Evolution of dust extinction and supernova cosmology

We have made a quantitative calculation for the systematic evolution of the average extinction by interstellar dust in host galaxies of high-redshift Type Ia supernovae by using a realistic model of photometric and chemical e volution of galaxies and supernova rate histories in various galaxy types. We find that the average B-band extinction [A(B)] at z similar to 0.5 is ty pically 0.1-0.2 mag larger than the present value, under a natural assumpti on that dust optical depth is proportional to gas column density and gas me tallicity. This systematic evolution causes average reddening with E(B-V) s imilar to 0.025-0.05 mag with the standard extinction curve, and this is co mparable with the observational uncertainty of the reddening of high-redshi ft supernovae. Therefore, our result does not contradict the observations t hat show no significant reddening in high-z supernovae. However, the differ ence in apparent magnitude between an open universe and a Lambda-dominated flat universe is only similar to 0.2 mag at z similar to 0.5, and hence thi s systematic evolution of extinction should be taken into account in a reli able measurement of cosmological parameters. Considering this uncertainty, we show that it is difficult to discriminate between open and Lambda-domina ted flat cosmologies from the current data.