  In spherical symmetry, the total energy-momentum tensor near the apparent
horizon is identified up to a single function of time from two assumptions: a
trapped region forms at a finite time of a distant observer, and values of two
curvature scalars are finite at its boundary. In general relativity, this
energy-momentum tensor leads to the unique limiting form of the metric. The
null energy condition is violated across the apparent horizon and is satisfied
in the vicinity of the inner apparent horizon. As a result, homogenous collapse
models cannot describe the formation of a black hole. Properties of matter
change discontinuously immediately after formation of a trapped region.
Absolute values of comoving density, pressure, and flux coincide at the
apparent horizon. Thus, collapse of ideal fluids cannot lead to the formation
of black holes. Moreover, these three quantities diverge at the expanding
apparent horizon, producing a regular (i.e., finite curvature) firewall. This
firewall is incompatible with quantum energy inequalities, implying that
trapped regions, once formed at some finite time of a distant observer, cannot
grow.
