  We derive the five-dimensional metrics which describe a non-singular boundary
brane collision in the ekpyrotic scenario in the context of general relativity,
taking into account brane tension. We show that the metrics constrain matter
created in the collision to have negative energy density or pressure. In
particular, the minimal field content of heterotic M-theory leads to negative
energy density. We also consider bulk brane-boundary brane collisions and show
that the collapse of the fifth dimension is an artifact of the four-dimensional
effective theory.
