2025/11/14 by Ziyang Zheng, Zheng, Ziyang, Schneider, Malte +2
Physics and Astronomy · #Astronomy and Astrophysical Research #Cosmology and Gravitation Theories #Cosmology and Nongalactic Astrophysics (astro-ph.CO) #FOS: Physical sciences #Galaxies: Formation, Evolution, Phenomena
paper · pdf · doi:10.48550/arxiv.2511.11554
openalex publication_date 2025/11/14 · openalex created_date 2025/11/18 · openalex updated_date 2026/08/01
We investigate how the cosmological Euler equation can be tested in the presence of viscous dark matter, violations of the equivalence principle (EP), and modifications of gravity, while relying on minimal theoretical assumptions. Extending the previous analysis, we generalize the observable EP, which quantifies EP violation, to EP, discuss the degeneracy between bulk and shear viscosities and EP-violating effects, and explicitly show that the EP can still be tested in the small-viscosity limit. In addition, we identify a model-independent observable, C\rm vis,0, which characterizes the present-day dark matter viscosity and can be measured from relativistic galaxy number counts by cross-correlating two galaxy populations. We perform forecasts for three forthcoming Stage-IV surveys: DESI, Euclid, and SKA Phase 2 (SKA2), and find that C\rm vis,0 can be tightly constrained, at the level of O(10-6) or better in all cases. Among these surveys, SKA2 provides the tightest constraint, with a 1σ uncertainty of 1.08 × 10-7 on C\rm vis,0.