The Inflationary Brane-Antibrane Universe

TL;DR

This paper explores how brane-antibrane dynamics in extra dimensions can lead to inflationary expansion of the universe, with specific conditions for sufficient inflation and implications for primordial fluctuations.

Contribution

It demonstrates that brane-antibrane motion can produce inflation under certain conditions, and discusses the resulting density fluctuations and reheating mechanisms in string theory models.

Findings

Inflation can occur from brane-antibrane motion in extra dimensions.

Primordial fluctuation spectrum has index n slightly less than 1.

Reheating involves tachyonic instability and brane cascade processes.

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.