Foundations of Modern Cosmology

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Chapter 14: Dark Matter and Large Scale Structure

Chapter Summary

The matter density 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.

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 by an invisible cloud of objects such as neutron stars or small black holes. How and when these bodies formed is still a mystery.

At larger scales, measurements of Omega rise until they typically level off within a range of 0.1 to 0.3, with the middle of this range (about 0.2) currently seeming to be most likely. This is far greater than can be accommodated by the abundances of primordial elements such as helium and deuterium. 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. 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 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, such as the Hercules cluster, on the left below. 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 Mp distant.

Galaxies are not found only in clusters. Even apparently "empty" regions of the sky contain huge numbers of galaxies. The HST produced the Hubble deep field image, on the right above, by taking a very long exposure of a patch of sky in which no galaxies had been seen with ground-based telescopes. 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 extends to superclusters, huge clumps of matter extending over millions of parsecs. These enormous structures cannot be gravitationally bound for a universe of age 10 to 20 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 recent observations of apparently normal galaxies to redshifts of nearly 5, and of objects that could be protogalaxies to even larger redshift. Using the Einstein-de Sitter model to compute the lookback times and ages (a good approximation for the early universe), we find that such ancient galaxies must have formed scarcely a billion 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.

Points to Ponder
  • What does the mass to light ratio mean?
  • What is dark matter?
  • What are some candidates for the dark matter?
  • What is a MACHO? a WIMP?

You should understand how astronomers use gravitational interactions to estimate the value of the density in the universe (Omega). You should understand what figure 14.1 means. The nucleosynthesis results and the observed deuterium abundances place strong constraints on the density of ordinary matter (baryons). Other massive particles could provide the additional density needed to close the universe.

Potential Pitfalls 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). Sometimes missing mass means the the unaccounted for mass that would be required to make Omega=1. Of course, if it turns out that Omega is less than one then the mass is missing completely!
Questions & Answers

Questions and Answers related to Chapter 14.

You may have heard news reports on the detection of massive neutrinos. Here is a brief description of the New Evidence for Massive Neutrinos.

Web World

Redshift surveys (Figure 14.5) provide a means to measure structure in the universe. Another example comes from the Las Campanas Redshift Survey.

The microwave background fluctuations provide a direct measure of the seeds of structure in the universe. The latest discoveries about the structure of the microwave background can be found at the home page of the BOOMERanG Project. The fluctuations observed are consistent with the flat universe model.

Numerical simulations on supercomputers provide a way to test the consequences of various assumptions (massive particles, Omega, etc). Comparison between the two (data and model) may lead to a determination of whether or not ghostly WIMPs or relatively ordinary MACHOs make up the dark matter. Figure 14.7 shows the results of a cosmological simulation done by Michael Norman's group at the University of Illinois. This model contains hot dark matter, cold dark matter, and baryons. Considerable filamentary structure has formed by the end of the simulation. The red regions are dense areas where galaxy clusters are forming.

Here are some Web sites for numerical simulations: First the current research and associated links, as well as images from a recent cold plus hot dark matter model (discussed above) done at the University of Illinois. Another research group at the University of Washington has put together a picture gallery of cosmological data and simulations. Here is an example picture from a simulation done at the University of Washington. It shows a box with colors indicating where the mass is. Each of the subsequent frames is a zoom-in showing finer and finer detail. The complete story of cosmological simulations can be found beginning with the Cosmos in a Computer Map available from NCSA. Some additional examples can be found in the Los Alamos Cosmology Simulations.

The Virgo Consortium consists of a group of British astronomers modeling large scale structure formation in cosmological models.

Check out the latest on MACHOs from the MACHO home page.

Dark Matter.