Speaker
Description
Hydrogen "snow clouds" - which are so cold and dense that they contain particles of solid or liquid molecular hydrogen - are a well motivated form of baryonic dark matter. Theoretical models of snow cloud structure prefer cloud masses in the planetary range, but radii as large as planetary orbits. Increasingly there is evidence that such gas plays an important role in our own Galaxy, but the total mass in this form is poorly constrained by the available data. Within the $\Lambda$CDM cosmological model the baryonic content of the universe is gauged by the amplitude of the microwave background fluctuations, and in that model the sum total of baryons is inferred to be a small fraction of all the mass. More generally the low amplitude of the microwave background fluctuations, together with the rich structure we observe in matter in the low-redshift universe, tells us that our universe contains more than just baryons that are diffuse (tightly coupled to the radiation field) at recombination. But baryons that are already collapsed at recombination may suffice. I'll highlight some key elements of snow cloud physics in the Galactic context, and sketch out some possible aspects of a Universe that is dominated by planetary mass lumps from very early times.
