LIGO, LISA, and ultralight axionlike dark matter

A coherent cosmic background of axion-like particles (ALPs) coupled to photons can produce a small periodic differential phase or polarization modulation for photons traversing separate arms in gravitational wave interferometers, peaked at a frequency associated with the particle mass, and suppressed whenever the dark-matter coherence length $λ_{\rm coh}$ exceeds the arm length~$L$. For the LIGO audio frequency band the sensitivity to cosmic ALPs is below current bounds. For LISA, however, the natural mass range $m_a \sim 4\times10^{-19}$--$4\times10^{-16}$~eV -- corresponding to a sideband frequency in LISA's science band of $0.1$~mHz--$0.1$~Hz -- may be observable. A 1-year shot-noise-limited search projects a sensitivity $g_{aγγ} \lesssim 5\times 10^{-14}$~GeV$^{-1}$ across most of the band, reaching $\sim 7\times 10^{-15}$~GeV$^{-1}$ near $0.1$~Hz, which is $10^{3}$--$10^{4}$ below the CAST helioscope bound. An RF heterodyne photodetection upgrade -- to either detector -- might extend the search sensitivity to $g_{aγγ}\sim 6.5\times 10^{-14}$~GeV$^{-1}$ at $m_a\sim 3\times 10^{-7}$~eV for LIGO and $g_{aγγ}\sim 3.3\times 10^{-17}$~GeV$^{-1}$ at $m_a\sim 5\times 10^{-13}$~eV for LISA. Dark-matter substructure can affect the signal in a several interesting ways.