Pyrotechnic Universe
Abstract
One of the central points of the recently proposed ekpyrotic cosmological scenario based on Horava-Witten theory is that we live on a negative tension brane. However, the tension of the visible brane in HW theory is positive. To improve the ekpyrotic scenario one should change the signs of the brane tensions as well as other parameters. We show that the mechanism for the generation of density perturbations in this scenario is not brane-specific and give several examples of this mechanism in terms of the theory of tachyonic preheating. The spectrum of perturbations can be flat, red, or blue, depending on the choice of the bulk brane potential. However, to make this mechanism 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}. If this mechanism can be made to work, then, unlike inflation, it 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^{-30} 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 the homogeneity problem.Keywords
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