Why isn't everything expanding in an expanding universe?

[PeterDonis]

If dark energy continues to become ever stronger in proportion to gravity

What makes you think it is? The average density of dark energy (which is constant as far as we can tell--see below) is becoming larger in proportion to the average density of matter and radiation; but that's not the same thing as dark energy becoming stronger in proportion to gravity. The gravity of a locally bound system, such as a solar system or a galaxy, is not determined by the average density of matter in the entire universe; it's determined by the density of matter in the locally bound system. The ratio of that density to the density of dark energy doesn't change as the universe expands; that's what "locally bound system" means.

shouldn't the vacuum energy density be steadily decreasing?

Not if it's a cosmological constant. In other models, it could change from place to place or change in time; but the change wouldn't necessarily be a decrease.

 

[newjerseyrunner]

Oh, I understand now.