Evolution of the average energy density as a function of the proper time of the system in hydrodynamic calculations. The green curve represents the average energy density profile from hydrodynamic model calculation from . The blue line the grey bands represent the energy density estimated from the total transverse energy, $E_{\mathrm{T}}$, measured in 0-5$\%$ central \PbPb\ collisions at $\sqrtSnn = 2.76$ TeV  using Eq. \ref{eq:bjorkenest} for the free-streaming expansion and Eq. \ref{eq:hydro_eps_evol} for hydrodynamical expansion. The hydrodynamical expansion is matched to the free-streaming behaviour at $\tau = 6$ and $12 \;\mathrm{fm}/c$.