A "Short" History Of The Dark Side - Part 2

In the previous post we saw how in certain circumstances it is possible to explain supposedly missing mass in terms of standard physics. I'm now moving onto a case where it appears that this isn't possible, the strange "Dark Matter" that seems to permeate space.


Its been known since the 1930s that if you add up the mass seen in clusters of galaxies (like that shown above) there isn't enough mass to account for the motion of the galaxies in the cluster. This is exactly analogous to the situation in GCs, where instead of individual stars appearing to be moving to fast its now entire galaxies moving too quickly. Over the years some of this missing mass has been found, in the form of very tenuous hot gas that resides between the galaxies in the cluster. The image below shows the Centaurus cluster as seen in X-Rays, the X-Rays are produced by the hot gas in between galaxies in the cluster. The gas itself is also of great use in determining the mass in the cluster, this is because we can measure the temperature and density of the gas and from this infer the gravity that must be present to stop the gas expanding out of the cluster and into intercluster space. Although this gas is very diffuse when its mass is added up it still adds up to more than the mass contained in the galaxies in the cluster but still it only makes up a small fraction of the total mass we know must be in the cluster from the motions of the galaxies.


Over time other manifestations of this missing mass has been seen, it was observed that spiral galaxies rotated too quickly to be explained by just the visible mass, then it was noticed that the stars in elliptical galaxies where also speeding around too quickly, finally that the GCs and dwarf galaxies around normal galaxies were themselves moving too quickly to be explained by the luminous mass of the parent galaxy. Other effects were noticed that do not rely on the kinematics of objects, it was observed that the bending of light due to the gravity of galaxy clusters and individual galaxies was too severe to be explained by the visible mass.

As it became clear that this invisible mass was a real phenomenon and not due to some problems with our models people attempted to explain this missing mass. Initially people attempted to explain this missing mass in terms of stellar remnants (like in the GCs) and/or gas and dust that doesn't emit light strongly. With our increasing ability to observe at different wavelengths of light where we would expect gas and dust to be emitting radiation it became clear that this gas and dust could only explain a small fraction of the missing mass. Similarly stellar population modeling showed that it was almost impossible to explain the missing mass as being due to stellar remnants, it would require far too many stars to have already died by now, this would only be possible if initially most stars that formed in a galaxy were very massive. This is not observed in nearby galaxies and from what we know of star formation is not expected to be the case in most situations.

So what is the solution to the puzzle? Well the one that is most popular is inclusion of some matter which is not made of the same material as normal atoms, this Dark Matter is non-baryonic and only interacts with luminous matter through the force of gravity, if it did interact in any other way we would be able to see it. This solution seems like a fudge, except that it can be used to fit all of the problems I have listed above and more, something which the other contending model has difficulty with. This other approach (called Modified Newtonian Dynamics or MOND) is to assume that gravity behaves differently on different scales, this approach can reproduce many of the observed effects but not all, and is generally distrusted because gravity in general and general relativity in particular is seen to work so well in every observation we have to date. At the present much work is being done to investigate and try and detect a Dark Matter particle, if one is found it will be the crowning achievement of modern astronomy, if it is not observed then we have major problems.

However just as the astronomical community was reaching consensus on the existence of DM another set of observations appeared that has led to another dark substance. We will look at this in my next post in the series.