Orateur
Description
Evolving dark energy is back at the center of cosmology, and several scenarios under active discussion---sign-switching $\Lambda_{\rm s}$CDM, AdS-to-dS transitions, and modified-gravity backgrounds recast in GR like form---feature an effective dark-energy density that is negative in the past and crosses zero at some redshift $z_\dagger$. Our standard language quietly assumes this never happens: $w_{\rm de}<-1/3$ signals repulsion only while $\rho_{\rm de}>0$, the phantom divide $w_{\rm de}=-1$ stops separating anything once the density changes sign, and at $\rho_{\rm de}=0$ the ratio $w_{\rm de}$ does not exist at all, even though nothing physical is singular there. In this talk we ask what defines dark energy when $w$ cannot, and answer with the two combinations the field equations actually use: the active gravitational mass density $\mathcal{M}=\rho+3p$, which drives Raychaudhuri defocusing, and the inertial mass density $\mathcal{I}=\rho+p$, whose zero is the null-energy-condition boundary. Three clean results follow: any smooth sign switch is necessarily phantom-like and repulsive at the crossing; the accompanying pole in $w_{\rm de}$ is purely kinematic, with a universal residue fixed only by $z_\dagger$ and the order of the zero; and repulsion begins strictly before the switch, at a distinct redshift $z_{\rm rep}>z_\dagger$, with sufficiently sharp transitions opening a transient acceleration window at intermediate redshift. We illustrate these results with an analytic profile, $f(T)$ gravity, and the minimal phantom brane, and close with the practical moral for reconstructions: across $\rho_{\rm de}=0$, the regular targets are $(\mathcal{I},\mathcal{M})$, not $w_{\rm de}$.