[russdill]

There is an addition of a inflation field. Observations show that such an inflation happened, and there's several ideas of inflation fields that are compatible with the standard model and match observations. The equations that make the observed expansion happen in the presence of such a field the friedmann equations, which are GR solutions.

The field isn't just something that magically expands things, it just effects energy density. The rapid expansion is then a consequence of GR.