How do we know dark matter exists, if we can't see it?

[Craig]

BBC Video: http://www.bbc.co.uk/science/space/universe/questions_and_ideas/dark_matter/#p009vbr0

 

Oh, and... http://www.space.com/20490-dark-matter-discovery-space-experiment.html (Dark Matter Possibly Found by $2 Billion Space Station Experiment)

[dawson]

When they talk about movement and the effect of gravity from the dark matter, what is moving or orbiting?

[Guest Steve]

Stars in Galaxies. Galaxies rotate with the stars moving in a fairly uniform speed from the centre out to the rim. But using standard physics you'd expect the stars in the middle to rotate more quickly. So galaxies appear bulked out to give them additional 'stiffness'.

So the 'fix' for this is to postulate that there is additional mass in the galaxy structure that we can't see. Dark Matter.

 

I believe that recent reports indicate that WIMPS (Weakly Interacting Massive Particles) that might fit the bill might have been detected - but it is right at the edge of what can be measured.

[Daz Type-R]

I don't think they have detected the WIMPs, I thought they had detected the remains of the collision of 2 or more WIMPs, I.e, the positrons and the electrons?

[dawson]

So can stars in galaxies be seen to move in the visible soectrum of light? If so, why does the shape of galaxies not change more quickly?

James

[Guest Steve]

I guess that is one for the grown up astronomers. I think the motion is so slow compared to the separation of individual stars that the actual motion of within the context of a galaxy is inferred.

 

But there again I was gobsmacked by one of the methods they use to detect planets. They use the wobble induced in a stars motion to detect the presence of planets - but they use variation in the light coming from the planet to do this - the gravity from the planet will change the motion of the planet by a few meters per second. They can measure the change in the spectra - red shift/blue shift i.e. the Doppler effect as demonstrated by passing ambulances, but applied to the light received from the distant star. So over millions of light years they can detect minute changes in the velocity of stars. Mind boggling.

 

And I guess the same sort of techniques can be applied to stars in galaxys. Maybe using standard candles??? (Help me some of these people who have done Astonomy 101 - I'm floundering)!

[Brantuk]

Try "Cepheid variables" - commonly used as standard candles to measure astronomical distances.

 

Classical Cepheids are used to determine distances to galaxies within the Local Group and beyond, and are a means by which the Hubble constant can be established. Classical Cepheids have also been used to clarify many characteristics of our galaxy, such as the Sun's height above the galactic plane and the Galaxy's local spiral structure.

[Tweedledee]

I gather that our Milky Way galaxy rotates roughly once every 225 million years. Also various clusters and larger groups of stars have slightly differing motions within this more general rotation.

 

Most other galaxies will rotate very roughly over a similar time scales give or take a few tens of millions of years, so it would take a very long time to actually see much change in another galaxy.

 

Other methods are available to detect these motions and infer orbits and rotations etc.