Cosmic expansion with perturbed Friedmann equations

Abstract

A time-delayed Friedmann equation can accommodate inflationary scenarios that naturally end without violating energy conditions or invoking scalar fields. But robust data-driven constraints on the theory are yet to be established. As a step forward, we take first- and second-order Taylor expansions of the delayed Friedmann equation as phenomenological models. We use these perturbed models to obtain predictions of the Hubble expansion rate H(z) and compute the best-fit parameters. For the first-order model, we find that a delay 𝜏 = 0.51^+2.53_‒0.40 Gyr is most consistent with the data and for the second-order model, 𝜏 = 0.25^+0.45_‒0.14 Gyr. We also find moderately strong statistical preference for the first- and second-order models over the standard ΛCDM model.