Helical Inflation and Cosmic Strings

TL;DR

The paper investigates string-inspired, axion-based mechanisms to achieve large-field inflation, introducing a two-axion helical model that yields an effective single cosine potential and, in the super-Planckian limit, behaves like $V(\phi)\approx \tfrac{1}{2}m^2\phi^2$ (chaotic inflation). It shows that the periodic axion potential slightly lowers the tensor-to-scalar ratio $r$ relative to chaotic inflation, quantified by the parameter ${\hat{\Delta}}=16\Delta$ with $\Delta=(r+4(n_s-1))/16$, and provides analytic expressions for observables in terms of $r$ and $\Delta$. The work derives bounds on ${\hat{\Delta}}$ (e.g., ${\hat{\Delta}}\lesssim -0.03$ for $f\ge14\,M_{pl}$) and discusses a quartic-corrected extension, illustrating how the cosine form modifies slow-roll parameters. Additionally, it argues that cosmic strings are natural in axion/string settings and could contribute to B-mode polarization, while outlining observational constraints (Planck and pulsar timing) and possibilities to dilute or accommodate strings via inflation.

Abstract

Recent BICEP2 detection of low-multipole B-mode polarization anisotropy in the cosmic microwave background radiation supports the inflationary universe scenario and suggests a large inflaton field range. The latter feature can be achieved with axion fields in the framework of string theory. We present such a helical model which naturally becomes a model with a single cosine potential, and which in turn reduces to the (quadratic) chaotic inflation model in the super-Planckian limit. The slightly smaller tensor/scalar ratio $r$ of models of this type provides a signature of the periodic nature of an axion potential. We present a simple way to quantify this distinctive feature. As axions are intimately related to strings/vortices and strings are ubiquitous in string theory, we explore the possibility that cosmic strings may be contributing to the B-mode polarization anisotropy observed.