OzDES: Spectroscopy for the Dark Energy Survey

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Observations of distant Type Ia supernovae revealed that the expansion of the Universe is currently accelerating, fueled by some mysterious substance dubbed "Dark Energy". Following this surprising discovery, the current challenge for astrophysicists is to characterise the behaviour of Dark Energy in precise detail, in order to gain some clues as to its physical origin. Significant effort and funding is now being devoted to this end, including major UK investment in the upcoming Large Synoptic Survey Telescope (LSST) which is poised to revolutionise our understanding of Dark Energy.One major challenge to this effort is the astrophysical diversity of the supernovae we use as "standard candles". Our ability to use these cataclysmic events to measure distances is dependent upon the intrinsic uniformity of their brightnesses. Recent work has indicated that the type of galaxy where a supernova occurs can cause a subtle shift in its final brightness, and this trend appears consistent with a slight dependence of the supernova brightness on the age of the star system which produced it. This presents a major difficulty for measuring the expansion history of the Universe because the light from very distant supernovae was produced at an earlier epoch of the Universe when stars were on average younger. Thus we could be comparing apples and oranges when comparing the brightnesses of nearby and distant supernovae to infer the expansion history of the Universe.This project aims to definitively address the evolving impact of this "age bias" on the brightnesses of Type Ia supernovae, so that we may use them precisely and accurately to determine the true expansion history of the Universe and thus the detailed nature of Dark Energy. We will use data from the OzDES spectroscopy programme and the Dark Energy Survey (DES) imaging programme to characterise the host galaxies of thousands of supernovae -- a factor of ten more than previous surveys. With these supernova host galaxies, we will precisely measure how the ages of supernovae affect their brightnesses at all epochs of cosmic time. This will enable the most precise measurement of the properties of Dark Energy ever made.This project will also serve as a perfect test bed for the techniques UK astronomers will employ for supernovae discovered by LSST. While DES represents a factor of ten increase in sample size over the previous generation of supernova experiments, LSST will outpace the DES supernova count by a factor of nearly one hundred. Thus astronomers must take the opportunity now to refine the programs and techniques we will employ on the massive flow of data that will arrive with the advent of LSST, and this project will do precisely that.