Abstract
Abstract It is widely believed that axions are ubiquitous in string theory and could be dark matter. The peculiar features of axion dark matter are coherent oscillations and a coupling to the electromagnetic field through the Chern–Simons term. In this letter, we study the consequences of these two features of axions with mass in the range $10^{-13}\,{\rm eV}$ to $10^{3}\,{\rm eV}$. First, we study the parametric resonance of electromagnetic waves induced by the coherent oscillation of the axion. Since the resonance frequency is determined by the mass of the axion dark matter, if we detect this signal, we can get information on the mass of the axion dark matter. Second, we study the velocity of light in the background of the axion dark matter. In the presence of the Chern–Simons term, the dispersion relation is modified and the speed of light will oscillate in time. It turns out that the change in the speed of light would be difficult to observe. We argue that future radio wave observations of the resonance can give rise to a stronger constraint on the coupling constant and/or the density of the axion dark matter.
Highlights
According to string theory, axions are ubiquitous in the universe, dubbed the string axiverse [1,2,3,4,5]
The electrodynamics in the presence of the axion is called the axion electrodynamics [25] and has the Chern–Simons coupling between the axion and the gauge field. We see this that interaction induces the parametric resonance of electromagnetic waves and yields an oscillation of the speed of light in time. We study these two effects to give rise to a new way to explore axion dark matter in the mass range 10−13 –103 eV corresponding to the observable frequency range of electromagnetic waves of 10Hz–105 THz
Except for α = ±π/2, circular polarization appears. These results show that axion dark matter can produce parity violation in electromagnetic waves
Summary
Axions are ubiquitous in the universe, dubbed the string axiverse [1,2,3,4,5]. There are other methods proposed for detecting axion dark matter, for example the super-radiance instability of the axion field in rotating black holes constraining the mass range to 10−20 –10−10 eV [2,17,18,19], gravitational wave interferometers for probing axions with mass 10−22 –10−20 eV [20], the dynamical resonance of binary pulsars probing the mass range. Since electromagnetic waves are often used to explore the universe, it is worth studying the phenomena in detail. The electrodynamics in the presence of the axion is called the axion electrodynamics [25] and has the Chern–Simons coupling between the axion and the gauge field We see this that interaction induces the parametric resonance of electromagnetic waves and yields an oscillation of the speed of light in time.
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