Theoretical outlook

In addition to observations designed to test the proposed picture, there is a need to develop the model more fully. The most blatant deficiency of the current picture is that the origin of the putative clouds is unspecified. As described in §5.2, there are some hints that the clouds formed in the very early Universe, prior to synthesis of the light elements, and this scenario could usefully be pursued theoretically. Unfortunately the physics appropriate to this era involves non-perturbative QCD and is currently unclear, so definitive investigations are not possible at present.

The physics relevant to the present day clouds is not subject to these difficulties, and theoretical models of their structure can be constructed. For the central, hydrostatic core it is especially important to determine the density profile; this profile affects the typical kinetic energy dissipated during collisions, as well as the gas- and gravitational-lensing properties of the clouds. Similarly, a better understanding of the structure of the photo-evaporated wind would improve the predictive power of radio-wave lensing calculations -- a critical area at present, given that radio-wave lensing phenomena currently provide much of our information on the clouds' properties.

It would be very useful to acquire a better picture of how the clouds evolved, from some (assumed) initial conditions right up to the present day. While the correct initial conditions are unknown at present, because the origin of the clouds is murky, such evolutionary calculations would help to clarify which formation scenarios are plausible. The properties of the cloud population prior to the epoch of recombination are particularly interesting, because these can be related to the measurements of Cosmic Microwave Background anisotropies. If the clouds are sufficiently dense that they decouple from the CMB well before recombination, then the development of large-scale structure in the Universe will presumably be qualitatively similar to that of the Cold Dark Matter (CDM) model, as the clouds are expected to be dynamically cold at decoupling. Some of the theoretical ``machinery'' currently used with CDM - notably the numerical simulations of structure formation - could then be usefully applied to the cold cloud picture. As described in §5.4, however, the effects of cloud-cloud collisions on the dark matter distribution function would have to be incorporated in any simulations.

© Copyright Astronomical Society of Australia 1997