Let's look at this from a different perspective.

Instead of considering the universe, let's consider a chunk of raw space - a cubic parsec, say. When I say "raw", I mean I am abstracting it from its contents (or its contents from it).

I have never seen why gravity should come into the rate of expansion of the universe, because I don't see why raw space should be affected by it.

Raw space seems to have a propensity to expand. In fact, that is one of its most curious properties. Question is: is this property a constant relative to time? It is one of the most fundamental properties in physics. It would please the Rev Occam if it were a constant.

We may judge (on the basis of observation) that our cubic parsec expands by 10% linear in a given number of years, and so (being three dimensional) becomes about one and a third cubic parsecs.

In a following period of the same duration, given that the propensity to expand is a constant, it expands by a further 10%. However, there is now more of the original chunk of raw space to expand. The overall linear expansion is 21% and the volume has grown by more than three-quarters.

Point is, we have the effect of compound interest. A constant rate of return on your investment results in an acceleration of your wealth. And a constant propensity of space to expand results in a continually increasing value for the Hubble variable.

The conclusion of cosmic acceleration deduced from the observation of supernova "standard candles" is entirely consistent with a constant propensity of raw space to expand - at least at first glance. The interesting question is whether this remains true at second glance - or indeed after much more careful scrutiny.

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"True skepticism encompasses not dismissing evidence because it seems to defy rational explanation."