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The fact that SOME galaxies are inconsistent with the theory of Dark Matter actually strengthens that theory. If the difference in rotation curves, the usual s
by petschge 7y ago
The fact that SOME galaxies are inconsistent with the theory of Dark Matter actually strengthens that theory.
If the difference in rotation curves, the usual signature of dark matter at the galaxy level, from what GR predicts based on the mass of luminous objects was due to a modification in the way gravity works, then that should apply to ALL galaxies. Modified Newtowian Dynamics (MOND) (or the relativistic TeVeS models) should work the same for all galaxies, at least all of roughly same mass and same depth of the gravitational potential.
The fact that we can find similar sized and aged galaxies that behave like they have different amounts of dark matter is therefore NOT caused by the way gravity works. The next best explanation is that most galaxies contain dark matter, but some loose it through their evolution history, e.g. due to interaction with other galaxies.
- whatshisface 7y agoTo be clear, they're not inconsistent with the theory of dark matter, they're inconsistent with those galaxies specifically having dark matter. The author was wording that in a confusing way.
- Koshkin 7y agoThe way I understood it, they are not inconsistent with anything, they just happen to have no DM in them.
- petschge 7y agoThat's right they are inconsistent with the combination of 1.) dark matter is the correct theory and 2.) they have the usual amount of dark matter for a galaxy of that size. Surprisingly this is does not make the theory of dark matter (#1) less likely, but actually makes it more likely. Because the alternative is #1b "gravity works differently than we think". But that would have to hold for all galaxies.
- slowmovintarget 7y agoTo be even more clear, "dark matter" isn't a theory. It is a name for something we need theories about. I wish the article had made that apparent.
- eaim 7y agoSorry but that’s an intellectual dodge. A verbal fiddle. Saying it is a “name for something” doesn’t make the fact that it’s a hypothesis disappear. One is simply positing the need for more “Russian dolls” inside the Russian doll one has hypothesised to begin with.
- astrophysician 7y agoWhy is this a dodge? We have observations that need an explanation; for example: rotation velocities of stars in our galaxy imply a large amount of mass that if it were visible would certainly be detected. There are several competing hypotheses, one being a cold, weakly-interacting (i.e. it interacts via the weak force) type of new matter but all of them are claiming to explain the same observations, and the collection of observations connected to this phenomenon have a place-holder name until the problem is conclusively resolved. There’s no dodge here...
- hannasanarion 7y agoDark matter is a hypothesis, and one that makes pretty strong predictions, several of which have come true, so it's not inappropriate to call it theory. The phenomenon of galaxies demonstrating significanty more mass than their sum of normal matter is the observation that is explained by dark matter.