ΛCDM does seem to be validated on the largest scales of the cosmic web and superclusters, but some of its predictions seem to deviate seriously from observations on smaller scales, from clusters of galaxies, down to galaxies themselves. Specifically, theory predicts that, even after small clumps of dark matter have merged to form ever-larger structures, many of the small clumps should survive intact, embedded within the merged halos. Yet observations appear to indicate that the dark matter in halos is much less “lumpy” than predicted by the straightforward calculations. Direct constraints on the dark matter distribution can be derived from observations of gravitational lenses, both weak and strong. The panel therefore concluded:

  • It is most important to obtain Hubble Space Telescope (HST)-like imaging to determine the morphologies, sizes, density profiles, and substructure of dark matter, on scales from galaxies to clusters, by means of weak and strong gravitational lensing, in lens samples at least an order-of-magnitude larger than currently available. HST can make an important start on this problem, but to develop large statistical samples will require a much larger field of view or more observing time than HST affords.

The best current calculations of cluster formation suggest that gas in the densest regions should cool more than is observed, and that more stars should form in cluster cores, especially in the richest clusters. Perhaps the physical processes that affect baryons in clusters need to be better understood, or perhaps extra energy is injected from supernovae, an active nucleus, or some other source. One critical missing piece of information concerns the dynamics of the hot intracluster gas: how turbulent is the gas, how does it flow through the cluster, what is its ionization and velocity structure, and how do these properties depend on cluster richness and cosmic epoch (redshift)? The panel concluded:

  • High-energy-resolution, high-throughput X-ray spectroscopic studies of groups and clusters to z ~ 2 are most important for understanding the dynamics, ionization and temperature structure, and metallicity of the hot intracluster gas, as well as for studying the growth of structure and the evolution of correlations among cluster properties.

Much is still not known about how galaxies were assembled. The well-defined correlations observed among the shapes, sizes, velocity structures, and compositions of galaxies, observed mainly in the local universe, are poorly understood. A Sloan Digital Sky Survey (SDSS)-size spectroscopic survey at z ~ 1-3 would provide essential information about the evolution of galaxy correlations and should provide essential clues to the process of galaxy formation and evolution. The panel concluded:



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