The Swampland: Consistency constraints on effective theories from quantum gravity

Eran Palti (Max Planck & Ben-Gurion)

General relativity is expected to break down at the so-called Planck length, to be replaced with some theory of quantum gravity. String theory, our best candidate for a quantum theory of gravity, appears to lead to a huge number of effective theories as its possible low-energy limits, the so-called Landscape. This has led to the idea that perhaps any apparently consistent effective field theory can arise as a low energy limit of string theory, making it completely un-predictive. In this talk, I will argue that this is far from the truth, and the Landscape of effective theories from string theory is surrounded by a much vaster Swampland of seemingly consistent theories that nonetheless could never be compatible with quantum gravity. Over the past few years there has been an active program attempting to chart the spectrum of conditions on effective theories which are requirements to not lie in this Swampland of inconsistent theories. I will explain the methodology of this program and give an overview of some of the most interesting proposals. These include the ideas that in any consistent theory gravity must be the weakest force, and that cosmic acceleration cannot be due to a cosmological constant.