Squeezing Full-Shape Dynamical Dark Energy Constraints with Galaxy Alignments

Recent $2-4\sigma$ deviations from the Cosmological Constant $\Lambda$ suggest that dark energy (DE) may be dynamical, based on baryon acoustic oscillations and full-shape galaxy clustering (FS GC) analyses. This calls for even tighter DE constraints to narrow down its true nature. In this Letter, we explore how galaxy intrinsic alignments (IA) can enhance the FS GC-based DE constraints, using Fisher forecasts on various extensions of dynamical DE models, including scenarios with curvature, massive neutrinos, and modified gravity. Incorporating IA improves the DE Figure-of-Merit by $42-57\%$ and tightens the primordial power spectrum amplitude constraints by $17-19\%$. Our findings highlight IA's potential as a valuable cosmological probe complementary to GC.