| Terminology nitpick: MOND is a Dark Matter theory. Dark Matter is whatever causes the observed phenomena that galaxy rotation rates are all wonky. If it explains galaxy rotation rates, which MOND does, it's dark matter. I'll use the term CDM ("Cold Dark Matter", meaning non-relativistic matter like dark matter) which is a catchall for Axions, WIMPs, and MACHOs when I'm talking about the prevailing theory of dark matter. (note that MACHOs have been falsified by observation, even though it was originally the leading hypothesis for a while.) > and extremely intuitive for the physicist, No it isn't. We haven't gotten here because it's intuitive, we've gotten here because it's the only hypothesis which is consistent with the data. > Unfortunately it seems that the physicist community (outside of a small subset) is unwilling to research in the MOND space; it has the taint. This is false. MOND was a leading theory up until it was falsified by the Bullet Cluster. MOND was later falsified by baryon acoustic oscillations. Then again by ultra diffuse galaxies. Most MOND theories predicted gravitational waves traveling slower than the speed of light, all of which were falsified by GW 170817, the neutron star-neutron star merger a few years ago. MOND has little support in 'orthodox' physics and cosmology because it keeps getting falsified. > So physicists just pile on a bunch of extra variables to make Dark Matter fit certain observations, when MOND describes those observations very simply. MOND is viable because we add more variables to it every time the latest observation comes along that falsifies it. It's true that the original, first incarnation of MOND is simpler than CDM, but every time we learn more about the universe, CDM still stands but MOND is invalidated, and so another variable has to be added to it. CDM frontloads all its warts and complexity. "We propose new, as yet undetected and possibly undetectable particles." Everything else is just running the numbers. MOND frontloads its simplicity. "F = m a^2 / a_0". Everything else adds more variables. You want it to be compatible with GR? You need to add new variables to Einstein's equations. You want it to have stars in that are stable? You need to add new variables. You want it to be consistent with gravitational waves traveling near or at the speed of light? New variables. You want it to be compatible with BAO observations? New variables. You want it to be compatible with the bullet cluster and ultra diffuse galaxies? Now you need to add CDM. We are now at the point where viable MOND theories still include CDM, just less CDM than pure CDM. Ballpark numbers are that pure CDM predicts 80% of galaxy rotation curves are caused by CDM and 20% by normal matter, leading viable MOND predicts 20% by normal unadjusted gravity on normal matter, 40% by CDM, and 40% by the modified gravity equations. If you're just looking for the simpler model, MOND ain't it. |