Pyrotechnic Universe Typeset Using Revt E X 1

One of the central points of the ekpyrotic cosmological scenario based on Hor̆ava-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 nonstandard 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 φ describing the position of the brane, this potential must take a very unusual form V(φ) = −10−22M4 p exp (