Cosmology seminar: Iain Alastair Brown: Frame and Gauge Dependences in Cosmological Averaging

The issue of averaging in cosmology has received significant interest
in the last decade in the hope that corrections to the Hubble rate
from inhomogeneities in the recent universe could behave as a dark
energy. However, an averaging procedure is sensibly defined only in a
volume-preserving coordinate system. An evaluation of the corrections
also naturally requires a definition of the Hubble rate. We construct
two (comoving) volume-preserving coordinate systems and analyse the
corrections to two common definitions of the Hubble rate.