Testing time-delayed cosmology
  Motivated by the proposed time-delayed cosmology in the primordial
inflationary era, we consider the application of the delayed Friedmann equation
in the late-time Universe and explore some of its observable consequences. We
study the background evolution predicted by the delayed Friedmann equation and
determine the growth of Newtonian perturbations in this delayed background. We
reveal smoking-gun imprints of time-delayed cosmology that can be traced to
derivative discontinuities generic in delay differential equations. We show
that a late-time cosmic delay is statistically consistent with Hubble expansion
rate and growth data. Based on these observables, we compute a nonzero best
estimate for the time delay parameter and find that the Bayesian evidence does
not strongly rule out a late-time time delay but warrants the subject further
study.
