Dark energy from the string axiverse

TL;DR

This paper proposes a string axiverse model where numerous axion-like fields contribute to dark energy, addressing the timing of cosmic acceleration without super-Planckian fields and predicting observable consequences.

Contribution

It introduces a novel axion-based quintessence model with all dimensionless parameters of order unity, explaining dark energy onset and avoiding super-Planckian excursions.

Findings

Dark energy onset occurs randomly over cosmic time.

Approximately 1% of universes resemble ours.

Modified axion potentials eliminate the need for rapid decay of axion fields.

Abstract

String theories suggest the existence of a plethora of axion-like fields with masses spread over a huge number of decades. Here we show that these ideas lend themselves to a model of quintessence with no super-Planckian field excursions and in which all dimensionless numbers are order unity. The scenario addresses the "why now" problem, i.e., why has accelerated expansion begun only recently, by suggesting that the onset of dark-energy domination occurs randomly with a slowly decreasing probability per unit logarithmic interval in cosmic time. The standard axion potential requires us to postulate a rapid decay of most of the axion fields that do no become dark energy. The need for these decays is averted, though, with the introduction of a slightly modified axion potential. In either case, a Universe like ours arises in roughly 1 in 100 universes. The scenario may have a host of observable consequences.