Effective equation of state for running vacuum: "mirage" quintessence
  and phantom dark energy
  Past analyses of the equation of state (EoS) of the Dark Energy (DE) were not
incompatible with a phantom phase near our time. This has been the case in the
years of WMAP observations, in combination with the remaining cosmological
observables. Such situation did not completely disappear from the data
collected from the Planck satellite mission. In it the EoS analysis may still
be interpreted as suggesting w<-1, and so a mildly evolving DE cannot be
discarded. In our opinion the usual ansatzs made on the structure of the EoS
for dynamical DE models (e.g. quintessence and the like) are too simplified. In
this work we examine in detail some of these issues and suggest that a general
class of models with a dynamical vacuum energy density could explain the
persistent phantom anomaly, despite there is no trace of real phantom behavior
in them. The spurious or "mirage" effect is caused by an attempt to describe
them as if the DE would be caused by fundamental phantom scalar fields.
Remarkably, the effective DE behavior can also appear as quintessence in
transit to phantom, or vice versa.
