Nonlocal Gravity, Restricted CPL Dynamics and DESI Constraints

Abstract: Nonlocal modifications of gravity provide a theoretically motivated framework in which cosmic acceleration can emerge dynamically without introducing a fundamental cosmological constant. We revisit the cosmological predictions of the Maggiore--Mancarella nonlocal $R\,\Box<sup>{-2}R$ model, with particular emphasis on its background dynamics and observational viability in light of recent DESI baryon acoustic oscillation measurements. A key feature of the theory is its predictive character: for fixed standard cosmological parameters, each value of the nonlocal parameter determines a specific effective pair $(w_0,w_a)$, so that the model traces a restricted trajectory in the Chevallier--Polarski--Linder plane rather than spanning the full phenomenological parameter space. We construct this mapping explicitly and confront the model with observations by implementing its cosmological dynamics in a Boltzmann solver and performing a Markov chain Monte Carlo analysis using Cosmic Microwave Background, supernova, and DESI BAO data. We find that current datasets constrain the nonlocal parameter close to the $Λ$CDM limit, with no statistically significant preference for nonlocal deviations.