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Dark-matter/Dark energy might not apply to galaxies - Input Junkie
February 28th, 2011
10:10 am


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Dark-matter/Dark energy might not apply to galaxies
A few theories that would modify our understanding of gravity have been proposed. One of these is Modified Newtonian Dynamics (MOND), which was hypothesized in 1983 by Moti Milgrom a physicist at the Weizmann Institute of Science in Rehovot, Israel. One of MOND's predictions specifies the relative relationship between the mass of any galaxy and its flat rotation velocity. However, uncertainties in the estimates of masses of stars in star-dominated spiral galaxies (such as our own Milky Way) previously had precluded a definitive test.

To avoid this problem, McGaugh examined gas rich galaxies, which have relatively fewer stars and a preponderance of mass in the form of interstellar gas. "We understand the physics of the absorption and release of energy by atoms in the interstellar gas, such that counting photons is LIKE counting atoms. This gives us an accurate estimate of the mass of such galaxies," McGaugh said.

Using recently published work that he and other scientists had done to determine both the mass and flat rotation velocity of many gas rich galaxies, McGaugh compiled a sample of 47 of these and compared each galaxy's mass AND rotation velocity with the relationship expected by MOND. All 47 galaxies fell on or very close to the MOND prediction. No dark matter model performed as well.

"I find it remarkable that the prediction made by Milgrom over a quarter century ago performs so well in matching these findings for gas rich galaxies," McGaugh said. "

Almost everyone agrees that on scales of large galaxy clusters and up, the Universe is well described by dark matter - dark energy theory. However, according to McGaugh this cosmology does not account well for what happens at the scales of galaxies and smaller.

If this pans out, then we have another different forces take effect at different scales regime-- it would be like relativity vs. Newtonian forces, or quantum effects vs. the world as we experience it directly.

And pans out or not, in the good old days there would be science fiction about refining black matter out of the infinitesmal amounts to be found in the solar system, and it would turn out to be magic! Or maybe there would be dark matter found in a lost intergalactic probe. Or it turns out that different forces take effect every time you multiply distance by a thousand. I like hard science fiction, but I also like the kind where the author just makes things up. Fantasy doesn't scratch quite the same itch.

Link thanks to shadesong.

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Date:March 1st, 2011 07:14 pm (UTC)
Interesting. I found the information on the collision I mentioned yesterday. It turns out to have actually been a collision of two clusters of galaxies rather than two single galaxies.
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