15 September, 2006

Science in Action

Astronomy text books have been re-written a few times since I last took an astronomy class (back when the Steady State Theory was leading - OK so I’m dating myself, nothing new, move along now) one of the enduring unsolved questions that has interesting implications (as well as more than a few tenures and seats at various institutions of higher learning) is how galaxies maintain their shape.

Galactic rotation curves

I won’t go in to the messy math details but it boils down to this, either our understanding of gravitational effects at galactic distances is inadequate or there must be several times more mass distributed throughout (and beyond) a given galaxy than can be inferred from observation in order for the galaxies to maintain their spiral shape for more than a rotation or two.

According to our understanding of gravity as described by Newton, the further a given star is from its galactic center the slower its orbital velocity should be. This holds true at the solar system level, where from Mercury out each planet’s (and minor planet’s) solar orbital velocity diminishes according to F=ma, when these velocities are plotted they produce a smooth diminishing curve. However, when the motion of stars orbiting at different distances from a galactic center are plotted something strange happens, at some point the curve flattens and the stars near the edge of the galactic disk have the same orbital velocity as those closer to the center maintaining the spiral structure over many galactic rotations.

Dark Matter

The current leading theory invokes something called dark matter, a so far invisible, undetectable theoretical construct that (allegedly) makes up at least 80% of the matter in the universe – the implication of this is that we can only account for 1 out of 5 parts of the universe around us – that means for every star we see, every planet we can infer there is the equivalent of at least 4 times as much mass somehow hiding in plain sight.


One heretical alternative named MOND (for Modified Newtonian Dynamics) was explored back in 83 when Mordehai Milgrom published 3 papers in volume 270 of The Astrophysical Journal that were received with thunderous…


Since then it has been attacked and ridiculed but no one has so far proven it wrong, it has recently attracted some renewed interest and has been expanded by Jacob Bekenstien into a MOND theory called TeVeS – acronym for Tensor, Vector and Scalar that describes how matter and energy react with space and time and can explain the phenomenon of gravitational lensing. First predicted by Einstein and given as the first proof of general relativity when during a solar eclipse the apparent position of a star ‘behind’ the sun was displaced by the amount predicted.

In the next few years one of these two theories will be consigned to the “nice try” bin along with luminiferous aether (although a modern replication of the Michelson Morley experiment with current state of the art instruments would be interesting) and a few academic careers – but that’s what happens in science as opposed to faith.

And the debate continues, a recent paper suggests that the x-ray halo surrounding the colliding galaxies 1E0657-556 (the bullet cluster) is direct evidence of dark matter. The two galaxies are ‘colliding’ (passing through each other) at 10 Million Mph and there are some interesting lensing effects – that some researchers infer as caused by dark matter.

Personally, I have a gut bias against invoking something (dark matter) that can’t be seen or detected, on the other hand Newton’s laws hold up remarkably well and by invoking dark matter can explain the rotation problem - however, if someone tells you that we know all about gravity and the electromagnetic spectrum gently ask them to explain the two slit problem.

However the answer turns out - that is the beauty of the scientific method

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