Probing the early Universe with axion physics 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.

Dark matter targets for axionlike particle searches

“…ALPs in these alternative cosmologies have also been considered recently in a number of papers. This includes work on using experimental results to use ALPs as a means to probe early-universe physics and distinguish between different cosmological scenarios [42,43], constraining ALP properties with gravitational waves [43,44], the implications of early-matter domination for small-scale DM structures such as ALP miniclusters [45,46] and solving axion problems associated with axion overproduction [47]. 1 This paper builds on this previous work and extends it with a focus on the implications for future experiments, including the provision of benchmarks.…”

Imprints of the early Universe on axion dark matter substructure

“…where the first factor is given by Eq. (26). For thermal DM, an analogous cutoff in the power on small scales occurs in the presence of non-zero DM velocities and the resulting free-streaming [28].…”

“…[22] performed an early study of axion dark matter with a period of EMD; more recent studies include Refs. [23][24][25][26] and projections for a wide range of proposed experiments for this scenario were given in Ref. [10].…”

Axion dark matter from Higgs inflation with an intermediate H*

“…Here, the subscript "osc" refers to the moment when the QCD axion field starts to oscillate, so that the numerical result is well approximated by computing the temperature T osc by setting m A (T osc ) ≈ 3H(T osc ). In order to compute the value of H(T osc ), the temperature-dependence of the axion mass [96,106], the particle content of the underlying theory [107][108][109], and the underlying cosmology [27,30,37,42] play a role. Assuming that around temperature T osc the evolution of the Universe is described by the standard cosmological model and that the mass of the QCD axion scales as m A (T ) ∝ T −4 , we obtain the present axion abundance as [27]…”

“…Light QCD axions that spectate inflation have recently received attention both from the theory perspective [23-27, 29, 30, 34] and from the perspective of detection in proposed and ongoing experiments [35]. In such a scenario, topological defects are washed out by inflation, and considerable additional effort to assess inhomogeneities like axion strings [16,36,37] or axion miniclusters [38][39][40][41][42] is not required. However, the challenge in this scenario relies on building a consistent theory of inflation that leads to a low enough scale of inflation, so that the axion can constitute all dark matter and simultaneously avoid the current bounds from the non-detection of dark matter isocurvature fluctuations.…”