Pyrotechnic Universe

TL;DR

The paper critically evaluates the ekpyrotic universe model rooted in Horava-Witten theory, focusing on the feasibility of a negative-tension visible brane and a finely-tuned bulk-brane potential to generate perturbations via tachyonic instability. It demonstrates that achieving the necessary potential form requires extreme fine-tuning and raises serious issues with brane stabilization, SUSY/BPS considerations, and producing a scale-invariant spectrum without inflation. The authors argue that inflation remains the robust mechanism for obtaining a flat spectrum and addressing major cosmological problems, highlighting that the tachyonic density-perturbation mechanism in brane setups is not sufficiently generic or stable. Through 4D toy models and 5D brane analyses, they illustrate the substantial theoretical and phenomenological hurdles to a string-theory–consistent, noninflationary alternative.

Abstract

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 $φ$ describing the position of the brane, this potential must take a very unusual form $V(φ)\sim e^{-{5000 φ\over M_p}}$. 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.

Figures (2)

• Figure 1: Sketch of the ekpyrotic scenario. We live on a brane with negative energy density. The Big Bang occurs when the bulk brane hits our brane. The bulk brane has potential energy $V(Y)$ which is postulated to have a very specific form: it is negative everywhere except on our brane, and its absolute value decreases exponentially at large $Y$. An important feature of this scenario is that the volume of space, controlled by the metric $D(Y)$, decreases near our brane, which makes the spectrum of perturbations blue.
• Figure 2: Sketch of the pyrotechnic scenario. We live on a brane with positive energy density. The volume of space controlled by the metric $D(Y)$ decreases near our brane, which gives a red tilt to the spectrum of perturbations. The mechanism for the generation of fluctuations $\delta Y_k$ in this scenario, as well as in the ekpyrotic scenario, amplifies all inhomogeneities, including classical inhomogeneities $\Delta Y$ of the bulk brane.