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| Natura: | Preprint |
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2025
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| Accesso online: | https://arxiv.org/abs/2511.11554 |
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| _version_ | 1866912950027026432 |
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| author | Zheng, Ziyang Schneider, Malte Amendola, Luca |
| author_facet | Zheng, Ziyang Schneider, Malte Amendola, Luca |
| contents | 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 $E_P$, which quantifies EP violation, to $\tilde{E}_P$, 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 $\mathcal{O}(10^{-6})$ or better in all cases. Among these surveys, SKA2 provides the tightest constraint, with a $1σ$ uncertainty of $1.08 \times 10^{-7}$ on $C_{\rm vis,0}$. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2511_11554 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Testing the cosmological Euler equation: viscosity, equivalence principle, and gravity beyond general relativity Zheng, Ziyang Schneider, Malte Amendola, Luca Cosmology and Nongalactic Astrophysics 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 $E_P$, which quantifies EP violation, to $\tilde{E}_P$, 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 $\mathcal{O}(10^{-6})$ or better in all cases. Among these surveys, SKA2 provides the tightest constraint, with a $1σ$ uncertainty of $1.08 \times 10^{-7}$ on $C_{\rm vis,0}$. |
| title | Testing the cosmological Euler equation: viscosity, equivalence principle, and gravity beyond general relativity |
| topic | Cosmology and Nongalactic Astrophysics |
| url | https://arxiv.org/abs/2511.11554 |