Monday, May 23, 2011

More Evidence For Weirdness In Weak Gravitational Fields

The single most interesting feature of attempts to replace dark matter with a modification of gravity is Milgrom’s discovery that in a wide variety of galaxies, there’s a unique place where ordinary gravity plus ordinary matter stops working: when the acceleration due to gravity (as Newton would have calculated it) drops below a fixed value a0 ≈ 10^−10 m/s2. This is the basis of MOND, but the pattern itself is arguably more interesting than any current attempt to account for it. . . but in any event it should be explained somehow.

From here

A clever new test of gravitational weak fields finds the same effect in a pre-print whose abstract appears below:

The Breakdown of Classical Gravity? X. Hernandez, M. A. Jimenez, C. Allen

Assuming Newton’s gravity and GR to be valid at all scales, leads to the dark matter hypothesis as a forced requirement demanded by the observed dynamics and measured baryonic content at galactic and extra galactic scales. Alternatively, one can propose a contrasting scenario where gravity exhibits a change of regime at acceleration scales less than $a_{0}$, and obtain just as good a fit to observations across astrophysical scales. A critical experiment in this debate is offered by wide orbit binary stars. Since for $1 M_{\odot}$ systems the acceleration drops below $a_{0}$ at scales of around 7000 AU, an statistical survey of relative velocities and binary separations reaching beyond $10^{4}$ AU should yield a conclusive answer to the above debate. By performing such a study we show Kepler’s third law to fail precisely beyond $a \approx a_{0}$ scales, precisely as predicted by modified gravity theories designed not to require any dark matter at galactic scales and beyond.

From a sociology of science perspective, it is particularly notable that this empirical data point is (1) predicted by a decades old theory, (2) found at a time when the Bullet Cluster example strongly disfavors a MOND mechanism, and (3) looks at a very different type of physical system than the one that formed a basis for the original theory.

To echo Sean in the first blockquote, the really important point is that the MOND cutoff is a simple, consistent cutoff that appears in a wide variety of circumstances and has repeatedly predicted new discoveries before empirical evidence was in place to test those predictions. Whatever its cause, the empirical phenomena is real and any theory of dark matter or quantum gravity that is consistent with empirical data has to reproduce this effect over a wide range of phenomena.

UPDATE: Motl plays with a variant on the MOND/TeVeS line of reasoning for modified gravity to produce a similar effect that is derived from the wave-like character of matter, the speed of light, the size of the universe and the holographic principle, that he called "HOND."

One interesting nuance of his idea is that in cosmology, the modified gravity effect would change as the universe gets bigger. The effect would have been stronger in the early universe than it is now.

UPDATE May 24, 2011: See also X-ray observaton evidence for low density, high temperature filaments that are part of the large scale structure of the universe.

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