[@Gyulassy:2003mc; @Baier:2000mf] refinements in the theory since then have opened the possibility of using the observed jet quenching pattern as a tomographic tool [@TOMO] to probe the parton densities in a QGP. The right panel shows a recent jet tomographic analysis [@Vitev:2002pf] of the PHENIX $\pi^0$ data [@Adcox:2001jp; @Adcox:2002pe] based on the GLV opacity formalism [@Gyulassy:2000er]. Vitev and I concluded from Fig.6b that the initial gluon rapidity density required to account for the observed jet quenching pattern must be $dN_g/dy\sim 1000\pm 200$.

This jet tomographic measure of the initial $dN_g/dy$ is in remarkable agreement with three other independent sources: (1) the initial entropy deduced via the Bjorken formula from the measured multiplicity, (2) the initial condition of the QGP required in hydrodynamics to produce the observed elliptic flow, and (3) the estimate of the maximum gluon rapidity density bound from the CGC gluon saturated initial condition [@Eskola:2002qz].

These four independent measures makes it possible to estimate the maximal initial energy density in central collisions $$\epsilon_0 = \epsilon(\tau \sim 1/p_0) \approx \frac{p_0^2}{\pi R^2}\frac{dN_g}{dy}
\approx 20 \frac{{\rm GeV}}{{\rm fm}^3} \sim 100 \times \epsilon_{A}
\label{eps0}$$ where $p_0\approx Q_{sat}\approx 1.0-1.4$ GeV is the mean transverse momentum of the initial produced gluons from the incident 