Effective equation of state for running vacuum: "mirage" quintessence and phantom dark energy

TL;DR

This paper explores a class of dynamical vacuum energy models that can mimic phantom or quintessence dark energy behaviors, explaining the apparent phantom signals in observational data without actual phantom fields.

Contribution

It introduces a general class of models with dynamical vacuum energy that can produce effective dark energy equations of state mimicking phantom or quintessence phases, clarifying observational ambiguities.

Findings

Dynamical vacuum models can explain the phantom anomaly without real phantom fields.

Effective DE behavior can transit between quintessence and phantom regimes.

The apparent w<-1 signals may be 'mirage' effects from model assumptions.

Abstract

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.