  Hoyle and Narlikar (HN) in the 1960's developed a theory of gravitation which
was completely Machian and used both retarded and advanced waves to communicate
gravitational influence between particles. The advanced waves, which travel
backward in time, are difficult to visualize and although they are
mathematically allowed by relativistic wave equations, they never really caught
on. The HN theory reduced to Einstein's theory of gravity in the smooth fluid
approximation and a transformation into the rest frame of the fluid.
Unfortunately the theory has been ignored by much of the General Relativity
community since it was developed with the static universe in mind. However, it
is easy to drop the static universe condition (by dropping the "C"-field matter
creation terms) and then you have a perfectly good theory of gravitation.
Hawking in 1965 pointed out a possible flaw in the theory. This involved
integrating out into the distant future to account for all the advanced waves
which might influence the mass of a particle here and now. Hawking used
infinity as his upper time limit and showed the integral was divergent. We
would like to point out that since the universe is known to be expanding, and
accelerating, the upper limit in the advanced wave time integral should not be
infinite but is bounded by the Cosmic Event Horizon. This event horizon $H_e$
represents a barrier between future events that can be observed and those which
cannot. We maintain that the advanced integral is in fact finite when the
cosmic event horizon is taken into account, since the upper limit of the
advanced wave integral becomes $H_e/c$. Hawking's objection is no longer valid
and the HN theory becomes a working theory once again.
