Tuesday, November 7, 2017

Dark Matter Day

Image result for dark matter

History of Science
Nature of Science
Edward Hessler

Aaarrgghh! Late again. As usual.

In addition to Halloween, October 31 was also International Dark Matter day. I noticed, with some relief that the date was "on or about" but wonder whether November 11 is close to about.

Symmetry magazine provides some pics from the celebration as well as a poster you can download.

Cosmologist Sean Carroll says that the following are the important facts about dark matter: 
It’s dark. Doesn’t interact with electromagnetism, at least not with anywhere near the strength that ordinary charged particles do. 
It’s cold. Individual dark matter particles are moving slowly and have been for a while, otherwise they would have damped perturbations in the early universe. 
There’s a goodly amount of it. About 25% of the energy density of the current universe, compared to only about 5% in the form of ordinary matter. 
It’s stable, or nearly so. The dark matter particle has to be long-lived, or it would have decayed away a long time ago. 
It’s dissipationless, or nearly so. Ordinary matter settles down to make galaxies because it can lose energy through collisions and radiation; dark matter doesn’t seem to do that, giving rise to puffy halos rather than thin galactic discs. 
Here is a short animation on dark matter and the force (Star Wars), with attention to some of their similarities.

For a short primer about dark matter see here.

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