  The discovery that the expansion of the universe is accelerating in time is a
major discovery which still awaits adequate explanation. It is generally agreed
that this implies a cosmic repulsion as a result of the existence of a
cosmological constant . However, estimates of the cosmological constant, based
on calculations of the zero-point fluctuations of quantum fields are too large
by over a hundred orders of magnitude. This result is obtained by summing the
zero-point energies up to a large cutoff energy, based on the Planck scale.
Since there is no compelling reason for this choice, we argue that since all
known quantum electrodynamic (QED) effects involves interaction with matter, a
preferred choice should be based on causality and other considerations, leading
to a much lower value for the cosmological constant .
