Cosmology of the Tachyon in Brane Inflation

TL;DR

This paper explores the cosmological effects of tachyon condensation in brane inflation, highlighting its limited role in inflation duration, its impact on density perturbations, and implications for non-gaussianities and tensor/scalar ratios.

Contribution

It provides a detailed analysis of tachyon dynamics in brane inflation, identifying conditions under which tachyons influence inflation and perturbation spectra.

Findings

Tachyon contributes to inflation only in a small parameter space.

Non-adiabatic density perturbations are generated at inflation's end.

Relativistic brane motion enhances tensor/scalar ratio and non-gaussianities.

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.