Thursday, February 21, 2013

Dark Matter? Maybe Not.

Great orthodoxies in astrophysics are like the Tower of Siloam (link). They grow cracks and begin to crumble in almost unnoticed ways—until, one day, they crash down with a roar. I became aware of another little crumble this morning (ht to Montag). It takes the form of a press release from Case Western Reserve University reporting on a study which appears to legitimize Modified Newtonian Dynamics (MOND) and thus throws doubt on the existence of dark matter (link).

MOND, a theory introduced by Mordehai Milgrom, the Israeli astrophysicist, in 1983, suggests that Newton’s theory of gravity may not operate uniformly at all stellar distances, hence Newton’s theory needs “modification.” Wikipedia’s article on MOND (link) brings a graphic that neatly illustrates the problem.

Expected (A) and observed (B) star velocities as
a function of distance from the galactic center.
The dotted blue line illustrates the expected rotational velocities of stars at increasing distances from a galactic core. The red line illustrates the actually observed velocities. In order to explain that gap, between velocities predicted by Newton’s law of gravitation and what is actually seen, astrophysics has invented dark matter to supply sufficient amounts of mass to justify the observations—while keeping Newton’s laws untouched. The problem, of course, is that dark matter has not been physically observed. It is simply a projection of gravity-producing mass into a void which may be empty.

The current study is headed by Stacy McGaugh and Mordehai Milgrom and deals with the rotational behavior of seventeen dwarf galaxies that circle around Andromeda. This sort of thing interests me—because I feel quite uneasy about a cosmological picture in which virtually the whole universe (96% of it) is supposedly made up of dark energy and dark matter—of which we have not even the slightest knowledge in a touch-and-feel sense.

The word mond in German means “moon.” Well, it isn't full moon yet in astrophysics, but the light is certainly increasing.


  1. This is what makes the whole dark matter/dark energy/cosmological constant discussion so interesting. Nature isn't behaving the way our previous theories said it would, and nobody quite knows why.

    I started having an easier time understanding both dark matter and dark energy when I realized that they're really shorthand for "there's this thing going on when we measure how the Universe actually behaves that we don't understand and that our current theories don't account for."

    In other words the "dark" doesn't just stand for a darkness of vision and measurement, it's also a darkness of theoretical understanding.

  2. Has something to do with the "professionalization" of science. People dare not say "we don't understand why." If they did, perhaps funding agencies would be less willing to make grants. Three, four hundred years of modern science? We haven't even scratched the surface...

  3. Scratching that itching surface to see more clearly, is exactly what we should and must continue to do.


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