Dark energy with $w \rightarrow -1$: Asymptotic $Λ$ versus pseudo-$Λ$
If the dark energy density asymptotically approaches a nonzero constant, $ρ_{DE} \rightarrow ρ_0$, then its equation of state parameter $w$ necessarily approaches $-1$. The converse is not true; dark energy with $w \rightarrow -1$ can correspond to either $ρ_{DE} \rightarrow ρ_0$ or $ρ_{DE} \rightarrow 0$. This provides a natural division of models with $w \rightarrow -1$ into two distinct classes: asymptotic $Λ$ ($ρ_{DE} \rightarrow ρ_0$) and pseudo-$Λ$ ($ρ_{DE} \rightarrow 0$). We delineate the boundary between these two classes of models in terms of the behavior of $w(a)$, $ρ_{DE}(a)$, and $a(t)$. We examine barotropic and quintessence realizations of both types of models. Barotropic models with positive squared sound speed and $w \rightarrow -1$ are always asymptotically $Λ$; they can never produce pseudo-$Λ$ behavior. Quintessence models can correspond to either asymptotic $Λ$ or pseudo-$Λ$ evolution, but the latter is impossible when the expansion is dominated by a background barotropic fluid. We show that the distinction between asymptotic $Λ$ and pseudo-$Λ$ models for $w> -1$ is mathematically dual to the distinction between pseudo-rip and big/little rip models when $w < -1$.