Evidence for Dark Matter


The Dark Matter Universe

The above image is a computer simulation of the distribution of mass in a Dark Matter dominated Universe. The simulation produces much filamentary structure and is filled with voids. Structure (\eg density enhancements) clearly forms at the intersection of these filaments.

How does this compare with observations?

Qualitatively the agreement is good as can be seen by comparing against the results of the Las Campanas Redshift Survey:

Quantitative comparison, however, to the simulation reveals the following points:


What is Dark Matter?

Theory: if the Universe is critical then all the luminous matter seen in galaxies contributes only 0.5% of the mass required to eventually halt the expansion (note that the recent WMAP results have substantially lowered the amount of dark matter required to reach a flat/critical Universe).

How do we know its there as a function of scale size?

The general idea is to infer the existence of gravitating matter from perturbations in the motions of objects. In general, this requires application of the Virial Theorem.

Galactic Scales:

Clusters of Galaxies:

Historically, this was the first set of observations that hinted at dark matter. However, substructure can fool you.

The case of the Cancer cluster:

Cancer is a spiral rich cluster of about 100 bright galaxies. Below is the velocity distribution of galaxies in the Cancer cluster:

IF you treat the above distribution as belonging to one system and just apply the virial theorem you determine that the Cancer Cluster has M/L = 1000 (!). If this value is representative of clusters of galaxies then

What's going on in Cancer? To answer that question required a complete redshift survey (see Figure above) coupled with an analysis of the spatial distribution of the galaxies.

Plotting contours of Galaxy density in Cancer suggested that the distribution of galaxies was not relaxed; that is secondary density maxima appeared. These are labelled A thru E below:

This brought up the issue if redshift and position were correlated in this cluster. That is, do galaxies in Group A have different velcoities than galaxies in Group B or C? After a thorough analysis it was shown that these two were highly correlated and that the Cancer cluster could in fact be decomposed into individual groups. In redshift space these groups are shown here:

There is a clear separation of these components in mean velocity and each component has an internal velocity dispersion of about 300 km/s. The M/L of these individual components is 200--300. Furthermore, the components themselves are not gravitationally bound. That is, the Cancer cluster is really an unbound collection of groups:

While Cancer is an extreme case, most all clusters do show some evidence of substructure. Hence, the measured velocity dispersion in these cases does not apply to one dynamical system but to, perhaps, several. Failure to adequately account for substructure in clusters of glaxies leads to systematic overestimates of M/L.

The observation that most clusters have significant substructure implies that cluster formation is still, in essence, occurring as small groups are assimilated into the cluster core. This has implications regarding structure formation scenarios.

Observations of galaxy rotation curves and cluster velocity dispersions suggest that M/L associated with these structures is in the range 10--500; where 500 is fairly extreme. At most, these structures then contribute 20% of the closure mass density. Hence we are driven to the following inescapable conclusion: