Dark matter moving 9 km per second (BBC)

[Martin]

http://news.bbc.co.uk/1/hi/sci/tech/4679220.stm

 

clouds of dark matter which are not massive enough (less than 30 million solar mass) or which are too small----less than 1000 lightyear diameter----apparently disperse by random motion of the particles, because astronomers do not find them below that size.

 

the assumption is smaller less massive clouds don't have enough gravity to hold themselves together given the random motion characteristics of DM.

 

this allows one to infer the AVERAGE SPEED of dark matter particles, or to put it another way their TEMPERATURE

 

so the estimate is that DM particles are moving on average at a speed of 9 kilometers per second

[Daecon]

What's Brownian motion?

[Royston]

It's the random movement of particles in a fluid...first discovered when pollen was seen to 'jump' randomly when suspended in water.

 

It was discovered by the botanist Robert Brown (hence the name) but it was called Brownian 'movement' before being accepted by physicists. I think the same effect can be observed with dust particles in the air.

 

IIRC we used smoke as an example when I was doing my GCSE's...many, many moons ago. I remember being baffled how you could model such movement.

 

This may seem a daft question, but how is dark matter 'hot' if it gives off no radiation ?

[swansont]

Wiki entry on Brownian motion.

 

Not too hard to find, eh?

[Severian]

I am a bit surprised that more has not been made of this. If this is correct, it would rule out cold dark matter, which previously was the favoured form. This result implies that the dark matter particle has really low mass, such as an axino, and rules out most traditional supersymmetry scenarios.

[Martin]

Hi Severian,

right down your research alley!

 

as of yesterday the technical paper preprint is now available on arxiv

just search under the co-author name Gilmore

 

============

EDIT: I did the search. The link is

http://arxiv.org/abs/astro-ph/0602186

 

General comment for others besides Severian, sometimes you can't trust the BBC reporting and it is clearer to go to the journal article.

 

Severian, I would be really interested in your further comment. I was wondering about the mass calculation. If you set the temperature in Kelvin at CMB, say, then what does the particle mass turn out to be? Incredibly tiny, no? I believe you were saying axino a couple of years ago---gave a paper about DM candidates.

[sunspot]

There is another way to interpret the data. If one was to take regular chemical matter coming off a star, after it has cooled and continues into semi-empy space, a simple continued entropy expansion would be endothermic and would not give off any heat signiture. Any particle interaction that gives off energy would be absorded into the endothermic.

[insane_alien]

sunspot: the emmisions of stars are still hotter than background so we can measure their temperature. also we can detect the absorbtion of starlight as it passes through the gases. matter interacts with light. darkmatter doesn't.