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Alternate theory poses dark matter challenge

Bullet_cluster.jpgThe predictions of a theory proposed as an alternative to dark matter have been verified in a new class of objects, according to a study currently in press. The results seem unlikely to convince astrophysicists to abandon dark matter as one of the cornerstones of the standard model of cosmology; but suggest a direction in which new work is urgently needed.

The alternative theory is Modified Newtonian Dynamics, or MOND, proposed in 1983 by astrophysicist Moti Milgrom of the Weizmann Institute of Science in Israel. Like dark matter, MOND was hypothesized to explain why galaxies remain in one piece when observations of the rotation speeds and estimates of the mass of their luminous contents suggest they should fly apart. Rather than assuming the universe contains 5/6 invisible dark matter, Milgrom proposed a modification to Newton’s laws that kicked in at the low acceleration scales of stars in galaxies, and that strengthened the attractive force of gravity, having the effect of dark matter without imagining a mysterious extra substance. MOND successfully predicted the observed rotation speeds of galaxies and gained supporters but its adherents dropped off dramatically after observations of the cosmic microwave background (CMB) – the so-called “echo of the Big Bang” — first by the balloon experiment BOOMERanG and then by NASA’s Wilkinson Anisotropy Probe (WMAP), provided stunning confirmation of predictions from the dark matter theory.

One astronomer who still hasn’t given up on MOND is Stacy McGaugh of the University of Maryland in College Park. In his latest paper, in press at Physical Review Letters, McGaugh compares the prediction of the theory to the rotation speeds of spiral galaxies that are gas-rich. While estimates of the mass of most galaxies are uncertain; because it’s not known exactly how to convert measurements of the light from the stars into the mass of the galaxy, the mass of gas-rich galaxies can be measured precisely from the intensity of a radio emission from the hydrogen atoms that make up the gas. That enabled McGaugh to test MOND more rigorously than has ever been done before. He found that, in detail, the galaxies obey the Tully-Fischer relation, the observation that more massive galaxies rotate proportionally faster than less massive ones. “MOND predicted this naturally ahead of time,” says McGaugh. In contrast a dark matter model that assumed the proportion of dark matter in the galaxies matched the cosmic value of 5/6 gave a prediction that was some way off the observations.

Cosmologist Anthony Aguirre of the University of California, Santa Cruz, says the result is no surprise given earlier work by McGaugh and others showing that the relation was predicted by MOND for other classes of galaxies. But the application to gas-rich galaxies improves the result, he says. “The basic result was already in place but the errors are smaller,” he says. The work is unlikely to lead to the acceptance of MOND, because that still isn’t confirmed by observations of the CMB, but it should motivate further work on more sophisticated dark matter models that would also match the observed Tully-Fischer relation, Aguirre says.

Cosmologist Priyamvada Natarajan of Yale University agrees. She emphasizes that, MOND is unable to explain observations of clusters of galaxies, including the bullet cluster (pictured), a collision of two clusters with a pattern of bright matter that matches exactly what would be expected in the dark matter model. But she agrees McGaugh’s work highlights a regularity in the behavior of galaxies that hasn’t been explained yet by processes involving dark matter. “It reveals there are physical processes we don’t understand,” she says. These almost certainly involve interaction between dark matter and normal matter that can result in a deviation of the proportion of dark matter to ordinary matter from the cosmic average value of 5/6. “We don’t fully understand how baryons [ordinary matter] and dark matter interplay. I see this as a challenge. It points the way forward,” she says.

McGaugh accepts that his work apparently verifying a prediction of MOND is now most likely to be interpreted within the dark matter paradigm, but says he’s OK with that. “I think it’s telling us something about nature. Maybe it’s telling us something about the nature of dark matter,” he says.

Astrophysicist Danilo Marchesini of Tufts University says he thinks McGaugh’s approach is very clever, and that the next step is to put a bigger effort into estimating the masses of galaxies containing stars rather than gas, to see if the result still holds.

Image: The Bullet Cluster/ NASA


  1. Report this comment said:

    If MOND still supports the phony dogma religion of the big-bang theory, which requires huge missing dark matter gravity particles, and cannot present a better cosmology like Plasma Filaments proven to connect superclusters and form the actual cosmic web, then dump MOND too! Obviously the big-bang gravity beginning is wrong, and they invented dark matter, and 99.9% of outer space is plasma. The WHIM is proven discovered by Taotao Fang to connect galaxy clusters together. obviously common sense of larger structures then superclusters must exist, but HYPERCLUSTERS are denied recognition, because they are older then the big-bang event. dump MOND like all the other papers if they are afraid of criticism from arrogant wrong big-bangers who manipulate and control science for personal gains, grants, fundings.

  2. Report this comment

    James T. Dwyer said:

    The requirement for dark matter actually became established among astrophysicists during the mid-1970s when astronomers, notably Vera Rubin, clearly demonstrated that spiral galaxies do not rotate like the Solar system. They had presumed that spiral galaxies must comply with the laws of planetary motion – specifically that Keplerian rotational curves (illustrating that the rotational velocities of planets orbiting the Sun diminish as their distance from the Sun increases) must also apply to individual stars within the discs of spiral galaxies.

    That stars in the discs of spiral galaxies’ velocities remained relatively flat as their distance from the galactic center increased was taken as convincing evidence that there must be some additional undetected mass accelerating the disc periphery and preventing peripheral stars from being expelled by their greater than expected velocities.

    It should have been simply considered that, as Newton proved in his Principia, Kepler’s equations, empirically derived from observation of the Solar system, were reliant on that system’s distribution of mass (the Sun contains 99.86% of total system mass) and that disperse small body planets effectively do not perturb one another’s orbits.

    From the perspective of a star at the periphery of a distributed mass spiral galaxy, its motion is not primarily determined by any center of mass located perhaps hundreds of light years away but more by much nearer comparably massive objects that almost surround it. The discs of spiral galaxies rotate as a loosely bound structure, not as individual stars independently orbiting any central mass (some spiral galaxies do not even include a central bulge. Please refer to:

    Feng & Gallo, (2010), “Rotating thin-disk galaxies through the eyes of Newton”,

    The perceived requirement for galactic dark matter to explain their rotational characteristics very simply resulted from the misapplication of Kepler’s equations describing specific characteristics of sparse planets independently orbiting the dominatingly massive Sun.

  3. Report this comment

    Curt Sandberg said:

    Maybe this has already been proposed? Perhaps “dark matter” and/or “dark energy” is a manifestation of an extra-universian’s ability to extract energy from nearby multiple universes so that they don’t have to use up their “local energy”?

  4. Report this comment

    Mike Landis said:

    I’m not a string theorist by any stretch of the imagination, but … is it possible that acceleration in the expansion of space is a consequence of some sort of balance between the visible and invisible dimensions, i.e. that expansion of visible dimensions somehow distorts the invisible dimensions and that distortion causes a ‘push back’ effect on the visible dimensions, loosely analogous to an optical lens or the Venturi effect. In short, space itself is responsible for the acceleration in the expansion of space – no dark energy required.

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