The Cloudy Universe

Mark Walker, Mark Wardle, PASA, 16 (3), 262.
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Contents Page: Volume 16, Number 3

The Cloudy Universe

Mark Walker & Mark Wardle

Special Research Centre for Theoretical Astrophysics, School of Physics,
University of Sydney, NSW 2006
M.Walker/M.Wardle@physics.usyd.edu.au

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 and X-ray flashes arising from cloud-cloud collisions, ultraviolet extinction, and three varieties of lensing phenomena.

Keywords: dark matter -- galaxies: halos -- ISM: clouds





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