Scalar ultralight dark matter coupled to photons: detect or exclude
In plain words
A scalar dark matter field (a field with no direction, like temperature) oscillating at its mass frequency would make $\alpha$ oscillate. Networks of clocks can look for this oscillation over a huge range of masses.
Precise statement
For scalar dark matter $\phi$ of mass $m_{\phi}$ in 1e-23 to 1e-15 eV with local density $0.4\,\mathrm{GeV}\,\mathrm{cm}^{-3}$ and dimensionless linear coupling $d_{\gamma}$ to the photon (called $d_e$ in the Damour-Donoghue convention, unrelated to the electron EDM), determine whether $d_{\gamma}$ is nonzero below present limits from clock comparisons and the MICROSCOPE equivalence-principle bound, by searching for oscillations of optical-clock frequency ratios at angular frequency $m_{\phi} c^2/\hbar$.
What would settle it
Clock-ratio time series (optical, nuclear or highly charged ion clocks, including space clocks) that improve on the MICROSCOPE-derived $d_{\gamma}$ limit across the mass range, or show a coherent oscillation seen at two sites.
Related problems
- Special case of What is dark matter made of? · Scalar ultralight dark matter is one candidate answer to dark matter identity