Cosmological imprints of string axions in plateau

TL;DR

This paper explores how string axions with plateau potentials can leave observable cosmological signatures, including effects on the matter power spectrum and potential gravitational wave signals, offering new insights into axion physics and cosmology.

Contribution

It introduces the study of cosmological imprints of string axions with plateau potentials, highlighting the effects of delayed oscillations and parametric resonance on observable phenomena.

Findings

Parametric resonance can enhance the power spectrum below the Jeans scale.

Long-lasting resonance may produce detectable gravitational waves.

Ultralight axions can alleviate Lyman-alpha forest tensions.

Abstract

We initiate a study on various cosmological imprints of string axions whose scalar potentials have plateau regions. In such cases, we show that a delayed onset of oscillation generically leads to a parametric resonance instability. In particular, for ultralight axions, the parametric resonance can enhance the power spectrum slightly below the Jeans scale, alleviating the tension with the Lyman $\alpha$ forest observations. We also argue that a long-lasting resonance can lead to an emission of gravitational waves at the frequency bands which are detectable by gravitational wave interferometers and pulsar timing arrays.