Supernovas and cosmology

Hubble redshift-magnitude diagram an important cosmological tool expressing the rate of expansion of the Universe in the past 

At first glance, the spectral properties, absolute magnitudes (intrinsic luminosities) and shapes of the light curves of the majority of type la supernovas (SNIa) are remarkably similar. Only a few rather subtle photometric and spectrometric differences can be discerned from one object to another. 

Hydrogen shines by its absence and the optical spectra of SNIa events feature spectral lines of neutral and once ionised elements (Ca, Mg+, S+ and 0+) at the minimum of the light curve. This indicates that the outer layers are composed of intermediate mass elements. SNIa events reach their maximum luminosity after about 20 days. This luminous peak is followed by a sharp drop amounting to three magnitudes per month. Later the light curve falls exponentially at the rate of one magnitude per month. 

The important datum for cosmology is precisely the luminosity at the peak of the light curve. It is crucial to be able to establish this maximum value in order to use the SNIa event as a distance indicator. Correctly cahbrated and reproducible hght curves from type la supernovas have become a major tool for determining the local expansion rate and geometrical structure of the Universe (Fig. A2.1). A great deal of effort has been put into producing adequate models of these events over the past few years. 

Through a well-established tradihon, relahvely close SNIa events (z 0.1) have been used to measure the current local value of the Hubble parameter. Supernova-based cosmology has seen a recent upturn in achvity. The local expansion rate is something hke 60 km s per megaparsec, which corresponds to 18 km s per million light-years. 

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