Tessa Baker: Agnostic Tests of Gravity

Tessa Baker, a cosmologist from Oxford's astrophysics department, introduces her research on developing agnostic tools for testing the nature of gravity. She emphasizes the importance of questioning the validity of Einstein's general relativity on all scales and environments in the universe. Motivated by the accelerating expansion of the universe and the theoretical issues with the cosmological constant, she aims to explore whether extensions to general relativity could explain this phenomenon. The talk is structured into three sections: an overview of alternative gravity theories, the development of an agnostic framework for testing these theories, and the potential of future data to measure parameters within this framework.

The speaker provides an overview of the space of modified gravity theories that have emerged over the past two decades. She discusses Lovelock's theorem, which states that the only second-order local gravitational field equations derivable from an action based solely on the four-dimensional metric of spacetime are those of Einstein's field equations with a cosmological constant. To construct alternative gravity theories, one must break one of the implicit clauses of this theorem, leading to five categories of theories, including additional field content, extra dimensions, higher-order theories, non-local theories, and those derived from actions in a non-standard way. The speaker briefly describes each category and its implications for gravity theories.

The speaker highlights the challenge in testing the numerous modified gravity theories that have been proposed. Due to the complexity of calculating predictions for cosmic microwave background, gravitational lensing, and large-scale structure of the universe for each theory, most theories have not been rigorously tested against observational data. The speaker and her collaborators have developed agnostic methods to test these theories within a single framework, aiming to make the process more efficient and comprehensive. The idea is to parameterize the vast space of theories in such a way that any particular theory's predictions can be recovered by adjusting the parameters of the framework.

The speaker introduces the effective field theory of cosmological perturbations, a framework developed by her and her collaborators for testing modified gravity theories. Inspired by effective field theory techniques used in particle physics, the framework involves writing down the most general action consistent with certain symmetries and physical principles. The speaker explains the process of constructing this parameterization, which involves expanding the action in linear perturbations of the metric and additional fields, and then using the symmetries of the theory to reduce the number of free coefficients in the action.

The speaker discusses the potential of future cosmological experiments to measure the parameters of the effective field theory framework. She acknowledges the current limitations in constraining the parameters due to degeneracies and the complexity of measuring functions of time with existing data. However, with upcoming experiments such as the Dark Energy Survey, EUCLID, the Large Synoptic Survey Telescope (LSST), and the Square Kilometer Array (SKA), the speaker is optimistic that more precise measurements will be possible. These measurements could help rule out certain patches of the modified gravity landscape and provide a more general test of the nature of gravity.

In the final part of the script, the speaker addresses concerns about the parameterization approach and its implications for theory assessment in cosmology. She discusses the potential for parameter degeneracies and the challenge of distinguishing between different hypotheses when multiple theories can fit the data. The speaker emphasizes the importance of the next generation of detectors and surveys in improving measurements and potentially detecting discrepancies from the standard model of general relativity. She also highlights the availability of public tools and codes for those interested in engaging with the theoretical framework she has described.