Ernest Rutherford Fellowship: New cosmology measurements with voids in next generation surveys

At present, our best understanding of the Universe is encapsulated in the standard Lambda Cold Dark Matter model. Although it successfully describes current data, this model requires a dark energy component that is not well understood. A new generation of Stage-IV galaxy surveys are about to start collecting data that will provide an order of magnitude better measurements of the nature of dark energy, and could reveal the existence of a new fundamental field driving dark energy, the breakdown of General Relativity, or require a complete rethinking of our understanding of the vacuum energy. These surveys will also measure the sum of the neutrino masses for the first time, and add new data at intermediate redshifts that could clarify or resolve the discrepancy in measurement of the Hubble constant.

I have led the development of a powerful new technique for the analysis of data from these surveys through the multipoles of the cross-correlation of galaxy positions with cosmic voids. Our analysis of the completed BOSS galaxy survey using this method measured the angular diameter distance scale and the expansion rate each to sub-percent precision, and the growth rate to less than 5 percent. These are the best measurements currently available from any data and demonstrate that the new technique leads to an information gain of about 50%, equivalent to quadrupling the data volume from a galaxy survey at no additional cost. This leads to significant improvements in dark energy constraints, including when combined with complementary probes such as Type Ia supernovae.

The scientific aims of my proposal are:
1. To apply this new void-galaxy technique, in conjunction with standard BAO and RSD methods, to new data from the DESI and Euclid spectroscopic galaxy surveys, which are starting operations in 2020 and 2022 respectively. Using this method, I will lead measurements of distances, expansion history and growth rate at a range of redshifts, which will be the most precise cosmological results for the next decade. These results will lead to a factor of 2 improvements in the measurement of the dark energy equation of state and the spatial curvature of the Universe over the results otherwise possible through standard methods alone.
2. To use the vast improvement to measurements of the expansion rate from BAO using the method above to improve the determination of the Hubble constant by the inverse distance-ladder method that is independent of both CMB and local distance ladder methods. This will provide a precise and independent observational window on the big discrepancy between Hubble constant measurements from these two techniques, that is an outstanding problem in cosmology today.
3. To improve the measurement of the sum of neutrino masses achievable with DESI and Euclid through the use of cosmic voids, including through better measurements of the expansion rate and growth rate as functions of redshift through the void-galaxy cross-correlation, statistics of void populations, and through constraining scale-dependence of the void bias.
4. To test modified gravity theories and possible deviations from General Relativity using DESI, Euclid and LSST: constraining generic deviations from GR through more precise measurement of the redshift-dependence of the growth rate through the void-galaxy cross-correlation, and specific models of screened modified gravity through void lensing.
Stage-IV surveys are about to start data collection from 2020 onwards. The techniques outlined here will vastly increase the information that can be extracted from this data, producing the gold standard cosmology measurements of the next 5 years and addressing fundamental questions about the history and constituents of our Universe, the nature of dark energy, and gravity. This is an ambitious but achievable proposal with far-reaching consequences for cosmology, based on new methodological advances that have been proven on current datasets.

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 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 the upcoming Stage-IV survey 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 Vera Rubin Observatory.

Together, these projects will dramatically improve our knowledge of dark energy, neutrino masses, the Hubble constant and alternative theories of gravity over the duration of this Fellowship