  It is argued that cosmic chronometers yield estimates of the spatially
averaged expansion rate even in a universe that is not well described by a
global FLRW model - as long as the Universe is statistically homogeneous and
isotropic with a sufficiently small homogeneity scale. On the other hand,
measurements of the expansion rate based on observations of redshift drift will
not in general yield estimates of the spatially averaged expansion rate - but
it will in the case where the universe is described well by a single FLRW model
on large scales. Therefore, a disagreement between measurements of the
expansion rate based on cosmic chronometers versus redshift drift is an
expected signal of non-negligible cosmic backreaction.

Cosmology and Nongalactic Astrophysics