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MoND versus Dark Matter: Battle to the Death in Low Surface-Brightness Galaxies

Today, I decided to exercise my artistic skill and create a 12-layer .gif. This .gif, which you will see very soon, demonstrates the reason scientists have postulated the existence of dark matter.

See, in the solar system, the farther you get from the Sun (where most of the system's mass is concentrated), the slower the planets orbit, as they experience less gravitational pull from the Sun. The same should be true of galaxies, where the mass is concentrated in the center. If this were true, we would observe the work of art labeled as 'expected' in the .gif, where velocity goes down as distance goes up. However, that is not what we see. What we see is that orbital velocity pretty much stays the same as you get farther from the center of the galaxy.

So astronomers said, "Whoa. There is mass missing. This missing mass must not emit light, making it 'missing,' and it must be distributed away from the galactic center in order for it to have the gravitational influence we're seeing, namely making things far from the see-able mass rotate quickly." This, by the way, is a direct quote from all astronomers.

Anyway, here's the .gif:

Most astronomers, scientists in general, and the general public accept, and are even excited by, the existence of dark matter, not least because of its suave name.

But there is another idea that can explain a flat galactic rotation curve, and it has an unsuave name: MoND, or Modified Newtonian Dynamics. MoND introduces a new factor, "a_o," into the Universal Gravitational Equation, in the form of a/a_o.

In situations like the solar system's, where acceleration (a) due to gravity is relatively high, a>>a_o approaches a, and Newton's equations are a perfect approximation. But in situations like the galaxy's, where the distance from the central mass is large and the gravitational acceleration thus small, a_o>>a, and gravitational behavior totally changes.

So Stacy McGaugh of the University of Maryland decided to do something to help resolve this debate between the MoNDistas and the Dark Matternians.

The Baryonic Tully-Fischer Relation (BTRF) is an empirical relation between baryonic mass (which in this case mostly means not-dark, and includes both gas and stars) and rotational velocity. Mass is assumed to be proportional to luminosity, because the brighter a galaxy is, the more stars it has--but there are problems with the specifics of the proportionality, namely that there are large errors introduced by uncertainties in stellar evolution and the stellar mass function. Calculating the gas-mass of a galaxy is much easier, because the gas is made of neutral hydrogen atoms, and each atom always emits a photon at a specific wavelength and a specific brightness. So if we know what the total brightness is, we can calculate how many atoms it would take to make that brightness, and then we can use our knowledge of the periodic table to find out how much mass is in that many H atoms.

So what if there was a galaxy that was made mostly of gas, so that we could calculate its mass very precisely, without the confusion of a bunch of stars? That's what McGaugh did. He found 47 "late-type, low surface-brightness galaxies," calculated their masses, calculated the rotational velocity profile that the BTRF dictates the galaxy should have, and then compared the predictions of the BTRF to MoND's and dark-matter's predictions for this same amount of mass.

This figure shows the mass/velocity of the 47 galaxies as neon spaceships. As you can see, the spaceships hover close to the MoND prediction, whereas the dark matter prediction is cold and alone.

Now, the author is careful not to say that MoND is The True Theory. There are things we observe that MoND doesn't predict, predictions MoND makes that we don't observe. However, it is one of the few 'fringe' theories that has experienced any 'predictive success.'

While this paper neither proves MoND correct nor dark matter incorrect, it does say, "Look, MoND is doing something right, and dark matter is doing something wrong. Stand up and take notice, because these are actual data, and you can't ignore the problems they present for your model." This paper shows that Dark Matternians have some explaining to do, which will, in the end, either make their theory stronger or destroy it in a flaming burst of scientific mind-death.

Data supporting a fringe theory will force the dominant theory to explain itself, to alter itself to fit the data or, if that is not possible, to cease and desist. I like it when the underdog undermines the dominant paradigm. And, as a side note, I like to think the underdog is the correct dog. In the end, though, I will cheer for whatever idea actually lines up with the way those crazy galaxies are spinning.


Stacy McGaugh (2011). A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies Physical Review Letters

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