Equivalence between Horndeski and beyond Horndeski theories and
  imperfect fluids
  In this paper we show that an equivalence between Horndeski and beyond
Horndeski theories and general relativity with an effective imperfect fluid can
be formally established. The formal equivalence is discussed for several
particular cases of interest. Working in the cosmological framework, it is
shown that, while the effective stress-energy tensor of viable Horndeski
theories is formally equivalent to that of an imperfect fluid with anisotropic
stresses and vanishing heat flux vector, the effective stress-energy tensor of
beyond Horndeski theories is equivalent to the one of a perfect fluid instead.
