# Probing the Early Universe with Axion Physics and Gravitational Waves

## TL;DR

This paper evaluates the potential of upcoming experiments to detect axion dark matter and gravitational waves, considering non-standard early universe histories, and highlights the complementary nature of axion searches and gravitational wave observations.

## Contribution

It provides new predictions for axion detection prospects and gravitational wave signals under various cosmological models with non-standard thermal histories.

## Key findings

- Certain axion mass ranges are accessible to upcoming experiments.
- Gravitational wave spectra from axionic strings could be detectable by future observatories.
- Non-standard cosmologies can enhance the detectability of axions and gravitational waves.

## Abstract

We show results for the expected reach of the network of experiments that is being set up globally with the aim of detecting the "invisible" axion, in light of a non-standard thermal history of the universe. Assuming that the axion is the dark matter, we discuss the reach of a successful detection by a given experimental setup in a particular axion mass window for different modifications of the cosmological background before primordial nucleosynthesis occurred. Results are presented both in the case where the present energy budget in cold axions is produced through the vacuum realignment mechanism alone, or in the case in which axionic strings also provide with additional contributions to the axion energy density. We also show that in some cosmological models, the spectrum of gravitational waves from the axionic string network would be within reach of the future network of detectors like LISA and DECIGO-BBO. We conclude that some scenarios describing the early universe can be probed jointly by the experimental efforts on axion detection and by gravity wave multi-messenger astronomy.