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Presented By: Leinweber Center for Theoretical Physics

HET Seminar | The Gravity of Light Scalars (Naturally)

Cliff Burgess (McMaster/Perimeter)

We live in remarkable times: the recent advent of gravitational-wave observations allows testing gravity in a strongly relativistic regime. We also have plausible candidates for UV physics that reconciles General Relativity with Quantum Mechanics. But there is also Bad News: Decoupling - which beautifully explains why low-energy measurements are largely insensitive to UV details - seems a central organizing feature of Nature that thwarts the extraction of fundamental insights about UV physics from astrophysical or cosmological observations. This talk argues that all is not lost because some UV features can penetrate the decoupling barrier in interesting ways. In particular generic accidental symmetries can robustly point to the existence of scalars in the low-energy effective theory (and these are not just axions). Normally we are taught that naturalness arguments preclude these scalars from being light enough or too weakly coupled to be important for tests of gravity, but I argue that the additional information that the observed Dark Energy is so small puts us in a regime where some scalars are pseudo-dilatons (ie naturally light with Brans-Dicke couplings to matter). The question of why these scalars are not already detected motivates more detailed studies of whether screening mechanisms exist that could have hidden them from present-day tests of gravity. Crucially they must do so in a way consistent with other properties of UV completions of gravity (in a way that standard screening mechanisms - like Chameleons - are not). The talk describes new proposals for such UV-consistent screening mechanisms and why they thread a blind spot in current theoretical approaches to testing gravity. If time permits I will also explore other implications these models might have, including possible relevance to other problems like the Hubble tension.

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