The universe is expanding faster and faster, and the standard explanation is a positive cosmological constant, that is, a de Sitter vacuum. String theory, which aspires to be the theory of quantum gravity, should be able to describe such a vacuum, but after decades of effort, no reliable construction has emerged. This work takes a more radical step: not just a difficulty, but a fundamental prohibition on de Sitter in quantum gravity.
The authors formulate a swampland criterion: the gradient of a scalar potential V must satisfy the inequality |∇V| ≥ c·V, with a constant c of order one. In other words, the potential cannot be too flat. If it is almost flat, the theory falls into the swampland of forbidden theories. Tests on concrete examples from string theory confirm this criterion.
Observational data on dark energy give c ≲ 0.6, only slightly below the theoretical estimates. The authors call this closeness of numbers "tantalizingly close." The work turns scattered no-go theorems into a single principle: de Sitter vacua fall into the swampland.
If the criterion is correct, dark energy must be quintessence, not a vacuum. Instead of fine-tuning parameters, we get a natural explanation for its smallness through the slope of the potential. This changes our view of the cosmological constant and opens a path to selecting realistic models of inflation and dark energy.
The string theory swampland is not just a metaphor but a working tool for selection. If de Sitter vacua are forbidden, the landscape of string vacua narrows dramatically, and the anthropic principle loses its footing. The swampland criterion links cosmology with constraints on effective field theories, which may shed light on the nature of dark energy.
The authors call the coincidence between the experimental bound c ≲ 0.6 and the theoretical estimates c ~ 1 'tantalizingly close.' If the value were slightly larger, accelerated expansion would be impossible within quintessence, and we would have to look for entirely different explanations.
The idea of a ban on stable vacuum echoes Greg Egan's novel 'Schild's Ladder,' where physicists discover that the vacuum is unstable and could decay. The paper deals with a milder but related restriction: de Sitter vacuum cannot be a resting point, but only a slowly rolling state.