Framed Wilson Operators, Fermionic Strings, and Gravitational Anomaly in 4d

Understand

We study gapped systems with anomalous time-reversal symmetry and global gravitational anomaly in three and four spacetime dimensions.

• These systems describe topological order on the boundary of bosonic Symmetry Protected Topological (SPT) Phases.
• Our description of these phases is via the recent cobordism proposal for their classification.
• In particular, the behavior of these systems is determined by the geometry of Stiefel-Whitney classes.