Null Impact of the Null Energy Condition in Current Cosmology

Abstract

We clarify the role of the oft-misunderstood Null Energy Condition (NEC) in the context of the current cosmological data. In particular, the NEC implies the sum of the total energy density and pressure satisfies $\rho_{tot}+P_{tot} \ge 0$; the energy conditions do not apply separately to individual components of the cosmological fluid. Consequently, we show that under the current best-fit cosmological model no violation of the NEC takes place, past or future. Further, growth in the energy density of an individual component cannot be used to signal violation of the NEC. We illustrate these points with a worked example whereby misestimation of the matter density leads to a phase during which $\rho_{de} + P_{de} < 0$ for the effective dark energy, followed by a phantom crossing and subsequent $\rho_{de} + P_{de} > 0$. At no time is the NEC violated. We also introduce ``elephant'' and ``chimera'' classes of physics for crossing $w_{de}=-1$.