The universe is expanding faster and faster. This is usually explained by a constant energy of empty space, but building such energy in string theory has proved impossible. A new paper proposes a simple rule: the energy of a field cannot be too flat. If it barely changes, the theory falls into a swamp of forbidden options.
The rule was tested on three different examples from string theory. In every case, the slope of the energy turned out to be no less than the energy itself, with a coefficient of order one. For one example, the result was 1.6; for another, 3.5; for the third, between 0.8 and 3.7. All these numbers are greater than √2, which means accelerated expansion is impossible.
If the rule holds, dark energy should not be constant but slowly changing. This explains why it is so small without fine-tuning parameters. The string theory swamp becomes a selection tool: overly quiet vacua sink, and only those where energy moves survive.
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.