  Assuming spherical symmetry and the semi-classical Einstein equation, we
prove that, for the observers on top of the trapping horizon, the vacuum
energy-momentum tensor is always that of an ingoing negative energy flux at the
speed of light with a universal energy density ${\cal E} \simeq - 1/(2\kappa
a^2)$, (where $a$ is the areal radius of the trapping horizon), which is
responsible for the decrease in the black hole's mass over time. This result is
independent of the composition of the collapsing matter and the details of the
vacuum energy-momentum tensor. The physics behind the universality of this
quantity ${\cal E}$ and its surprisingly large magnitude will be discussed.
