Dark matter in galaxies and MOND phenomenology

7 problems

• Can a relativistic MOND theory fit clusters and the CMB without dark matter?

  Open

  MOND explains galaxy rotation well but still leaves missing mass in galaxy clusters and has trouble with the pattern of the cosmic microwave background. The question is whether any complete version of the theory fixes this without adding unseen matter.

• What is the intrinsic scatter of the radial acceleration relation?

  Contested

  If the universal curve has no spread at all beyond measurement errors, that would favor modified gravity. If it has a real spread that depends on galaxy history, that would favor dark matter.

• Why does galaxy dynamics depend only on the visible matter distribution?

  Contested

  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.

• Why do similar dwarf galaxies have such different inner rotation curves?

  Open

  Two small galaxies with the same overall mass can have very different speeds of rotating gas near their centers. Cold dark matter simulations predict much more uniform behavior and dense central cusps instead of the flat cores often seen.

• Are thin co-orbiting planes of satellite galaxies consistent with cold dark matter?

  Contested

  Many small galaxies orbiting the Milky Way and Andromeda lie in thin flat planes, and many move around in the same direction. Simulations rarely produce such planes unless they are short-lived chance alignments.

• Does the halo mass function continue below $1e8\,\text{solar masses}$?

  Open

  Cold dark matter predicts huge numbers of small dark clumps with no stars in them. They can be detected only by how they bend light from distant objects or disturb thin streams of stars.

• Do wide binary stars show a MOND-like boost in orbital velocity?

  Contested

  Pairs of stars orbiting each other thousands of times farther apart than the Earth and Sun feel very weak mutual gravity. If MOND is right, they should move noticeably faster than Newton predicts.