FattyCatty
Picky V. Nitty
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- Aug 22, 2010
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I saw this article in Science and wondered how common belief in MOND is as opposed to belief in dark matter. Also, does belief in MOND rule out belief in dark matter?
The paper by Stacy McGaugh referred to seems, from what little I could understand, to be more ambiguous and less either or on the existence of dark matter than the Science article implied.
[QUOTE
The current cosmological paradigm, ΛCDM, requires that the mass-energy of the universe be dominated by invisible components: dark matter and dark energy. An alternative to these dark components is that the law of gravity be modified on the relevant scales. A test of these ideas is provided by the Baryonic Tully-Fisher Relation (BTFR), an empirical relation between the observed mass of a galaxy and its rotation velocity. Here I report a test using gas rich galaxies for which both axes of the BTFR can be measured independently of the theories being tested and without the systematic uncertainty in stellar mass that affects the same test with star dominated spirals. The data fall precisely where predicted a priori by the modified Newtonian dynamics (MOND). The scatter in the BTFR is attributable entirely to observational uncertainty. This is consistent with the action of a single effective force law but poses a serious fine-tuning problem for ΛCDM.
<snip>
If MOND is essentially correct in that it is pointing towards an extension of gravitational theory, then the experiments seeking to detect dark matter will find null results. This would also be the case if dark matter has a different nature than currently presumed. If dark matter is detected, then the MOND formula is still useful as a phenomenological constraint on the effective force law in spiral galaxies. In any case, the predictive power of the simple formula proposed by Milgrom [2] is telling us something profound about Nature.[/QUOTE]
Thousands of physicists, astrophysicists, and astronomers are searching for dark matter, mysterious stuff whose gravity seems to hold the galaxies together. However, an old and highly controversial theory that simply changes the law of gravity can explain a key property of galaxies better than the standard dark-matter theory, one astronomer reports. That claim isn't likely to win over many skeptics, but even some theorists who favor the standard theory say the analysis hands them a homework problem they should solve.
"The standard theory should explain this, and it doesn't yet. That's fair to say," says Simon White, a cosmologist at the Max Planck Institute for Astrophysics in Garching, Germany, who was not involved in the current analysis.
<snip>
For the past 28 years, Milgrom's idea, known as Modified Newtonian Dynamics (MOND) has generated a long-simmering debate. Many researchers argue that ever more evidence from clusters of galaxies, the largest scale structure of the universe, and the afterglow of the big bang points to the existence of dark matter. Still, a few researchers counter that when they look at the details, MOND does a better job—at least on the galactic scale.
Now, in the latest shot from the MOND side, Stacy McGaugh, an astronomer at the University of Maryland, College Park, reports that MOND can explain an observed correlation between the mass and the rotation speed of galaxies—that is, the speed of those outer stars—called the baryonic Tully-Fisher relation. MOND researchers had tried to do this before, but for their models to work, they had to make an untested assumption about the relationship between a star's mass and the amount of light it puts out. That assumption introduces a large uncertainty, weakening the argument.
To avoid that problem, McGaugh gathered data from various sources on 47 galaxies that contain more hydrogen gas than stars. The mass of the gas can then be estimated directly. McGaugh made a plot of visible mass versus rotation speed for the galaxies. He then plotted the prediction that comes straight out of MOND in a few lines of algebra. The MOND line went right through the data. "You draw the line and the data fall right on it," McGaugh says. "No muss, no fuss." He reports the result in a paper in press at Physical Review Letters.
The paper by Stacy McGaugh referred to seems, from what little I could understand, to be more ambiguous and less either or on the existence of dark matter than the Science article implied.
[QUOTE
]A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies
Stacy S. McGaugh
Department of Astronomy, University of Maryland, College Park, MD 20742-2421
(Dated: February 22, 2011)
Stacy S. McGaugh
Department of Astronomy, University of Maryland, College Park, MD 20742-2421
(Dated: February 22, 2011)
The current cosmological paradigm, ΛCDM, requires that the mass-energy of the universe be dominated by invisible components: dark matter and dark energy. An alternative to these dark components is that the law of gravity be modified on the relevant scales. A test of these ideas is provided by the Baryonic Tully-Fisher Relation (BTFR), an empirical relation between the observed mass of a galaxy and its rotation velocity. Here I report a test using gas rich galaxies for which both axes of the BTFR can be measured independently of the theories being tested and without the systematic uncertainty in stellar mass that affects the same test with star dominated spirals. The data fall precisely where predicted a priori by the modified Newtonian dynamics (MOND). The scatter in the BTFR is attributable entirely to observational uncertainty. This is consistent with the action of a single effective force law but poses a serious fine-tuning problem for ΛCDM.
<snip>
If MOND is essentially correct in that it is pointing towards an extension of gravitational theory, then the experiments seeking to detect dark matter will find null results. This would also be the case if dark matter has a different nature than currently presumed. If dark matter is detected, then the MOND formula is still useful as a phenomenological constraint on the effective force law in spiral galaxies. In any case, the predictive power of the simple formula proposed by Milgrom [2] is telling us something profound about Nature.[/QUOTE]