Fine-Tuning in DBI Inflationary Mechanism

TL;DR

This paper identifies a fundamental fine-tuning problem in DBI inflation related to the warp factor, which challenges the viability of the mechanism due to geometric and dynamical constraints.

Contribution

It reveals a model-independent fine-tuning issue in DBI inflation linked to warp factor behavior and Einstein equations, connecting it to the eta-problem in slow-roll inflation.

Findings

The warp factor h must be small for slow inflaton motion.

Einstein equations deform the warp space, causing the h-problem.

The h-problem is related to the eta-problem via AdS/CFT duality.

Abstract

We show a model-independent fine-tuning issue in the DBI inflationary mechanism. DBI inflation requires a warp factor h small enough to sufficiently slow down the inflaton. On the other hand, the Einstein equation in extra dimensions under the inflationary background deforms the warp space in the IR side. Generically these two locations coincide with each other, spoiling the DBI inflation. The origin and tuning of this ``h-problem'' is closely related, through the AdS/CFT duality, to those of the well-known ``eta-problem'' in the slow-roll inflationary mechanism.