  Dark energy could actually be the manifestation of a modification to the
Friedmann equation arising from new physics (e.g., extra dimensions). Writing
the correction as $(1-\Omega_M)H^\alpha /H_0^{\alpha -2}$, we explore the
phenomenology and detectability of such. We show that: (i) $\alpha$ must be
$\la 1$; (ii) such a correction behaves like dark energy with equation-of-state
$w_{\rm eff} = -1 + {\alpha \over 2}$ in the recent past ($10^4> z\gg 1$) and
$w=-1$ in the distant future and can mimic $w<-1$ without violating the
weak-energy condition; (iii) $w_{\rm eff}$ changes, $dz/dw|_{z\sim 0.5} \sim
{\cal O}(0.2)$, which is likely detectable; and (iv) a future supernova
experiment like SNAP that can determine $w$ with precision $\sigma_w$, could
determine $\alpha$ to precision $\sigma_\alpha \approx 2 \sigma_w$.
