A new theoretical study proposes that the observed cosmological constant is not determined by microscopic vacuum energy scales, but by the deepest infrared scale of the present universe. This result emerges when progressively taking into account the quantum fluctuations of the gravitational field, suggesting a screening mechanism that reconciles the discrepancy between theoretical predictions of vacuum energy and the measured value of the cosmological constant.

The researchers used an Einstein-Hilbert+$R^2$ truncation of gravity to analyze the renormalization group flow. They found that in a wide region of the parameter space, the theory develops instabilities that give rise to large amplitude quantum fluctuations. These fluctuations dominate the renormalization flow, causing the effective cosmological constant to progressively lose memory of its initial boundary conditions as the flow moves towards the infrared.

The running cosmological constant, Λk (where k is the running scale), is driven towards a universal infrared scaling: Λk ~ λIR k², with λIR = 1/(2(π-2)). At the deepest infrared scale, k ~ H₀ (the current Hubble scale), the cosmological constant is then screened to its observed value, ΛIR ~ H₀². This model offers insight into how fundamental properties of spacetime at large scales could be intrinsically linked to quantum gravitational phenomena, providing a possible solution to one of the most persistent problems in modern physics.