How do galaxies form? How much dark matter is there in the Universe? How does the Universe evolve? Rotation curve fitting gives astronomers a handle on these questions. It's still very much a work-in-progress, and new results come daily, with quite a bit of arguing going on about how to interpret the results. But here's a bit of a flavor:
How do galaxies form?The results from the rotation curve fitting indicate that the dark matter fraction in galaxies can vary quite a bit. If the dark matter and normal matter (i.e. stars + gas) is mixed evenly throughout the Universe, why do some galaxies (like low surface brightness galaxies) have a very largeHow much dark matter is there in the Universe?(M/L)total , while other galaxies have a relatively small(M/L)total? Obviously something differs in the way these two types of galaxies have formed and/or evolved with time. Some astronomers have suggested that galaxies do in fact form with different amounts of dark and normal matter; others have argued that they start with similar amounts but that some galaxies expel a great deal of their normal material through a phenomenon known as "starburst winds." Because we don't "see" out in radius far enough to see the halo stop, other models claim that the amount of dark matter one infers for a galaxy depends on the physical size of the disk -- disks that extend further reveal more massive halos. All these models have their strengths and weaknesses, and active research in this field continues.A lot. Given thatHow will the Universe evolve?(M/L)total for galaxies fall in the range 10-50 (and remember there may be even more, since we don't see the drop in rotation velocity which means we've seen the edge of the halo), there is easily a factor of 10 or more dark matter than normal matter. By observing the density of starlight in the Universe (i.e., the luminosity in a given volume of space), and then multiplying by a typical mass-to-light ratio, astronomers can derive an estimate of the density of matter -- both normal and dark matter -- in the Universe. This density is typically characterized by a parameter called OmegaM, which is the density of matter in the Universe divided by the critical density needed to halt the expansion of the Universe. Typical numbers are about OmegaM ~ 0.3 -- that is, the density of matter is about 30% of the critical density. Other arguments based on the abundances of light elements in the Universe suggest that the Omegab, the density of normal mass, is about 0.03, again arguing that there is at least 10x more dark matter than normal matter.Given that these arguments (and others as well) indicate that the Universe only has 30% of the mass necessary to halt the expansion, the Universe should keep expanding. In fact, there is new tentative evidence that the expansion may be even be accelerating -- but that's a different stort!Are we completely nuts?At this point it would be incomplete to leave out a comment on a very controversial theory to explain galaxy rotation curves. A few astronomers have argued that we don't need to invent massive dark matter halos to explain the flatness of rotation curves. Instead, they argue, perhaps we don't understand all the fine details of the law of gravity. One of these theories is called MOND -- "modified Newtonian dynamics" -- and it says that when the force of gravity is very weak, like it is in the outer parts of spiral galaxies, it does not fall off is rapidly as under "normal" conditions (like in the inner regions of galaxies, or around stars and planets, etc). In this case, the rotation curves could be explained entirely by the mass we see -- the stars and gas in galaxies.Again, this theory is very controversial and few astronomers give it much credence. But there is good data to support the theory, and although many claims have been made that MOND has been disproved, that is not yet true. Either way, we are missing something fundamental -- either galaxies and the Universe is filled with mysterious dark matter which we cannot see and don't understand, or else the very laws of gravity need to be revised. Interesting times indeed...
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