New opportunities for axion dark matter searches in nonstandard cosmological models

Abstract

We study axion dark matter production from a misalignment mechanism in scenarios featuring a general nonstandard cosmology. Before the onset of Big Bang nucleosynthesis, the energy density of the universe is dominated by a particle field φ described by a general equation of state ω. The ensuing enhancement of the Hubble expansion rate decreases the temperature at which axions start to oscillate, opening this way the possibility for axions heavier than in the standard window. This is the case for kination, or in general for scenarios with ω > 1/3. However, if ω < 1/3, as in the case of an early matter domination, the decay of φ injects additional entropy relative to the case of the standard model, diluting this way the preexisting axion abundance, and rendering lighter axions viable. For a misalignment angle 0.5 < θ i < π/ √ 3, the usual axion window becomes expanded to 4 × 10 −9 eV m a 2 × 10 −5 eV for the case of an early matter domination, or to 2 × 10 −6 eV m a 10 −2 eV for the case of kination. Interestingly, the coupling axion-photon in such a wider range can be probed with next generation experiments such as ABRACADABRA, KLASH, ADMX, MADMAX, and ORGAN. Axion dark matter searches may therefore provide a unique tool to probe the history of the universe before Big Bang nucleosynthesis.

Axions in String Theory and the Hydra of Dark Radiation

“…For the sake of simplicity and concreteness, we will ignore some of the O(1) parameters by fixing them to unity. The more general analysis including the dependence on these parameters is sketched in Appendix E. We will also again make use of the approximate analytical formulae of [70,73] to obtain order-of-magnitude estimates for the most important quantities.…”

“…In this section, we use standard cosmological constraints (cf., e.g., [58,[70][71][72][73] for detailed studies), i.e. measurements of the dark matter abundance and isocurvature bounds, to set the scene and to obtain restrictions concerning the volume, the decay constant etc.…”

“…Assuming a standard cosmological history, this onset of oscillations occurs during radiation domination. As is well known, the axion relic density is given approximately by [58,70,73]…”

“…Such an Early Matter Dominated (EMD) phase may have significant impact on the predictions for axion dark matter [70,72,73,81]. In particular, if a modulus φ decays very late, the axion may begin to oscillate during a phase of matter domination.…”

“…As is commonly done, we will also refer to T end as the reheating temperature, T r ≡ T end . However, it is important to remember that the SM sector itself thermalizes already much earlier and at a higher temperature [70,72,73] 10 , based on the energy input from early φ-decays. Because of this, one can use the formula for the axion potential valid at temperatures above the QCD phase transition underlying (3.16).…”