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Headlining Can Be a Dark Matter
By Cindy Merrick and Rebecca Goldin, Ph.D., April 29, 2011
Is dark matter's "demise" a phantom menace? Recent headlines challenging dark matter suggest that science is being shaken to its core but did the media misrepresent the story?

With frequency, headlines turn scientific inquiry into warfare among different ideological camps. Recent headlines suggest upheaval in the fundamental conclusions of physics, and specifically, cosmology. “Dark Matter Theory Challenged,” says the BBC, and “More Evidence Against Dark Matter” boasts Science Magazine. We expect an exciting story to follow, one in which pro-dark-matter physicists are pitted against those denying its existence.  So it seems, science is being shaken at its foundations.

In the latest skirmish, we might conclude from media headlines, a theory without dark matter prevailed handily against the defenders of gravity and dark matter. Using a “new gravity” theory, a physicist accurately modeled the ratios of mass to rotational velocity in 47 spiral galaxies. The plain-old-gravity-plus-dark-matter model seems to have been vanquished; the argument among physicists is a battle for the soul of the universe.

Unfortunately, the headlines obscure a far more nuanced story, only part of which was revealed in the articles themselves. The theory of dark matter is not challenged, and while important questions are raised, nothing is presented as evidence against its existence.

In the study, which appeared in the March issue of Physical Review Letters, University of Maryland physicist Stacy McGaugh found, to his apparent chagrin, that for 47 mostly-gas (that is, having few stars) galaxies, a controversial theory called MOND, which suggests that the theory of gravitation be modified, offered a predictive model that fits almost perfectly with observed data, while currently-used techniques involving the presumed behavior of dark matter required significant assumptions about unknown parameters and calculation adjustments to match the observed data.

Modified Newtonian Dynamics, or MOND for short, was proposed decades ago by Mordehai Milgrom of Israel’s Weizmann Institute, and has since won fewer supporters than skeptics. In Newtonian mechanics, the acceleration due to gravity that one object feels from another shrinks as the square of the distance between them. MOND proposes a twist; perhaps, when acceleration is small enough, a force such as gravity may be proportional to the square of the object’s acceleration.

McGaugh only studied 47 of one type of galaxy, those that are gas-rich and have comparably less mass. The effect (and purpose) of this choice was to diminish the high margin of error that occurs when measuring the mass of a galaxy containing a large number of stars. And while a restriction to this small number of gas-rich galaxies available for his analysis limited his data set, this set also represented the most likely candidates for which it was possible to make direct measurements of mass and velocity with the greatest possible accuracy. These measurements were what McGaugh needed to make his eye-opening comparison between models involving dark matter and MOND. The BBC laudably noted the variance involved with mass estimation; Science Magazine was silent on the issue, and consequently implied that these galaxies were perhaps representative of the astronomical ideas to be studied.

Rotational galaxies are known to have an empirically-derived relation between mass and rotation which contradicts expectations developed by smaller-scale results. For example, in our own solar system, Pluto, far from the sun, our system’s gravitational center, requires 248 Earth years to make one complete orbit of the sun, while the Venus accomplishes this in less than 8 months. In general, Newton’s law of gravitation predicts such a drop-off of rotational velocities with increased distance from a gravitational center. However, rotating galaxies flout this model consistently, by displaying a rotation effect on parts of galaxies increasingly far-flung from the center, in which their rotational velocities are not smaller than that of objects closer in. This effect is commonly attributed to dark matter: fast-moving objects on the edge of a rotating galaxy must be held in orbit by the gravitation of as-yet-unobserved sources of gravitational mass.

Milgrom developed the MOND model based on the possibility that modified gravitation, and not dark matter, explains the faster-than-expected rotation of spiral galaxies. Rather than acceleration due to gravity between two masses (such as a star and the center of a galaxy) being a simple ratio that gets smaller with the square of the distance between them, MOND introduces a new value that only has an effect on a body when its acceleration is very small, as with objects on the outer edges of galaxies. However, McGaugh freely explains (in his research paper) the inability of MOND, for example, to predict the dynamics of larger-scale objects such as galaxy clusters. For news outlets, this was a missed opportunity.  Neither the BBC nor Science Magazine attempted to address the serious failings of MOND.

Despite BBC’s headline implying that dark matter theory has received a significant challenge, McGaugh himself puts forth a variety of possible explanations for his results. In particular, in describing several known shortcomings of the MOND theory, he bluntly states “a theory [MOND] that seeks to eliminate the need for cosmic dark matter itself suffers a missing mass problem.” Furthermore, in a spectacularly clarifying blog by California Institute of Technology physicist Sean Carroll, dark matter is seen to be the only model that fits cosmic microwave background (the radiation echoes remaining from the big bang), and that applies more broadly to other astronomical bodies besides McGaugh’s gassy galaxies.  The real story behind McGaugh’s analysis, according to Carroll, is that MOND “provides a good fit to a certain class of gas-rich galaxies.” And, most importantly, we still don’t know why. While, McGaugh concedes, it is possible dark matter does not exist, if dark matter is the prevailing theory, “the challenge is to understand the empirical systematics encapsulated in the simple MOND formula.” To McGaugh, “what is surprising in the case of MOND is that it continues to enjoy predictive successes at all.”

Despite Science Magazine’s headline’s claim to be providing “More Evidence Against Dark Matter,” not only does the ensuing text fail to describe the original evidence that this is “more” of, but the recent results at most, as Science Magazine’s subsequent text puts it, hand physicists “a homework problem they should solve.”

In short, McGaugh’s research has given physics a really good head-scratcher. It’s a good story, full of subtlety, and worthy of in-depth discussion complete with good headlines. The kind that indicate what you’re really about to read.

Special thanks to Jessica Rosenberg, Ph.D., Dept. of Physics and Astronomy, George Mason University and Gerald Goldin, Ph.D., Physics Department, Rutgers University for discussions on physics and dark matter.



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