The String Landscape, the Swampland, and the Missing Corner

TL;DR

This paper reviews the string landscape and swampland conjectures, compares holography in superstring and topological string theories, and discusses the need for a direct quantum gravity definition beyond holographic dualities.

Contribution

It highlights the gap in string dualities and proposes the search for a direct quantum gravity formulation independent of holographic duals.

Findings

Comparison of holography in superstring and topological string theories

Identification of a missing corner in string dualities

Proposal for a direct quantum gravity definition

Abstract

We give a brief overview of the string landscape and techniques used to construct string compactifications. We then explain how this motivates the notion of the swampland and review a number of conjectures that attempt to characterize theories in the swampland. We also compare holography in the context of superstrings with the similar, but much simpler case of topological string theory. For topological strings, there is a direct definition of topological gravity based on a sum over a "quantum gravitational foam." In this context, holography is the statement of an identification between a gravity and gauge theory, both of which are defined independently of one another. This points to a missing corner in string dualities which suggests the search for a direct definition of quantum theory of gravity rather than relying on its strongly coupled holographic dual as an adequate substitute (Based on TASI 2017 lectures given by C. Vafa).