Dark Matterby Cyrus Ance
Dark Matterby Cyrus Ance
I mentioned dark matter back in the column on the cosmic microwave background. Its existence was first hinted at in 1933 when Fred Zwicky noticed that galaxies in a large cluster were rotating about each other faster than could be explained by the amount of glowing stars that could be seen in the galaxies. Something moving fast around an orbit requires a larger mass to hold it gravitationally in orbit than something moving slow. Thus by looking at the speeds of things moving in orbit we can estimate the mass of the thing holding them in orbit. We can do this as function of distance from the center of the orbit and measure the distribution of mass as function of this distance. Both the speed of the orbiting galaxies and the distribution of the mass as function of distance pointed to, quoting Zwicky who coined the term dark matter , "...we arrive at the astonishing conclusion that dark matter is present with a much greater density than luminous matter." (1) Zwicky's observation was confirmed by Smith in 1936 in a different galaxy cluster.
This is astonishing because it is at variance from what we observe in the solar system. The glowing sun accounts for 99.8% of the mass of the solar system. Looking at the nearby stars we do not see any systems with stars rotating about large dark bodies and the amount of dust and gas between the stars is tiny. Since a galaxy looks like a bunch of stars we expect that like the solar system most of the mass of a galaxy to be glowing. That the observation seems to show that dark matter predominates over luminous matter is a complete surprise. Smith speculated that dark matter was non-glowing material between the stars, while Zwicky suggested that there could be something extra going along with the luminous matter that makes the glowing density higher than in the solar system.
Babcock in 1939 was the first to look at the rotation of stars within the Andromeda galaxy and found that the matter seemed to be evenly distributed with the non-glowing matter dominating over the luminous matter in the outer part of the galaxy. No one put this together with the earlier Zwicky/Smith observation. Oort made a similar observation in another galaxy in 1940. In 1959 Kahn and Woltjer looked at the motion of the local group of galaxies dominated by Andromeda and the Milky Way. They found that most of the mass was invisible and speculated that it was very hot gas. Again no one made a connection with the Zwicky/Smith observation.
In the 1970's Babcock's observation's of Andromeda was improved by first Rubin and Ford and later Roberts and Whitehurst. Again no connection was made with the Zwicky/Smith observation. Roberts and Whitehurst suggested that the outer reaches of the Andromeda galaxy had a vast population of dim stars. Roberts recalls, the observation "...was, at best, received with skepticism in many colloquia and meeting presentations." The tide turned in the mid 70's as theoretical work (many including Ostriker and Peebles) on the stability of galaxies suggested that large mass was needed in the outer reaches of galaxies to give rise to their observed structure. This combined with the quality and simplicity of the observation from Roberts and Whitehurst and rediscovery of the Zwicky/Smith observation led to the acceptance that dark matter made up a large fraction of the mass of galaxies and thus the universe. The question then turned to the nature of dark matter, although there continues to be speculation that we may be observing some sort of break down in gravity at large distance scales. Dark Energy may be related to this last speculation.
There are basically two possibilities for dark matter, hot or cold. Very light stuff that moves around near light speed is called hot dark matter. The favorite candidate for hot dark matter is neutrinos, which continue to wait for a future column devoted to them. As early as 1979 the evidence was against neutrinos being a large component of dark matter as dark matter seems to clump up in galaxies and galaxy clusters which hot dark matter would not do. WMAP results in 2003 made it clear that hot dark matter can only be a small fraction of the dark matter total.
Cold dark matter comes in two forms, baryonic or non-baryonic. Baryonic is ordinary stuff (protons and neutrons are the best known baryons). Basically baryonic dark matter would have to be non-glowing things wandering around the void between the stars going by all kinds of names depending on their mass (black holes for very heavy, white dwarfs for star remnants, brown dwarfs for almost stars, Jupiters for more planet-like things). Generically that are called MACHO's for MAssive Compact Halo Objects. (2) The term was invented by Griest in 1991. These can be looked for using gravitational lensing which is a consequence of Einstein's relativity prediction that light should also be bent by gravity. Thus if a dark object passes between a viewer and a distant star, the star will briefly brighten as its light is focused by the object. In 1986 Paczynski suggested that this would be a way to search for baryonic dark matter in the halo of the Milky Way. Gravitational lensing events were seen in the early 90's, and the measurements seem to indicate that while there are a bunch of MACHO's, they are only a small fraction of the dark matter that is needed to keep a galaxy together.
Non-baryonic cold dark matter on the other hand is stuff we do not know what it might be. There are speculations about things called axions which are probably too strange for discussion even in this column. The favorite candidate is a Weak Interacting Massive Particle (WIMP) (3). These are the favorite candidate at the moment as WIMP's naturally arise out of extensions to the current model of particle physics, needing a future article to explain, and do not contradict observed features of the universe. There is even a recent positive observation by the DAMA collaboration and a contradictory non-observation by the CDMS collaboration. The jury is still out on WIMP's, even though it seems dark matter is dominated by the cold sort. Combined with the MACHO observations and the many searches for WIMP's it could be that an observation of WIMP's is just around the corner.