  Color Glass Condensate (CGC) provides a classical description of dense gluon
matter at high energies. Using the McLerran-Venugopalan (MV) model we calculate
the initial energy density \epsilon(\tau) in the early stage of the
relativistic nucleus-nucleus collision. Our analytical formula reproduces the
quantitative results from lattice discretized simulations and leads to an
estimate \epsilon(\tau=0.1fm)=40-50GeV/fm^3 in the Au-Au collision at RHIC
energy. We then formulate instability with respect to soft fluctuations that
violate boost invariance inherent to hard CGC backgrounds. We find unstable
modes arising as a result of ensemble average over the initial CGC fields.
