Probing the Early Universe with Axion Physics and Gravitational Waves
  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.
