Dust versus Supernova Cosmology

Here we present some critical discussions about the systematic uncertainty by dust extinction in the recent cosmological results of high-redshift Type Ia supernovae. First we argue that the currently available data do not robustly exclude the cosmologically significant extinction either by the reddening check or the dispersion argument because of the observational uncertainties, even in the case of ordinary dust that reddens. Then we discuss two theoretical possibilities that high-z supernovae have larger extinction and hence they are fainter than local supernovae: the intergalactic dust and host galaxy evolution. Optical and near-infrared observations for a large number of supernovae with improved photometric accuracy are required to reject these possibilities and derive a compelling cosmological result.