Dark matter in galaxies
In the early 1930's (pretty much the dawn of modern cosmology) two astronomers, Fritz Zwicky and Jan Oort, independently suggested that much of the matter which comprises our Universe remains to be detected.
Their proposals were based on observations of the motions of visible stars in the disk of our Galaxy and of the motions of galaxies within gravitationally-bound clusters of galaxies. In both cases the stars or galaxies appeared to be moving much too fast to remain bound to the system they were observed to be in, yet they clearly had been confined to that system over much of the lifetime of the Universe - it seemed that some unseen or dark matter must be providing the necessary additional gravitational energy to keep the stars and galaxies bound together.
The dark matter problem raises three important questions:
- Where is the dark matter located?
- How much more dark matter is there than visible matter?
- What is it made of?
The answers to these questions promise to help cosmologists understand the future dynamics of the Universe as well as the evolution of structure in the Universe. The dark matter problem may also unlock vital clues to the highly complicated process of star formation, as well as help particle physicists to gain an insight into the characteristics of exotic fundamental particles which are otherwise difficult or impossible to analyse in the laboratory.
Answers to the first and second questions are now at hand. Dark matter is inferred to exist both within and between galaxies and its distribution has been "mapped" in more than 1000 galaxies from measurements of the rotation speed of clouds of atomic hydrogen within the galaxies. Its distribution has also been determined within many gravitationally-bound clusters of galaxies using a variety of techniques including measuring the temperature profile of hot X-ray emitting cluster gas and measuring the distortion of background galaxies due to gravitational lensing.
Within galaxies the amount of dark matter appears to exceed the amount of visible matter by a factor of 10 to 1 in some cases, and even more than this for a few galaxies. Within galaxy clusters the ratio of dark to visible matter appears to be even larger still and a general result is that the ratio of dark to visible matter appears to increase as one observes on larger scales. On the very largest scales of all (the scale of the observable Universe) visible matter may account for less than 1% of all the matter in the Universe.
The nature of dark matter
More than 60 years since it was first postulated, dark matter still remains unidentified. Not because it is too remote for us to observe it, but simply because it emits little or no radiation. In general terms the dark matter may be either baryonic,
like ordinary matter, and in a form which makes it much dimmer than normal stars, or else it may comprise a more exotic species of non-baryonic matter. This latter option is of special interest to particle physicists. Or it could be a mixture of the two. In either case there are no shortage of plausible candidates, though there are a host of observational and theoretical constraints which have been used to narrow down the range of possibilities.
Among the more plausible of the baryonic candidates are dim low-mass stars, perhaps one-tenth the mass of the Sun, white dwarfs or very massive black holes (VMOs) larger than about 200 Solar masses. White dwarfs and black holes are remnants of stars which have exhausted all their nuclear fuel by blowing themselves apart. In the case of a black hole the remnant collapses in on itself, forming an object so dense that not even light can escape from it. Alternatively, the dark matter may comprise sub-stellar objects, called brown dwarfs, which have masses below about 0.09 solar masses - too light to initiate the nuclear reactions which make stars shine. Baryonic dark matter is often collectively referred to as MACHOs - massive compact halo objects - since it is a prime candidate for explaining the dark matter within galactic halos.
Favourite among non-baryonic contenders from particle physics are the light neutrino, the axion (predicted by one of the more promising "next generation" theories of particle physics), and the lightest stable particle (LSP) predicted by super-symmetric particle theories.
Exotic particles are almost inevitable
Theoretical considerations concerning the ratio of hydrogen, helium and other light elements formed in the early history of the Universe indicate that the total amount of baryonic matter can be no more than about ten times the amount of visible matter. This is sufficient to explain the dark matter on the scale of galaxies but not enough to explain the dark matter in galaxy clusters and on larger scales. The existence of non-baryonic matter therefore appears inescapable. Non-baryonic matter is often placed into two catagories:
- Hot dark matter (HDM) is particles which were relativistic (moving at velocities close to the speed of light) at the early epoch when the Universe was so dense that it was opaque to light.
- Cold dark matter (CDM) which was non-relativistic at that epoch (some CDM particles are also often referred to as WIMPS - weakly interacting massive particles).
Predictions for how structures in the Universe begin to form radically differ depending on whether the dark matter is dominated by HDM or CDM. Comparison of computer simulations for both types of matter with the observed structure of clusters of galaxies indicates that the dark matter on the largest scales is likely dominated by CDM, though best agreement between the two require both CDM and HDM in the ratio of about 2 to 1. CDM could also in principle explain the dark matter problem within individual galaxies.
Alternatives to dark matter
Alternatives to dark matter have been suggested, including the existence of a non-zero and positive value for the Cosmological Constant (an extra term allowed within the Einstein Equation of General Relativity theory, providing a global repulsive force to counterbalance the effect of gravity). The Cosmological Constant could conceivably explain the dynamics of the Universe on the largest scales but cannot provide an explanation for dark matter in galaxies or in galaxy clusters, since these are local not global phenomena.
It has also been suggested that magnetic fields generated by visible matter may be affecting the dynamics of the matter, giving the false impression of the existence of dark matter. However, observations imply that dark matter exists between galaxies, over scales much larger than the extent of the visible matter, so it is inconceivable that magnetic fields associated with visible matter could be affecting such observations.
One other explanation is that maybe one requires a modification of the law of gravity for systems which undergo only tiny accelarations in their motion (as is genarally the case for galaxies and larger systems). The small modification required to explain the dark matter problem in normal galaxies and galaxy clusters would be completely unmeasurable within our Solar System or on Earth. However, even this subtle modification does not work for some dwarf galaxies which are inferred to have a relatively high dark matter content and which undergo relatively large accelarations in motion. In short, no alternative explanation can simultaneously explain the dark matter problem on all scales, so the conclusion that dark matter exists appears unavoidable.