The background Friedmannian Hubble constant in relativistic inhomogeneous cosmology and the age of the Universe

Understand

In relativistic inhomogeneous cosmology, structure formation couples to average cosmological expansion.

• A conservative approach to modelling this assumes an Einstein--de Sitter model (EdS) at early times and extrapolates this forward in cosmological time as a "background model" against which average properties of today's Universe can be measured.
• This requires adopting an early-epoch--normalised background Hubble constant $H_1^{bg}$.
• Here, we show that the $\Lambda$CDM model can be used as an observational proxy to estimate $H_1^{bg}$ rather than choose it arbitrarily.