Opening the 1$\,$Hz axion window

Understand

An axion-like particle (ALP) with mass $m_\phi \sim 10^{-15}$eV oscillates with frequency $\sim$1 Hz.

• This mass scale lies in an open window of astrophysical constraints, and appears naturally as a consequence of grand unification (GUT) in string/M-theory.
• However, with a GUT-scale decay constant such an ALP overcloses the Universe, and cannot solve the strong CP problem.
• In this paper, we present a two axion model in which the 1 Hz ALP constitutes the entirety of the dark matter (DM) while the QCD axion solves the strong CP problem but contributes negligibly to the DM relic density.