DBI in the Sky

TL;DR

The paper analyzes density perturbations in the D-cceleration (DBI) inflation framework, where strong coupling dynamics generate a DBI-like inflaton action with a sound speed $c_s=1/\gamma$ and a velocity-imposed speed limit. It shows that Gaussian perturbations acquire a power spectrum with scalar tilt $n_s-1 = O(\epsilon^2)$ and a tensor-to-scalar ratio $r = 16 \epsilon / \gamma$, while non-Gaussianity is bounded from below as $f_{NL} \approx -0.32 \gamma^2$, tying observable non-Gaussian signals to the model’s parameters. Constraints from observations imply $\gamma \lesssim 17$, which in turn enforces lower bounds on tensor power and places strong, testable predictions on the inflationary scale and the required throat geometry (large $\lambda$). The framework is falsifiable with Planck-like data, and the authors discuss tuning prospects, reheating, and possible generalizations of the warp geometry, highlighting a novel link between horizon physics and microscopic degrees of freedom in a holographic setting.

Abstract

We analyze the spectrum of density perturbations generated in models of the recently discovered "D-cceleration" mechanism of inflation. In this scenario, strong coupling quantum field theoretic effects sum to provide a DBI-like action for the inflaton. We show that the model has a strict lower bound on the non-Gaussianity of the CMBR power spectrum at an observable level, and is thus falsifiable. This in particular observationally distinguishes this mechanism from traditional slow roll inflation generated by weakly interacting scalar fields. The model also favors a large observable tensor component to the CMBR spectrum.