  Following a short discussion of some unresolved issues in the standard model
of cosmology (considered to be a generic LCDM model with flat geometry and an
early period of inflation), an update on the current state of research
regarding the problem of negative energy is provided. Arguments are then given
to the effect that traditional assumptions concerning the behavior of
negative-action matter give rise to violations of both the principle of
relativity and the principle of inertia. An alternative set of axioms is
proposed that would govern the behavior of negative-action matter if it is to
be considered a viable element of physical theories upon which cosmological
models are build. A simple framework, based on general relativity and the
proposed axioms, is elaborated which enables the formulation of quantitative
predictions concerning the interaction between positive and negative-action
bodies. Based on those developments, a solution is proposed to the problem of
the discrepancy between current experimental and theoretical values of vacuum
energy density (in any cosmological model), which may also constitute a
solution to the problem of the unexplained coincidence between the
(model-dependent) experimental values of vacuum energy density and present-day,
average, matter energy density. It is also shown how irreversibility naturally
arises in cosmological models derived in this context.
