The Cosmological Dynamics of String Theory Axion Strings

TL;DR

This paper explores the unique behavior of string theory axions in the early universe, showing they generally do not form strings and overproduce dark matter unless specific conditions like large warping are met, with implications for gravitational wave signals.

Contribution

It demonstrates that string theory axions differ from field theory axions in string formation and dark matter production, highlighting scenarios where axion strings can form and impact cosmological observations.

Findings

String theory axions rarely form strings in the early universe.

QCD axion strings tend to overproduce dark matter, ruling them out in most scenarios.

Heavier axion eigenstates can alter string network evolution and produce gravitational waves.

Abstract

The quantum chromodynamics (QCD) axion may solve the strong CP problem and explain the dark matter (DM) abundance of our Universe. The axion was originally proposed to arise as the pseudo-Nambu Goldstone boson of global $\mathrm{U}(1)_{\rm PQ}$ Peccei-Quinn (PQ) symmetry breaking, but axions also arise generically in string theory as zero modes of higher-dimensional gauge fields. In this work we show that string theory axions behave fundamentally differently from field theory axions in the early Universe. Field theory axions may form axion strings if the PQ phase transition takes place after inflation. In contrast, we show that string theory axions do not generically form axion strings. In special inflationary paradigms, such as D-brane inflation, string theory axion strings may form; however, their tension is parametrically larger than that of field theory axion strings. We then show that such QCD axion strings overproduce the DM abundance for all allowed QCD axion masses and are thus ruled out, except in scenarios with large warping. A loop-hole to this conclusion arises in the axiverse, where an axion string could be composed of multiple different axion mass eigenstates; a heavier eigenstate could collapse the network earlier, allowing for the QCD axion to produce the correct DM abundance and also generating observable gravitational wave signals.