# Flat JT Gravity and the BMS-Schwarzian

Abstract : We consider Minkowskian Jackiw-Teitelboim (JT) gravity in Bondi gauge at finite temperature, with non-zero vacuum energy. Its asymptotic symmetries span an extension of the warped Virasoro group, dubbed "BMS$_2$", which we investigate in detail. In particular, we show that this extension has a {\it single} coadjoint orbit when central charges are real and non-zero. The ensuing BMS-Schwarzian action has no saddle points, and only coincides with the boundary action functional of flat JT gravity up to a crucial dilatonic zero-mode that ensures the existence of a well-defined bulk variational principle. We evaluate the corresponding gravitational partition function, which turns out to be one-loop exact precisely thanks to the presence of such a zero-mode.
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https://hal.archives-ouvertes.fr/hal-03520943
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Submitted on : Tuesday, January 11, 2022 - 12:24:22 PM
Last modification on : Friday, January 14, 2022 - 6:30:54 PM

### Citation

Hamid Afshar, Blagoje Oblak. Flat JT Gravity and the BMS-Schwarzian. 2022. ⟨hal-03520943⟩

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