A ghost-free gravity theory stumbles on CMB data

Two modified gravity theories — ghost-free non-local gravity and a generalized exponential F(R) model — were pit against the latest supernova, DESI, BAO, and CMB datasets. Ghost-free non-local gravity holds its own against most observations but breaks down once CMB data is added, largely because it only mimics quintessence-like dark energy rather than the phantom-to-quintessence transition the data seem to prefer. The exponential F(R) model outperforms even the standard cosmological constant across all datasets combined, suggesting the universe's dark energy may have a more dynamic history.