Chapter 15
Through space the universe grasps me and swallows me up like a speck; through thought I grasp it..
Blaise Pascal


Determining the contents of the universe is one of the fundamental problems of cosmology. Possible constituents include ordinary baryonic matter, other forms of matter (e.g., massive neutrinos), radiation, and something that behaves like a cosmological constant.

The matter density of the universe relative to the critical value (OmegaM) is one of the fundamental parameters of the universe; in the standard (Lambda = 0) models, the matter density determines the geometry of the cosmos. An accurate determination of this quantity is thus of great cosmological importance. Many methods have been developed to try to measure the matter density. Most rely upon detecting the orbits of visible matter and using Kepler's laws to compute the mass; these dynamical methods are probably the most widely employed. At all scales, we have found that the amount of matter revealed through gravitational interactions is greater than can be explained by the mass of the visible stars. The nature of this unseen dark matter is one of the most important outstanding cosmological problems.

People (including astronomers) sometimes use the terms "dark matter" and "missing mass" interchangeably and at other times use them to mean different things. Here we have defined dark matter to mean simply matter which does not emit detectable light. This includes both ordinary matter (baryons) in the form of planets, or compact stars and possible exotic forms of mass such as "hot" or "cold" dark matter (weakly interacting massive particles, or WIMPs). We generally have tried to avoid the term missing mass, but missing mass sometimes is used to refer to mass you know must be there but can't account for (e.g. rotation curves provide evidence for mass but we don't know what it is).

At the scale of individual galaxy disks, much of the dark matter can be attributed to stellar ashes such as white dwarfs, neutron stars, and black holes, as well as to extremely faint objects such as very low-mass stars and brown dwarfs. However, at larger scales, the total aggregation of known baryonic matter cannot account for the mass that orchestrates gravitational interactions. Galaxies, including the Milky Way, appear to be surrounded by huge spheroidal dark halos . The composition of the dark halos is unknown, but observations have indicated that at least some of the mass must take the form of compact objects called MACHOs. Since we believe that only baryonic (i.e. ordinary) matter can form compact objects, this suggests that galaxies are surrounded in part by an invisible cloud of objects such as neutron stars or small black holes.

At larger scales, measurements of Omega rise until they typically level off within a range of 0.1 to 0.3. This is far greater than can be accommodated by the abundances of primordial elements such as helium and deuterium as determined by big bang nucleosynthesis. Thus we conclude that some 90% of the matter of the universe is not only invisible, but is nonbaryonic. If it is not baryons, what is it? It must consist of some type of WIMP (weakly interacting massive particle). What this particle might be remains unknown, though particle physicists can provide plenty of possibilities. One obvious candidate is the neutrino. Evidence has grown over the last decade that the neutrino actually has an extremely tiny mass. There is no theoretical reason that the neutrino must be massless; indeed, one hypothesis for the dearth of solar neutrinos observed over the past 30 years is that neutrinos are massive, and the kind that can be observed is converted into a kind that is invisible to most neutrino detectors on the journey from the Sun. Laboratory evidence is beginning to support this hypothesis. (Here is a brief description of the New Evidence for Massive Neutrinos.) If neutrinos have even a small mass per particle, they could add considerably to the cosmic matter density because they are approximately as abundant as photons. However, the experimental limits on the neutrino mass preclude it from providing enough matter density to close the universe. Plenty of more exotic WIMPS have been suggested, but as yet none has been detected. A great deal of experimental and theoretical ingenuity and effort is being devoted to identifying the elusive "missing matter."

The nature of the dark matter is also inextricably connected with the formation of galaxies and galaxy clusters, since it must dominate the gravity of the universe. It is well known that galaxies tend to occur in clusters. Galaxy clusters are gravitationally bound; that is, the galaxies within a cluster orbit one another. The Milky Way is a member of a small cluster of a few dozen members, dominated by itself and the Andromeda galaxy; this cluster is called the Local Group. The nearest large cluster to the Local Group is the Virgo cluster, about 20 Mpc distant.

Galaxies are not found only in clusters. Even apparently "empty" regions of the sky contain huge numbers of galaxies. The Hubble deep field image, and the Hubble ultradeep field image were obtained by taking a very long exposure of patches of sky that were relatively devoid of galaxies, at least as had been seen with ground-based telescopes.Hubble Deep Field

Here is a copy of the Hubble Deep Field image. Can you determine which are spiral type galaxies? Do you see any ellipticals? A huge number of faint (and very distant) galaxies is revealed. Try counting the galaxies in some small area. Almost every spot of light is a separate galaxy. (The bright object with the diffraction "spikes" in the left center of the image is a star.)

Structure does not stop at the level of galaxy clusters, but continues to superclusters, huge clumps of matter extending over millions of parsecs. These enormous structures cannot be gravitationally bound for a universe of age 14 billion years; they must have been somehow present in the early conditions, or else they must be in the process of formation.

What, then, is the dark matter, and how is it related to structure? Models that attempt to explain these phenomena fall into two main categories based upon the average energy of the WIMP they invoke. Models of structure formation are complex and must be solved computationally; the results are computer-generated plots of density that can be compared to observations. Hot dark matter models assume some high-energy particle. The most likely hot WIMP is the massive neutrino; the neutrino has the definite advantage that it indubitably exists, and its mass seems to be on the way to being established. Unfortunately, hot dark matter models have difficulty forming the kinds of structure observed. The structures they create are too large and form too late in the history of the universe. At the opposite extreme are cold dark matter models, which assume a heavy, slow particle is dominant. Cold dark matter models form small structures, such as galaxies, first; these smaller structures develop quite early in the history of the model. This seems consistent with observations of apparently normal galaxies to redshifts of nearly 10, and of objects that could be protogalaxies to even larger redshift. Indeed, the WMAP data indicate that the first stars formed only a few hundred million years after the big bang. Structure formation in the early universe is one of the most active fields of current cosmological research, so answers to our many questions may be forthcoming in the foreseeable future.

For more information see Questions and Answers related to Chapter 15.

UK Dark Matter Collaboration describes various experiments designed to search for the presence of dark matter WIMPs.

The Next Generation Space Telescope web site provides information on NASAs planned telescope that would study the epoch of galaxy formation.

Original content © 2005 John F. Hawley