| Abstract: Ultralight dark matter (ULDM) that induces variations in fundamental constants is well motivated by a variety of UV models and provides an excellent target for rapidly advancing precision measurements. While linear interactions have been extensively studied, quadratic scalar couplings remain much less constrained and arise naturally in many theories. Importantly, they can generate qualitatively new signatures through background-induced effects. In this talk, I will introduce two major classes of background-induced effects: wake-type forces and oscillatory-type forces.
For wake-type forces, I will present a complete framework that unifies previous approaches developed for different regions of parameter space. I will then apply this framework to the MICROSCOPE satellite, a leading space-based test of the equivalence principle. The Earth's screening effect redistributes the directional dark matter wind, while the satellite's orbital motion modulates the resulting signal, imprinting a characteristic frequency-band structure with an annual modulation that provides a distinctive fingerprint and substantially improves sensitivity. For oscillatory-type forces, I will discuss two distinct signatures in gravitational-wave observatories: a coherent signal at the second harmonic of the ULDM oscillation and a stochastic beat signal arising from interference among different ULDM modes. I will focus on pulsar timing arrays (PTAs), which currently provide leading sensitivity, and the future flagship space-based observatory, the Laser Interferometer Space Antenna (LISA), where screening is absent. Together, these two classes of background-induced effects provide complementary precision probes of quadratically coupled ultralight dark matter through late-Universe measurements. |