Abstract
The axion-gravity Chern-Simons coupling is well motivated but is relatively weakly constrained, partly due to difficult measurements of gravity. We study the sensitivity of LIGO measurements of chirping gravitational waves (GWs) on such coupling. When the frequency of the propagating GW matches with that of the coherent oscillation of axion dark matter field, the decay of axions into gravitons can be stimulated, resonantly enhancing the GW. Such a resonance peak can be detected at LIGO as a deviation from the chirping waveform. Since all observed GWs will undergo similar resonant enhancement from the Milky-Way (MW) axion halo, LIGO O1+O2 observations can potentially provide the strongest constraint on the coupling, at least for the axion mass $m_a = 5 \times 10^{-13} - 5 \times 10^{-12}$ eV. Along the course, we also emphasize the relevance of the finite coherence of axion fields and the ansatz separating forward and backward propagations of GWs. As a result, the parity violation of the Chern-Simons coupling is not observable from chirping GWs.
Highlights
The axion is an important candidate of dark matter
We study the sensitivity of LIGO measurements of chirping gravitational waves (GWs) on such coupling
Since all observed GWs will undergo similar resonant enhancement from the Milky Way (MW) axion halo, LIGO O1 þ O2 observations can potentially provide the strongest constraint on the coupling, at least for the axion mass ma 1⁄4 5 × 10−13 − 5 × 10−12 eV
Summary
The axion is an important candidate of dark matter. Axions are not restricted to the QCD axion, but a variety of axions are predicted from stringy setups [1]. A whole new variety of axion detection experiments and astrophysical probes has been proposed, mainly based on its lightness (due to the pseudo Goldstone nature) and the coherent oscillation (due to the nonrelativistic dark matter nature) [4]. They can constrain the axion couplings to photons and electrons, for example through supernova cooling,. The modification of the EM wave propagating through the coherent axion dark matter field can produce a sharp resonance peak in the frequency spectrum [16,17,18,19,20] or can even produce an echo coming back to us [21].
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