Is Brane Inflation Eternal?

TL;DR

This paper analyzes whether eternal inflation can arise within brane inflation frameworks. It finds that random-walk eternal inflation is generically absent in both slow-roll KKLMMT and DBI scenarios because the finite bulk size and warp factors bound the inflaton's range and fluctuations, preventing the quantum diffusion needed for self-reproduction. However, eternal inflation of the false-vacuum type can occur in multi-throat or multi-brane setups via tunneling processes, leading to bubble nucleation and distinct cosmological predictions. The work emphasizes that different brane-inflation realizations yield different CMB signatures, offering a pathway to empirically discriminate among scenarios and to understand the role of eternal inflation in the string landscape.

Abstract

In this paper, we show that eternal inflation of the random walk type is generically absent in the brane inflationary scenario. Depending on how the brane inflationary universe originated, eternal inflation of the false vacuum type is still quite possible. Since the inflaton is the position of the D3-brane relative to the anti-D3-brane inside the compactified bulk with finite size, its value is bounded. In DBI inflation, the warped space also restricts the amplitude of the scalar fluctuation. These upper bounds impose strong constraints on the possibility of eternal inflation. We find that eternal inflation due to the random walk of the inflaton field is absent in both the KKLMMT slow roll scenario and the DBI scenario. A more careful analysis for the slow-roll case is also presented using the Langevin equation, which gives very similar results. We discuss possible ways to obtain eternal inflation of the random walk type in brane inflation. In the multi-throat brane inflationary scenario, the branes may be generated by quantum tunneling and roll out the throat. Eternal inflation of the false vacuum type inevitably happens in this scenario due to the tunneling process. Since these scenarios have different cosmological predictions, more data from the cosmic microwave background radiation will hopefully select the specific scenario our universe has gone through.