Intergalactic Dust and Observations of Type Ia Supernovae

Estimates of the cosmic star formation rate and of cluster metallicities independently imply that at z < 0.5 the gas in the universe has substantial average metallicity: 1/10 < Z/Z_solar < 1/3 for Omega_gas = 0.05. This metal density probably cannot be contained in known solar-metallicity galaxies of density parameter Omega_star ~ 0.004, implying significant enrichment of the intergalactic medium (IGM) by ejection of metals and dust from galaxies via winds, in mergers or in dust efflux driven by radiation pressure. Galaxies have a dust/metal ratio of ~ 0.5 in their interstellar media, but some fraction (1-f) > 0 of this must be destroyed in the IGM or during the ejection process. Assuming the Draine & Lee dust model and preferential destruction of small grains, I calculate the reddening and extinction of a uniform cosmological dust component in terms of (f) and the minimum grain size a_min. Very small grains provide most of the reddening but less than half of the opacity for optical extinction. For f > 0.3 and a_min > 0.1 microns, the intergalactic dust would be too grey to have been detected by its reddening, yet dense enough to be cosmologically important: it could account for the recently observed type Ia supernova dimming at z ~ 0.5 without cosmic acceleration. The importance of grey intergalactic dust of the described type can be tested by observations of z=0.5 supernovae in (rest) R-band or longer wavelengths and by the fluxes of a large sample of supernovae at z > 1. (Abridged)