Observations of type Ia supernovae and challenges for cosmology

Observations of Type la supernovae (SNe Ia) reveal correlations between their luminosities and light-curve shapes, and between their spectral sequence and photometric sequence. Assuming SNe la do not evolve at different redshifts, the Hubble diagram of SNe la may indicate an accelerating Universe, the signature of a cosmological constant or other forms of dark energy. Several studies raise concerns about the evolution of SNe la (e.g., the peculiarity rate, the rise time, and the color of SNe la at different redshifts), but all these studies suffer from the difficulties of obtaining high-quality spectroscopy and photometry for SNe la at high redshifts. There are also some troubling cases of SNe la that provide counterexamples to the observed correlations, suggesting that a secondary parameter is necessary to describe the whole SN la family. Understanding SNe Ia both observationally and theoretically will be the key to boosting confidence in the SN la cosmological results.