I’ve just been reading this BBC article about dark matter. It says that one of the early clues was the velocities of stars orbiting the galactic centre. In the solar system, the farther out you orbit, the slower you orbit. That’s not true for the galaxy.
But aren’t they missing a big difference? In the solar system, the Sun out-masses everything else by orders of magnitude. However, the black hole at the heart of the Milky Way is ‘only’ 4.5M solar masses but the Milky Way as a whole weighs 100B solar masses. So the situation is reversed. How is that not relevant?
The galactic core is not a point source. It’s an estimated 25,000 light years in diameter. That would be a large percentage of the mass in the Milky Way.
The mass distribution is factored into calculations of galactic motion, as are relativstic effects due to mass density concentrations and the overall galactic distances. Even with that factored in, we would expect the stars toward the rim to be moving at a significantly slower rotational rate, bit this clearly isn’t the case. Even more, we’ve seen in collisions between galaxies effects that could only be described as pseudoviscous behavior resulting from gravitation interaction between unseen mass distributed between stars, the “missing” or cold dark matter.
Although astronomers and particle physicists really don’t have a good model or explanation for the particle(s) that comprise dark matter, it is almost universally accepted that it is some kind of weakly interacting non-radiating mass. The only viable alternative is a major modification of general relativity, and so far no proposed hypothesis has survived more than cursory effort at falsification.
There is a famous theorem in gravitational mechanics that says that for a spherically-symmetric mass distribution (in the case of a galaxy, only in two-dimensions, but the same concept applies), the gravitational field felt by a mass is the same as the gravitational field that would be emitted by a point mass at the centre with the same mass as all of the matter that is *closer *to the centre than the particle we are considering. The effect of all of the mass that is *farther *from the centre nets to zero.
So, yes, this does predict a difference between the orbital profile of a galaxy and that of the solar system. For the solar system, the sun is effectively a point source and the planets all orbit the same mass, just at different radii. For a galaxy, since the mass is not all concentrated at a point, it is like the outer stars are orbiting a larger central mass than the inner ones. This can still result if a rotation curve that obeys the general rule “the further out you are, the slower you orbit”, but it also might not be true, depending on the distribution of mass in the system.
The problem comes in if you assume that the density of a region in a galaxy is proportional to the amount of “glow” (which seems reasonable at first blush, because most of the mass we see in our galaxy is stars,) we predict a rotation cure that is different that what is observed – we predict one where the rotation curve decays with radius, while we observe one that is basically flat. So one way (IMO the simplest) to resolve this contradiction is to reject the hypothesis that the density is proportional to amount of glow, and that there must be a lot of “non-glowing matter” in galaxies.
Is this really true? Wiki’s page on alternatives to general relativity mentions relativistic MOND and Moffat’s theories as two modern theories that seem to be still under consideration.
There are naturally proposed alternatives and modifications to general relativity; despite how well the theory has been tested and predicted phenomena before discovery, there are some pretty significant problems with relativity, not the least of which that the metric theory of GR is completely unharmonious with the quantum field theories that have become the accepted treatment for the interactions of all other fundamental forces. I think nearly all physicists acknowledge that relativity is, at best, an approximation for some deeper theory that unifies all of the forces, and may actually be a trivial expression of the true mechanism for gravitation. But proposed alternatives suchas MOND or Moffet–which are incomplete at best in comparison to GR–are far from unseating GR as the dominant model of gravitation and in various formulations have been repeatedly falsified.
The notion of a nonradiating, slow moving, weakly interacting mass comprising nearly a quarter of the bound mass-energy in thr visivle universe is pretty consistent with observation and works within the existing framework of GR. Like the luminiferous aether, it may turn out to be completely wrong, but for now it provides the most complete and accurate prediction of the behavior we see on cosmic scales, and also (along with similarly ambiguous “dark energy”) resolves some fundamental problems with inflation rates of the early universe. That it is a placeholder for a particle (or multiple paricles, or gauge fields) is an expression of our lack of detailed knowlege, but until another workable, falsifiable theory comes along that is more comprehensive than GR–and the most likely theories are some kind of quantum field theory of gravity or loop quantum gravity–the dark matter/dark energy hypothesis, such as it is, remains the best candidate for fitting to observation.
Really, it depends on what you mean by “under consideration”. There are plenty of physicists coming up with new and novel ways to attack MOND, so by that standard, it’s under plenty of consideration. And the model does have some interesting predictions which have panned out, so it’s impossible to sweep it completely under the rug. But most physicists believe that those interesting predictions are actually just some as-yet-unexplained emergent property of dark matter dynamics, and if you polled ten random physicists, you’d get nine of them saying that MOND doesn’t stand a chance, and that only if Milgrom happened to be one of the ten.