The Cloudy Universe

Abstract

Modelling of Extreme Scattering Events suggests that the Galaxy's dark matter is an undetected population of cold, AU-sized, planetary-mass gas clouds. None of the direct observational constraints on this picture -- thermal/non-thermal emission, extinction and lensing -- are problematic. The theoretical situation is less comfortable, but still satisfactory. Galactic clouds can survive in their current condition for billions of years, but we do not have a firm description for either their origin or their evolution to the present epoch. We hypothesise that the proto-clouds formed during the quark-hadron phase transition, thereby introducing the inhomogeneity necessary for compatibility with light element nucleosynthesis in a purely baryonic universe. We outline the prospects for directly detecting the inferred cloud population. The most promising signatures are cosmic-ray-induced H-alpha emission from clouds in the solar neighbourhood, optical flashes arising from cloud-cloud collisions, ultraviolet extinction, and three varieties of lensing phenomena.

16 pages, LaTeX, no figures, to appear in Pub. Ast. Soc. Aus

References in corpus (9)

• Big-bang Nucleosynthesis Enters the Precision Era
• EROS and MACHO Combined Limits on Planetary Mass Dark Matter in the Galactic Halo
• Extreme scattering events and Galactic dark matter
• Evidence for a Galactic gamma ray halo
• Lensing of Stars by Spherical Gas Clouds
• Collisional baryonic dark matter halos
• The Early History of Dark Matter
• How to find MACHOs in the Virgo Cluster
• Unambiguous quasar microlensing

Cited by in corpus (6)

• Molecular gas in NGC6946
• Can Microlensing Explain the Long-Term Optical Variability of Quasars?
• Dense gas clouds and the Unidentified EGRET sources
• High-redshift microlensing and the spatial distribution of dark matter in the form of MACHOs
• Interpretation of Extreme Scattering Events
• Supernova Remnants, Pulsars and the Interstellar Medium - Summary of a Workshop Held at U Sydney, March 1999