Speaker
Description
Ultralight axion-like particles (ALPs) generically induce cosmic birefringence through their coupling to photons, but existing measurements based on integrated polarization rotation are primarily sensitive to the overall amplitude of the effect and do not directly probe the spatial coherence of the underlying field. We show that wide-field measurements of galaxy polarization provide a direct probe of the three-dimensional coherence structure of ALPs in the late Universe by exploiting the two-point correlation of the polarization-rotation angle as a function of the physical pair separation $(r)$. If the ALP behaves as nonrelativistic dark matter or a subdominant ultralight component, spatial fluctuations in its oscillating phase generate a correlation signal characterized by a coherence length $(r_{\rm coh}\propto(m_a v)^{-1})$, corresponding to the de~Broglie wavelength of the field. This geometric observable enables a direct and model-independent determination of the ALP mass, effectively breaking the degeneracy between coupling strength and field amplitude inherent in one-point statistics. We construct a tomographic estimator based on three-dimensional pair separations and show that the coherence scale can be robustly extracted from wide-field data using an inverse-variance--weighted collapsed correlation function. For a survey with $(\sim10^6)$ polarized galaxies over a large sky fraction, a polarization rotation amplitude of order $(0.1^\circ)–(1^\circ)$ is detectable at $(5\sigma)$ over the mass range $(m_a\sim10^{-28})–(10^{-27}\,\mathrm{eV})$. The corresponding coherence scale can be measured with a precision $(\sigma(\log_{10} m_a)\lesssim 0.1)$ across a broader range $(m_a\sim10^{-28})–(10^{-26}\,\mathrm{eV})$, provided that the signal is detected. At higher masses $((m_a\sim10^{-26}\,\mathrm{eV}))$, detection becomes challenging for the fiducial survey assumptions, but would be enabled by larger galaxy samples or improved inference of the intrinsic polarization position angles of the sources (e.g., from galaxy morphology). This method provides a direct geometric probe of the coherence scale of ultralight fields, complementary to CMB birefringence and structure-formation constraints.