Two new diagnostics of dark energy

We introduce two new diagnostics of dark energy (DE). The first, $Om$, is a combination of the Hubble parameter and the cosmological redshift and provides a null test of dark energy being a cosmological constant $\ensuremath{\Lambda}$. Namely, if the value of $Om(z)$ is the same at different redshifts, then $\mathrm{DE}\ensuremath{\equiv}\ensuremath{\Lambda}$, exactly. The slope of $Om(z)$ can differentiate between different models of dark energy even if the value of the matter density is not accurately known. For DE with an unevolving equation of state, a positive slope of $Om(z)$ is suggestive of phantom ($w<\ensuremath{-}1$) while a negative slope indicates quintessence ($w>\ensuremath{-}1$). The…