One of the central points of the ekpyrotic cosmological scenario based on Hořava-Witten (HW) 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 nonstandard embedding. To make the 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 $\mathcal{V}\left(\phi \right)=-{10}^{-22}{M}_p^4\exp \left(-5000\phi {/M}_p\right).\mathrm{}$ 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.

• Received 26 April 2001

DOI:https://doi.org/10.1103/PhysRevD.64.123523