  It will be argued here that the cosmological constant problem exists because
of the way the vacuum is defined in quantum field theory. It has been known for
some time that for QFT to be gauge invariant certain terms--such as part of the
vacuum polarization tensor--must be eliminated either explicitly or by some
form of regularization followed by renormalization. It has recently been shown
that lack of gauge invariance is a result of the way the vacuum is defined, and
redefining the vacuum so that the theory is gauge invariant may also offer a
solution to the cosmological constant problem.
