitations in generic media, which are described in terms of spontaneous symmetry breaking. In particular, all media spontaneously break at least part of the fundamental space-time symmetries. The Goldstone modes associated to this breaking are gapless, and correspond to the collective excitations of the system. I will first introduce the general theoretical framework, with particular emphasis on the effective theory for superfluids. After that, I will show how these techniques are now mature enough to be successfully used in various phenomenological contexts.
Roberto Bonciani