• Andrea Minotti

• Title: “Cosmic Birefringence from a pseudo-scalar field”
• Abstract: “When polarized electromagnetic radiation propagates through a region of space permeated by a pseudo-scalar field, the plane of linear polarization undergoes a rotation. The rotation angle depends on the field’s values when the radiation is emitted and when it is detected and does not depend on the frequency of the radiation. This phenomenon is called Cosmic Birefringence (CB) and is usually studied for CMB photons in the context of axion dark matter and axion dark energy. First, we present an analysis of several pseudo-scalar DE models and derive constraints on the parameters of their potentials by combining observations of the background expansion history with measurements of CB. Subsequently, we propose a new explanation in terms of a stiff (w=1) pseudo-scalar field that constitutes a now-subdominant component of the total energy density of the Universe.”

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• Xucheng Gan

• TITLE: “Ultralight Dark Matter Background: Waking, Shaking, and Detecting “
• 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.