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Abstract content <br> (Max 300 words)
Understanding dark energy, the mysterious component which comprises 70% of the energy density of the Universe, is arguably the most important open question in physics. The observed number of galaxy clusters of a given mass and its evolution with cosmic time, N(M,z), provides a powerful means to measure the equation of state, w, of dark energy.
In contrast to geometric tests of cosmology (such as those using Type Ia Supernovae), N(M,z) depends on both geometry and growth factor, making it potentially more sensitive. In this talk I will describe an optical survey of ~10 000 galaxy clusters designed to measure N(M,z) over half the age of the Universe, and extensive on-going work to calibrate our mass proxy, optical richness. I will present first results from Multi-Object Spectroscopy (MOS) using the Robert Stobie Spectrograph on SALT to make dynamical mass measurements of these clusters.