  In 1998 astronomers discovered that the expansion of the universe is
accelerating. Somehow, something must have made gravity repulsive on
cosmological scales. This something was called dark energy; it is described by
Einstein's cosmological constant; and it amounts to about 70% of the total mass
of the universe. It has been conjectured that the cosmological constant is a
form of vacuum energy, but its prediction from quantum field theory has failed
by many orders of magnitude, until recently. Informed by empirical evidence on
Casimir forces, Lifshitz theory has not only produced the correct order of
magnitude, but is quantitatively consistent with the astronomical data.
Moreover, the theory appears to resolve the tension between the measured and
the predicted Hubble constant. There is therefore a good chance that Casimir
physics explains dark energy. This article introduces cosmology for
practitioners of vacuum forces as part of "The State of the Quantum Vacuum:
Casimir Physics in the 2020s" edited by K. A. Milton. It may also be
interesting for other physicists and engineers who wish to have a concise
introduction to cosmology.
