Measuring the properties of extragalactic dust and implications for the Hubble diagram

Scattering and absorption of light by a homogeneous distribution of intergalactic large dust grains has been proposed as an alternative, non-cosmological explanation for the faintness of Type Ia supernovae at z ∼ 0.5. We investigate the differential extinction for high-redshift sources caused by extragalactic dust along the line of sight. Future observations of Type Ia supernovae up to z ∼ 2, e.g. by the proposed SNAP satellite, will allow the measurement of the properties of dust over cosmological distances. We show that 1% relative spectrophotometric accuracy (or broadband photometry) in the wavelength interval 0.7–1.5 μm is required to measure the extinction caused by “grey” dust down to δm = 0.02 magnitudes. We also argue that the presence of grey dust is not necessarily inconsistent with the recent measurement of the brightness of a supernova at z = 1.7 (SN 1997ff), in the absence of accurate spectrophotometric information of