Cosmology of the Tachyon in Brane Inflation

TL;DR

This work analyzes the cosmological role of the tachyon at the end of brane inflation in a warped D3–anti-D3 setup. Using a combined tachyon–inflaton action and the delta-N formalism, it shows that tachyon-driven inflation is highly constrained and typically contributes only a small number of e-folds, unless parameters lie in a fine-tuned slow-roll regime. However, density perturbations generated at the end of inflation due to additional light fields can be substantial in the relativistic (DBI) regime, potentially accounting for up to about half of the total curvature perturbation and enhancing non-Gaussian signatures and the tensor-to-scalar ratio. These findings tighten constraints on DBI-like string models and highlight the importance of end-of-inflation physics for observable cosmology, reheating, and cosmic strings, while suggesting avenues for further study of multi-field dynamics and bispectrum shapes.

Abstract

In certain implementations of the brane inflationary paradigm, the exit from inflation occurs when the branes annihilate through tachyon condensation. We investigate various cosmological effects produced by this tachyonic era. We find that only a very small region of the parameter space (corresponding to slow-roll with tiny inflaton mass) allows for the tachyon to contribute some e-folds to inflation. In addition, non-adiabatic density perturbations are generated at the end of inflation. When the brane is moving relativistically this contribution can be of the same order as fluctuations produced 55 e-folds before the end of inflation. The additional contribution is very nearly scale-invariant and enhances the tensor/scalar ratio. Additional non-gaussianities will also be generated, sharpening current constraints on DBI-type models which already predict a significantly non-gaussian signal.

Figures (2)

• Figure 1: To calculate the density perturbation, we must add the contribution from the end of inflation. The dashed lines represent surfaces of uniform density while the curved line is the surface inflation ends on. The arrow indicates the direction of time. Between horizon exit and A, the superhorizon modes do not evolve. However, to calculate the size of the perturbation that will remain after inflation (line C), we must calculate the contribution from the shaded region.
• Figure 2: A schematic illustration of the brane position on a simple space $\mathbb{R}^{1}\times S^1$. The anti-brane prefers energetically to sit at $\tau=0$, while the brane moves toward it along the surface of the cylinder (along the dashed blue line connecting the branes). In calculating the brane/anti-brane separation the angular position of the brane is at least as important as the radial position, since the radius (and therefore the circumference) of the $S^1$ is larger than the red-shifted string length.