Variable sound speed in interacting dark energy models
  We consider a self-consistent and physical approach to interacting dark
energy models described by a Lagrangian, and identify a new class of models
with variable dark energy sound speed. We show that if the interaction between
dark energy in the form of quintessence and cold dark matter is purely momentum
exchange this generally leads to a dark energy sound speed that deviates from
unity. Choosing a specific sub-case, we study its phenomenology by
investigating the effects of the interaction on the cosmic microwave background
and linear matter power spectrum. We also perform a global fitting of
cosmological parameters using CMB data, and compare our findings to
$\Lambda$CDM.
