Nonlocal Cosmology

TL;DR

This paper proposes nonlocal gravity corrections, inspired by quantum loop effects, as a mechanism for late-time cosmic acceleration. It uses the retarded inverse d'Alembertian $\mathcal{G}$ acting on the Ricci scalar $R$ to construct a nonlocal distortion $f(\mathcal{G}[R])$ that modifies the Einstein tensor and yields a delayed acceleration after the radiation-to-matter transition at $z\sim2300$, aiming to match observations without fine tuning. A concrete model with $f(x)=C e^{-\frac{3}{4}k x}$ and order-one parameters produces the desired onset around $z\sim0.7$, and a reconstruction framework allows tailoring $f$ to reproduce arbitrary expansion histories while preserving causality and conservation. The work suggests a viable phenomenology with negligible solar-system effects and potential implications for black hole information, while remaining primarily phenomenological and open to further refinement.

Abstract

We explore nonlocally modified models of gravity, inspired by quantum loop corrections, as a mechanism for explaining current cosmic acceleration. These theories enjoy two major advantages: they allow a delayed response to cosmic events, here the transition from radiation to matter dominance, and they avoid the usual level of fine tuning; instead, emulating Dirac's dictum, the required large numbers come from the large time scales involved. Their solar system effects are safely negligible, and they may even prove useful to the black hole information problem.