The inflationary brane-antibrane universe

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 inflation being the inter -brane separation. Although most of our discussion refers to arbitrary p-br anes, 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 ar ising in string theory, the inter-brane potentials are too steep to in ate the universe for acceptably long times. However, for special regions of the parameter space of brane-antibrane positions the brane motion is slow enou gh 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 line ar size 1/r similar tol 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 wit hin which higher-dimension branes annihilate into lower-dimension ones. We argue that such a cascade naturally stops with the production of 3-branes i n 10-dimensional string theory.