Difference between revisions of "Dark matter"

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Latest revision as of 21:52, 4 July 2016

The dark matter hypothesis is the current popular explanation of the galaxy rotation-curve problem.

the shape of galaxy spiral arms suggested that matter within a galaxy was more cohesive than expected according to GR1960 (and NM).


galaxy rotation-curve anomaly

The galaxy rotation-curve problems appears under General relativity and alsio under Newtonian mechanics.

The nature of the problem is that if we imagine a rotatign disc, the rotaito nspeed and the outward force increases as a simple functon f the distance form the centre. matter further form the centre makes afewer revolutions per epoch, partly because ther eis no obvious reason why matter at the periphery woudl have a rotationsal speed high enough to circle in step with slower-moving more central matter, and partly becuase stars at the eriphery were movign that fast, they'd tend to be flung free. This means that a "bar-shaped glaxy, when rotating, tends to "wind itself up and take on the shpae fo a double-spiral.

We can predict the shapes of these accumulayed distortionsm but when we compare the predicted curve fo spiral galaxy arms, we find the the the curve is not as "wound up" as expected - it seems that the outer material is circling faster than it ought to.

While the problem was known for some decades, it was not until 19xxx that we had enough data to be sure that the effect was real - it might instead have been that the armsrepresented a sort fo compression wave circling the system, in which the stars were first clumped and then dispersed, so that the wave speed was not the same as the average star speed.

However, in the 19xxx's if it was concluded that the rotation curve anomaly was real, which led to one of two possibilities:

  • GR 1960 is wrong, and has the wrong gravitaitonal equations. The real physics is more nonlinear, leadign to enhanced interaction inside a galaxy and attaentuated interactions with outside mass this woudl allow galaxy systems to behave a little more like "island universes" with .
  • GR is right, in which case the backgroundn field strength cannot drop below aan extrapolated floor level, and the known amount of matter in a galaxy must somehow be associayted with an enhanced amount of gravity.

This second interpretation led to the dark matter hypothesis.