Constraining gravity with cosmology

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The recent past has witnessed phenomenal successes in cosmology, constraining the structure, content and evolution of the Universe with tremendous accuracy. Consequently, a current focus of cosmology is to gain an understanding of the agents behind this evolution, identifying the fundamental physics behind the accelerated expansion of the Universe at early and late times and the nature of dark energy and dark matter. Key to this enterprise is thoroughly understanding gravity itself - the least well-understood of the known forces in nature. Specifically, this involves testing, constraining (and potentially detecting deviations from) our current best working theory: General Relativity. With the emergence of new observational windows (e.g. the first detections of gravitational waves) in addition to other well-established probes of gravity (e.g. observations of the cosmic microwave background), we presently have a remarkable and continuously growing amount of data allowing us to precisely measure gravitational phenomena. Simultaneously, recent theoretical advances in understanding what features a consistent theory of gravity must possess, enable us to impose additional strong restrictions on how gravity can behave and and how it interacts with matter in the universe. Altogether this means we currently have at our disposal an unprecedented amount of powerful instruments to precisely identify and constrain the nature of gravity. Given the different areas of physics they originate from, these instruments have not been used jointly so far, yet the qualitatively different ways in which they operate means they in fact ideally complement one another, testing and probing gravity in a variety of different ways. This research program therefore aims to take full advantage of this complementarity, integrating methods ranging from the development of data-driven numerical codes to the implementation of theoretical bounds from high-energy particle physics in order to obtain the most systematic and tight bounds on gravity yet. In this way, we will be able to thoroughly probe the nature of gravity, testing several of its fundamental properties in the process.