  Cosmic rays produced or deposited at sites in hot cluster gas are thought to
provide the pressure that forms X-ray cavities. While cavities have a net
cooling effect on cluster gas, young, expanding cavities drive shocks that
increase the local entropy. Cavities also produce radial filaments of thermal
gas and are sources of cluster cosmic rays that diffuse through cavity walls,
as in Virgo where a radio lobe surrounds a radial thermal filament. Cosmic rays
also make the hot gas locally buoyant, allowing large masses of low entropy gas
to be transported out beyond the cooling radius. Successive cavities maintain a
buoyant outflow that preserves the cluster gas temperature and gas fraction
profiles and dramatically reduces the cooling rate onto the central black hole.
