Chameleon Effects on Small Scale Structure and the Baryonic Jeans Mass

In the framework of Newtonian cosmology or general relativity it is simple to derive a mass scale below which collapsed structures are relatively devoid of baryons. We examine how the inclusion of a chameleon scalar field affects this baryonic Jeans mass, bearing in mind both the canonical case of a gravitational-strength coupling between the scalar field and matter, as well as the strong coupling regime wherein the coupling is very large. We find that baryon effects persist down to smaller scales in a chameleon theory than they do in ordinary general relativity, especially in the case of strong coupling. Several potentially observable consequences of this are identified.