  One of the central points of the ekpyrotic cosmological scenario based on
Horava-Witten theory is that we live on a negative tension brane. However, the
tension of the visible brane is positive in the usual HW phenomenology with
stronger coupling on the hidden brane, both for standard and non-standard
embedding. To make ekpyrotic scenario realistic one must solve the problem of
the negative cosmological constant on the visible brane and fine-tune the bulk
brane potential with an accuracy of $10^{-50}$. In terms of a canonically
normalized scalar field $\phi$ describing the position of the brane, this
potential must take a very unusual form $V(\phi)\sim e^{-{5000 \phi\over
M_p}}$. We describe the problems which appear when one attempts to obtain this
potential in string theory. The mechanism for the generation of density
perturbations in this scenario is not brane-specific; it is a particular
limiting case of the mechanism of tachyonic preheating. Unlike inflation, this
mechanism exponentially amplifies not only quantum fluctuations, but also
initial inhomogeneities. As a result, to solve the homogeneity problem in this
scenario, one would need the branes to be parallel to each other with an
accuracy better than $10^{-60}$ on a scale $10^{30}$ times greater than the
distance between the branes. Thus, at present, inflation remains the only
robust mechanism that produces density perturbations with a flat spectrum and
simultaneously solves all major cosmological problems.
