BMS Characters and Modular Invariance

Understand

We construct the characters for the highest weight representations of the 3d Bondi-Metzner-Sachs (BMS$_3$) algebra.

• We then use these to construct the partition function and show how to use BMS modular transformations to obtain a density of primary states.
• The entropy thus obtained accounts for the principle part of the entropy obtained from the BMS-Cardy formula.
• This suggests that BMS primaries capture most of the entropy of Flat Space Cosmologies, which are the flatspace analogues of BTZ black holes in AdS$_3$.