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Title: Why does galaxy dynamics depend only on the visible matter distribution?
Plain: In rotating galaxies the actual pull of gravity at each radius can be predicted from the stars and gas alone through one universal curve. That is natural in MOND but surprising if invisible dark matter dominates the mass.
Precise: The radial acceleration relation $g_{\mathrm{obs}} = g_{\mathrm{bar}} / (1 - \exp(-\sqrt{g_{\mathrm{bar}} / a0}))$ with $a0 \sim 1.2e-8\ \mathrm{cm}/\mathrm{s}^{2}$ fits about 150 SPARC rotation curves with small scatter, and the baryonic Tully-Fisher relation $M_{b}$ proportional to $V_{\mathrm{flat}}^{4}$ holds over 5 decades in mass. Explain whether LCDM galaxy formation produces one relation with a single a0 and the observed small scatter, or whether the relation signals modified dynamics or a dark matter coupling to baryons.
Settled by: Cosmological hydrodynamic simulations reproducing the relation, its scatter and its residual correlations, or a theory that predicts it while also fitting clusters and the CMB.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: contested
Status note: (empty)
Posed since: 2016