Nonlocal action fluctuations and the Hubble tension

Le résumé fourni par la source

The persistent discrepancy between early- and late-universe measurements of the Hubble constant, known as the Hubble tension, motivates exploring modifications to General Relativity. We introduce Functors of Actions Theories (FATs), a novel framework in which the gravitational action is modified by a functional transformation, giving rise to a new degree of freedom—the actionion—that encodes the global past of the universe through an integral of the Ricci scalar. We derive the modified Einstein field equations for both quadratic and exponential FAT models and solve the resulting integro-differential equations numerically in a flat FLRW universe. A foundational analysis establishes that the theory is nonlocal yet causal when temporal integration limits are restricted to the past, preserving general covariance and recovering GR in the weak-coupling limit. Our numerical solutions reveal a time-dependent damping of the late-time expansion rate, driven by the actionion integral. This suppression increases the comoving angular diameter distance to recombination for a given Hubble constant, allowing the CMB data to be reconciled with a higher value of \(H_0\)—precisely the shift needed to bridge the \(\sim 5\sigma\) tension between Planck and local supernovae measurements. Moreover, the modified Friedmann equation admits a reinterpretation as an effective, dynamical dark energy density, offering an emergent origin for cosmic acceleration without a bare cosmological constant. The predicted redshift-dependent deviation in \(H(z)\) provides a clear observational test for upcoming surveys such as Euclid and DESI. This work establishes FATs as a promising framework for addressing the Hubble tension while providing a novel perspective on the nature of dark energy.

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