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 λcoh 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 mₐ ~ 4×10⁻¹⁹--4×10⁻¹⁶~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 gaγγ 5× 10⁻¹⁴~GeV⁻¹ across most of the band, reaching ~ 7× 10⁻¹⁵~GeV⁻¹ near $0.1$~Hz, which is 10³--10⁴ below the CAST helioscope bound. An RF heterodyne photodetection upgrade -- to either detector -- might extend the search sensitivity to gaγγ~ 6.5× 10⁻¹⁴~GeV⁻¹ at mₐ~ 3× 10⁻⁷~eV for LIGO and gaγγ~ 3.3× 10⁻¹⁷~GeV⁻¹ at mₐ~ 5× 10⁻¹³~eV for LISA. Dark-matter substructure can affect the signal in a several interesting ways.