New cosmology measurements with voids in next-generation galaxy surveys

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In 1998, astronomers made the surprising discovery that the expansion of the universe is accelerating. This result led to a revolution in cosmology and the introduction of a mysterious 'dark energy' in our model of the universe. A theoretical understanding of this dark energy remains the biggest open problem not just in cosmology, but perhaps in all of physics. The explanation for dark energy will require either a new understanding of the quantum field theory of vacuum energy, the discovery of a new fundamental field, or a modification of the theory of gravity on very large scales.Guidance as to the correct theoretical approach must be taken from observation. A new generation of galaxy surveys - DESI, Euclid and LSST - will start to come online from 2020 onwards, mapping the 3D distribution of millions of galaxies in space. These surveys will provide an unprecedented dataset with which to measure the expansion rate, capable of providing an order of magnitude improvement in our knowledge of dark energy as well testing Einstein's General Relativity on cosmological scales. The same data can also be used to measure the mass of neutrinos better than can be done in laboratory experiments on earth, to determine the curvature of space, and even to weigh the total mass of the Universe. To me, this makes it the most exciting field of contemporary cosmology.But collecting this vast amount of data is expensive and time-consuming, and it is imperative to process it as efficiently as possible to extract the maximum information. I have developed a new method for measuring cosmological quantities from the galaxy distribution that is based on the properties of the large empty regions, or voids, between them. I led an analysis of current data using this method, demonstrating that it leads to the most precise cosmological measurements from any existing galaxy survey. The information gain over the use of conventional methods alone is equivalent to quadrupling the number of galaxies observed, all while requiring no additional cost or observational time. These improved measurements already greatly reduce the uncertainty in dark energy properties, including when combined with other types of data such as Type Ia supernovae and weak lensing. When applied to Stage-IV data the benefits will be even greater. I will lead cosmological measurements by applying this new technique to data from the DESI and Euclid spectroscopic surveys that are starting operations in 2020 and 2022 respectively. This will increase their information content and provide the gold standard of cosmological measurements. I will also continue to develop additional new analyses using cosmic voids in other ways, for application to photometric surveys and data from the LSST experiment, which is also expected to start in 2020.Together, these projects will dramatically improve our knowledge of dark energy, neutrino masses, the Hubble constant and alternative theories of gravity over the 5-year duration of this Fellowship.