5.1. Cluster Morphology

Clusters are expected to be more evolved in a low-_m universe, i.e., be more spherical and show less substructure than clusters in an _m = 1 universe which must still be undergoing significant collapse and merger activity [52].

New Developments: X-ray maps from ROSAT provide a useful measure of morphology for many clusters.

Pro: The method relies on a simple feature distinguishing the different cosmologies by the rate of evolution of structure at different epochs. Qualitative results can be obtained by visual inspection of maps.

Con: The method requires high-resolution simulations with gas dynamics and many clusters in order to beat the cosmic scatter. Substructure is hard to quantify. The substructure depends also on the fluctuation power spectrum, on _m, and on the various other dark matter species.

Current Results: Dissipative simulations [53] agree with dissipationless simulations [54] that a significant effect is expected. However, these papers disagree about its strength and about the conclusions to be drawn from comparison with the clusters observed by ROSAT. The current situation is somewhat confused.