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.
Categories: hep-th astro-ph gr-qc hep-ph