The Inflationary Brane-Antibrane Universe

TL;DR

This work proposes that inflation can be driven by the slow, inter-brane motion of branes and antibranes in extra dimensions, with the four-dimensional inflaton identified with their separation. It shows that slow-roll inflation generically requires very weak inter-brane couplings or special initial conditions, since string-theory couplings are typically too steep; in special setups (e.g., y ∼ r_{ot} on a torus) the potential flattens enough to yield sufficient e-foldings and a slightly red-tilted spectrum consistent with observations. Reheating proceeds via tachyon condensation, triggering a hybrid-like transition and a cascade of brane annihilations that preferentially leaves behind 3-branes aligned with the large spatial dimensions. The cascade mechanism provides a natural dynamical route to a final 3-brane universe, with implications for the distribution and evolution of brane populations in string cosmology and for the origin of the observed 3+1-dimensional spacetime.

Abstract

We show how the motion through the extra dimensions of a gas of branes and antibranes can, under certain circumstances, produce an era of inflation as seen by observers trapped on a 3-brane, with the inflaton being the inter-brane separation. Although most of our discussion refers to arbitrary p-branes, when we need to be specific we assume that they are D-branes of Type II or Type I string theory. For realistic brane couplings, such as those arising in string theory, the inter-brane potentials are too steep to inflate the universe for acceptably long times. However, for special regions of the parameter space of brane-antibrane positions the brane motion is slow enough for there to be sufficient inflation. Inflation would be more generic in models where the inter-brane interactions are much weaker. The spectrum of primordial density fluctuations predicted has index n slightly less than 1, and an acceptable amplitude, provided that the extra dimensions have linear size 1/r ~ 10^{12} GeV. Reheating occurs as in hybrid inflation, with the tachyonic instability of the brane-antibrane system taking over for small separations. The tachyon field can induce a cascade mechanism within which higher-dimension branes annihilate into lower-dimension ones. We argue that such a cascade naturally stops with the production of 3-branes in 10-dimensional string theory.