  I consider certain renormalization effects in curved spacetime quantum field
theory. In the very early universe these effects resemble those of a
cosmological constant, while in the present universe they give rise to a
significant finite renormalization of the gravitational constant. The relevant
renormalization term and its relation to elementary particle masses was first
found by Parker and Toms in 1985, as a consequence of the ``new partially
summed form'' of the propagator in curved spacetime. The significance of the
term is based on the contribution of massive particles to the vacuum. In the
present universe, this renormalization term appears to account for a large part
or even all of the Newtonian gravitational constant. This conjecture is
testable because it relates the value of Newton's constant to the elementary
particle masses.
