🚀 go-pugleaf

General relativity gives a geometric explanation to gravitational
phenomena; thus, we say that an object deflected by a gravitational
field is moving "straight" (on a gedesic) in a curved spacetime.
Also, we say that space is expanding.

However, for weak and not rapidly varying fields, a Newtonian
description is also correct.  For cosmology, this limit applies for
distances which are small with respect to the Hubble distance (the
distance light has traveled since the start of the universe), i.e. the
vast majority of what we can see (and everything for which the simple
Hubble law, v = H r, is well defined).

Thus, you can understand Hubble expansion correctly as galaxies flying
away from themselves because of their momentum, with the rate of
separation slowing due to their mutual gravitational attraction.

The same limit applies in the early universe, as long as you replace
the gravitating mass density by the energy density plus three times
the pressure.

-- Ethan