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That's exactly right, and there's really nothing more to it than that, conceptually anyway. Now, one possible alternative to dark matter is that gravity itself

That's exactly right, and there's really nothing more to it than that, conceptually anyway. Now, one possible alternative to dark matter is that gravity itself doesn't behave the way we think it does. There is a group of theories that we generally call MOND (MOdified Newtonian Mechanics), which propose that gravity is stronger than we think at very low levels of acceleration. When is acceleration very low? Well, it's low when an object is very far away from any other sources of gravity, and this actually describes most stars in galaxies. If a star isn't too near the center of a galaxy, the gravitational force on it is extremely small -- much smaller, for example, than the pull of the sun on the earth. Most stars are traveling in nearly a straight line, bending only ever so slightly on an enormous circular path. That bending is caused by a teeny tiny acceleration -- a gravitational pull -- toward the center of the galaxy's mass. Which is always more or less right in the middle. MOND theories propose that as objects get farther away from each other and experience less gravity, the gravity still remains stronger than is predicted by classical newtonian mechanics. Thus, the gravity experienced by most stars in a galaxy is greater than we think, and therefore they must rotate faster for the galaxy to maintain a stable shape. These are attractive theories IMO, and they explain some things that dark matter has difficulty explaining. However, the galaxy described in the article is a serious problem for MOND, because MOND says that it should be rotating as fast as all other galaxies of its size. But it isn't. It is rotating at the rate predicted by the classic newtonian model of gravity, which would naturally seem to rule out any other modified model of gravity.

tynpeddler 9y ago
This is a good explanation, but there's a bit more to be said about MOND. MOND only argues that dark matter is unnecessary, it does not argue against the existence of undetected matter. If you look at a galaxy, you can measure the amount of visible mass using star luminosity, and you can calculate the total mass of the galaxy under standard cosmology using orbits, radial velocities and so forth. Comparing the visible mass to the calculated mass reveals a huge discrepancy. Some of the undetected matter can be normal, baryonic matter, but there is to much missing matter for it all to be normal matter. So we postulate the existence of an unknown type of matter that interacts gravitationally but not by radiation (light). Using MOND, when you calculate the the mass of a galaxy and compare it to the visible mass, you still get a discrepancy! This discrepancy is much smaller however. Under standard cosmology it's a factor of 10-100 times, under MOND, the mass discrepancy is usually in the neighborhood of 2. In other words, under MOND, there's still matter in the universe that we can't detect by its light, but the missing mass is small enough that it can be explained by normal matter that's to dim to detect. This is why things like the bullet cluster are actually expected under MOND. I don't know the particulars of this exact galaxy, but it's possible that the radial velocities that astronomers measured for its stars have a high enough margin of error such that it still fits within MOND's predictions. Something I've noticed is that MOND is still fringe enough that you can't necessarily trust non-MOND physicists when they talk about it. One thing is for sure however, this galaxy is as puzzling for Dark Matter as it is for MOND.
- nate_meurer 9y ago
  This is great, thank you.

tynpeddler 9y ago

This is a good explanation, but there's a bit more to be said about MOND. MOND only argues that dark matter is unnecessary, it does not argue against the existence of undetected matter. If you look at a galaxy, you can measure the amount of visible mass using star luminosity, and you can calculate the total mass of the galaxy under standard cosmology using orbits, radial velocities and so forth. Comparing the visible mass to the calculated mass reveals a huge discrepancy. Some of the undetected matter can be normal, baryonic matter, but there is to much missing matter for it all to be normal matter. So we postulate the existence of an unknown type of matter that interacts gravitationally but not by radiation (light). Using MOND, when you calculate the the mass of a galaxy and compare it to the visible mass, you still get a discrepancy! This discrepancy is much smaller however. Under standard cosmology it's a factor of 10-100 times, under MOND, the mass discrepancy is usually in the neighborhood of 2. In other words, under MOND, there's still matter in the universe that we can't detect by its light, but the missing mass is small enough that it can be explained by normal matter that's to dim to detect. This is why things like the bullet cluster are actually expected under MOND. I don't know the particulars of this exact galaxy, but it's possible that the radial velocities that astronomers measured for its stars have a high enough margin of error such that it still fits within MOND's predictions. Something I've noticed is that MOND is still fringe enough that you can't necessarily trust non-MOND physicists when they talk about it. One thing is for sure however, this galaxy is as puzzling for Dark Matter as it is for MOND.

nate_meurer 9y ago
This is great, thank you.