A non-relativistic limit of NS-NS gravity

Unification of Decoupling Limits in String and M Theory

We study and extend the duality web unifying different decoupling limits of type II superstring theories and M theory. We systematically build connections to different corners, such as matrix theories, nonrelativistic string and M theory, tensionless (and ambitwistor) string theory, Carrollian string theory, and spin matrix limits of AdS/CFT. We discuss target space, world sheet, and worldvolume aspects of these limits in arbitrary curved backgrounds.

Identifying Riemannian Singularities with Regular Non-Riemannian Geometry

Admitting non-Riemannian geometries, double field theory extends the notion of spacetime beyond the Riemannian paradigm. We identify a class of singular spacetimes known in general relativity with regular non-Riemannian geometries. The former divergences merely correspond to coordinate singularities of the generalized metric for the latter. Computed in the string frame, they feature an impenetrable non-Riemannian sphere outside of which geodesics are complete with no singular deviation. Approaching the non-Riemannian points, particles freeze and strings become (anti)chiral.

Nonrelativistic Expansion of Closed Bosonic Strings

We develop a novel approach to nonrelativistic closed bosonic string theory that is based on a string 1/c^{2} expansion of the relativistic string, where c is the speed of light. This approach has the benefit that one does not need to take a limit of a string in a near-critical Kalb-Ramond background. The 1/c^{2}-expanded Polyakov action at next-to-leading order reproduces the known action of nonrelativistic string theory provided that the target space obeys an appropriate foliation constraint. We compute the spectrum in a flat target space, with one circle direction that is wound by the string, up to next-to-leading order and show that it reproduces the spectrum of the Gomis-Ooguri string.