The standard model of particle physics is a quantum field theory (QFT) that has been spectacularly successful in explaining the electroweak and strong interactions between particles. Yet many puzzles remain concerning the inputs to the model, such as masses and coupling strengths, which seem to have been precisely selected from a nearly infinite set of possibilities. To explain this fine-tuning, researchers have explored a wide range of QFTs that go beyond the standard model. However, recent work in string theory, the prime candidate for a quantum theory of gravity, suggests that some of the alternative QFTs being considered are incompatible with gravity. In 2005, an effort began to delineate the conditions that a QFT must satisfy to be consistent with a quantum theory of gravity [1]. QFTs not meeting those conditions are said to reside in the “swampland.” This sorting of QFTs by their consistency with gravity has become an unexpectedly powerful theoretical tool, offering potential solutions to the problems of fine-tuning. Moreover, by mapping out the swampland, string theorists have uncovered testable predictions in cosmology related to the nature of dark energy and to the dynamics of the very early Universe.

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