Unstable Nambu-Goldstone modes
  Nambu-Goldstone (NG) modes for 0-form and higher-form symmetries can become
unstable in the presence of background fields. The examples include the
instability of a photon with a time-dependent axion background or with a
chirality imbalance, known as the chiral plasma instability, and the
instability of a dynamical axion with a background electric field. We show that
all these phenomena can be universally described by a symmetry algebra for
0-form and higher-form symmetries. We prove a counting rule for the number of
unstable NG modes in terms of correlation functions of broken symmetry
generators. Based on our unified description, we further give a simple new
example where one of the NG modes associated with the spontaneous 0-form
symmetry breaking $U(1) \times U(1) \to \{1\}$ becomes unstable.
