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 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 aₘin. Very small grains provide most of the reddening but a minority of the opacity for optical extinction. For f > 0. 3 and aₘin > 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. It would also have implications for galaxy counts and evolutionary studies, and would contribute significantly to the cosmic infrared background (CIB). 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.