world line, sources of $F(i)$ lie on the null cone with apex at $O$. The null cone and the system of particles on it are called the absorber of $i$ at $O$. It was shown by Hogarth[@Hogarth] that a static, infinite Minkowski universe, as used by Wheeler and Feynman, leads to an indeterminate solution for the absorber, whereas a non-static (expanding or contracting) universe can determine whether the absorber is ideal or non-ideal. Here, ideal or non-ideal refer to whether the absorber can produce either of the solutions, Eqs. (\[retarded\]) or (\[advanced\]). The conformal invariance of Maxwell’s field equations can provide solutions of the inhomogeneous equations in a conformally flat universe. Combined with the fact that essentially all homogeneous and isotropic solutions of Einstein’s gravitational equations are conformally flat, we can test the different absorber models for certain classes of conformally flat expanding universes, including the Einstein-deSitter universe.

The interactions of the particles in the absorber can be described by assuming propagation through a medium of refractive index. Wheeler and Feynman used the well-known formula for the refractive index of a medium consisting of unbound charged particles. Hoyle and Narlikar[@Hoyle] used the theory in which the imaginary part of the 